Ambac Financial (AMBC) 10-K1 Mar 212020 FY Annual reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2020	Feb. 26, 2021	Jun. 30, 2020	Dec. 31, 2019	Apr. 30, 2013
Entity Information [Line Items]
Entity File Number	1-10777
Document Quarterly Report	true
Document Transition Report	false
City Area Code	(212)
Entity Incorporation, State or Country Code	DE
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Voluntary Filers	No
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	AMBAC FINANCIAL GROUP, INC.
Entity Central Index Key	0000874501
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		45,850,468
Entity Public Float			$ 655,986,870
Common Stock, Par or Stated Value Per Share	$ 0.01			$ 0.01	$ 0.01
Entity Tax Identification Number	13-3621676
Entity Address, Address Line One	One World Trade Center
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10007
Local Phone Number	658-7470
ICFR Auditor Attestation Flag	true
Common Stock [Member]
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	AMBC
Security Exchange Name	NYSE
Warrant [Member]
Entity Information [Line Items]
Title of 12(b) Security	Warrants
Trading Symbol	AMBC WS
Security Exchange Name	NYSE

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Estimated Fair Value	$ 2,949	$ 3,314
Other Investments	595	478
Investments, Total	3,544	3,792
Cash and cash equivalents	20	24
Restricted Cash	13	55
Premiums Receivable, Net	370	416
Deferred ceded premium	70	82
Subrogation recoverable	2,156	2,029
Derivative assets	93	75
Income Taxes Receivable, Current	0	11
Intangible assets	409	427
Other assets	114	95
Total assets	13,220	13,320
Liabilities:
Unearned premiums	456	518
Loss and loss expense reserves	1,759	1,548
Ceded premiums payable	27	29
Deferred Income Tax Liabilities, Net	24	32
Taxes Payable, Current	6	0
Long-term debt	2,739	2,822
Accrued interest payable	517	441
Derivative liabilities	114	90
Other liabilities	106	93
Variable interest entity liabilities:
Total liabilities	$ 12,074	$ 11,783
Preferred Stock, Shares Outstanding	0	0
Preferred stock, shares authorized	20,000,000	20,000,000
Stockholders’ equity:
Preferred Stock, Value, Issued	$ 0	$ 0
Common Stock, Value, Issued	0	0
Additional paid-in capital	242	232
Accumulated other comprehensive income (loss)	79	42
Retained earnings	759	1,203
Treasury Stock, Value	(1)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,080	1,477
Nonredeemable noncontrolling interest	60	60
Total stockholders’ equity	1,140	1,536
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	13,220	13,320
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Uncollectable premium receivables	17
Reinsurance Recoverables, Including Reinsurance Premium Paid	33	$ 26
Reinsurance Recoverable, Allowance for Credit Loss	$ 0
Preferred stock, par value	$ 0.01	$ 0.01
Common stock, shares issued	45,865,081	45,571,743
Common stock, shares authorized	130,000,000	130,000,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Fixed income securities, amortized cost	$ 2,807	$ 3,187
Xchange
Stockholders’ equity:
Redeemable Noncontrolling Interest, Equity, Carrying Amount	7
Variable Interest Entity [Member]
Assets:
Estimated Fair Value	3,354	3,121
Restricted Cash	2	2
Loans, at fair value	2,998	3,108
Derivative assets	41	52
Other assets	2	3
Total assets	6,398	6,286
Variable interest entity assets:
Loans, at fair value	2,998	3,108
Liabilities:
Long-term debt	4,493	4,554
Accrued interest payable	0	1
Derivative liabilities	1,835	1,657
Variable interest entity liabilities:
Total liabilities	6,328	6,212
Stockholders’ equity:
Long-term Debt	4,324	4,351
Fixed Income Securities [Member]
Assets:
Estimated Fair Value	2,317	2,577
Investment income due and accrued	10
Stockholders’ equity:
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Fixed income securities, amortized cost	2,175	2,450
Short-term Investments [Member]
Assets:
Estimated Fair Value	492	653
Collateral Pledged [Member]
Assets:
Estimated Fair Value	140
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Assets:
Estimated Fair Value	15	85
Stockholders’ equity:
Fixed income securities, amortized cost	15	0
Collateral Pledged [Member] \| Short-term Investments [Member]
Assets:
Estimated Fair Value	125	85
Stockholders’ equity:
Fixed income securities, amortized cost	$ 125	$ 85

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fixed income securities, amortized cost	$ 2,807	$ 3,187
Other Investments	$ 595	$ 478
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred Stock, Shares Outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued	45,865,081	45,571,743
Common Stock, Shares, Outstanding	45,809,139
Treasury stock, shares	55,942	16,343
Fixed Income Securities [Member]
Fixed income securities, amortized cost	$ 2,175	$ 2,450
Equity investments in pooled funds [Member]
Other Investments	543	432
Other Investment Not Readily Marketable, Fair Value	544	432
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Fixed income securities, amortized cost	15	0
Parent Company [Member]
Fixed income securities, amortized cost	76	71
Other Investments	54	46
Short-term Investments [Member]
Fixed income securities, amortized cost	492	653
Short-term Investments [Member] \| Fixed Income Securities [Member]
Fixed income securities, amortized cost		653
Short-term Investments [Member] \| Parent Company [Member]
Fixed income securities, amortized cost	$ 229	$ 318

Consolidated Statements of Tota

Consolidated Statements of Total Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Net premiums earned	$ 54,000	$ 66,000	$ 111,000
Net investment income:
Securities available-for-sale and short-term	103,000	196,000	271,000
Net investment income	122,000	227,000	273,000
Realized Investment Gains (Losses)	22,000	81,000	108,000
Derivative, Gain (Loss) on Derivative, Net	(50,000)	(50,000)	7,000
Net realized gains (losses) on extinguishment of debt	0	0	3,000
Other income	3,000	134,000	5,000
Income (loss) on variable interest entities	5,000	38,000	3,000
Total revenues	156,000	496,000	511,000
Expenses:
Losses and loss expenses (benefit)	225,000	13,000	(224,000)
Intangible amortization	57,000	295,000	107,000
Operating expenses	92,000	103,000	112,000
Interest expense	222,000	269,000	242,000
Total expenses	596,000	680,000	238,000
Pre-tax income (loss)	(440,000)	(183,000)	273,000
Provision (benefit) for income taxes	(3,000)	32,000	5,000
Net income (loss)	(437,000)	(216,000)	267,000
Net income (loss) attributable to common stockholders	(437,000)	(216,000)	186,000
Other comprehensive income (loss), after tax:
Net income (loss)	(437,000)	(216,000)	267,000
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax	1,000	0	1,000
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(3,000)	(1,000)	(2,000)
Total other comprehensive income (loss), net of income tax	37,000	91,000	6,000
Total comprehensive income (loss)	(400,000)	(125,000)	274,000
Less: comprehensive (loss) gain attributable to the noncontrolling interest:
Total comprehensive income (loss) attributable to common stockholders	(400,000)	(125,000)	192,000
AMPS Exchange Loss Based on Carry Value	$ 0	$ 0	$ 82,000
Basic	$ (9.47)	$ (4.69)	$ 4.07
Diluted	$ (9.47)	$ (4.69)	$ 3.99
Weighted Average Number of Shares Outstanding, Basic	46,147,062	45,954,908	45,665,883
Weighted Average Number of Shares Outstanding, Diluted	46,147,062	45,954,908	46,559,835

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended			66 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2017
Proceeds From Issuance of Tier II Notes										$ 0	$ 0	$ 240,000
Cash flows from operating activities:
Net income (loss) attributable to common stockholders	$ (14,000)	$ (108,000)	$ (35,000)	$ (280,000)	$ (110,000)	$ 66,000	$ (128,000)	$ (43,000)		(437,000)	(216,000)	186,000
AMPS Exchange Loss Based on Carry Value										0	0	82,000
Net income (loss)										(437,000)	(216,000)	267,000
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization										1,000	0	1,000
Amortization of bond premium and discount										(15,000)	(63,000)	(137,000)
Share-based compensation										11,000	12,000	12,000
Deferred income taxes										(9,000)	1,000	7,000
Current income taxes										17,000	35,000	(35,000)
Unearned premiums, net										(48,000)	(132,000)	(163,000)
Losses and loss expenses, net										76,000	(364,000)	(1,633,000)
Ceded premiums payable										(3,000)	(4,000)	(5,000)
Premium receivables										44,000	77,000	91,000
Accrued interest payable										93,000	87,000	9,000
Amortization of insurance intangible assets	16,000	14,000	14,000	13,000	15,000	17,000	226,000	36,000		57,000	295,000	107,000
Net realized investment gains	(2,000)	(2,000)	(10,000)	(8,000)	(9,000)	(18,000)	(36,000)	(17,000)		(22,000)	(81,000)	(108,000)
Net realized gains (losses) on extinguishment of debt										0	0	(3,000)
Other, net										59,000	79,000	68,000
Net cash used in operating activities										(175,000)	(311,000)	(1,543,000)
Cash flows from investing activities:
Proceeds from sales of bonds										1,109,000	1,212,000	1,248,000
Proceeds from matured bonds										137,000	379,000	432,000
Purchases of bonds										(975,000)	(959,000)	(528,000)
Proceeds from sales of other invested assets										374,000	81,000	159,000
Purchases of other invested assets										(475,000)	(137,000)	(140,000)
Change in short-term investments										158,000	(218,000)	127,000
Change in cash collateral receivable										0	100,000	(58,000)
Proceeds From Financial Guaranty Variable Interest Entity Assets										178,000	543,000	349,000
Payments to Acquire Businesses, Net of Cash Acquired										(74,000)
Other, net										1,000	(2,000)	0
Net cash provided by investing activities										432,000	1,000,000	1,588,000
Cash flows from financing activities:
Proceeds from Sale of Senior Surplus Notes										0	0	24,000
Paydown of Ambac Note										(121,000)	(178,000)	(214,000)
Paydown Of Variable Interest Entity Secured Borrowing										0	0	74,000
Payments for extinguishment of surplus notes										0	0	(191,000)
Payments of Debt Issuance Costs										0	0	(9,000)
AMPS Proceeds										0	19,000	0
AMPS Exchange Cash Paid									$ (11,000)	0	0	(11,000)
Payment, Tax Withholding, Share-based Payment Arrangement										(3,000)	(3,000)	(1,000)
Payments for Repurchase of Warrants													$ (8,000)
Paydown of Financial Guaranty Variable Interest Entity Liabilities										(178,000)	(542,000)	(349,000)
Net Cash Provided by (Used in) Financing Activities										(303,000)	(691,000)	(585,000)
Effect of Exchange Rate on Cash and Cash Equivalents										0	0	0
Cash and Cash Equivalents, Period Increase (Decrease)										(46,000)	(2,000)	(541,000)
Cash and cash equivalents end of period	20,000				24,000					20,000	24,000	63,000	20,000
Supplemental disclosure of cash flow information
Income Taxes Paid										11,000	21,000	35,000
Cash payments related to reorganization items:
Variable interest entity activities	(3,000)	$ 0	$ 0	$ (3,000)	(7,000)	$ (11,000)	$ (3,000)	$ (16,000)		(5,000)	(38,000)	(3,000)
Increase (Decrease) in Derivative Assets and Liabilities										6,000	(1,000)	(17,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	35,000				81,000					35,000	81,000	83,000	35,000	$ 625,000
Proceeds From Issuance of Ambac UK Debt										0	12,000	0
Ambac Financial Group, Inc Parent Company Only [Member]
Cash flows from operating activities:
Net income (loss) attributable to common stockholders										(437,000)	(216,000)	186,000
Net income (loss)										(437,000)	(216,000)	186,000
Adjustments to reconcile net income to net cash used in operating activities:
Amortization of bond premium and discount										(6,000)	(6,000)	(7,000)
Share-based compensation										11,000	12,000	12,000
Current income taxes										30,000	15,000	(15,000)
Net realized investment gains										1,000	2,000	2,000
Other, net										(10,000)	(6,000)	0
Net cash used in operating activities										11,000	16,000	32,000
Cash flows from investing activities:
Proceeds from matured bonds										46,000	86,000	230,000
Purchases of bonds										(45,000)	(2,000)	(137,000)
Change in short-term investments										89,000	(125,000)	(123,000)
Payments to Acquire Businesses, Net of Cash Acquired										(74,000)
Net cash provided by investing activities										16,000	(22,000)	(21,000)
Cash flows from financing activities:
AMPS Exchange Cash Paid										0
Cash and Cash Equivalents, Period Increase (Decrease)										(2,000)	(6,000)	11,000
Cash and cash equivalents end of period	$ 7,000				$ 9,000					$ 7,000	9,000	15,000	$ 7,000
Supplemental disclosure of cash flow information
Income Taxes Paid											$ 1,000	$ 4,000

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Common Stock Held in Treasury, at Cost [Member]	Noncontrolling Interest [Member]	Parent Company [Member]	Parent Company [Member]Retained Earnings [Member]	Parent Company [Member]AOCI Attributable to Parent [Member]	Parent Company [Member]Additional Paid-in Capital [Member]	Parent Company [Member]Common Stock Held in Treasury, at Cost [Member]
Beginning balance at Dec. 31, 2017	$ 1,645	$ 1,234	$ (52)	$ 0	$ 0	$ 200	$ 0	$ 264
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	274	267	6	0	0	0	0	0	$ 192	$ 186	$ 6
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0								0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification \| Accounting Standards Update 2016-01 [Member]		3	(3)
Cost of warrants acquired	12	0	0	0	0	12	0	0	12			$ 12
Issuance of common stock	(1)	(1)	0	0	0	0	0	0	(1)	(1)			$ 0
Ending balance at Dec. 31, 2018	1,633	1,421	(49)	0	0	219	0	41
Increase (Decrease) in Stockholders' Equity [Roll Forward]
AMPS Impact on Stockholders Equity	(297)	(82)	0	0	0	8	0	(223)	8			8
Total comprehensive income (loss)	(125)	(216)	91	0	0	0	0	0	(125)	(216)	91
Cost of warrants acquired	12	0	0	0	0	12	0	0	12			12	0
Issuance of common stock	(3)	(3)	0	0	0	0	0	0	(3)	(3)			0
Ending balance at Dec. 31, 2019	1,536	1,203	42	0	0	232	0	60
Increase (Decrease) in Stockholders' Equity [Roll Forward]
AMPS Impact on Stockholders Equity	19	0	0	0	0	0	0	19
Redeemable Noncontrolling Interest, Equity, Other, Fair Value	0
Total comprehensive income (loss)	(400)	(437)	37	0	0	0	0	0	(400)	(437)	$ 37
Cost of warrants acquired	11	0	0	0	0	11	0	0	11			$ 11
Issuance of common stock	(3)	(2)	0	0	0	0	(1)	0	$ (3)	$ (2)			$ (1)
Ending balance at Dec. 31, 2020	1,140	759	$ 79	$ 0	$ 0	$ 242	$ (1)	$ 60
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Noncontrolling Interest, Increase from Subsidiary Equity Issuance	7
Cumulative Effect of New Accounting Principle in Period of Adoption \| Accounting Standards Update 2016-13 [Member]	(4)	$ (4)
Redeemable Noncontrolling Interest, Equity, Other, Fair Value	$ 7

Leases Statement

Leases Statement - USD ($) $ in Millions	12 Months Ended
Leases Statement - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessor, Operating Lease, Term of Contract	9 years
Operating Leases, Future Minimum Payments Receivable, Remainder of Fiscal Year	$ 1
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	5
Lessee, Operating Lease, Liability, Payments, Due Year Two	5
Lessee, Operating Lease, Liability, Payments, Due Year Three	5
Lessee, Operating Lease, Liability, Payments, Due Year Four	5
Lessee, Operating Lease, Liability, Payments, Due Year Five	5
Lessee, Operating Lease, Liability, Payments, Due after Year Five	18
Lessee, Operating Lease, Liability, Payments, Due	41
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	(11)
Operating Lease, Right-of-Use Asset	25	$ 25	$ 14
Operating Lease, Payments	4	6
Operating Lease, Expense	4	7
Variable Lease, Cost	0	0
Sublease Income	(1)	(1)
Lease, Cost	4	7
Operating Lease, Liability	$ 30	$ 29
Operating Lease, Weighted Average Remaining Lease Term	8 years 8 months 12 days	9 years 10 months 24 days
Operating Lease, Weighted Average Discount Rate, Percent	7.70%	7.90%
Operating Leases, Future Minimum Payments Receivable, in Two Years	$ 1
Operating Leases, Future Minimum Payments Receivable, in Three Years	1
Operating Leases, Future Minimum Payments Receivable, in Four Years	1
Operating Leases, Future Minimum Payments Receivable, in Five Years	1
Operating Leases, Future Minimum Payments, Due Thereafter	5
Lessor, Operating Lease, Payments to be Received	11
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 0	$ 30
Minimum [Member]
Lessee Operating Lease Remaining Lease Term	1 year
Maximum [Member]
Lessee Operating Lease Remaining Lease Term	9 years

Comprehensive Income

Comprehensive Income	12 Months Ended
Comprehensive Income	Dec. 31, 2020
Equity [Abstract]
Comprehensive Income	5. COMPREHENSIVE INCOME The following tables detail the changes in the balances of each component of accumulated other comprehensive income for the affected periods: Unrealized Gains (1) Amortization of (1) Gain (Loss) on (1) Credit Risk Changes of Fair Value Option Liabilities (1) (2) Total Year Ended December 31, 2020: Beginning Balance $ 151 $ 8 $ (116) $ (2) $ 42 Other comprehensive income (loss)before reclassifications 36 (2) 23 — 58 Amounts reclassified from accumulated other comprehensive income (loss) (21) (1) — 1 (21) Net current period other comprehensive income (loss) 15 (3) 23 1 37 Balance at December 31, 2020 $ 166 $ 5 $ (92) $ — $ 79 Year ended December 31, 2019: Beginning Balance $ 86 $ 9 $ (142) $ (2) $ (49) Other comprehensive income before reclassifications 142 1 26 — 168 Amounts reclassified from accumulated other comprehensive income (76) (1) — — (78) Net current period other comprehensive income (loss) 65 (1) 26 — 91 Balance at December 31, 2019 $ 151 $ 8 $ (116) $ (2) $ 42 (1) All amounts are net of tax and noncontrolling interest. Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income. (2) Represents the changes in fair value attributable to instrument-specific credit risk of liabilities for which the fair value option is elected. The following table details the significant amounts reclassified from each component of accumulated other comprehensive income, shown in the above rollforward tables, for the affected periods: Details about Accumulated Other Amount Reclassified from Accumulated Affected Line Item in the Year Ended December 31, 2020 2019 Unrealized Gains (Losses) on Available-for-Sale Securities (1) $ (22) $ (81) Net realized investment gains (losses) 1 4 Provision for income taxes $ (21) $ (76) Net of tax and noncontrolling interest Amortization of Postretirement Benefit Prior service cost $ (1) $ (1) Other income Actuarial gains (losses) — — Other income (1) (1) Total before tax — — Provision for income taxes $ (1) $ (1) Net of tax and noncontrolling interest Credit Risk Changes of Fair Value Option Liabilities $ 2 $ — Credit risk changes of fair value option liabilities — — Provision for income taxes 1 — Net of tax and noncontrolling interest Total reclassifications for the period $ (21) $ (78) Net of tax and noncontrolling interest

Fair Value Measurements

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	10. FAIR VALUE MEASUREMENTS The Fair Value Measurement Topic of the ASC establishes a framework for measuring fair value and disclosures about fair value measurements. Fair Value Hierarchy The Fair Value Measurement Topic of the ASC specifies a fair value hierarchy based on whether the inputs to valuation techniques used to measure fair value are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect Company-based assumptions. The fair value hierarchy prioritizes model inputs into three broad levels as follows: l Level 1 Quoted prices for identical instruments in active markets. Assets and liabilities classified as Level 1 include US Treasury and other foreign government obligations traded in highly liquid and transparent markets, certain highly liquid pooled fund investments, exchange traded futures contracts, variable rate demand obligations and money market funds. l Level 2 Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets. Assets and liabilities classified as Level 2 generally include investments in fixed maturity securities representing municipal, asset-backed and corporate obligations, certain interest rate swap contracts and most long-term debt of variable interest entities consolidated under the Consolidation Topic of the ASC. l Level 3 Model derived valuations in which one or more significant inputs or significant value drivers are unobservable. This hierarchy requires the use of observable market data when available. Assets and liabilities classified as Level 3 include credit derivative contracts, certain uncollateralized interest rate swap contracts, equity interests in Ambac sponsored special purpose entities and certain investments in fixed maturity securities. Additionally, Level 3 assets and liabilities generally include loan receivables, and certain long-term debt of variable interest entities consolidated under the Consolidation Topic of the ASC. The Fair Value Measurement Topic of the ASC permits, as a practical expedient, the estimation of fair value of certain investments in funds using the net asset value per share of the investment or its equivalent (“NAV”). Investments in funds valued using NAV are not categorized as Level 1, 2 or 3 under the fair value hierarchy. The following table sets forth the carrying amount and fair value of Ambac’s financial assets and liabilities as of December 31, 2020 and 2019, including the level within the fair value hierarchy at which fair value measurements are categorized. As required by the Fair Value Measurement Topic of the ASC financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Carrying Total Fair Fair Value Measurements Categorized as: Level 1 Level 2 Level 3 December 31, 2020: Financial assets: Fixed maturity securities: Municipal obligations $ 358 $ 358 $ — $ 358 $ — Corporate obligations 1,077 1,077 4 1,073 — Foreign obligations 98 98 98 — — U.S. government obligations 106 106 106 — — Residential mortgage-backed securities 302 302 — 302 — Collateralized debt obligations 74 74 — 74 — Other asset-backed securities 303 303 — 225 78 Fixed maturity securities, pledged as collateral: U.S. government obligations 15 15 15 — — Short-term 125 125 125 — — Short term investments 492 492 415 76 — Other investments (1) 595 597 91 — 53 Cash, cash equivalents and restricted cash 33 33 32 2 — Derivative assets: Interest rate swaps—asset position 93 93 — 9 85 Other assets - equity in sponsored VIE 1 1 — — 1 Other assets-Loans 3 3 — — 3 Variable interest entity assets: Fixed maturity securities: Corporate obligations 3,215 3,215 — — 3,215 Fixed maturity securities: Municipal obligations 139 139 — 139 — Restricted cash 2 2 2 — — Loans 2,998 2,998 — — 2,998 Derivative assets: Currency swaps-asset position 41 41 — 41 — Total financial assets $ 10,071 $ 10,073 $ 888 $ 2,299 $ 6,433 Financial liabilities: Long term debt, including accrued interest $ 3,255 $ 3,071 $ — $ 2,670 $ 401 Derivative liabilities: Interest rate swaps—liability position 114 114 — 114 — Liabilities for net financial guarantees written (2) (740) 539 — — 539 Variable interest entity liabilities: Long-term debt (includes $4,324 at fair value) 4,493 4,504 — 4,349 155 Derivative liabilities: Interest rate swaps—liability position 1,835 1,835 — 1,835 — Total financial liabilities $ 8,958 $ 10,063 $ — $ 8,968 $ 1,095 Carrying Total Fair Fair Value Measurements Categorized as: Level 1 Level 2 Level 3 December 31, 2019: Financial assets: Fixed maturity securities: Municipal obligations $ 215 $ 215 $ — $ 215 $ — Corporate obligations 1,430 1,430 — 1,430 — Foreign obligations 44 44 44 — — U.S. government obligations 156 156 156 — — Residential mortgage-backed securities 248 248 — 248 — Commercial mortgage-backed securities 50 50 — 50 — Collateralized debt obligations 146 146 — 146 — Other asset-backed securities 287 287 — 215 72 Fixed maturity securities, pledged as collateral: Short-term 85 85 85 — — Short term investments 653 653 598 55 — Other investments (1) 478 493 136 — 61 Cash and cash equivalents and restricted cash 79 79 70 9 — Derivative assets: Interest rate swaps—asset position 75 75 — 8 67 Other assets - equity in sponsored VIE 3 3 — — 3 Other assets-loans 10 13 — — 13 Variable interest entity assets: Fixed maturity securities: Corporate obligations 2,957 2,957 — — 2,957 Fixed maturity securities: Municipal obligations 164 164 — 164 — Restricted cash 2 2 2 — — Loans 3,108 3,108 — — 3,108 Derivative assets; Currency swaps-asset position 52 52 — 52 — Total financial assets $ 10,242 $ 10,260 $ 1,091 $ 2,593 $ 6,281 Financial liabilities: Long term debt, including accrued interest $ 3,262 $ 3,274 $ — $ 2,829 $ 445 Derivative liabilities: Credit derivatives — — — — — Interest rate swaps—liability position 89 89 — 89 — Liabilities for net financial guarantees written (2) (863) 284 — — 284 Variable interest entity liabilities: Long-term debt (includes $4,351 at fair value) 4,554 4,567 — 4,408 159 Derivative liabilities: Interest rate swaps—liability position 1,657 1,657 — 1,657 — Total financial liabilities $ 8,699 $ 9,872 — 8,983 889 (1) Excluded from the fair value measurement categories in the table above are investment funds of $453 and $296 as of December 31, 2020 and 2019, respectively, which are measured using NAV as a practical expedient. (2) The carrying value of net financial guarantees written includes the following balance sheet items: Premium receivables; Reinsurance recoverable on paid and unpaid losses; Deferred ceded premium; Subrogation recoverable; Insurance intangible asset; Unearned premiums; Loss and loss expense reserves; Ceded premiums payable, premiums taxes payable and other deferred fees recorded in Other liabilities. Determination of Fair Value When available, Ambac uses quoted active market prices specific to the financial instrument to determine fair value and classifies such items within Level 1. The determination of fair value for financial instruments categorized in Level 2 or 3 involves judgment due to the complexity of factors contributing to the valuation. Third-party sources from which we obtain independent market quotes also use assumptions, judgments and estimates in determining financial instrument values and different third parties may use different methodologies or provide different values for financial instruments. In addition, the use of internal valuation models may require assumptions about hypothetical or inactive markets. As a result of these factors, the actual trade value of a financial instrument in the market, or exit value of a financial instrument position by Ambac, may be significantly different from its recorded fair value. Ambac’s financial instruments carried at fair value are mainly comprised of investments in fixed maturity securities, equity interests in pooled investment funds, derivative instruments, certain variable interest entity assets and liabilities and interests in Ambac sponsored special purpose entities. Valuation of financial instruments is performed by Ambac’s finance group using methods approved by senior financial management with consultation from risk management and portfolio managers as appropriate. Preliminary valuation results are discussed with portfolio managers quarterly to assess consistency with market transactions and trends as applicable. Market transactions such as trades or negotiated settlements of similar positions, if any, are reviewed to validate fair value model results. However many of the financial instruments valued using significant unobservable inputs have very little or no observable market activity. Methods and significant inputs and assumptions used to determine fair values across portfolios are reviewed quarterly by senior financial management. Other valuation control procedures specific to particular portfolios are described further below. Fixed Maturity Securities The fair values of fixed maturity investment securities are based primarily on market prices received from broker quotes or alternative pricing sources. Because many fixed maturity securities do not trade on a daily basis, pricing sources apply available market information through processes such as matrix pricing to calculate fair value. Such prices generally consider a variety of factors, including recent trades of the same and similar securities. In those cases, the items are classified within Level 2. For those fixed maturity investments where quotes were not available or cannot be reasonably corroborated, fair values are based on internal valuation models. Key inputs to the internal valuation models generally include maturity date, coupon and yield curves for asset-type and credit rating characteristics that closely match those characteristics of the specific investment securities being valued. Items valued using valuation models are classified according to the lowest level input or value driver that is significant to the valuation. Thus, an item may be classified in Level 3 even though there may be significant inputs that are readily observable. Longer (shorter) expected maturities or higher (lower) yields used in the valuation model will, in isolation, result in decreases (increases) in fair value. Generally, lower credit ratings or longer expected maturities will be accompanied by higher yields used to value a security. At December 31, 2020, approximately 2%, 95%, and 3% of the fixed maturity investment portfolio (excluding variable interest entity investments) was valued using broker quotes, alternative pricing sources and internal valuation models, respectively. At December 31, 2019, approximately 4%, 94%, and 2% of the fixed maturity investment portfolio (excluding variable interest entity investments) was valued using broker quotes, alternative pricing sources and internal valuation models, respectively. Ambac performs various review and validation procedures to quoted and modeled prices for fixed maturity securities, including price variance analyses, missing and static price reviews, overall valuation analysis by portfolio managers and finance managers and reviews associated with our ongoing impairment analysis. Unusual prices identified through these procedures will be evaluated further against alternative third-party quotes (if available, internally modeled prices and/or other relevant data, and the pricing source values will be challenged as necessary. Price challenges generally result in the use of the pricing source’s quote as originally provided or as revised by the source following their internal diligence process. A price challenge may result in a determination by either the pricing source or Ambac management that the pricing source cannot provide a reasonable value for a security or cannot adequately support a quote, in which case Ambac would resort to using either other quotes or internal models. Results of price challenges are reviewed by portfolio managers and finance managers. Information about the valuation inputs for fixed maturity securities classified as Level 3 is included below: Other asset-backed securities: This security is a subordinated tranche of a securitization collateralized by Ambac-insured military housing bonds. The fair value classified as Level 3 was $78 and $72 at December 31, 2020 and 2019, respectively. Fair value was calculated using a discounted cash flow approach with expected future cash flows discounted using a yield consistent with the security type and rating. Significant inputs for the valuation at December 31, 2020 and 2019 include the following: December 31, 2020: a. Coupon rate 5.97% b. Average Life 14.83 years c. Yield 10.50% December 31, 2019: a. Coupon rate 5.97% b. Average Life 15.58 years c. Yield 11.75% Other Investments Other investments primarily relate to investments in pooled investment funds. The fair value of pooled investment funds is determined using dealer quotes or alternative pricing sources when such investments have readily determinable fair values. When fair value is not readily determinable, pooled investment funds are valued using NAV as a practical expedient as permitted under the Fair Value Measurement Topic of the ASC. Refer to Note 11. Investments for additional information about such investments in pooled funds that are reported at fair value using NAV as a practical expedient. Other investments also includes Ambac's equity interest in a non-consolidated VIE created in connection with Ambac's monetization of AAC junior surplus notes. This equity interest is carried under the equity method. Fair value for the non- consolidated VIE equity interest is internally calculated using a market approach and is classified as Level 3. Derivative Instruments Ambac’s derivative instruments primarily comprise interest rate swaps, credit default swaps and exchange traded futures contracts. Fair value is determined based upon market quotes from independent sources, when available. When independent quotes are not available, fair value is determined using valuation models. These valuation models require market-driven inputs, including contractual terms, credit spreads and ratings on underlying referenced obligations, yield curves and tax-exempt interest ratios. The valuation of certain derivative contracts also require the use of data inputs and assumptions that are determined by management and are not readily observable in the market. Under the Fair Value Measurement Topic of the ASC, Ambac is required to consider its own credit risk when measuring the fair value of derivatives and other liabilities. Factors considered in estimating the amount of any Ambac credit valuation adjustment ("CVA") on such contracts include collateral posting provisions, right of set-off with the counterparty, the period of time remaining on the derivative and the pricing of recent terminations. The aggregate Ambac CVA impact reduced the fair value of derivative liabilities by less than a million dollars at both December 31, 2020 and 2019. Interest rate swaps that are not centrally cleared are valued using vendor-developed models that incorporate interest rates and yield curves that are observable and regularly quoted. These models provide the net present value of the derivatives based on contractual terms and observable market data. Generally, the need for counterparty (or Ambac) CVAs on interest rate derivatives is mitigated by the existence of collateral posting agreements under which adequate collateral has been posted. Certain of these derivative contracts entered into with financial guarantee customers are not subject to collateral posting agreements. Counterparty credit risk related to such customer derivative assets is included in our determination of their fair value. Ambac's credit derivatives ("CDS") are valued using an internal model that uses traditional financial guarantee CDS pricing to calculate the fair value of the derivative contract based on the reference obligation's current pricing, remaining life and credit rating and Ambac's own credit risk. The model calculates the difference between the present value of the projected fees receivable under the CDS and our estimate of the fees a financial guarantor of comparable credit quality would charge to provide the same protection at the balance sheet date. Unobservable inputs used include Ambac's internal reference obligation credit ratings and expected life, estimates of fees that would be charged to assume the credit derivative obligation and Ambac's CVA. Ambac is party to only one remaining credit derivative with an internal credit rating of AA at December 31, 2020. Ambac has not made any significant changes to its modeling techniques or related model inputs for the periods presented. Financial Guarantees Fair value of net financial guarantees written represents our estimate of the cost to Ambac to completely transfer its insurance obligation to another market participant of comparable credit worthiness. In theory, this amount should be the same amount that another market participant of comparable credit worthiness would hypothetically charge in the marketplace, on a present value basis, to provide the same protection as of the balance sheet date. This fair value estimate of financial guarantees is presented on a net basis and includes direct and assumed contracts written, net of ceded reinsurance contracts. Long-term Debt Long-term debt includes AAC surplus notes and junior surplus notes, the Ambac Note and Tier 2 Notes issued in connection with the Rehabilitation Exit Transactions and the Ambac UK debt issued in connection with the Ballantyne commutation. The fair values of surplus notes, the Ambac Note and Tier 2 Notes are classified as Level 2. The fair value of junior surplus notes and Ambac UK debt are classified as Level 3. Other Financial Assets and Liabilities Included in Other assets are Loans and Ambac’s equity interest in an Ambac sponsored VIE established to provide certain financial guarantee clients with funding for their debt obligations. The fair values of these financial assets are estimated based upon internal valuation models and are classified as Level 3. Variable Interest Entity Assets and Liabilities The financial assets and liabilities of FG VIEs consolidated under the Consolidation Topic of the ASC consist primarily of fixed maturity securities and loans held by the VIEs, derivative instruments and notes issued by the VIEs which are reported as long-term debt. As described in Note 4. Variable Interest Entities, these FG VIEs are securitization entities which have liabilities and/or assets guaranteed by AAC or Ambac UK. The fair values of FG VIE long-term debt are based on price quotes received from independent market sources when available. Such quotes are considered Level 2 and generally consider a variety of factors, including recent trades of the same and similar securities. For those instruments where quotes were not available or cannot be reasonably corroborated, fair values are based on internal valuation models. Comparable to the sensitivities of investments in fixed maturity securities described above, longer (shorter) expected maturities or higher (lower) yields used in the valuation model will, in isolation, result in decreases (increases) in fair value liability measurement for FG VIE long-term debt. FG VIE derivative asset and liability fair values are determined using vendor-developed valuation models, which incorporated observable market data related to specific derivative contractual terms including interest rates, foreign exchange rates and yield curves. The fair value of FG VIE fixed maturity securities and loan assets are based on Level 2 market price quotes received from independent market sources when available. Typically, FG VIE asset fair values are not readily available from market quotes and are estimated internally. Internal valuation of each FG VIE’s fixed maturity securities or loan assets are derived from the fair values of the notes issued by the respective VIE and the VIE’s derivatives, determined as described above, adjusted for the fair values of Ambac’s financial guarantees associated with the VIE. The fair value of financial guarantees consist of: (i) estimated future premium cash flows discounted at a rate consistent with that implicit in the fair value of the VIE’s liabilities and (ii) estimates of future claim payments discounted at a rate that includes Ambac’s own credit risk. Estimated future premium payments to be paid by the VIEs were discounted at a par-weighted average rate of 2.4% and 2.7% at December 31, 2020 and 2019, respectively. At December 31, 2020, the range of these discount rates was between 1.8% and 3.9%. Additional Fair Value Information for Financial Assets and Liabilities Accounted for at Fair Value The following tables present the changes in the Level 3 fair value category for the periods presented in 2020, 2019 and 2018. Ambac classifies financial instruments in Level 3 of the fair value hierarchy when there is reliance on at least one significant unobservable input to the valuation model. In addition to these unobservable inputs, the valuation models for Level 3 financial instruments typically also rely on a number of inputs that are readily observable either directly or indirectly. Thus, the gains and losses presented below include changes in the fair value related to both observable and unobservable inputs. Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2020 Investments Other (1) Derivatives Investments Loans Long-term Total Balance, beginning of period $ 72 $ 3 $ 66 $ 2,957 $ 3,108 $ — $ 6,207 Total gains/(losses) realized and unrealized: Included in earnings 1 (2) 25 183 98 — 306 Included in other comprehensive income 6 — — 109 83 — 198 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (1) — (7) (35) (290) — (334) Balance, end of period $ 78 $ 1 $ 84 $ 3,215 $ 2,998 $ — $ 6,376 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (2) $ 25 $ 183 $ 98 $ — $ 304 The amount of total gains/(losses) included in other comprehensive income attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ 109 $ 83 $ — $ 192 Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2019 Investments Other Derivatives Investments Loans Long-term Total Balance, beginning of period $ 72 $ 5 $ 46 $ 2,737 $ 4,288 $ (217) $ 6,930 Total gains/(losses) realized and unrealized: Included in earnings 2 (2) 25 138 287 (15) 436 Included in other comprehensive income — — — 116 74 8 199 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (2) — (5) (35) (690) — (731) Deconsolidations of VIEs — — — — (851) 223 (627) Balance, end of period $ 72 $ 3 $ 66 $ 2,957 $ 3,108 $ — $ 6,207 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (2) $ 25 $ 138 $ 215 $ — $ 376 (1) Other assets carried at fair value and classified as Level 3 relate to an equity interest in an Ambac sponsored VIE. Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2018 Investments Other Derivatives Investments Loans Long-term Total Balance, beginning of period $ 809 $ 6 $ 61 $ 2,914 $ 11,529 $ (2,758) $ 12,561 Total gains/(losses) realized and unrealized: Included in earnings 36 (1) (9) 16 (201) 189 30 Included in other comprehensive income (53) — — (158) (470) 91 (590) Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (714) — (6) (35) (624) 23 (1,356) Transfers out of Level 3 (5) — — — — — (5) Deconsolidation of VIEs — — — — (5,946) 2,237 (3,709) Balance, end of period $ 72 $ 5 $ 46 $ 2,737 $ 4,288 $ (217) $ 6,930 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (1) $ (10) $ 16 $ (63) $ 47 $ (11) The tables below provide roll-forward information by class of investments and derivatives measured using significant unobservable inputs. Level-3 Investments by Class 2020 2019 Year Ended December 31, Other Asset Non-Agency RMBS Total Other Asset Non-Agency RMBS Total Balance, beginning of period $ 72 $ — $ 72 $ 72 $ — $ 72 Total gains/(losses) realized and unrealized: Included in earnings 1 — 1 2 — 2 Included in other comprehensive income 6 — 6 — — — Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (1) — (1) (2) — (2) Balance, end of period $ 78 $ — $ 78 $ 72 $ — $ 72 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ — $ — $ — Level-3 Investments by Class Year Ended December 31, 2018 Other Asset Non-Agency RMBS Total Balance, beginning of period $ 73 $ 736 $ 809 Total gains/(losses) realized and unrealized: Included in earnings 1 35 36 Included in other comprehensive income (1) (52) (53) Purchases — — — Issuances — — — Sales — — — Settlements (1) (713) (714) Transfers out of Level 3 — (5) (5) Balance, end of period $ 72 $ — $ 72 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — Level-3 Derivatives by Class 2020 2019 Year Ended December 31, Interest Credit Total Interest Credit Total Balance, beginning of period $ 67 $ — $ 66 $ 47 $ (1) $ 46 Total gains/(losses) realized and unrealized: Included in earnings 25 — 25 24 2 25 Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (7) — (7) (4) — (5) Balance, end of period 85 — 84 67 — 66 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date 25 — 25 24 1 25 Level-3 Derivatives by Class Year Ended December 31, 2018 Interest Credit Total Balance, beginning of period $ 61 $ (1) $ 61 Total gains/(losses) realized and unrealized: Included in earnings (9) (1) (9) Purchases — — — Issuances — — — Sales — — — Settlements (5) — (6) Balance, end of period $ 47 $ (1) $ 46 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ (9) $ (1) $ (10) Invested assets and VIE long-term debt are transferred into Level 3 when internal valuation models that include significant unobservable inputs are used to estimate fair value. All such securities that have internally modeled fair values have been classified as Level 3. Derivative instruments are transferred into Level 3 when the use of unobservable inputs becomes significant to the overall valuation. Invested assets transferred out of Level 3 into Level 2 in 2018 consisted of an Ambac-insured re-REMIC collateralized by distressed mortgage-backed securities. There were no other transfers of financial instruments into or out of Level 3 in the periods disclosed. Gains and losses (realized and unrealized) relating to Level 3 assets and liabilities included in earnings for the affected periods are reported as follows: Net Investment Income Net Gains Income (Loss) on Variable Interest Entities Other Year Ended December 31, 2020 Total gains (losses) included in earnings for the period $ 1 $ 25 $ 281 $ (2) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — 25 281 (2) Year Ended December 31, 2019 Total gains (losses) included in earnings for the period $ 2 $ 25 $ 410 $ (2) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — 25 353 (2) Year Ended December 31, 2018 Total gains (losses) included in earnings for the period $ 36 $ (9) $ 4 $ (1) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — (10) — (1)

Leases (Notes)

Leases (Notes)	12 Months Ended
Leases (Notes)	Dec. 31, 2020
Leases of Lessee Disclosure [Text Block]	16. LEASES Ambac adopted the New Lease Standard, as defined and further described in Note 2. Basis of Presentation and Significant Accounting Policies on January 1, 2019. Ambac is the lessee and lessor for certain lease agreements further described below. Lessee information Ambac is the lessee in operating leases for corporate offices, a data center and equipment. Ambac's purchase of Xchange resulted in additional office and equipment leases. Leases in effect at December 31, 2020, have remaining lease terms ranging from less than 1 year to 9 years. Our data center lease has an automatic renewal of one-year unless either party elects to terminate by providing 120 days notice prior to the renewal. This renewal feature is not recognized in the lease liability or right-of-use asset as it is not reasonably certain we will elect to renew. An office lease related to Xchange includes a one-time early termination provision. The lease liability and right-of-use asset on this lease consider its full term as Ambac does not reasonably expect to exercise the early termination option. No other leases contain extension or termination provisions. Lease costs are included in operating expenses on the Consolidated Statement of Total Comprehensive Income (Loss). The components of lease costs, net of sub-lessor income, is as follows: Year Ended December 31, 2020 2019 Operating lease cost $ 4 $ 7 Variable lease cost — — Sublease income (1) (1) Total lease cost $ 4 $ 7 Ambac is required to make variable lease payments under certain leases which primarily relates to variable costs of the lessor, such as taxes, insurance, maintenance and electricity. Supplemental information related to leases is as follows: Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 4 $ 6 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) (1) — 30 (1) Includes right-of-use assets of $14 for the year ended December 31, 2019 for leases which existed prior to the New Lease Standard implementation date of January 1, 2019. Supplemental balance sheet information related to leases is as follows: December 31, 2020 2019 Operating leases: Operating lease right of use assets $ 25 $ 25 Operating lease liabilities 30 29 Weighted average remaining lease term: Operating leases 8.7 years 9.9 years Weighted average discount rate: Operating leases 7.7 % 7.9 % Operating lease right of use assets and operating lease liabilities are included in Other assets and Other liabilities, respectively, on the consolidated balance sheet. Future undiscounted lease payments, gross of sublease receipts, to be made are as follows: As of December 31, 2020 Operating Leases 2021 $ 5 2022 5 2023 5 2024 5 2025 5 Thereafter 18 Total lease payments 41 Less: imputed interest (11) Total $ 30 Lessor information Ambac is the lessor in one operating sublease of corporate office space which has a remaining term of 9.0 years. There are no extension or termination provisions. Future undiscounted lease payments to be received are as follows: As of December 31, 2020 Operating Leases 2021 $ 1 2022 1 2023 1 2024 1 2025 1 Thereafter 5 Total lease receipts $ 11

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Cash Flow Supplemental Information [Line Items]
Schedule of Supplemental Cash Flow Information [Table Text Block]	Supplemental Disclosure of Cash Flow Information Year Ended December 31, 2020 2019 2018 Cash paid during the period for: Income taxes $ 11 $ 21 $ 35 Interest on long-term debt 107 143 232 Non-cash investing and financing activities: Increase in long-term debt in exchange for AMPS — — 187 Exchange of investments in Puerto Rico COFINA bonds for new bonds issued in the Plan of Adjustment — 510 — Decrease in long-term debt as a result of an exchange for investment securities — — — Rehabilitation exit transaction discharge of all Deferred Amounts and cancellation of certain senior surplus notes — — 1,919 December 31, 2020 2019 2018 Reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets to the Consolidated Statements of Cash Flow: Cash and cash equivalents $ 20 $ 24 $ 63 Restricted cash 13 55 19 Variable Interest Entity Restricted cash 2 2 1 Total cash, cash equivalents, and restricted cash shown on the Consolidated Statements of Cash Flows 35 81 83
Cash Flow, Supplemental Disclosures [Text Block]	Supplemental information related to leases is as follows: Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 4 $ 6 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) (1) — 30 (1) Includes right-of-use assets of $14 for the year ended December 31, 2019 for leases which existed prior to the New Lease Standard implementation date of January 1, 2019.

Comprehensive Income (Tables)

Comprehensive Income (Tables)	12 Months Ended
Comprehensive Income (Tables)	Dec. 31, 2020
Equity [Abstract]
Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income	The following tables detail the changes in the balances of each component of accumulated other comprehensive income for the affected periods: Unrealized Gains (1) Amortization of (1) Gain (Loss) on (1) Credit Risk Changes of Fair Value Option Liabilities (1) (2) Total Year Ended December 31, 2020: Beginning Balance $ 151 $ 8 $ (116) $ (2) $ 42 Other comprehensive income (loss)before reclassifications 36 (2) 23 — 58 Amounts reclassified from accumulated other comprehensive income (loss) (21) (1) — 1 (21) Net current period other comprehensive income (loss) 15 (3) 23 1 37 Balance at December 31, 2020 $ 166 $ 5 $ (92) $ — $ 79 Year ended December 31, 2019: Beginning Balance $ 86 $ 9 $ (142) $ (2) $ (49) Other comprehensive income before reclassifications 142 1 26 — 168 Amounts reclassified from accumulated other comprehensive income (76) (1) — — (78) Net current period other comprehensive income (loss) 65 (1) 26 — 91 Balance at December 31, 2019 $ 151 $ 8 $ (116) $ (2) $ 42 (1) All amounts are net of tax and noncontrolling interest. Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income. (2) Represents the changes in fair value attributable to instrument-specific credit risk of liabilities for which the fair value option is elected.
Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income	The following table details the significant amounts reclassified from each component of accumulated other comprehensive income, shown in the above rollforward tables, for the affected periods: Details about Accumulated Other Amount Reclassified from Accumulated Affected Line Item in the Year Ended December 31, 2020 2019 Unrealized Gains (Losses) on Available-for-Sale Securities (1) $ (22) $ (81) Net realized investment gains (losses) 1 4 Provision for income taxes $ (21) $ (76) Net of tax and noncontrolling interest Amortization of Postretirement Benefit Prior service cost $ (1) $ (1) Other income Actuarial gains (losses) — — Other income (1) (1) Total before tax — — Provision for income taxes $ (1) $ (1) Net of tax and noncontrolling interest Credit Risk Changes of Fair Value Option Liabilities $ 2 $ — Credit risk changes of fair value option liabilities — — Provision for income taxes 1 — Net of tax and noncontrolling interest Total reclassifications for the period $ (21) $ (78) Net of tax and noncontrolling interest

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
Fair Value Measurements (Tables)	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities	The following table sets forth the carrying amount and fair value of Ambac’s financial assets and liabilities as of December 31, 2020 and 2019, including the level within the fair value hierarchy at which fair value measurements are categorized. As required by the Fair Value Measurement Topic of the ASC financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Carrying Total Fair Fair Value Measurements Categorized as: Level 1 Level 2 Level 3 December 31, 2020: Financial assets: Fixed maturity securities: Municipal obligations $ 358 $ 358 $ — $ 358 $ — Corporate obligations 1,077 1,077 4 1,073 — Foreign obligations 98 98 98 — — U.S. government obligations 106 106 106 — — Residential mortgage-backed securities 302 302 — 302 — Collateralized debt obligations 74 74 — 74 — Other asset-backed securities 303 303 — 225 78 Fixed maturity securities, pledged as collateral: U.S. government obligations 15 15 15 — — Short-term 125 125 125 — — Short term investments 492 492 415 76 — Other investments (1) 595 597 91 — 53 Cash, cash equivalents and restricted cash 33 33 32 2 — Derivative assets: Interest rate swaps—asset position 93 93 — 9 85 Other assets - equity in sponsored VIE 1 1 — — 1 Other assets-Loans 3 3 — — 3 Variable interest entity assets: Fixed maturity securities: Corporate obligations 3,215 3,215 — — 3,215 Fixed maturity securities: Municipal obligations 139 139 — 139 — Restricted cash 2 2 2 — — Loans 2,998 2,998 — — 2,998 Derivative assets: Currency swaps-asset position 41 41 — 41 — Total financial assets $ 10,071 $ 10,073 $ 888 $ 2,299 $ 6,433 Financial liabilities: Long term debt, including accrued interest $ 3,255 $ 3,071 $ — $ 2,670 $ 401 Derivative liabilities: Interest rate swaps—liability position 114 114 — 114 — Liabilities for net financial guarantees written (2) (740) 539 — — 539 Variable interest entity liabilities: Long-term debt (includes $4,324 at fair value) 4,493 4,504 — 4,349 155 Derivative liabilities: Interest rate swaps—liability position 1,835 1,835 — 1,835 — Total financial liabilities $ 8,958 $ 10,063 $ — $ 8,968 $ 1,095 Carrying Total Fair Fair Value Measurements Categorized as: Level 1 Level 2 Level 3 December 31, 2019: Financial assets: Fixed maturity securities: Municipal obligations $ 215 $ 215 $ — $ 215 $ — Corporate obligations 1,430 1,430 — 1,430 — Foreign obligations 44 44 44 — — U.S. government obligations 156 156 156 — — Residential mortgage-backed securities 248 248 — 248 — Commercial mortgage-backed securities 50 50 — 50 — Collateralized debt obligations 146 146 — 146 — Other asset-backed securities 287 287 — 215 72 Fixed maturity securities, pledged as collateral: Short-term 85 85 85 — — Short term investments 653 653 598 55 — Other investments (1) 478 493 136 — 61 Cash and cash equivalents and restricted cash 79 79 70 9 — Derivative assets: Interest rate swaps—asset position 75 75 — 8 67 Other assets - equity in sponsored VIE 3 3 — — 3 Other assets-loans 10 13 — — 13 Variable interest entity assets: Fixed maturity securities: Corporate obligations 2,957 2,957 — — 2,957 Fixed maturity securities: Municipal obligations 164 164 — 164 — Restricted cash 2 2 2 — — Loans 3,108 3,108 — — 3,108 Derivative assets; Currency swaps-asset position 52 52 — 52 — Total financial assets $ 10,242 $ 10,260 $ 1,091 $ 2,593 $ 6,281 Financial liabilities: Long term debt, including accrued interest $ 3,262 $ 3,274 $ — $ 2,829 $ 445 Derivative liabilities: Credit derivatives — — — — — Interest rate swaps—liability position 89 89 — 89 — Liabilities for net financial guarantees written (2) (863) 284 — — 284 Variable interest entity liabilities: Long-term debt (includes $4,351 at fair value) 4,554 4,567 — 4,408 159 Derivative liabilities: Interest rate swaps—liability position 1,657 1,657 — 1,657 — Total financial liabilities $ 8,699 $ 9,872 — 8,983 889 (1) Excluded from the fair value measurement categories in the table above are investment funds of $453 and $296 as of December 31, 2020 and 2019, respectively, which are measured using NAV as a practical expedient. (2) The carrying value of net financial guarantees written includes the following balance sheet items: Premium receivables; Reinsurance recoverable on paid and unpaid losses; Deferred ceded premium; Subrogation recoverable; Insurance intangible asset; Unearned premiums; Loss and loss expense reserves; Ceded premiums payable, premiums taxes payable and other deferred fees recorded in Other liabilities.
Summary of Changes in Level 3 Fair Value Category	The following tables present the changes in the Level 3 fair value category for the periods presented in 2020, 2019 and 2018. Ambac classifies financial instruments in Level 3 of the fair value hierarchy when there is reliance on at least one significant unobservable input to the valuation model. In addition to these unobservable inputs, the valuation models for Level 3 financial instruments typically also rely on a number of inputs that are readily observable either directly or indirectly. Thus, the gains and losses presented below include changes in the fair value related to both observable and unobservable inputs. Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2020 Investments Other (1) Derivatives Investments Loans Long-term Total Balance, beginning of period $ 72 $ 3 $ 66 $ 2,957 $ 3,108 $ — $ 6,207 Total gains/(losses) realized and unrealized: Included in earnings 1 (2) 25 183 98 — 306 Included in other comprehensive income 6 — — 109 83 — 198 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (1) — (7) (35) (290) — (334) Balance, end of period $ 78 $ 1 $ 84 $ 3,215 $ 2,998 $ — $ 6,376 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (2) $ 25 $ 183 $ 98 $ — $ 304 The amount of total gains/(losses) included in other comprehensive income attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ 109 $ 83 $ — $ 192 Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2019 Investments Other Derivatives Investments Loans Long-term Total Balance, beginning of period $ 72 $ 5 $ 46 $ 2,737 $ 4,288 $ (217) $ 6,930 Total gains/(losses) realized and unrealized: Included in earnings 2 (2) 25 138 287 (15) 436 Included in other comprehensive income — — — 116 74 8 199 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (2) — (5) (35) (690) — (731) Deconsolidations of VIEs — — — — (851) 223 (627) Balance, end of period $ 72 $ 3 $ 66 $ 2,957 $ 3,108 $ — $ 6,207 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (2) $ 25 $ 138 $ 215 $ — $ 376 (1) Other assets carried at fair value and classified as Level 3 relate to an equity interest in an Ambac sponsored VIE. Level-3 Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Year Ended December 31, 2018 Investments Other Derivatives Investments Loans Long-term Total Balance, beginning of period $ 809 $ 6 $ 61 $ 2,914 $ 11,529 $ (2,758) $ 12,561 Total gains/(losses) realized and unrealized: Included in earnings 36 (1) (9) 16 (201) 189 30 Included in other comprehensive income (53) — — (158) (470) 91 (590) Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (714) — (6) (35) (624) 23 (1,356) Transfers out of Level 3 (5) — — — — — (5) Deconsolidation of VIEs — — — — (5,946) 2,237 (3,709) Balance, end of period $ 72 $ 5 $ 46 $ 2,737 $ 4,288 $ (217) $ 6,930 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (1) $ (10) $ 16 $ (63) $ 47 $ (11) The tables below provide roll-forward information by class of investments and derivatives measured using significant unobservable inputs. Level-3 Investments by Class 2020 2019 Year Ended December 31, Other Asset Non-Agency RMBS Total Other Asset Non-Agency RMBS Total Balance, beginning of period $ 72 $ — $ 72 $ 72 $ — $ 72 Total gains/(losses) realized and unrealized: Included in earnings 1 — 1 2 — 2 Included in other comprehensive income 6 — 6 — — — Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (1) — (1) (2) — (2) Balance, end of period $ 78 $ — $ 78 $ 72 $ — $ 72 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ — $ — $ — Level-3 Investments by Class Year Ended December 31, 2018 Other Asset Non-Agency RMBS Total Balance, beginning of period $ 73 $ 736 $ 809 Total gains/(losses) realized and unrealized: Included in earnings 1 35 36 Included in other comprehensive income (1) (52) (53) Purchases — — — Issuances — — — Sales — — — Settlements (1) (713) (714) Transfers out of Level 3 — (5) (5) Balance, end of period $ 72 $ — $ 72 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — Level-3 Derivatives by Class 2020 2019 Year Ended December 31, Interest Credit Total Interest Credit Total Balance, beginning of period $ 67 $ — $ 66 $ 47 $ (1) $ 46 Total gains/(losses) realized and unrealized: Included in earnings 25 — 25 24 2 25 Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (7) — (7) (4) — (5) Balance, end of period 85 — 84 67 — 66 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date 25 — 25 24 1 25 Level-3 Derivatives by Class Year Ended December 31, 2018 Interest Credit Total Balance, beginning of period $ 61 $ (1) $ 61 Total gains/(losses) realized and unrealized: Included in earnings (9) (1) (9) Purchases — — — Issuances — — — Sales — — — Settlements (5) — (6) Balance, end of period $ 47 $ (1) $ 46 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ (9) $ (1) $ (10)
Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings	Gains and losses (realized and unrealized) relating to Level 3 assets and liabilities included in earnings for the affected periods are reported as follows: Net Investment Income Net Gains Income (Loss) on Variable Interest Entities Other Year Ended December 31, 2020 Total gains (losses) included in earnings for the period $ 1 $ 25 $ 281 $ (2) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — 25 281 (2) Year Ended December 31, 2019 Total gains (losses) included in earnings for the period $ 2 $ 25 $ 410 $ (2) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — 25 353 (2) Year Ended December 31, 2018 Total gains (losses) included in earnings for the period $ 36 $ (9) $ 4 $ (1) Changes in unrealized gains (losses) relating to financial instruments still held at the reporting date — (10) — (1)

Leases (Tables)

Leases (Tables)	12 Months Ended
Leases (Tables)	Dec. 31, 2020
Lessor, Operating Lease, Payments to be Received, Maturity [Table Text Block]	Future undiscounted lease payments to be received are as follows: As of December 31, 2020 Operating Leases 2021 $ 1 2022 1 2023 1 2024 1 2025 1 Thereafter 5 Total lease receipts $ 11
Lease, Cost [Table Text Block]	The components of lease costs, net of sub-lessor income, is as follows: Year Ended December 31, 2020 2019 Operating lease cost $ 4 $ 7 Variable lease cost — — Sublease income (1) (1) Total lease cost $ 4 $ 7 Ambac is required to make variable lease payments under certain leases which primarily relates to variable costs of the lessor, such as taxes, insurance, maintenance and electricity. Supplemental information related to leases is as follows: Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 4 $ 6 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) (1) — 30 (1) Includes right-of-use assets of $14 for the year ended December 31, 2019 for leases which existed prior to the New Lease Standard implementation date of January 1, 2019. Supplemental balance sheet information related to leases is as follows: December 31, 2020 2019 Operating leases: Operating lease right of use assets $ 25 $ 25 Operating lease liabilities 30 29 Weighted average remaining lease term: Operating leases 8.7 years 9.9 years Weighted average discount rate: Operating leases 7.7 % 7.9 %
Cash Flow, Supplemental Disclosures [Text Block]	Supplemental information related to leases is as follows: Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 4 $ 6 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) (1) — 30 (1) Includes right-of-use assets of $14 for the year ended December 31, 2019 for leases which existed prior to the New Lease Standard implementation date of January 1, 2019.
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	Future undiscounted lease payments, gross of sublease receipts, to be made are as follows: As of December 31, 2020 Operating Leases 2021 $ 5 2022 5 2023 5 2024 5 2025 5 Thereafter 18 Total lease payments 41 Less: imputed interest (11) Total $ 30
Schedule of Supplemental Cash Flow Information [Table Text Block]	Supplemental Disclosure of Cash Flow Information Year Ended December 31, 2020 2019 2018 Cash paid during the period for: Income taxes $ 11 $ 21 $ 35 Interest on long-term debt 107 143 232 Non-cash investing and financing activities: Increase in long-term debt in exchange for AMPS — — 187 Exchange of investments in Puerto Rico COFINA bonds for new bonds issued in the Plan of Adjustment — 510 — Decrease in long-term debt as a result of an exchange for investment securities — — — Rehabilitation exit transaction discharge of all Deferred Amounts and cancellation of certain senior surplus notes — — 1,919 December 31, 2020 2019 2018 Reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets to the Consolidated Statements of Cash Flow: Cash and cash equivalents $ 20 $ 24 $ 63 Restricted cash 13 55 19 Variable Interest Entity Restricted cash 2 2 1 Total cash, cash equivalents, and restricted cash shown on the Consolidated Statements of Cash Flows 35 81 83

Background and Business Descrip

Background and Business Description Rehabilitation Exit Support Agreement (Details)	Feb. 12, 2018USD ($)
Deferred Payment Obligation [Member]
Discount Amount	$ 0.065
Surplus Notes [Member]
Cash Payment Amount	0.40
Payment Amount in New Secured Notes	0.41
Payment Amount in Existing Surplus Notes	0.125
satisfaction and discharge of segregated account deferred amounts	3,857,000,000
Cancellation of Surplus Notes Principal	552,000,000
Cancellation of Surplus Notes Accrued Interest	257,000,000
Ambac Assurance [Member]
Secured Notes Received from Ambac LSNI	644,000,000
Ambac [Member] \| Deferred Payment Obligation [Member]
One Dollar Outstanding	1
Discount Amount	0.09
Payment Amount in New Secured Notes	0.91
AFG [Member]
Secured Notes Received from Ambac LSNI	125,000,000
Ambac LSNI Secured Note [Domain]
AMBAC LSNI Secured Note	$ 2,154,000,000

Background and Business Descr_2

Background and Business Description Tier 2 Commitment (Details) $ in Millions	Feb. 12, 2018USD ($)
Tier 2 Note Security in RMBS Litigation Settlement Proceeds	$ 1,600

Background and Business Descr_3

Background and Business Description AMPS Exchange (Details) $ in Thousands	Aug. 03, 2018USD ($)$ / sharesshares
AMPS Liquidation Preference Value Per Share \| $ / shares	$ 25,000
AMPS Exchange Surplus Notes Issued Per Share \| $ / shares	$ 13,875
AMPS Exchange Shares Tendered \| shares	22,096
AMPS Exchange Cash Paid Per Share \| $ / shares	$ 500
AMPS Exchange Warrants Issued Per Share \| $ / shares	$ 37.3076
AMPS Exchange Percent Acquired	0.844
AMPS Exchange Shares Acquired \| shares	22,296
AMPS Exchange Aggregated Liquidation Preference	$ 557,000
AMPS Exchange Nominal Discount	227,000
AMPS Exchange Nominal Discount at Fair Value	253,000
AMPS Exchange Total Surplus Notes Issued	213,000
AMPS Exchange Interest Included in Surplus Notes Issued	$ 98,000
AMPS Exchange Total Warrants Issued	824,307
AFG [Member]
AMPS Exchange Aggregated Liquidation Preference	$ 35,000

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Jul. 01, 2019	Jun. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Operating Lease, Liability	$ 30	$ 30
Income Taxes Paid		$ 11
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common stock, shares issued	45,865,081	45,865,081
Warrant Exercised During Period Shares		34
Cash and cash equivalents	$ 20	$ 20
Restricted Cash	13	13
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	35	35
UK Income Tax Rate			19.00%	17.00%
Restricted Cash	$ 13	$ 13
Weighted average period of future premiums		8 years 3 months 18 days
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%	2.20%
Business Combination, Redeemable Noncontrolling Percentage	20.00%	20.00%
Xchange
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	14 years
Restricted Cash	$ 4	$ 4
Restricted Cash	$ 4	$ 4
Minimum [Member] \| Furniture and Fixtures [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		3 years
Maximum [Member] \| Furniture and Fixtures [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Long-term Debt [Member]
Finite-Lived Intangible Assets [Line Items]
Interest Paid, Including Capitalized Interest, Operating and Investing Activities		$ 107
Variable Interest Entity, Primary Beneficiary [Member]
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%	2.40%

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies Goodwill (Details) $ in Millions	Dec. 31, 2020USD ($)
Intangible assets	$ 409
Goodwill, Impaired, Accumulated Impairment Loss	0
Insurance Intangible Asset [Member]
Intangible assets	$ 373

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies Noncontrolling Interest (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)shares	Jun. 30, 2018USD ($)shares
Stockholders' Equity Attributable to Noncontrolling Interest \| $	$ 60	$ 60	$ 264
Preferred Stock, Shares Outstanding \| shares	0	0
Ambac Assurance Corporation [Member]
Preferred Stock, Shares Outstanding \| shares	5,501	5,501	26,411
Preferred Stock, Liquidation Preference, Value \| $	$ 138	$ 138	$ 660
London Interbank Offered Rate (LIBOR) [Member]
Dividend Rate on Auction Rate Preferred Shares	200

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies Incentive Compensation (Details)	12 Months Ended
	Dec. 31, 2020
Twelve Months from Award Date [Domain]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	50.00%
Twenty Four Months from Award Date [Domain]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	50.00%

Basis of Presentation and Sig_6

Basis of Presentation and Significant Accounting Policies Adoption of New Accounting Standards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Operating Lease, Liability		$ 30	$ 29
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 0

Basis of Presentation and Sig_7

Basis of Presentation and Significant Accounting Policies Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 03, 2018
Income Taxes Paid	$ 11	$ 21	$ 35
AMPS Exchange Carrying Value of Surplus Notes Issued				$ 187
Non Cash Exchange of Puerto Rico Bonds		510
Non Cash Impact of Rehabilitation Exit Transaction			1,919
Cash and cash equivalents	20	24	63
Restricted Cash	13	55	19
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	35	81	83
Long-term Debt [Member]
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	$ 107	$ 143	$ 232

Variable Interest Entities Vari

Variable Interest Entities Variable Interest Entity - Change in Fair Value of Assets and Liabilities (Details) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Deconsolidated VIE Assets	$ 0		$ 0
Fixed income securities, amortized cost	2,807	$ 3,187	2,807	$ 3,187
Long-term debt	2,739	2,822	2,739	2,822
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	149	132	149	132
Variable Interest Entity Change in Fair Value of Assets and Liabilities			(1)	13	$ 3
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax					(1)
Realized Investment Gains (Losses)	2	9	22	81	108
Variable Interest Entity, Initial Consolidation, Gain (Loss)			0	15	0
Variable Interest Entities Deconsolidation Gain Loss			0	(2)	2
Variable Interest Entity, Measure of Activity, Income or Loss before Tax			5	$ 38	$ 3
Variable Interest Entity, Consolidated, Carrying Amount, Assets	292		292
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	364		364
Deconsolidated VIE Liabilities	0		$ 0
Consolidated Entities [Member]
Number of DeConsolidated Variable Interest Entities			1	1	4
Variable Interest Entities [Member]
Loans, Unpaid principal balance	2,546	2,618	$ 2,546	$ 2,618
Long-term debt, Unpaid principal balance	3,769	3,800	3,769	3,800
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax			(3)	14	$ 2
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax			(1)	0
Variable Interest Entity [Member]
Long-term debt	$ 4,493	$ 4,554	4,493	4,554
Interest Expense, Long-term Debt			(6)	(11)	0
Other Expenses			0	(1)	0
Variable Interest Entity [Member] \| Available-for-sale Securities [Member]
Gross Investment Income, Operating			7	10	0
Realized Investment Gains (Losses)			$ 8	$ 13	$ 0

Variable Interest Entities Va_2

Variable Interest Entities Variabile Interest Entities - Carry Value of Secured Notes from LSNI (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Long-term debt	$ 2,739	$ 2,822
Ambac [Member]
Carry Value Secured Notes from LSNI	465	535
Ambac Assurance Corporation [Member]
Long-term debt	2,739	2,822
Ambac Note Post Exit From Rehabilitation [Member] \| Ambac Assurance Corporation [Member]
Long-term debt	$ 1,641	$ 1,763

Comprehensive Income - Schedule

Comprehensive Income - Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 42	$ (49)
Other comprehensive income before reclassifications	(58)	(168)
Amounts reclassified from accumulated other comprehensive income	(21)	(78)
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ 0
Income Tax Expense (Benefit)	3	(32)	(5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	37	91
Ending Balance	79	42	(49)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	151	86
Other comprehensive income before reclassifications	(36)	(142)
Amounts reclassified from accumulated other comprehensive income	(21)	(76)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	15	65
Ending Balance	166	151	86
Accumulated Translation Adjustment [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(116)	(142)
Other comprehensive income before reclassifications	(23)	(26)
Amounts reclassified from accumulated other comprehensive income	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	23	26
Ending Balance	(92)	(116)	(142)
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(2)	(2)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	1	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1	0
Ending Balance	0	(2)	(2)
Other Postretirement Benefits Plan [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	8	9
Other comprehensive income before reclassifications	2	(1)
Amounts reclassified from accumulated other comprehensive income	(1)	(1)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(3)	(1)
Ending Balance	$ 5	$ 8	$ 9

Comprehensive Income - Schedu_2

Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ 3	$ (32)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(21)	(78)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains	(22)	(81)
Net of tax and noncontrolling interest	(21)	(76)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(21)	(76)
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	(1)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1	0
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, before Tax	2	0
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax and noncontrolling interest	(21)	(78)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	(1)	(4)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	0
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Accumulated Gain (Loss), Financial Liability, Fair Value Option, Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)		0
Other Postretirement Benefits Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(1)	(1)
Other Postretirement Benefits Plan [Member] \| Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	0	0
Prior service cost	(1)	(1)
Actuarial gains (losses)	0	0
Total before tax	(1)	(1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (1)	$ (1)

Financial Guarantee Insurance C

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	$ 11	$ 7	$ 2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(430)	(436)	$ (130)	$ 4,073
Loss and loss expense reserves	1,759	1,548
Subrogation recoverable	(2,156)	(2,029)
Claim liability reported on Balance Sheet, before reinsurance	(397)	(482)
Present Value of Expected Net Cash Flows- Claims and Loss Expenses [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	2,060	1,835
Subrogation recoverable	100	131
Claim liability reported on Balance Sheet, before reinsurance	2,160	1,966
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(229)	(233)
Subrogation recoverable	(2,256)	(2,160)
Claim liability reported on Balance Sheet, before reinsurance	(2,486)	(2,394)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(72)	(54)
Subrogation recoverable	0	0
Claim liability reported on Balance Sheet, before reinsurance	$ (72)	$ (54)

Financial Guarantee Insurance -

Financial Guarantee Insurance - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	100.00%
Net unsecured reinsurance recoverable	$ 34
Assured Guaranty Re Ltd [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	46.00%
Net unsecured reinsurance recoverable	$ 0
Sompo Japan Nipponkoa Holdings, Inc [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	3.00%
Net unsecured reinsurance recoverable	$ 0
Assured Guaranty Corporation [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	8.00%
Net unsecured reinsurance recoverable	$ 4
Build America Mutual Assurance Company [Domain]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	43.00%
Net unsecured reinsurance recoverable	$ 30

Financial Guarantee Insurance_2

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts Earned Premiums by Geographic Location (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ 54	$ 66	$ 111
Premiums Receivable, Gross	387
Public Finance [Member]
Premiums Receivable, Gross	185
Structured Finance [Member]
Premiums Receivable, Gross	59
International Finance [Member]
Premiums Receivable, Gross	144
Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	168
Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	17
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	22
Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	16
Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	14
Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	7
Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	108
Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	31
Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	5
Surveillance Category One [Member]
Premiums Receivable, Gross	302
Surveillance Category One [Member] \| Public Finance [Member]
Premiums Receivable, Gross	157
Surveillance Category One [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	27
Surveillance Category One [Member] \| International Finance [Member]
Premiums Receivable, Gross	118
Surveillance Category One [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	155
Surveillance Category One [Member] \| Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	2
Surveillance Category One [Member] \| Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	3
Surveillance Category One [Member] \| Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	3
Surveillance Category One [Member] \| Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	14
Surveillance Category One [Member] \| Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	7
Surveillance Category One [Member] \| Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	82
Surveillance Category One [Member] \| Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	31
Surveillance Category One [Member] \| Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	5
IA [Member]
Premiums Receivable, Gross	40
IA [Member] \| Public Finance [Member]
Premiums Receivable, Gross	27
IA [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| International Finance [Member]
Premiums Receivable, Gross	13
IA [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	13
IA [Member] \| Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	15
IA [Member] \| Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	13
IA [Member] \| Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
IA [Member] \| Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
II [Member]
Premiums Receivable, Gross	3
II [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	3
II [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	1
II [Member] \| Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	2
II [Member] \| Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
II [Member] \| Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
III [Member]
Premiums Receivable, Gross	27
III [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	14
III [Member] \| International Finance [Member]
Premiums Receivable, Gross	13
III [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	3
III [Member] \| Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	11
III [Member] \| Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	13
III [Member] \| Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
III [Member] \| Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
IV [Member]
Premiums Receivable, Gross	15
IV [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	15
IV [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Other Public Finance Sectors [Member] \| Public Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	15
IV [Member] \| Student Loan [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Structured Insurance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Other Structured Finance [Member] \| Structured Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Investor-owned and Public Utilities [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
IV [Member] \| Other International [Member] \| International Finance [Member]
Premiums Receivable, Gross	0
Reportable Geographical Components [Member] \| UNITED STATES
Premiums Earned, Net, Financial Guarantee Insurance Contracts	32	55	88
Reportable Geographical Components [Member] \| UNITED KINGDOM
Premiums Earned, Net, Financial Guarantee Insurance Contracts	24	17	19
Reportable Geographical Components [Member] \| Other International [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ (2)	$ (6)	$ 5

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Amount [Member]
Financial assets:
Other Investments	$ 89,000	$ 136,000
Cash	33,000	79,000
Loans	3,000	10,000
Other assets	1,000	3,000
Total financial assets	10,071,000	10,242,000
Financial liabilities:
Liabilities for net financial guarantees written	(740,000)	(863,000)
Long-term Debt	3,255,000	3,262,000
Total financial liabilities	8,958,000	8,699,000
Carrying Amount [Member] \| Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans, at fair value	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	4,493,000	4,554,000
Carrying Amount [Member] \| Other Debt Obligations [Member]
Financial assets:
Other Investments	595,000	478,000
Carrying Amount [Member] \| Short-term Investments [Member]
Financial assets:
Short term investments	492,000	653,000
Carrying Amount [Member] \| Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Carrying Amount [Member] \| Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Carrying Amount [Member] \| Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Carrying Amount [Member] \| Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Carrying Amount [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Carrying Amount [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Total Fair Value [Member]
Financial assets:
Cash	33,000	79,000
Debt Securities, Available-for-sale	2,949,000	3,314,000
Other Investments	595,000	478,000
Derivative assets	93,000	75,000
Other assets	1,000	3,000
Financial liabilities:
Derivative liabilities	114,000	90,000
Credit valuation adjustment included in fair value of credit derivative liabilities	$ 32	$ 19
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%	2.40%
Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 3,354,000	$ 3,121,000
Loans, at fair value	2,998,000	3,108,000
Derivative assets	41,000	52,000
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Long-term Debt	$ 4,324,000	$ 4,351,000
Variable Interest Entity [Member]
Financial liabilities:
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%	2.70%
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 492,000
Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	$ 2,957,000
Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Other Assets [Member] \| Carrying Amount [Member] \| Interest Rate Swaps [Member]
Financial assets:
Derivative assets	93,000	75,000
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	453,000	296,000
Level 1 [Member]
Financial assets:
Cash	32,000	70,000
Total financial assets	888,000	1,091,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	0	0
Total financial liabilities	0	0
Level 1 [Member] \| Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans	0	0
Financial liabilities:
Long-term Debt	0	0
Level 1 [Member] \| Other Debt Obligations [Member]
Financial assets:
Other Investments	91,000	136,000
Level 1 [Member] \| Short-term Investments [Member]
Financial assets:
Short term investments	415,000	598,000
Level 1 [Member] \| Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 1 [Member] \| Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 1 [Member] \| Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 1 [Member] \| Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 1 [Member] \| Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 1 [Member] \| Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] \| Other Assets [Member] \| Interest Rate Swaps [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	2,000	9,000
Total financial assets	2,299,000	2,593,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	2,670,000	2,829,000
Total financial liabilities	8,968,000	8,983,000
Level 2 [Member] \| Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	0	0
Financial liabilities:
Long-term Debt	4,349,000	4,408,000
Level 2 [Member] \| Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 2 [Member] \| Short-term Investments [Member]
Financial assets:
Short term investments	76,000	55,000
Level 2 [Member] \| Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 2 [Member] \| Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 2 [Member] \| Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 2 [Member] \| Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Level 2 [Member] \| Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Level 2 [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Level 2 [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Level 2 [Member] \| Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] \| Other Assets [Member] \| Interest Rate Swaps [Member]
Financial assets:
Derivative assets	9,000	8,000
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	6,433,000	6,281,000
Financial liabilities:
Liabilities for net financial guarantees written	539,000	284,000
Long-term Debt	401,000	445,000
Total financial liabilities	1,095,000	889,000
Level 3 [Member] \| Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	155,000	159,000
Level 3 [Member] \| Other Debt Obligations [Member]
Financial assets:
Other Investments	53,000	61,000
Level 3 [Member] \| Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] \| Equity Method Investments [Member]
Financial assets:
Other assets	1,000	3,000
Level 3 [Member] \| Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Level 3 [Member] \| Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Level 3 [Member] \| Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 3 [Member] \| Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 3 [Member] \| Loans Receivable [Member]
Financial assets:
Loans	3,000	13,000
Level 3 [Member] \| Other Assets [Member] \| Interest Rate Swaps [Member]
Financial assets:
Derivative assets	85,000	67,000
Municipal Bonds [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Corporate Debt Securities [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	1,077,000	1,430,000
Debt Security, Government, Non-US [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
US Government Debt Securities [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Residential Mortgage-Backed Securities [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Residential Mortgage-Backed Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Collateralized Debt Obligations [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Asset-backed Securities [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	303,000	287,000
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	10,073,000	10,260,000
Financial liabilities:
Liabilities for net financial guarantees written	539,000	284,000
Long-term Debt	3,071,000	3,274,000
Total financial liabilities	10,063,000	9,872,000
Fair Value, Recurring [Member] \| Variable Interest Entity [Member]
Financial assets:
Restricted cash	2,000	2,000
Loans	2,998,000	3,108,000
Financial liabilities:
Long-term Debt	4,504,000	4,567,000
Fair Value, Recurring [Member] \| Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Fair Value, Recurring [Member] \| Other Debt Obligations [Member]
Financial assets:
Other Investments	597,000	493,000
Fair Value, Recurring [Member] \| Short-term Investments [Member]
Financial assets:
Short term investments	492,000	653,000
Fair Value, Recurring [Member] \| Equity Method Investments [Member]
Financial assets:
Other assets	1,000	3,000
Fair Value, Recurring [Member] \| Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities		0
Fair Value, Recurring [Member] \| Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	303,000	287,000
Fair Value, Recurring [Member] \| Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Fair Value, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Fair Value, Recurring [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Fair Value, Recurring [Member] \| Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,077,000	1,430,000
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	3,215,000	2,957,000
Fair Value, Recurring [Member] \| Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Fair Value, Recurring [Member] \| Municipal Bonds [Member] \| Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	139,000	164,000
Fair Value, Recurring [Member] \| Interest Rate Swaps [Member]
Financial liabilities:
Derivative liabilities	114,000	89,000
Fair Value, Recurring [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	1,835,000	1,657,000
Fair Value, Recurring [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Financial assets:
Derivative assets	41,000	52,000
Fair Value, Recurring [Member] \| Loans Receivable [Member]
Financial assets:
Loans	3,000	13,000
Fair Value, Recurring [Member] \| Other Assets [Member] \| Interest Rate Swaps [Member]
Financial assets:
Derivative assets	93,000	75,000
Fair Value, Recurring [Member] \| Level 1 [Member] \| Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		0
Fair Value, Recurring [Member] \| Level 1 [Member] \| Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 1 [Member] \| Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 1 [Member] \| Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 1 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	106,000	156,000
Fair Value, Recurring [Member] \| Level 1 [Member] \| Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	98,000	44,000
Fair Value, Recurring [Member] \| Level 1 [Member] \| Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	4,000	0
Fair Value, Recurring [Member] \| Level 1 [Member] \| Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 2 [Member] \| Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		50,000
Fair Value, Recurring [Member] \| Level 2 [Member] \| Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	225,000	215,000
Fair Value, Recurring [Member] \| Level 2 [Member] \| Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	74,000	146,000
Fair Value, Recurring [Member] \| Level 2 [Member] \| Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	302,000	248,000
Fair Value, Recurring [Member] \| Level 2 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 2 [Member] \| Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 2 [Member] \| Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	1,073,000	1,430,000
Fair Value, Recurring [Member] \| Level 2 [Member] \| Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	358,000	215,000
Fair Value, Recurring [Member] \| Level 3 [Member] \| Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale		0
Fair Value, Recurring [Member] \| Level 3 [Member] \| Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	78,000	72,000
Fair Value, Recurring [Member] \| Level 3 [Member] \| Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 3 [Member] \| Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 3 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 3 [Member] \| Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 3 [Member] \| Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] \| Level 3 [Member] \| Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member]
Financial assets:
Debt Securities, Available-for-sale	140,000
Collateral Pledged [Member] \| US Government Debt Securities [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] \| Short-term Investments [Member] \| Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 1 [Member] \| Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	125,000	85,000
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 1 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	15,000
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 2 [Member] \| Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 2 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 3 [Member] \| Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	0	$ 0
Collateral Pledged [Member] \| Fair Value, Recurring [Member] \| Level 3 [Member] \| US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 595,000	$ 478,000
Credit valuation adjustment included in fair value of credit derivative liabilities	32	19
Fair value of derivative liabilities	113,000
Derivative liabilities	$ 114,000	$ 90,000
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.20%	2.40%
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	2.00%	4.00%
Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Coupon rate	5.97%	5.97%
Minimum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	3.00%	2.00%
Maximum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	95.00%	94.00%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.40%	2.70%
Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 16,000	$ 16,000
Hedge Funds, Multi-strategy [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	196,000	65,000
Interest Rate Contract [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	78,000	176,000
Illiquid Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 65,000	51,000
Other Credit Derivatives [Member] \| Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities		$ 0

Fair Value Measurements - Infor

Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Investments	$ 595	$ 478
Debt Securities, Available-for-sale	$ 2,949	$ 3,314
Asset-backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Coupon rate	5.97%	5.97%
Fair Value Inputs Maturity	14 years 9 months 29 days	15 years 6 months 29 days
Yield	10.50%	11.75%
Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Investments	$ 16	$ 16
Hedge Funds, Multi-strategy [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Investments	196	65
Interest Rate Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Investments	78	176
Illiquid Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Investments	65	51
Fair Value, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	302	248
Fair Value, Recurring [Member] \| Residential Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 0	$ 0

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail) $ in Millions	Dec. 31, 2020USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value of derivative liabilities	$ 113

Fair Value Measurements - Sum_3

Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail2) $ in Millions	Dec. 31, 2020USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value of derivative liabilities	$ 113

Fair Value Measurements - Inf_2

Fair Value Measurements - Information about Valuation Inputs for Variable Interest Entity Assets and Liabilities Classified as Level 3 (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%
Variable Interest Entity [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated Future Premium Payments Minimum Discounted Rate	1.80%
Estimated Future Premium Payments Maximum Discounted Rate	3.90%
Asset-backed Securities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate	5.97%
Fair Value Inputs Maturity	14 years 9 months 29 days
Fair Value Inputs Yield	10.50%
Variable Interest Entity [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%

Fair Value Measurements - Sum_4

Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 78	$ 72	$ 72	$ 809
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		6	0	(53)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		1	2	36
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(1)	(2)	(714)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				5
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	$ 0	0		0
Credit Derivatives [Member] \| Credit Derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Other Credit Derivatives [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0	0	(1)	(1)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		0	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0	0	0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		0	(1)	1
Interest Rate Swaps [Member] \| Interest Rate Swaps [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		85	67	47	61
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		25	24
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(7)	(4)	(5)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(25)	(24)	9
Residential Mortgage-Backed Securities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			0	0	736
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)			0	(52)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings				35
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases			0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements			0	(713)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				5
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	0			0
Non-Agency RMBS [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		0
Asset-backed Securities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		78	72	72	73
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		6	0	(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		1	2	1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(1)	(2)	(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	$ 0	0		0
Derivative [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		84	66	46	61
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		25	25
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(7)	(5)	(6)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(25)	(25)	10
Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		6,376	6,207	6,930	12,561
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		306		30
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				(590)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		198	199
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings			436
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(334)	(731)	(1,356)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				5
Deconsolidation of VIEs			627	3,709
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(304)	(376)	11
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		192
Level 3 [Member] \| Investments Contract [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Deconsolidation of VIEs			0
Level 3 [Member] \| Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Deconsolidation of VIEs			851
Level 3 [Member] \| Long-term Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Deconsolidation of VIEs			(223)
Level 3 [Member] \| Investments [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		78	72	72	809
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		1		36
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				(53)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		6	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings			2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(1)	(2)	(714)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				5
Deconsolidation of VIEs			0	0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		0	0	0
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Level 3 [Member] \| Other Assets [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		1	3	5	6
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		(2)		(1)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings			(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Deconsolidation of VIEs			0	0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		2	2	1
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Level 3 [Member] \| Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Deconsolidation of VIEs			0
Level 3 [Member] \| Derivative Financial Instruments, Liabilities [Member] \| Derivative Financial Instruments, Liabilities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		84	66	46	61
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		25
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings			25
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(7)	(5)	(6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Deconsolidation of VIEs				0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(25)	(25)	10
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Variable Interest Entity, Primary Beneficiary [Member] \| Level 3 [Member] \| Long-term Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	8	91
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases		0	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues		0	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales		0	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements		0	0	(23)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Deconsolidation of VIEs				(2,237)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		0	0	(47)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		0	0	(217)	(2,758)
Variable Interest Entity, Primary Beneficiary [Member] \| Level 3 [Member] \| Loans Receivable [Member] \| Loans [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		2,998	3,108	4,288	11,529
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		98		(201)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				(470)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		83	74
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings			(15)	189
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		0	287
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(290)	(690)	(624)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Deconsolidation of VIEs				5,946
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(98)	(215)	63
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		83
Variable Interest Entity, Primary Beneficiary [Member] \| Level 3 [Member] \| Corporate Debt Securities [Member] \| Investments Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		3,215	2,957	2,737	$ 2,914
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		183		16
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)				(158)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		109	116
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings			138
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(35)	(35)	(35)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Deconsolidation of VIEs				0
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		(183)	$ (138)	$ (16)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		$ 109

Fair Value Measurements - Sum_5

Fair Value Measurements - Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period			$ 4
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held			0
Net Investment Income [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	$ 1	$ 2	36
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	0	0	0
Derivative Contract [Domain]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	25	25	(9)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	25	25	(10)
Income Loss On Variable Interest Entities [Member] \| Variable Interest Entity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	281	410
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	281	353
Other Income or (Loss) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	(2)	(2)	(1)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	$ (2)	$ (2)	$ (1)

Income Taxes Impact of New Tax

Income Taxes Impact of New Tax Legislation (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019
AMT Receivable Resulting from TCJA			$ 37,019
Change in Impact of AMT Repeal on Current Taxes	$ 1,902
Impact of AMT Repeal on Current Taxes		$ 29,581
AMT Related to Filing of 2017 Tax Return		$ 5,536

Schedule II - Condensed Financi

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued	45,865,081	45,571,743
Treasury stock, shares	55,942	16,343
Common Stock, Shares, Outstanding	45,809,139
Assets:
Debt Securities, Available-for-sale	$ 2,949	$ 3,314
Short-term investments, at cost (approximates fair value)	2,807	3,187
Other Investments	595	478
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investments, Total	3,544	3,792
Other assets	114	95
Total assets	13,220	13,320
Taxes Payable, Current	6	0
Tolling Payment Payable to AFG		28
Liabilities:
Total liabilities	12,074	11,783
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	0	0
Additional paid-in capital	242	232
Accumulated other comprehensive income (loss)	79	42
Retained earnings	759	1,203
Common stock held in treasury at cost	(1)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,080	1,477
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	13,220	13,320
Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	76	71
Other Investments	54	46
Debt Securities, Available-for-sale, Allowance for Credit Loss	2
Investments, Total	349	434
Cash	7	9
Investment in subsidiaries	714	993
Current taxes receivable	0	30
Other assets	13	12
Total assets	1,082	1,478
Taxes Payable, Current	0	0
Liabilities:
Accounts payable and other liabilities	2	2
Total liabilities	3	2
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	0	0
Additional paid-in capital	242	232
Accumulated other comprehensive income (loss)	79	42
Retained earnings	759	1,203
Common stock held in treasury at cost	(1)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,080	1,477
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	1,082	1,478
Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale	492
Short-term investments, at cost (approximates fair value)	492	653
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Short-term Investments [Member] \| Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	229	318
Fixed Income Securities [Member]
Assets:
Debt Securities, Available-for-sale	2,317	2,577
Short-term investments, at cost (approximates fair value)	2,175	2,450
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment income due and accrued	10
Fixed Income Securities [Member] \| Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Debt Securities, Available-for-sale	$ 66	70
Fixed Income Securities [Member] \| Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale		653
Short-term investments, at cost (approximates fair value)		$ 653

Schedule II - Condensed Finan_2

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statement of Total Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Other income	$ 3,000	$ 134,000	$ 5,000
Net realized investment gains	(22,000)	(81,000)	(108,000)
Total revenues	156,000	496,000	511,000
Expenses:
Pre-tax income (loss)	(440,000)	(183,000)	273,000
Income tax expense (benefit)	3,000	(32,000)	(5,000)
Net income (loss) attributable to common stockholders	(437,000)	(216,000)	186,000
Other comprehensive income, after tax:
Net income (loss)	(437,000)	(216,000)	267,000
Unrealized gains (losses) on securities, net of deferred income taxes of $0	15,000	65,000	55,000
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	23,000	26,000	(48,000)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax	1,000	0	1,000
Total other comprehensive income (loss), net of income tax	37,000	91,000	6,000
Total comprehensive income (loss) attributable to common stockholders	(400,000)	(125,000)	192,000
Ambac Financial Group, Inc Parent Company Only [Member]
Condensed Income Statements, Captions [Line Items]
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	1,000	(8,000)	2,000
Other Comprehensive Income (Loss), Foreign Currency Translation Gain (Loss) Arising During Period, Tax	0	0	0
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, Tax	0	0	0
Revenues:
Investment income	13,000	(1,000)	(2,000)
Net realized investment gains	1,000	2,000	2,000
Total revenues	12,000	18,000	26,000
Expenses:
Operating Expenses	19,000	16,000	8,000
Pre-tax income (loss)	(7,000)	2,000	17,000
Income tax expense (benefit)	0	5,000	11,000
Income before equity in undistributed net income (loss) of subsidiaries	7,000	(7,000)	(28,000)
Equity in undistributed net income (loss) of subsidiaries	(430,000)	(223,000)	157,000
Net income (loss) attributable to common stockholders	(437,000)	(216,000)	186,000
Other comprehensive income, after tax:
Net income (loss)	(437,000)	(216,000)	186,000
Unrealized gains (losses) on securities, net of deferred income taxes of $0	15,000	65,000	55,000
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	23,000	26,000	(48,000)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, after Tax	1,000	0	1,000
Changes to postretirement benefit, net of tax	(3,000)	(1,000)	(2,000)
Total other comprehensive income (loss), net of income tax	37,000	91,000	6,000
Total comprehensive income (loss) attributable to common stockholders	(400,000)	(125,000)	192,000
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect	$ 0	$ 0	$ 0

Schedule II - Condensed Finan_3

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Stockholders' Equity (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	$ 274
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Stock-based compensation	(12)
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	(297)
Ending balance	1,633
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	267
Stock-based compensation	0
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	(82)
Ending balance	1,421
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	6
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	(49)
Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	(12)
Issuance of common stock	0
AMPS Impact on Stockholders Equity	8
Ending balance	219
Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	(223)
Ending balance	41
Ambac Financial Group, Inc Parent Company Only [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	192
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Stock-based compensation	(12)
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	8
Stockholders' Equity Attributable to Parent	1,592	$ 1,381
Ambac Financial Group, Inc Parent Company Only [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	186
Issuance of common stock	(1)
Stockholders' Equity Attributable to Parent	1,421	1,234
Ambac Financial Group, Inc Parent Company Only [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	6
Stockholders' Equity Attributable to Parent	(49)	(52)
Ambac Financial Group, Inc Parent Company Only [Member] \| Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0
Ambac Financial Group, Inc Parent Company Only [Member] \| Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0
Ambac Financial Group, Inc Parent Company Only [Member] \| Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	(12)
AMPS Impact on Stockholders Equity	8
Stockholders' Equity Attributable to Parent	219	200
Ambac Financial Group, Inc Parent Company Only [Member] \| Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock	0
Stockholders' Equity Attributable to Parent	0	$ 0
Accounting Standards Update 2016-01 [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	3
Accounting Standards Update 2016-01 [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Accounting Standards Update 2016-01 [Member] \| Parent [Member] \| Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	3
Accounting Standards Update 2016-01 [Member] \| Parent [Member] \| AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ (3)

Schedule II - Condensed Finan_4

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 03, 2018
Cash flows from operating activities:
Net income (loss)	$ (437,000)	$ (216,000)	$ 267,000
Adjustments to reconcile net income loss to net cash used in operating activities:
Accretion (Amortization) of Discounts and Premiums, Investments	(15,000)	(63,000)	(137,000)
Net realized investment gains	(22,000)	(81,000)	(108,000)
Decrease in current income taxes payable	17,000	35,000	(35,000)
Share-based compensation	11,000	12,000	12,000
Other, net	59,000	79,000	68,000
Net cash used in operating activities	(175,000)	(311,000)	(1,543,000)
Cash flows from investing activities:
Proceeds from matured bonds	137,000	379,000	432,000
Purchases of bonds	(975,000)	(959,000)	(528,000)
Change in short-term investments	158,000	(218,000)	127,000
Proceeds from Sale and Maturity of Other Investments	374,000	81,000	159,000
Payments for (Proceeds from) Other Investing Activities	1,000	(2,000)	0
Net cash provided by investing activities	432,000	1,000,000	1,588,000
Capital Contribution to Subsidiaries	(29,000)
AMPS Proceeds	0	19,000	0
Cash flows from financing activities:
AMPS Exchange Cash Paid	0	0	(11,000)
Net Cash Provided by (Used in) Financing Activities	(303,000)	(691,000)	(585,000)
Cash and Cash Equivalents, Period Increase (Decrease)	(46,000)	(2,000)	(541,000)
Cash and cash equivalents at beginning of period	24,000	63,000
Cash and cash equivalents end of period	20,000	24,000	63,000
Cash paid during the period for:
Income taxes	11,000	21,000	35,000
Payments to Acquire Businesses, Net of Cash Acquired	(74,000)
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
AMPS Exchange Dollar Value Warrants Issued				$ 8,000
Parent Company [Member]
Cash flows from operating activities:
Net income (loss)	(437,000)	(216,000)	186,000
Adjustments to reconcile net income loss to net cash used in operating activities:
Equity in undistributed net (income) loss of non-debtor subsidiaries	423,000	223,000	(157,000)
Accretion (Amortization) of Discounts and Premiums, Investments	(6,000)	(6,000)	(7,000)
Net realized investment gains	1,000	2,000	2,000
Decrease in current income taxes payable	30,000	15,000	(15,000)
Share-based compensation	11,000	12,000	12,000
Decrease in other assets	(1,000)	(8,000)	12,000
Other, net	(10,000)	(6,000)	0
Net cash used in operating activities	11,000	16,000	32,000
Cash flows from investing activities:
Proceeds from matured bonds	46,000	86,000	230,000
Purchases of bonds	(45,000)	(2,000)	(137,000)
Change in short-term investments	89,000	(125,000)	(123,000)
Net cash provided by investing activities	16,000	(22,000)	(21,000)
Cash flows from financing activities:
AMPS Exchange Cash Paid	0
Cash and Cash Equivalents, Period Increase (Decrease)	(2,000)	(6,000)	11,000
Cash and cash equivalents at beginning of period	9,000	15,000	4,000
Cash and cash equivalents end of period	7,000	9,000	15,000
Cash paid during the period for:
Income taxes		1,000	4,000
Payments to Acquire Businesses, Net of Cash Acquired	(74,000)
Parent [Member]
Cash flows from investing activities:
Proceeds from Sale and Maturity of Other Investments	0	0	25,000
Payments for (Proceeds from) Other Investing Activities	0	0	(5,000)
AMPS Proceeds	$ 0	19,000	0
Cash flows from financing activities:
AMPS Exchange Cash Paid		$ 0	$ (11,000)

Schedule II - Condensed Finan_5

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) Schedule II - Condensed Financial Information of Registrant - Additional Information (Details)	Apr. 29, 1991
Entity Incorporation, Date of Incorporation	Apr. 29, 1991

Schedule II - Condensed Finan_6

Schedule II - Condensed Financial Information of Registrant (Parent Company Only) Schedule II - Condensed Financial Information of Registrant - Income Taxes (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Operating Loss Carryforwards	$ 3,639
Ambac Assurance [Member]
Operating Loss Generated in Current Year	$ 270
Minimum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
Maximum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2040
AFG [Member]
Operating Loss Carryforwards	$ 1,457
AFG [Member] \| Minimum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
AFG [Member] \| Maximum [Member]
Operating Loss Carryforwards, Expiration Date	Jan. 1, 2034
Ambac Assurance [Member] \| Minimum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
Ambac Assurance [Member] \| Maximum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2040

Background and Business Descr_4

Background and Business Description	12 Months Ended
Background and Business Description	Dec. 31, 2020
Accounting Policies [Abstract]
Background and Business Description	1. BACKGROUND AND BUSINESS DESCRIPTION Ambac Financial Group, Inc. (“AFG”), headquartered in New York City, is a financial services holding company incorporated in the state of Delaware on April 29, 1991. References to “Ambac,” the “Company,” “we,” “our,” and “us” are to AFG and its subsidiaries, as the context requires. Ambac's business operations include: • Financial Guarantee Insurance — Ambac Assurance Corporation ("Ambac Assurance" or "AAC") and its wholly owned subsidiary, Ambac Assurance UK Limited (“Ambac UK”), legacy financial guarantee businesses, both of which have been in runoff since 2008. Insurance policies issued by AAC and Ambac UK generally guarantee payment when due of the principal and interest on the obligations guaranteed. • Specialty Property & Casualty Program Insurance — Currently includes admitted insurer Everspan Insurance Company and excess and surplus lines insurer Everspan Indemnity Insurance Company (collectively, "Everspan" or the "Everspan Group"). This platform, which received an A- Financial Strength Rating from A.M. Best in February 2021, is expected to launch new underwriting programs in 2021. • Managing General Agency / Underwriting — Currently includes Xchange Benefits, LLC and Xchange Affinity Underwriting Agency, LLC (collectively, “Xchange”) a property and casualty Managing General Underwriter 80% of which AFG acquired on December 31, 2020. Refer to Note 3. Business Combination for further information relating to this acquisition. As of and for the year ended December 31, 2020, management reviewed financial information, allocated resources and measured financial performance on a consolidated basis and accordingly the Company had a single reportable segment. As a result of the acquisition of Xchange and the expected launch of the Everspan platform, segments will be re-evaluated in 2021. Limitations on Voting and Transfer of Common Stock AFG’s Amended and Restated Certificate of Incorporation limits voting and transfer rights of stockholders in significant ways. Article IV contains voting restrictions applicable to any person owning at least 10% of AFG's common stock so that such person (including any group consisting of such person and any other person with whom such person or any affiliate or associate of such person has any agreement, contract, arrangement or understanding with respect to acquiring, voting, holding or disposing of AFG’s common stock) shall not be entitled to cast votes in excess of one vote less than 10% of the votes entitled to be cast by all common stock holders, except as otherwise approved by the OCI (as defined below). Article XII contains substantial restrictions on the ability to transfer AFG’s common stock. In order to preserve certain tax benefits, subject to limited exceptions, any attempted transfer of common stock shall be prohibited and void to the extent that, as a result of such transfer (or any series of transfers of which such transfer is a part), either (i) any person or group of persons shall become a holder of 5% or more of the Company’s common stock or (ii) the percentage stock ownership interest in AFG of any holder of 5% or more of the Company’s common stock shall be increased (a “Prohibited Transfer”). These restrictions shall not apply to an attempted transfer if the transferor or the transferee obtains the written approval of AFG’s Board of Directors to such transfer. A purported transferee of a Prohibited Transfer shall not be recognized as a stockholder of AFG for any purpose whatsoever in respect of the securities which are the subject of the Prohibited Transfer (the “Excess Securities”). Until the Excess Securities are acquired by another person in a transfer that is not a Prohibited Transfer, the purported transferee of a Prohibited Transfer shall not be entitled with respect to such Excess Securities to any rights of stockholders of AFG, including, without limitation, the right to vote such Excess Securities and to receive dividends or distributions, whether liquidating or otherwise, in respect thereof, if any. Once the Excess Securities have been acquired in a transfer that is not a Prohibited Transfer, the securities shall cease to be Excess Securities. If the Board determines that a transfer of securities constitutes a Prohibited Transfer then, upon written demand by AFG, the purported transferee shall transfer or cause to be transferred any certificate or other evidence of ownership of the Excess Securities within the purported transferee’s possession or control, together with any distributions paid by AFG with respect to such Excess Securities, to an agent designated by AFG. Such agent shall thereafter sell such Excess Securities and the proceeds of such sale shall be distributed as set forth in the Amended and Restated Certificate of Incorporation. If the purported transferee of a Prohibited Transfer has resold the Excess Securities before receiving such demand, such person shall be deemed to have sold the Excess Securities for AFG’s agent and shall be required to transfer to such agent the proceeds of such sale, which shall be distributed as set forth in the Amended and Restated Certificate of Incorporation. Strategies to Enhance Shareholder Value The Company's primary goal is to maximize shareholder value through executing the following key strategies: • Active runoff of AAC and its subsidiaries through transaction terminations, commutations, restructurings, and reinsurance with a focus on our watch list credits and known and potential future adversely classified credits, that we believe will improve our risk profile, and maximizing the risk-adjusted return on invested assets; • Ongoing rationalization of Ambac's capital and liability structures; • Loss recovery through active litigation management and exercise of contractual and legal rights; • Ongoing review of the effectiveness and efficiency of Ambac's operating platform; and • Further expanding into specialty property and casualty program insurance, managing general agency/underwriting and potentially other insurance and insurance related businesses that will generate long-term shareholder value with attractive risk-adjusted returns and meet other preestablished criteria. The execution of Ambac’s strategy to increase the value of its investment in AAC is subject to the restrictions set forth in the Settlement Agreement, dated as of June 7, 2010 (the "Settlement Agreement"), by and among AAC, Ambac Credit Products LLC ("ACP"), AFG and certain counterparties to credit default swaps with ACP that were guaranteed by AAC, as well as the Stipulation and Order among the Office of the Commissioner of Insurance for the State of Wisconsin (“OCI”), AFG and AAC that became effective on February 12, 2018, as amended (the “Stipulation and Order”), and the indenture for the Tier 2 Notes (as defined below), each of which requires OCI and, under certain circumstances, holders of the debt instruments benefiting from such restrictions, to approve certain actions taken by or in respect of AAC. In exercising its approval rights, OCI will act for the benefit of policyholders, and will not take into account the interests of AFG. Opportunities for remediating losses on poorly performing insured transactions also depend on market conditions, including the perception of AAC’s creditworthiness, the structure of the underlying risk and associated policy as well as other counterparty specific factors. AAC's ability to commute policies or purchase certain investments may also be limited by available liquidity. The Segregated Account In March 2010, AAC established a Segregated Account pursuant to Wisc. Stat. §611.24 (2) (the “Segregated Account”) to segregate certain segments of AAC’s liabilities, and the Wisconsin Insurance Commissioner, acting as rehabilitator (the "Rehabilitator") commenced rehabilitation proceedings in the Dane County, Wisconsin Circuit Court (the “Rehabilitation Court”) with respect to the Segregated Account (the “Segregated Account Rehabilitation Proceedings”) in order to permit OCI to facilitate an orderly run-off and/or settlement of the liabilities allocated to the Segregated Account. On October 8, 2010, OCI filed a plan of rehabilitation for the Segregated Account (the “Segregated Account Rehabilitation Plan”) in the Rehabilitation Court, which was confirmed on January 24, 2011. On June 11, 2014, the Rehabilitation Court approved amendments to the Segregated Account Rehabilitation Plan and the Segregated Account Rehabilitation Plan, as amended, became effective on June 12, 2014. Policy obligations not allocated to the Segregated Account remained in the General Account of AAC, and such policies in the General Account were not subject to and, therefore, were not directly impacted by the Segregated Account Rehabilitation Plan. On February 12, 2018, the rehabilitation of the Segregated Account was concluded pursuant to an amendment to the Segregated Account Rehabilitation Plan (the "Second Amended Plan of Rehabilitation"). The conclusion of the rehabilitation followed the successful completion of Ambac's surplus note exchange offers and consent solicitation, which, together with the satisfaction of all conditions precedent to the effectiveness of the Second Amended Plan of Rehabilitation, including the discharge of all unpaid policy claims of the Segregated Account, including accretion amounts thereon ("Deferred Amounts"), completed the restructuring transactions (the "Rehabilitation Exit Transactions") . In exchange for an effective consideration package of 40% cash, 41% Secured Notes (as defined below) and 12.5% AAC's 5.1% surplus notes due 2020 ("senior surplus notes"), paid in respect of outstanding Deferred Amounts and senior surplus notes. AAC received the following benefits as a result of the completion of the Rehabilitation Exit Transactions: • Satisfaction and discharge of all outstanding Deferred Amounts (including accretion) of the Segregated Account, totaling $3,857; • Cancellation of $552 in principal amount outstanding, plus accrued and unpaid interest of $257 thereon, of senior surplus notes; and • An effective discount of 6.5% on Deferred Amounts (applied first against accretion) and on the outstanding amount of principal and accrued and unpaid interest on tendered senior surplus notes. AFG received $0.91 in principal amount of Secured Notes for each $1.00 of Deferred Amounts (including accretion) that it held, and provided a $0.09 discount in full satisfaction and discharge of its Deferred Amount claims. AFG did not participate in the voluntary surplus note exchange offers. The Secured Notes A newly formed special purpose entity, Ambac LSNI, LLC ("Ambac LSNI") issued $2,154 of new secured notes (the “Secured Notes”), secured by all assets of the special purpose entity, which include a note issued by AAC to the special purpose entity (the "Ambac Note"), which is secured by a pledge of AAC’s right, title and interest in up to the first $1,400 of proceeds (net of reinsurance) from certain litigations in which AAC seeks redress for breaches of representations and warranties and/or fraud related to residential mortgage-backed securitizations (the “RMBS Litigations”). In addition, the Ambac Note is secured by cash and securities having a market value of $178 as of December 31, 2020. AAC also pledged for the benefit of the holders of Secured Notes (other than AAC) the proceeds of the Secured Notes held by AAC from time to time, and issued a financial guaranty insurance policy to a trustee for the benefit of holders of Secured Notes irrevocably guarantying all principal and interest payments in respect of the Secured Notes as and when such payments become due and owing. Prior to the Rehabilitation Exit Transactions, AFG and AAC owned securities that were insured by AAC and allocated to the Segregated Account. As a result of the Rehabilitation Exit Transactions, AFG and AAC received $125 and $644, respectively, of par amount of Secured Notes issued by Ambac LSNI. The current holdings of these secured notes are reported in Investments in the Consolidated Balance Sheets at their fair value. Tier 2 Financing On the effective date of the Rehabilitation Exit Transactions, AAC issued $240 of senior notes (the “Tier 2 Notes”) secured by AAC’s rights, title and interest in the cash and non-cash proceeds (net of reinsurance) above $1,600 received in connection with the RMBS Litigations. The indenture for the Tier 2 Notes limits certain activities of AAC and its subsidiaries, such as issuing certain indebtedness; engaging in mergers and similar transactions; disposing of assets; making restricted payments; and creating or permitting liens (among other restrictions and limitations). The indenture for the Tier 2 Notes includes certain allowances with respect to these activities and generally requires the approval of OCI and, in some cases, holders of the Tier 2 Notes, for consents, waivers or amendments. Bank Settlement Agreement As part of the Rehabilitation Exit Transactions, AFG and AAC received sufficient consents from holders of senior surplus notes for a waiver and amendment (the "BSA Waiver and Amendment") of the Settlement Agreement. After giving effect to the BSA Waiver and Amendment, the Settlement Agreement continues to limit certain activities of AAC and its subsidiaries, such as issuing indebtedness; engaging in mergers and similar transactions; disposing of assets; making restricted payments; creating or permitting liens; engaging in transactions with affiliates; modifying or creating tax sharing agreements; and taking certain actions with respect to surplus notes (among other restrictions and limitations). The Settlement Agreement includes certain allowances with respect to these activities and generally requires the approval of OCI and, in some cases, holders of surplus notes issued pursuant to the Settlement Agreement, for consents, waivers or amendments. Stipulation and Order Upon consummation of the Rehabilitation Exit Transactions, the Stipulation and Order became effective. The Stipulation and Order includes affirmative covenants, as well as restrictions on certain business activities and transactions, of AFG and AAC. The Stipulation and Order has no fixed term and may be terminated or modified only with the approval of OCI. OCI reserved the right to modify or terminate the Stipulation and Order in a manner consistent with the interests of policyholders, creditors and the public generally. August 2018 AMPS Exchange At June 30, 2018, AAC had 26,411 shares of issued and outstanding AMPS with a liquidation preference of $660 (reported as nonredeemable noncontrolling interest of $264 on Ambac's balance sheet). On July 3, 2018, AFG and AAC commenced an offer to exchange (the “AMPS Exchange”) all of AAC’s outstanding AMPS for senior surplus notes and, from AFG, cash and warrants to purchase AFG's common stock. The senior surplus notes offered in the AMPS Exchange have the same terms as other outstanding surplus notes of AAC (other than junior surplus notes). The offering period for the AMPS Exchange expired on August 1, 2018 and the transaction closed on August 3, 2018 (the "Settlement Date"). In exchange for each AMPS share (i.e. $25 thousand of liquidation preference), holders received senior surplus notes with a total outstanding amount (including accrued and unpaid interest thereon through June 22, 2018 (the "Signing Date")) equal to $13.875 thousand (the “Repurchase”). AMPS holders who tendered on or before July 17, 2018, representing 22,096 shares of the AMPS, also received from AFG $0.500 in cash and 37.3076 warrants (rounded down to the nearest whole warrant) to purchase an equivalent number of shares of common stock of AFG at an exercise price of $16.67 per share (the “AFG Purchase” and, together with the Repurchase, the “Purchases”). As a result of the completion of the Purchases, Ambac: (1) Repurchased 84.4% or 22,296 AMPS with an aggregate liquidation preference of $557, including $35 in aggregate liquidation preference in the AFG Purchase; (2) Captured a nominal discount of approximately $227 (a discount of approximately $253 on a fair market value basis) on $557 of the total outstanding liquidation preference of AMPS; and (3) Issued, in aggregate, $213 in current principal amount of senior surplus notes with accrued interest thereon on Settlement Date of $98, issued 824,307 warrants and paid $11 in cash. The AMPS are reported on the balance sheet within nonredeemable non-controlling interests and are carried at their fair value at the date AFG emerged from bankruptcy in April 2013, which is lower than the fair value of the total consideration provided to the AMPS holders in the Purchases. The difference between the fair value of consideration provided to AMPS holders and the carrying amount of the AMPS was reflected as a reduction to Net income attributable to common stockholders in 2018 for approximately $82.

Basis of Presentation and Sig_8

Basis of Presentation and Significant Accounting Policies (Notes)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Significant Accounting Policies	2. BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES Ambac’s consolidated financial statements have been prepared on the basis of U.S. generally accepted accounting principles (“GAAP”). The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues, expenses and disclosures. Such estimates that are particularly susceptible to change are used in connection with certain fair value measurements, valuation of loss reserves for non-derivative insurance policies and the valuation allowance on the deferred tax asset, any of which individually could be material. Consolidation The consolidated financial statements include the accounts of AFG and all other entities in which AFG (directly or through its subsidiaries) has a controlling financial interest, including variable interest entities (“VIEs”) for which AFG or an AFG subsidiary is deemed the primary beneficiary in accordance with the Consolidation Topic of the Accounting Standards Codification ("ASC"). All significant intercompany balances have been eliminated. The usual condition for a controlling financial interest is ownership of a majority of the voting interests of an entity. However, a controlling financial interest may also exist in entities, such as VIEs, through arrangements that do not involve controlling voting interests. A VIE is an entity: a) that lacks enough equity investment at risk to permit the entity to finance its activities without additional subordinated financial support from other parties; or b) where the group of equity holders does not have: (1) the power, through voting rights or similar rights, to direct the activities of an entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb the entity’s expected losses; or (3) the right to receive the entity’s expected residual returns. The determination of whether a variable interest holder is the primary beneficiary involves performing a qualitative analysis of the VIE that includes, among other factors, its capital structure, contractual terms including the rights of each variable interest holder, the activities of the VIE, whether the variable interest holder has the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance, whether the variable interest holder has the obligation to absorb losses of the VIE that could potentially be significant to the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE, related party relationships and the design of the VIE. An entity that is deemed the primary beneficiary of a VIE is required to consolidate the VIE. Refer to Note 4. Variable Interest Entities , for a detailed discussion of Ambac’s involvement in VIEs, Ambac’s methodology for determining whether Ambac is required to consolidate a VIE and the effects of VIEs being consolidated. AFG Unconsolidated Financial Information Financial information of AFG is presented in Schedule II to this Form 10-K as of December 31, 2020 and 2019 and for the years ended December 31, 2020, 2019 and 2018. Investments in subsidiaries are accounted for using the equity method of accounting in Schedule II. Measurement of Credit Losses on Financial Instruments (CECL) On January 1, 2020 Ambac adopted ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) - Measurement of Credit Losses on Financial Instruments, subsequently amended by ASU 2018-19 , Codification Improvements to Topic 326, Financial Instruments - Credit Losses; ASU 2019-04 , Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments ; ASU 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief ; and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses (collectively the Current Expected Credit Loss standard or "CECL"). The new CECL standard affects how reporting entities measure credit losses for financial assets that are not accounted for at fair value through net income. For Ambac, these financial assets include available-for-sale debt securities and amortized cost assets, specifically premium receivables, reinsurance recoverables and loans. CECL does not apply to recoveries of previously paid losses on financial guarantee insurance contracts accounted for under ASC 944 nor does it apply to equity method investments accounted for under ASC 323. • For available-for-sale debt securities, credit losses under CECL are measured similarly to other-than-temporary impairments under prior GAAP. The updated guidance was applied prospectively. • For financial instruments measured at amortized cost, CECL replaces the "incurred loss" model, which generally delayed recognition of the full amount of credit losses until the loss was probable of occurring, with an "expected loss" model, which reflects an entity's current estimate of all expected lifetime credit losses. The estimate of expected lifetime credit losses should consider historical information, current information, as well as reasonable and supportable forecasts. Expected lifetime credit losses for amortized cost assets will be recorded as an allowance for credit losses, with subsequent increases or decreases in the allowance reflected in net income each period. The updated guidance was applied by a cumulative effect adjustment to the opening balance of retained earnings at January 1, 2020. This adjustment was not material to retained earnings or any individual balance sheet line item. Refer to the discussion below for each asset type. As a result of adopting CECL, management revised its policies and procedures around the credit impairment evaluation process. CECL also introduced new disclosures related to the credit impairment process, including certain accounting policy elections that Ambac made under the new standard. Investments The Investments - Debt Securities Topic of the ASC requires that all debt instruments be classified in Ambac’s Consolidated Balance Sheets according to their purpose and, depending on that classification, be carried at either cost or fair market value. Ambac’s non-VIE debt investment portfolio is accounted for on a trade-date basis and consists primarily of investments in fixed maturity securities that are considered available-for-sale as defined by the Investments - Debt Securities Topic of the ASC. Available-for-sale debt securities are reported in the financial statements at fair value with unrealized gains and losses, net of deferred taxes, reflected in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity and computed using amortized cost as the basis. For purposes of computing amortized cost, premiums and discounts are accounted for using the effective interest method over a future term of the security. For structured debt securities with a large underlying pool of homogenous loans, such as mortgage-backed and asset-backed securities, premiums and discounts are adjusted for the effects of actual and anticipated prepayments. For other fixed maturity securities, such as corporate and municipal bonds, discounts were amortized or accreted over the remaining term of the securities. Ambac adopted ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities, on January 1, 2019. ASU 2017-08 shortened the amortization period for the premium on callable debt securities to the earliest call date. Under previous GAAP, Ambac generally amortized the premium over the contractual life (i.e. maturity) of the debt security and if that debt security was called, we would record a loss equal to the unamortized premium. Ambac’s non-VIE investment portfolio also includes equity interests in pooled investment funds which are accounted for in accordance with the Investments - Equity Securities Topic of the ASC and reported as Other investments on the Consolidated Balance Sheet with income reported through Net investment income on the Statement of Total Comprehensive Income (Loss). Equity interests in the form of common stock or in-substance common stock are classified as trading securities and reported at fair value while limited partner interests in such funds are reported using the equity method. Fair value is based primarily on quotes obtained from independent market sources. When quotes for fixed maturity securities are not available or cannot be reasonably corroborated, valuation models are used to estimate fair value. These models include estimates, made by management, which utilize current market information. When fair value is not readily determinable for pooled investment funds, the investments are valued using net asset value ("NAV") as a practical expedient as permitted under the Fair Value Measurement Topic of the ASC. Investment valuations could differ materially from amounts that would actually be realized in the market. Realized gains and losses on the sale of investments are determined on the basis of specific identification. VIE investments in fixed maturity securities are carried at fair value as they are either considered as available for sale securities or under the fair value option election. For additional information about VIE investments, including fair value by asset-type, see Note 4. Variable Interest Entities . Ambac has a formal credit impairment review process for fixed maturity available-for-sale securities in its investment portfolio. Ambac conducts a review each quarter to identify and evaluate investments that have indications of impairment in accordance with the Investments - Debt Securities Topic of the ASC. • Prior to the adoption of CECL, factors considered to identify and assess securities for other than temporary impairment include: (i) fair values that have declined by 20% or more below amortized cost; (ii) market values that have declined by 5% or more but less than 20% below amortized cost for a continuous period of at least six months; (iii) recent downgrades by rating agencies; (iv) the financial condition of the issuer and financial guarantor, as applicable, and an analysis of projected defaults on the underlying collateral; (v) whether scheduled interest payments are past due; (vi) whether Ambac has the intent to sell the security; and (vii) whether it is more likely than not that Ambac will be required to sell a security before the anticipated recovery of its amortized cost basis. If we believed a decline in the fair value of a particular investment is not credit-related, we recorded the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If it was determined that a credit impairment existed, the credit impairment loss was recognized in earnings, and the other-than-temporary amount related to all other factors was recognized in other comprehensive income. For fixed maturity securities that have credit impairments in a period, the previous amortized cost of the security less the amount of the credit impairment recorded through earnings becomes the investment’s new amortized cost basis. Ambac accretes the new amortized cost basis to par or to the estimated future cash flows to be recovered over the expected remaining life of the security. • Under CECL, credit losses are evaluated and measured similarly, however the recognition of credit impairment losses for available-for-sale debt securities are recorded as an allowance for credit losses with an offsetting charge to net income, rather than as a direct write-down of the security as was required under prior GAAP. As a result, improvements to estimated credit losses for available-for-sale debt securities are recognized immediately in net income rather than as interest income over time. Furthermore, as required under CECL, Ambac no longer considers the length of time a security has continuously been in an unrealized loss in the credit impairment process. If we believe a decline in the fair value of a particular investment is not credit impaired, we record the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If management either: (i) has the intent to sell its investment in a debt security or (ii) determines that the Company more likely than not will be required to sell the debt security before its anticipated recovery of the amortized cost basis less any current period credit impairment, then an impairment charge is recognized in earnings, with the amortized cost of the security being written-down to fair value. The evaluation of securities for credit impairment is a quantitative and qualitative process, which is subject to risks and uncertainties and is intended to determine whether, and to what extent, declines in the fair value of investments should be recognized in current period earnings. The risks and uncertainties include changes in general economic conditions, the issuer’s or guarantor’s financial condition and/or future prospects, the impact of regulatory actions on the investment portfolio, the performance of the underlying collateral, the effects of changes in interest rates or credit spreads and the expected recovery period. With respect to Ambac insured securities owned, future cash flows used to measure credit impairment represents the sum of (i) the bond’s intrinsic cash flows and (ii) the estimated AAC claim payments. Ambac’s assessment about whether a decline in value is considered a credit impairment reflects management’s current judgment regarding facts and circumstances specific to a security and the factors noted above. If that judgment changes, Ambac may ultimately record a charge for other-than-temporary impairment in future periods. Ambac has made certain accounting policy elections related to accrued interest receivable ("AIR") for available-for-sale investments under CECL, which are consistent with past practices under prior GAAP. Elections include: i) not measuring AIR for credit impairment, instead AIR is written off when it becomes 90 days past due; ii) writing off AIR by reversing interest income; iii) presenting AIR separately in Other Assets on the balance sheet and iv) excluding AIR from amortized cost balances in required CECL disclosures found in Note 11. Investments. AIR at December 31, 2020 was $10. Refer to Note 11. Investments for further credit impairment disclosures. Net Premiums Gross premiums were received either upfront or in installments. For premiums received upfront, an unearned premium revenue (“UPR”) liability was established, which was initially recorded as the cash amount received. For installment premium transactions, a premium receivable asset and offsetting UPR liability was initially established in an amount equal to: (i) the present value of future contractual premiums due (the “contractual” method) or (ii) if the underlying insured obligation is a homogenous pool of assets which are contractually prepayable, the present value of premiums to be collected over the expected life of the transaction (the “expected” method). An appropriate risk-free rate corresponding to the weighted average life of each policy and currency is used to discount the future premiums contractually due or expected to be collected. For example, U.S. dollar exposures are discounted using U.S. Treasury rates while exposures denominated in a foreign currency are discounted using the appropriate risk-free rate for the respective currency. The weighted average risk-free rate at December 31, 2020 and 2019, was 2.2%. and 2.4%, respectively, and the weighted average period of future premiums used to estimate the premium receivable at December 31, 2020 and 2019, was 8.3 years and 8.5 years, respectively. Insured obligations consisting of homogeneous pools for which Ambac uses expected future premiums to estimate the premium receivable include residential mortgage-backed securities ("RMBS"). As prepayment assumptions change for homogenous pool transactions, or if there is an actual prepayment for a “contractual” method installment transaction, the related premium receivable and UPR are adjusted in equal and offsetting amounts with no immediate effect on earnings using new premium cash flows and the then current risk-free rate corresponding to the initial weighted average life of the related policy. For both upfront and installment premium policies, premium revenues are earned over the life of the financial guarantee contract in proportion to the insured principal amount outstanding at each reporting date (referred to as the level-yield method). For installment paying policies, the premium receivable discount, equating to the difference between the undiscounted future installment premiums and the present value of future installment premiums, is accreted as premiums earned in proportion to the premium receivable balance at each reporting date. • For financial guarantee contracts, the issuer's ability and willingness to pay its insured debt obligation impacts the payment of policy losses by Ambac as well as the receipt of premiums from the issuer. As such, management leverages its existing loss reserve estimation process to evaluate credit impairment for premium receivables. Key factors in assessing credit impairment include historical premium collection data, internal risk classifications, credit ratings and loss severities. For structured finance transactions involving special purpose entities, we further evaluate the priority of premiums paid to Ambac within the contractual waterfall, as required by bond indentures. Ambac has a formal quarterly credit impairment review process for premium receivables under financial guarantee insurance contracts. • Prior to the adoption of CECL, Ambac assessed collectability of premium receivables in accordance with ASC 944 and recorded an allowance for uncollectible premiums. • Under CECL, management utilizes either a discounted cash flow ("DCF") or probability of default/loss given default ("PD/LGD") approach to estimate credit impairment. The DCF approach utilizes expected cash flows developed by Ambac's Risk Management Group using the same (or similar) models used for estimating loss reserves where such models can identify shortfalls in premiums. Credit impairment using the DCF approach is equal to the difference between amortized cost and the present value of expected cash flows. Credit impairment under the PD/LGD approach is the product of (i) the premium receivable carrying value, (ii) internally developed default probability (considering internal ratings and average life), and (iii) internally developed loss severities. Refer to Note 8. Financial Guarantee Insurance Contracts for further credit impairment disclosures. AAC has reinsurance in place pursuant to surplus share treaty and facultative reinsurance agreements. Similar to gross premiums, premiums ceded to reinsurers were paid either upfront or in installments. For premiums paid upfront, a deferred ceded premium asset was established which is initially recorded as the cash amount paid. For installment premiums, a ceded premiums payable liability and offsetting deferred ceded premium asset were initially established in an amount equal to: i) the present value of future contractual premiums due or ii) if the underlying insured obligation is a homogenous pool of assets, the present value of expected premiums to be paid over the life of the transaction. An appropriate risk-free rate corresponding to the weighted average life of each policy and exposure currency is used to discount the future premiums contractually due or expected to be collected. Premiums ceded to reinsurers reduce the amount of premiums earned by Ambac from its financial guarantee insurance policies. For both upfront and installment premiums, ceded premiums written are primarily recognized in earnings in proportion to and at the same time as the related gross premium revenue is recognized. For premiums paid to reinsurers on an installment basis, Ambac records the present value of future ceding commissions as an offset to ceded premiums payable, using the same assumptions noted above for installment premiums. When a bond issue insured by Ambac has been retired early, typically due to an issuer call, any remaining UPR is recognized at that time to the extent the financial guarantee contract is legally extinguished, causing accelerated premium revenue. For installment premium paying transactions, we offset the recognition of any remaining UPR by the reduction of the related premium receivable to zero (as it will not be collected as a result of the retirement), which may cause negative accelerated premium revenue. Certain obligations insured by Ambac have been legally defeased whereby government securities are purchased by the issuer with the proceeds of a new bond issuance, or less frequently with other funds of the issuer, and held in escrow. The principal and interest received from the escrowed securities are then used to retire the Ambac-insured obligations at a future date either to their maturity date (a refunding) or a specified call date (a pre-refunding). Ambac has evaluated the provisions in policies issued on these obligations and determined those insurance policies have not been legally extinguished. For policies with refunding securities, premium revenue recognition is not impacted as the escrowed maturity date is the same as the previous legal maturity date. For policies with pre-refunding securities, the maturity date of the pre-refunded security has been shortened from its previous legal maturity. Although premium revenue recognition has not been accelerated in the period of the pre-refunding, it results in an increase in the rate at which the policy's remaining UPR is to be recognized. Loans Loans are reported at either their outstanding principal balance less unamortized discount or at fair value. • Loans not held by consolidated VIEs are reported at their outstanding principal balance less unamortized discount and are reported within Other assets on the Consolidated Balance Sheet. Interest income is earned using the effective interest method based upon interest accrued on the unpaid principal balance adjusted for accretion of discounts. A loan is considered impaired when, based on the financial condition of the borrower, it is probable that Ambac will be unable to collect all principal and interest due according to the contractual terms of the loan agreement. Under CECL, Ambac has a formal quarterly credit impairment review process for these loans. The key factors in assessing credit impairment are internal credit ratings and loss severities. Management utilizes a PD/LGD approach, similar to the one described above for premium receivables, which is applied to the loan carrying value. • Loans held by VIEs consolidated as required under the Consolidation Topic of the ASC are carried at fair value under the fair value option election with changes in fair value recorded in Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). Such loans are reported as Loans, at fair value within the Variable interest entity assets section of the Consolidated Balance Sheet. Derivative Contracts The Company has entered into derivative contracts to hedge certain economic risks inherent in its asset and liability portfolios. None of Ambac’s derivative contracts are designated as hedges under the Derivatives and Hedging Topic of the ASC. Ambac's derivatives consist primarily of interest rate swaps and futures contracts. • Ambac maintains a portfolio consisting primarily of interest rate swaps and futures contracts to economically hedge interest rate risk in the financial guarantee and investment portfolios. While this portfolio also includes certain legacy interest rate swaps executed in connection with financial guarantee client financings, the interest rate derivatives portfolio is managed on the basis of its net sensitivity to changes in interest rates. Changes in the fair value of these interest rate derivatives are recorded, along with changes in fair value of Ambac's remaining credit derivatives, within Net gains (losses) on derivative contracts on the Consolidated Statements of Total Comprehensive Income (Loss). • VIEs consolidated under the Consolidation Topic of the ASC entered into derivative contracts to meet specified purposes within their securitization structure. Changes in fair value of consolidated VIE derivatives are included within Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). All derivatives are recorded on the Consolidated Balance Sheets at fair value on a gross basis; assets and liabilities are netted by counterparty only when a legal right of offset exists. Variation payments on centrally cleared swaps and futures contracts are considered settlements of the associated derivative balances and are reflected as a reduction to derivative liabilities or assets on the Consolidated Balance Sheets. For other derivatives, Ambac has determined that the amounts recognized for the right to reclaim cash collateral or the obligation to return cash collateral may not be used to offset amounts due under the derivative instruments in the normal course of settlement. Therefore, such amounts are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement and are included in "Other assets" on the Consolidated Balance Sheets. Refer to Note 12. Derivative Instruments for further discussion of the Company’s use of derivative instruments and their impact of the consolidated financial statements. Refer to Note 10. Fair Value Measurements for further description of the methodologies used to determine the fair value of derivative contracts, including model inputs and assumptions where applicable. Goodwill Goodwill of $46 is attributable to the Xchange acquisition, further discussed in Note 3. Business Combination and represents the acquisition cost in excess of the fair value of net assets acquired, including identifiable intangible assets. Goodwill is assigned at acquisition to the applicable reporting unit of the acquired entity giving rise to the goodwill. Goodwill is not amortized but is subject to impairment testing. Goodwill impairment tests are performed annually or more frequently if circumstances indicate a possible impairment. If, after assessing qualitative factors, management believes it is more likely than not that the fair value of a reporting unit is less than its carrying amount, a quantitative impairment evaluation is performed. The quantitative goodwill test compares the estimated fair value of the reporting unit with its carrying value (including goodwill and identifiable intangible assets). An impairment is recognized for the excess of the carrying amount of the reporting unit over it estimated fair value. If the reporting unit’s estimated fair value exceeds its carrying value, goodwill is not impaired. As the Xchange acquisition occurred on December 31, 2020 no goodwill impairment evaluation was performed in 2020. Intangible Assets Financial Guarantee Insurance intangible: Upon Ambac's emergence from bankruptcy in 2013, an insurance intangible asset was recorded which represented the difference between the fair value and aggregate carrying value of the financial guarantee insurance and reinsurance assets and liabilities. The carrying values of our financial guarantee insurance and reinsurance contracts continue to be reported and measured in accordance with their existing accounting policies. Pursuant to the Financial Services-Insurance Topic of the ASC, the insurance intangible is to be measured on a basis consistent with the related financial guarantee insurance and reinsurance contracts. The insurance intangible asset carrying value is $373 at December 31, 2020 and is amortized using a level-yield method based on par exposure of the related financial guarantee insurance or reinsurance contracts and is applied to groups of contracts with similar characteristics. Acquired intangible assets: Ambac acquired $36 of identifiable intangible assets attributable to the Xchange acquisition, further discussed in Note 3. Business Combination . The intangible assets are primarily related to distribution relationships, non-compete agreements and trade names, all of which have finite lives and are amortized over their estimated useful lives using the straight-line method. The Company tests finite-lived acquired intangible assets for impairment if certain events occur or circumstances change indicating that the carrying amount of the intangible asset may not be recoverable. To the extent the carrying value of an asset or asset group exceeds the projected undiscounted cash flows expected to result from the use and eventual disposal of the asset or asset group, the Company determines the asset or asset group is impaired and records an impairment equal to the difference between the estimated fair value and the carrying value of the asset or asset group. In addition, we will recognize an impairment prior to the sale of an asset or asset group if the carrying value of the asset or asset group exceeds its estimated fair value. As the Xchange acquisition occurred on December 31, 2020, there was no amortization expense nor any impairment of the intangible assets in 2020. Restricted Cash Cash that we do not have the right to use for general purposes is recorded as restricted cash in our consolidated balance sheets. Restricted cash includes (i) consolidated variable interest entity cash restricted to support the obligations of the consolidated VIEs, (ii) cash held by AAC received from its investment in Secured Notes and pledged for the benefit of holders of Secured Notes (other than AAC) and (iii) fiduciary cash held by Xchange described below. Fiduciary Assets and Liabilities : In Xchange's capacity as a managing general agent, generally it collects premiums from insureds and remits the premiums to the respective insurance carriers, net of fees to other parties, including its commissions. Xchange also collects claims or refunds from carriers on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by Xchange in a fiduciary capacity. Since fiduciary assets are not available for corporate use, they are shown in the consolidated balance sheets as an offset to fiduciary liabilities, which are reported in Other liabilities. Restricted cash for net uncollected premiums and claims and the related fiduciary liabilities were $4 at December 31, 2020. Loss and Loss Expenses The loss and loss expense reserve (“loss reserve”) policy relates only to Ambac’s non-derivative insurance business for insurance policies issued to beneficiaries, including VIEs, for which we do not consolidate the VIE. Losses and loss expenses are based upon estimates of the ultimate aggregate losses inherent in the non-derivative financial guarantee portfolio as of the reporting date. The policy for derivative contracts is discussed in the “Derivative Contracts” section above. A loss reserve is recorded on the balance sheet on a policy-by-policy basis based upon the present value ("PV") of expected net claim cash outflows or expected net recovery cash inflows, discounted at risk-free rates. The estimate for future net cash flows consider the likelihood of all possible outcomes that may occur from missed principal and/or interest payments on the insured obligation. This estimate also considers future recoveries related to breaches of contractual representations and warranties by RMBS transaction sponsors, remediation strategies, excess spread and other contractual or subrogation-related cash flows. Ambac’s approach to resolving disputes involving contractual breaches by transaction sponsors or other third parties has included negotiations and/or pursuing litigation. Ambac does not estimate recoveries for litigations where its sole claim is for fraudulent inducement, since any remedies under such claims would be non-contractual. • Net claim cash outflow policies represent contracts where the PV of expected cash outflows are greater than the PV of expected recovery cash inflows. For such policies, a “Loss and loss expense reserves” liability is recorded for the exc

Business Combinations

Business Combinations	12 Months Ended
Business Combinations	Dec. 31, 2020
Business Combinations [Abstract]
Business Combination Disclosure	3. BUSINESS COMBINATION On December 31, 2020, Ambac completed the acquisition of 80% of the membership interests of Xchange for a purchase price of $81 in cash. Xchange, whose management principals retained the remaining 20% is a property and casualty Managing General Underwriter ("MGU"), specializing in accident and health insurance. Since its inception in 2010, Xchange's business has been supported by major insurers, reinsurers, third party administrators, brokers and producers. The acquisition has been accounted for as a business combination and advances Ambac's strategy of expanding into the MGU and Managing General Agent ("MGA") sectors. Based on the acquisition date and the complexity of the underlying valuation work, certain amounts included in the Company's Consolidated Financial Statements may be provisional and thus subject to further adjustments within the permitted measurement period as defined by ASC 805. The following table summarizes the consideration paid for Xchange and the estimated fair values of the aggregate assets and liabilities acquired, as well as the fair value of the noncontrolling interest, at the acquisition date: Fair Value Cash $ 2 Restricted cash 4 Intangible assets 36 Goodwill 46 Other assets 8 Total assets acquired $ 96 Other liabilities 8 Total liabilities assumed 8 Less: Redeemable noncontrolling interest 7 Total consideration $ 81 Goodwill was recorded to reflect the excess purchase consideration over net assets acquired and primarily consists of the future economic benefits that we expect to receive as a result of the acquisition, driven by the value of Xchange's potential future distribution and carrier relationships, and synergies with other Ambac business operations. Goodwill that is expected to be deductible for tax purposes amounts to approximately $36. The fair values assigned to tangible and identifiable intangible assets acquired and liabilities assumed are based on management’s estimates and assumptions at the time of acquisition and is subject to updating as more detailed analyses are completed and additional information about the fair value of assets acquired and liabilities assumed becomes available. The fair value of the redeemable non-controlling interest of $7 was estimated based on the non-controlling interest’s respective share of Xchange's enterprise value, adjusted for the value of Ambac's call option to purchase, and the minority owners' put option to sell to Ambac, respectively, the remaining 20% membership interests in Xchange. Please refer to the Noncontrolling Interests section of Note 2. Basis of Presentation and Significant Accounting Policies , for further information regarding the terms of the call and put option, as well as the redeemable noncontrolling interest balance sheet classification. The following table sets forth the estimated fair values of identifiable intangible assets acquired and their estimated useful lives as of the date of acquisition. Fair Weighted Distribution relationships $ 33 15.0 Non-compete agreements 1 5.0 Trade name 1 8.0 Total $ 36 The distribution relationships intangible represents existing relationships Xchange maintains with a variety of brokers and distributors across its product lines. It excludes the value of potential future distribution relationships that may be developed, which is included in goodwill. The non-compete agreements intangible relates to agreements entered into with certain key management personnel of Xchange. The trade name intangible represents the rights to the Xchange Group brand name which is well known in the marketplace Xchange competes in. The overall weighted average useful life of the identified amortizable intangible assets acquired is fourteen As of December 31, 2020, future annual amortization of finite-lived acquired intangible assets for the years 2021 through 2025 and thereafter is estimated to be: Year Estimated 2021 $ 3 2022 3 2023 3 2024 3 2025 3 Thereafter 22 Total $ 36 Because the acquisition occurred on last day of the reporting period, there were no revenues or earnings of Xchange included in Ambac's Consolidated Statement of Comprehensive Income for the period ended December 31, 2020. Pro forma information related to the acquisition has not been presented as the impact was not material to the Company’s financial results.

Net Income Per Share

Net Income Per Share	12 Months Ended
Net Income Per Share	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Income Per Share	6. NET INCOME PER SHARE As of December 31, 2020, 45,809,139 shares of AFG's common stock (par value $0.01) and warrants entitling holders to acquire up to 4,877,749 shares of new common stock at an exercise price of $16.67 per share were outstanding. For the three years ended December 31, 2020, 2019 and 2018, 34, 0 and 194 warrants were exercised, respectively, resulting in an issuance of 8, — and 194 shares of common stock, respectively. On June 30, 2015, the Board of Directors of AFG authorized the establishment of a warrant repurchase program that permits the repurchase of up to $10 of warrants. On November 3, 2016, the Board of Directors of AFG authorized a $10 increase to the warrant repurchase program. For the years ended December 31, 2020 and 2019, AFG did not repurchase any warrants. As of December 31, 2020, AFG had repurchased 985,331 warrants at a total cost of $8 (average cost of $8.21 per warrant). The remaining aggregate authorization at December 31, 2020 was $12. In connection with the AMPS Exchange, AFG issued 824,307 of the repurchased warrants at a price of $9.72 per warrant on August 3, 2018. Refer to Note 1. Background and Business Description for further discussion of the AMPS Exchange. The following table provides a reconciliation of the common shares used for basic net income per share to the diluted shares used for diluted net income per share: Year Ended December 31, 2020 2019 2018 Basic weighted average shares outstanding 46,147,062 45,954,908 45,665,883 Effect of potential dilutive shares (1) : Warrants — — 441,104 Stock options — — — Restricted stock units — — 77,572 Performance stock units (2) — — 375,276 Diluted weighted average shares outstanding 46,147,062 45,954,908 46,559,835 Anti-dilutive shares excluded from the above reconciliation Stock options 16,121 16,667 16,667 Warrants 4,877,754 4,877,783 — Restricted stock units 302,145 249,263 — Performance stock units (2) 1,002,501 872,258 — (1) For the years ended December 31, 2020 and 2019 , Ambac had a net loss and accordingly excluded all potentially dilutive securities from the determination of diluted loss per share as their impact was anti-dilutive. (2) Performance stock units are reflected based on the performance metrics through the balance sheet date. Vesting of these units is

Financial Guarantees in Force (

Financial Guarantees in Force (Notes)	12 Months Ended
Financial Guarantees in Force (Notes)	Dec. 31, 2020
Guarantees [Abstract]
Financial Guarantees in Force	7. FINANCIAL GUARANTEES IN FORCE Financial guarantees outstanding includes the exposures of policies that insure variable interest entities (“VIEs”) consolidated in accordance with ASC Topic 810, Consolidation. Financial guarantees outstanding includes the exposure of policies that insure capital appreciation bonds which are reported at the par amount at the time of issuance of the insurance policy as opposed to the current accreted value of the bonds. Financial guarantees outstanding exclude the exposures of policies that insure bonds which have been called, pre-refunded or refunded and excludes exposure of the policy that insures the notes issued by Ambac LSNI as defined in Note 1. Background and Business Description . The gross par amount of financial guarantees outstanding was $39,070 and $43,908 at December 31, 2020 and 2019, respectively. The par amount of financial guarantees outstanding, net of reinsurance, was $33,888 and $38,018 at December 31, 2020 and 2019, respectively. As of December 31, 2020, the aggregate amount of insured par ceded by AAC to reinsurers under reinsurance agreements was $5,182 with the largest reinsurer accounting for $2,398 or 6.1% of gross par outstanding at December 31, 2020. As of December 31, 2020 and 2019, the guarantee portfolio was diversified by type of guaranteed bond as shown in the following table: December 31, Net Par Outstanding December 31, 2020 2019 Public Finance: Housing revenue $ 5,855 $ 5,991 Lease and tax-backed revenue 4,179 5,102 General obligation 2,345 3,011 Transportation revenue 771 855 Higher education 747 885 Utility revenue 675 768 Other 925 1,041 Total Public Finance 15,497 17,653 Structured Finance: Mortgage-backed and home equity 3,635 4,423 Investor-owned utilities 1,617 1,675 Student loan 626 769 Structured Insurance 311 395 Asset-backed and other 148 246 Total Structured Finance 6,337 7,508 International Finance: Sovereign/sub-sovereign 5,270 5,264 Investor-owned and public utilities 3,899 4,436 Transportation 1,511 1,532 Asset-backed and other 1,374 1,625 Total International Finance 12,054 12,857 Total $ 33,888 $ 38,018 (1) Includes $5,575 and $5,654 of Military Housing net par at December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, the International Finance guaranteed portfolio by location of risk was as outlined in the table below: Net Par Outstanding December 31, 2020 2019 United Kingdom $ 9,711 $ 10,593 Italy 803 767 Austria 707 674 Australia 420 382 France 277 303 Other international (1) 136 138 Total International Finance $ 12,054 $ 12,857 (1) Other international may include components of U.S. exposure. Gross financial guarantees in force (principal and interest) were $61,895 and $69,826 at December 31, 2020 and 2019, respectively. Net financial guarantees in force (after giving effect to reinsurance) were $51,603 and $58,245 as of December 31, 2020 and 2019, respectively. In the United States, Colorado, California and New York were the states with the highest aggregate net par amounts in force, accounting for 7.0%, 6.2% and 5.4% of the total at December 31, 2020, respectively. No other state accounted for more than 5%. The highest single insured risk represented 2.9% of the aggregate net par amount guaranteed.

Financial Guarantee Insurance_3

Financial Guarantee Insurance Contracts	12 Months Ended
Financial Guarantee Insurance Contracts	Dec. 31, 2020
Insurance [Abstract]
Summary of Rollforward of RMBS Subrogation, by Estimation Approach	Below is the rollforward of RMBS R&W subrogation for the affected periods: Year ended December 31, 2020 2019 2018 Discounted RMBS subrogation recovery (gross of reinsurance) at beginning of year $ 1,727 $ 1,771 $ 1,834 All other changes (1) 23 (43) (64) Discounted RMBS subrogation recovery (gross of reinsurance) at end of year $ 1,751 $ 1,727 $ 1,771

Insurance Regulatory Restrictio

Insurance Regulatory Restrictions (Notes)	12 Months Ended
Insurance Regulatory Restrictions (Notes)	Dec. 31, 2020
Regulatory Assets and Liabilities Disclosure [Abstract]
Insurance Regulatory Restrictions	9. INSURANCE REGULATORY RESTRICTIONS United States AAC is domiciled in the State of Wisconsin and, as such, it is subject to the insurance laws and regulations of the State of Wisconsin (the “Wisconsin Insurance Laws”) and is regulated by the OCI. Everspan Indemnity Insurance Company ("Everspan Indemnity") and its wholly owned subsidiary, Everspan Insurance Company ("Everspan Insurance" and, together with Everspan Indemnity, "Everspan" or the “Everspan Group”), are domiciled in Arizona and are subject to the insurance laws and regulations of Arizona (the “Arizona Insurance Laws” and together with the Wisconsin Insurance Laws, the “State Insurance Laws”). Everspan is regulated by the Arizona Department of Insurance and Financial Institutions (“DIFI”). In addition, both Ambac Assurance and Everspan Insurance are subject to the insurance laws and regulations of the other jurisdictions in which they are licensed. Insurance laws and regulations applicable to insurers vary by jurisdiction, but the insurance laws and regulations applicable to our insurance carriers generally require them to maintain minimum standards of business conduct and solvency; to meet certain financial tests; and to file policy forms, premium rate schedules and certain reports with regulatory authorities, including information concerning capital structure, ownership, financial condition, corporate governance and enterprise risk. Regulated insurance companies are also required to file quarterly and annual statutory financial statements in each jurisdiction in which they are licensed. The level of supervisory authority that may be exercised by non-domiciliary insurance regulators varies by jurisdiction. Generally, however, non- domiciliary regulators are authorized to suspend or revoke the insurance license they issued and to impose restrictions on that license in the event that laws or regulations are breached by a regulated insurance company or in the event that continued or unrestricted licensing of the regulated insurance company constitutes a “hazardous condition” (or meets a similar standard) in the opinion of the regulator. The domiciliary regulators of Ambac Assurance and Everspan, OCI and DIFI, respectively, have primary regulatory authority, including with respect to the initiation and administration of rehabilitation or liquidation proceedings. Additionally, the accounts and operations of Ambac Assurance and Everspan are subject to periodic comprehensive financial examinations by, respectively, the OCI and DIFI. The State Insurance Laws require regulated insurance companies to maintain minimum standards of business conduct, maintain minimum surplus to policyholders, meet certain financial tests, and file certain reports, including information concerning their capital structure, ownership, financial condition, corporate governance and enterprise risk. The State Insurance Laws also require prior approval by OCI and DIFI, respectively, of certain transactions between AAC or Everspan, respectively, and their affiliates. Ambac Assurance, because it is a financial guarantee insurer is not subject to risk-based capital requirements. In December 2020, Everspan Insurance completed its re-domestication from Wisconsin to Arizona and obtained broad authority to write property and casualty insurance (while contemporaneously surrendering its authority to write financial guaranty insurance) in Arizona. Everspan Insurance is seeking similar amendments to its certificates of authority in all other states. Everspan Insurance is subject to risk-based capital requirements. Everspan Indemnity was formed in 2020 as a domestic surplus lines insurer in Arizona and, accordingly, is eligible to write property and casualty insurance as an excess and surplus lines insurer in all states by virtue of the U.S. Nonadmitted and Reinsurance Reform Act of 2010. Everspan Indemnity is subject to risk-based capital requirements. Neither Everspan Insurance nor Everspan Indemnity has yet issued any new policies. Ambac Assurance and Everspan are in compliance with the minimum capital and surplus levels required under the State Insurance Laws required to transact all business written to date. Xchange, like other managing general agents and program administrators, is subject to licensing requirements and regulation by insurance regulators in various states in which they conduct business. Every state and Washington, D.C. have enacted a version of the NAIC Model Managing General Agents Act, which governs licensing and the relationship between insurers and managing general agents. In addition to the legal restrictions applicable to AAC as described herein, pursuant to the terms of the Settlement Agreement, the Stipulation and Order and the indenture for the Tier 2 Notes, AAC must seek prior approval by OCI of certain corporate actions. The Settlement Agreement, Stipulation and Order and indenture for the Tier 2 Notes include covenants which restrict the operations of AAC. The Settlement Agreement will remain in force until the surplus notes that were issued pursuant to the Settlement Agreement have been redeemed, repurchased or repaid in full. The Stipulation and Order will remain in force for so long as OCI determines it to be necessary. The indenture for the Tier 2 Notes will remain in force until the Tier 2 Notes have been redeemed, repurchased or repaid in full. Certain of the restrictions in the Settlement Agreement and the indenture for the Tier 2 Notes may be waived with the approval of the OCI and/or the requisite percentage of holders of the related debt securities. Although not domiciled in New York, AAC is nevertheless subject to the New York insurance law governing financial guarantee insurers. New York’s comprehensive financial guarantee insurance law defines the scope of permitted financial guarantee insurance and governs the conduct of business of all financial guarantors licensed to do business in New York, including AAC. The New York financial guarantee insurance law also establishes single and aggregate risk limits with respect to insured obligations insured by financial guarantee insurers. Such single risk limits are specific to the type of insured obligation (for example, municipal or asset-backed). Under the aggregate limits, policyholders’ surplus and contingency reserves must at least equal a percentage of aggregate net liability that is equal to the sum of various percentages of aggregate net liability for various categories of specified obligations. At December 31, 2020, AAC is in compliance with applicable aggregate risk limits but not in compliance with applicable single risk limits. Through run-off of the portfolio, AAC will continue to seek the reduction in its exposure for compliance with applicable single and aggregate risk limits, but may not be able to do so. The financial statements of AAC and Everspan are prepared on the basis of accounting practices prescribed or permitted by the State Insurance Laws and OCI and DIFI actions thereunder. AAC and Everspan use such statutory accounting practices prescribed or permitted by the OCI and DIFI, respectively, for determining and reporting their financial condition and results of operations, including for determining solvency under the State Insurance Laws. Both Wisconsin and Arizona have adopted the National Association of Insurance Commissioners (“NAIC”) accounting practices and procedures manual (“NAIC SAP”) as a component of prescribed practices as codified in each state’s applicable law or regulation. Statutory policyholder surplus differs from stockholder's equity determined under GAAP principally due to statutory accounting rules that treat financial guarantee premiums and loss reserves, investments, consolidation of subsidiaries or variable interest entities and surplus notes differently. • AAC’s statutory policyholder surplus was $865 at December 31, 2020, as compared to $1,088 as of December 31, 2019. • Everspan Indemnity has statutory policyholder surplus of $26 as of December 31, 2020. At December 31, 2020, there were no significant differences from stockholder's equity under GAAP. Additionally, the OCI has prescribed additional practices and has permitted accounting practices for AAC. As a result of the prescribed and permitted practices discussed below, AAC’s statutory surplus at December 31, 2020 and 2019 was higher by $40 and lower by $12, respectively, than if AAC had reported such amounts in accordance with NAIC SAP. Everspan does not have any prescribed or permitted practices at December 31, 2020 or December 31, 2019. Prescribed Accounting Practices OCI has prescribed the following accounting practices that differ from NAIC SAP for AAC: • Paragraph 8 of Statement of Statutory Accounting Principles No. 60 “Financial Guaranty Insurance” allows for a deduction from loss reserves for the time value of money by application of a discount rate equal to the average rate of return on the admitted assets of the financial guaranty insurer as of the date of the computation of the reserve. The discount rate shall be adjusted at the end of each calendar year. Additionally, in accordance with paragraph 13.e of Statutory Accounting Principles No. 97 "Investments in Subsidiary, Controlled and Affiliated Entities" and paragraph 8 of Statutory Accounting Principles No. 5R “Liabilities, Contingencies and Impairments of Assets - Revised”, AAC records probable losses on its subsidiaries for which it guarantees their obligations. AAC also discounts probable losses on guarantees of subsidiary obligations using a discount rate equal to the average rate of return on its admitted assets. AAC’s average rates of return on its admitted assets at December 31, 2020 and 2019 were 4.56% and 5.43%, respectively. OCI has directed AAC to utilize a prescribed discount rate of 5.10% for the purpose of discounting both its loss reserves and its estimated impairment losses on subsidiary guarantees. • Paragraph 4 of Statement of Statutory Accounting Principles No. 41 “Surplus Notes” (“SSAP 41”) states that proceeds received by the issuer of surplus notes must be in the form of cash or other admitted assets having readily determinable values and liquidity satisfactory to the commissioner of the state of domicile. Under statutory accounting principles, surplus notes issued in conjunction with commutations or the settlement of obligations would be valued at zero upon issuance pursuant to paragraph 4, SSAP 41. OCI has directed the Company to record surplus notes issued in connection with commutations or the settlement of obligations at full par value upon issuance. The surplus notes issued have a claim against surplus senior to the preferred and common shareholders. • Paragraph 35 of Statement of Statutory Accounting Principles No. 43R ”Loan-backed and Structured Securities” states that when an other-than-temporary impairment ("OTTI") has occurred, the amount of the OTTI recognized as a realized loss shall equal the difference between the investment’s amortized cost basis and the present value of cash flows expected to be collected, discounted at the loan-backed or structured security’s effective interest rate. Beginning June 11, 2014, as a result of the amended Segregated Account Rehabilitation Plan, OCI has directed the Company to not evaluate investments in AAC insured securities with policies that were allocated to the Segregated Account for OTTI and require all such investments be reported at amortized cost regardless of its NAIC risk designation. This accounting determination was intended to recognize that AAC continues to maintain statutory loss reserves without adjustment for the economic effects of its ownership of the insured investment securities, improve transparency to the users of the statutory financial statements and to minimize operational risks. Effective February 12, 2018, with the Segregated Account's exit from Rehabilitation, this prescribed practice is no longer applicable for OTTI evaluations going forward. Permitted Accounting Practices OCI has allowed the following permitted practice for AAC: • Wisconsin accounting practices for changes to contingency reserves differ from NAIC SAP. Under NAIC SAP, contributions to and releases from the contingency reserve are recorded via a direct charge or credit to surplus. Under the Wisconsin Administrative Code, contributions to and releases from the contingency reserve are to be recorded through underwriting income. AAC received permission from OCI to record contributions to and releases from the contingency reserve, in accordance with NAIC SAP. United Kingdom The Prudential Regulatory Authority (“PRA”) and Financial Conduct Authority (“FCA”) (and their predecessor regulator the Financial Services Authority (“FSA”)) are the dual statutory regulator responsible for regulating the financial services industry in the United Kingdom, with the purpose of maintaining confidence in the U.K. financial system, providing public understanding of the system, securing the proper degree of protection for consumers and helping to reduce financial crime. In addition, until December 31, 2020, the regulatory regime in the United Kingdom must have complied with certain EU legislation binding on all EU member states. These regulators have exercised significant oversight of Ambac UK since 2008, after Ambac, AAC and Ambac UK began experiencing financial stress. In 2009, Ambac UK’s license to write new business was curtailed by the FSA and the insurance license was limited to undertaking only run-off related activity. As such, Ambac UK is authorized to run-off its credit, suretyship and financial guarantee insurance portfolio in the United Kingdom, and (until December 31, 2020) to do the same through a branch in Milan, Italy, and a number of other European Union (“EU”) countries. Until December 31, 2020, EU legislation had allowed Ambac UK to conduct business in EU states other than the United Kingdom through a “passporting” arrangement, which eliminated the necessity of additional licensing or authorization in those other EU jurisdictions. These passporting arrangements ended on December 31, 2020, when the U.K.’s Brexit transitional arrangements with the EU ended. Ambac UK closed its Milan branch and transferred it's remaining policy to the United Kingdom in December 2020. Ambac UK's remaining policies in the EU were either commuted or the benefits of those policies were transferred to United Kingdom entities in advance of December 31, 2020. The PRA requires that non-life insurance companies such as Ambac UK maintain a margin of solvency at all times in respect of the liabilities of the insurance company, the calculation of which depends on the type and amount of insurance business a company writes. These solvency requirements were amended on January 1, 2016, in order to implement the European Union's "Solvency II" directive on risk-based capital. Notwithstanding the foregoing, Ambac UK is deficient in terms of compliance with currently applicable regulatory capital requirements under Solvency II directive. The PRA and FCA are aware of the same, and dialogue between Ambac UK management and its regulators remains ongoing with respect to options for addressing the shortcoming, although such options remain few. Dividend Restrictions, Including Contractual Restrictions Due to losses experienced by AAC, it has been unable to pay ordinary dividends to AFG since 2008 and will be unable to pay common dividends in 2021 without the prior consent of the OCI, which is unlikely. AAC’s ability to pay dividends is further restricted by the Settlement Agreement (as described below), by the indenture for the Tier 2 Notes (as described below), by the terms of its AMPS (as described below) and by the Stipulation and Order. See Note 1. Background and Business Description for further information. AAC is not expected to make dividend payments to AFG for the foreseeable future. Everspan does not have sufficient earned surplus at this time to pay ordinary dividends under the Arizona Insurance Laws. Subject to the foregoing, pursuant to the State Insurance Laws, AAC and Everspan may declare dividends, subject to restrictions in their respective articles of incorporation, provided that, after giving effect to the distribution, such dividends would not violate certain statutory solvency, surplus and asset tests. Board action authorizing a shareholder distribution by AAC (other than stock dividends) must be reported to the OCI at least 30 days prior to payment, unless the distribution is no more than 15% larger than for the corresponding period in the previous year. Everspan similarly must report to the DIFI all dividends and other distributions to shareholders within five business days following their declaration and at least ten business days before payment of the dividend or distribution. In addition, Wisconsin Insurance Laws restrict the payment of extraordinary dividends, which is any distribution which, together with distributions in the prior 12 months, is greater than the lesser of (a) 10% of policyholders’ surplus as of the preceding December 31, and (b) the greater of (i) statutory net income (loss) for the calendar year preceding the date of the dividend, minus realized capital gains for that calendar year or (ii) the aggregate of statutory net income (loss) for three calendar years preceding the date of the dividend, minus realized capital gains for those calendar years and minus dividends paid or credited within the first two of the three preceding calendar years. Extraordinary dividends must be reported to OCI at least 30 days prior to payment and are subject to disapproval by the OCI. Arizona Insurance Laws also restrict the payment of extraordinary dividends, which is any dividend or distribution which together with other dividends or distributions made within the preceding twelve months exceeds the lesser of (a) 10% of policyholders’ surplus as of the preceding December 31 and (b) the net income for the twelve month period ending the preceding December 31. Extraordinary dividends must be reported to DIFI at least 30 days prior to payment, during which period DIFI may disapprove or approve such payment. UK law prohibits Ambac UK from declaring a dividend to its shareholders unless it has “profits available for distribution.” The determination of whether a company has profits available for distribution is based on its accumulated realized profits less its accumulated realized losses. While the UK insurance regulatory laws impose no statutory restrictions on a general insurer’s ability to declare a dividend, the PRA’s and FCA’s capital requirements in practice act as a restriction on the payment of dividends. Further, the FSA amended Ambac UK’s license in 2010 such that the PRA must specifically approve (“non-objection”) any transfer of value and/or assets from Ambac UK to AAC or any other Ambac group company, other than in respect of certain disclosed contracts between the two parties (such as in respect of a management services agreement between AAC and Ambac UK). Ambac UK is not expected to pay any dividends to AAC for the foreseeable future. Pursuant to the Settlement Agreement, AAC may not make any “Restricted Payment” (which includes dividends from AAC to Ambac) in excess of $5 in the aggregate per annum, other than Restricted Payments from AAC to Ambac in an amount up to $8 per annum solely to pay operating expenses of Ambac. Concurrent with making any such Restricted Payment, a pro rata amount of AAC's surplus notes would also need to be redeemed at par. The indenture for the Tier 2 Notes contains a similar restrictive covenant and further requires a proportional payment of the Tier 2 Notes (or interest thereon) when payments are made on the surplus notes. Under the terms of AAC’s AMPS, dividends may not be paid on the common stock of AAC unless all accrued and unpaid dividends on the AMPS for the then current dividend period have been paid, provided, that dividends on the common stock may be made at all times for the purpose of, and only in such amounts as are necessary for, enabling Ambac (i) to service its indebtedness for borrowed money as such payments become due or (ii) to pay its operating expenses. If dividends are paid on the common stock as provided in the prior sentence, dividends on the AMPS become cumulative until the date that all accumulated and unpaid dividends have been paid on the AMPS. The Stipulation and Order requires OCI approval for the payment of any dividend or distribution on the common stock of AAC.

Derivative Instruments

Derivative Instruments	12 Months Ended
Derivative Instruments	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	12. DERIVATIVE INSTRUMENTS The following tables summarize the gross fair values of individual derivative instruments and the impact of legal rights of offset as reported in the Consolidated Balance Sheets as of December 31, 2020 and 2019. Gross Gross Net Amounts Gross Amount Net Amount December 31, 2020: Derivative Assets: Interest rate swaps $ 93 $ — $ 93 $ — $ 93 Total non-VIE derivative assets $ 93 $ — $ 93 $ — $ 93 Derivative Liabilities: Credit derivatives $ — $ — $ — $ — $ — Interest rate swaps 114 — 114 113 1 Total non-VIE derivative liabilities $ 114 $ — $ 114 $ 113 $ 1 Variable Interest Entities Derivative Assets: Currency swaps $ 41 $ — $ 41 $ — $ 41 Total VIE derivative assets $ 41 $ — $ 41 $ — $ 41 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,835 $ — $ 1,835 $ — $ 1,835 Total VIE derivative liabilities $ 1,835 $ — $ 1,835 $ — $ 1,835 Gross Gross Net Amounts Gross Amount Net Amount December 31, 2019: Derivative Assets: Interest rate swaps $ 75 $ — $ 75 $ — $ 75 Total non-VIE derivative assets $ 75 $ — $ 75 $ — $ 75 Derivative Liabilities: Credit derivatives $ — $ — $ — $ — $ — Interest rate swaps 89 — 90 89 1 Total non-VIE derivative liabilities $ 90 $ — $ 90 $ 89 $ 1 Variable Interest Entities Derivative Assets: Currency swaps $ 52 $ — $ 52 $ — $ 52 Total VIE derivative assets $ 52 $ — $ 52 $ — $ 52 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,657 $ — $ 1,657 $ — $ 1,657 Total VIE derivative liabilities $ 1,657 $ — $ 1,657 $ — $ 1,657 Amounts representing the right to reclaim cash collateral or the obligation to return cash collateral are not offset against fair value amounts recognized for derivative instruments on the Consolidated Balance Sheets. The amounts representing the right to reclaim cash collateral and posted margin, recorded in “Other assets” were $1 and $36 as of December 31, 2020 and 2019, respectively. There were no amounts held representing an obligation to return cash collateral as of December 31, 2020 and 2019. The following tables summarize the location and amount of gains and losses of derivative contracts in the Consolidated Statements of Total Comprehensive Income (Loss) for the years ended December 31, 2020, 2019 and 2018: Location of Gain (Loss) Recognized Amount of Gain (Loss) Recognized in Consolidated Statement of Total Comprehensive Income (Loss) – 2020 2019 2018 Non-VIE derivatives: Credit derivatives Net gains (losses) on derivative contracts $ — $ 2 $ (1) Interest rate swaps Net gains (losses) on derivative contracts (9) (6) 1 Futures contracts Net gains (losses) on derivative contracts (41) (45) 7 Total non-VIE derivatives (50) (50) 7 Variable Interest Entities: Currency swaps Income (loss) on variable interest entities (6) (12) 11 Interest rate swaps Income (loss) on variable interest entities (138) (20) 493 Total Variable Interest Entities (144) (32) 505 Total derivative contracts $ (193) $ (82) $ 512 Credit Derivatives Credit derivatives, which are privately negotiated contracts, provide the counterparty with credit protection against the occurrence of a specific event such as a payment default or bankruptcy relating to an underlying obligation. Credit derivatives issued are insured by AAC. The outstanding credit derivative transaction at December 31, 2020, does not include ratings based collateral-posting triggers or otherwise require Ambac to post collateral regardless of Ambac’s ratings or the size of the mark to market exposure to Ambac. Our credit derivatives were written on a “pay-as-you-go” basis. Similar to an insurance policy, pay-as-you-go provides that Ambac pays interest shortfalls on the referenced transaction as they are incurred on each scheduled payment date, but only pays principal shortfalls upon the earlier of (i) the date on which the assets designated to fund the referenced obligation have been disposed of and (ii) the legal final maturity date of the referenced obligation. Ambac maintains internal credit ratings on its guaranteed obligations, including credit derivative contracts, solely to indicate management’s view of the underlying credit quality of the guaranteed obligations. The principal notional outstanding for credit derivative contracts was $257 and $280 as of December 31, 2020 and 2019, respectively, which had internal Ambac ratings of AA in both periods: Interest Rate Derivatives Ambac, through its subsidiary Ambac Financial Services (“AFS”), uses interest rate swaps, US Treasury futures contracts and other derivatives, to provide a partial economic hedge against the effects of rising interest rates elsewhere in the Company, including on Ambac’s financial guarantee exposures. Additionally, AFS provided interest rate swaps to states, municipalities and their authorities, asset-backed issuers and other entities in connection with their financings. As of December 31, 2020 and 2019, the notional amounts of AFS's derivatives are as follows: Notional - December 31, Type of Derivative 2020 2019 Interest rate swaps—pay-fixed/receive-variable $ 726 $ 1,261 US Treasury futures contracts—short 240 755 Interest rate swaps—receive-fixed/pay-variable 195 332 Derivatives of Consolidated Variable Interest Entities Certain VIEs consolidated under the Consolidation Topic of the ASC entered into derivative contracts to meet specified purposes within the securitization structure. The notional for VIE derivatives outstanding as of December 31, 2020 and 2019, were as follows: Notional - December 31, Type of VIE Derivative 2020 2019 Interest rate swaps—receive-fixed/pay-variable $ 1,233 $ 1,194 Interest rate swaps—pay-fixed/receive-variable 1,151 1,176 Currency swaps 308 329 Credit derivatives — 9 Contingent Features in Derivatives Related to Ambac Credit Risk Ambac’s over-the-counter interest rate swaps are centrally cleared when eligible. Certain interest rate swaps remain with professional swap-dealer counterparties and direct customer counterparties. These non-cleared swaps are generally executed under standardized derivative documents including collateral support and master netting agreements. Under these agreements, Ambac is required to post collateral in the event net unrealized losses exceed predetermined threshold levels. Additionally, given that AAC is no longer rated by an independent rating agency, counterparties have the right to terminate the swap positions. As of December 31, 2020 and 2019, the net liability fair value of derivative instruments with contingent features linked to Ambac’s own credit risk was $113 and $89, respectively, related to which Ambac had posted cash and securities as collateral with a fair value of $130 and $109, respectively. All such ratings-based contingent features have been triggered requiring maximum collateral levels to be posted by Ambac while preserving counterparties’ rights to terminate the contracts. Assuming all such contracts terminated at fair value on December 31, 2020, settlement of collateral balances and net derivative liabilities would result in a net receipt of cash and/or securities by Ambac. If counterparties elect to exercise their right to terminate, the actual termination payment amounts will be determined in accordance with derivative contract terms, which may result in amounts that differ from market values as reported in Ambac’s financial statements.

Long-Term Debt (Notes)

Long-Term Debt (Notes)	12 Months Ended
Long-Term Debt (Notes)	Dec. 31, 2020
Debt Disclosure [Abstract]
Long-term Debt	13. LONG-TERM DEBT Long-term debt outstanding, excluding VIE long-term debt, was as follows: December 31, 2020 2019 Par Value Unamortized Discount Carrying Value Par Value Unamortized Discount Carrying Value Ambac Assurance: 5.1% surplus notes $ 531 $ — $ 531 $ 531 $ (14) $ 517 5.1% junior surplus notes 365 (118) 247 365 (113) 252 Ambac note 1,641 — 1,641 1,763 — 1,763 Tier 2 notes 306 — 306 281 (4) 278 Ambac UK debt 41 (27) 14 41 (28) 13 Long-term debt $ 2,884 $ (145) $ 2,739 $ 2,980 $ (159) $ 2,822 Aggregated annual maturities of non-VIE long-term debt obligations (based on scheduled maturity dates as further discussed below) are as follows: 2021 $ 531 (1) 2022 — 2023 1,641 2024 — 2025 — Thereafter 712 (2) Total $ 2,884 (1) Includes $531 related to surplus notes that were not approved for payment by OCI on their stated June 7, 2020, maturity date (2) Includes $365 of junior surplus notes that were acquired in January and February of 2021 in exchanges for an aggregate of $279 of surplus notes. Refer to "2021 Surplus Note Exchanges" in in Note 1. Background and Business Description. Surplus Notes Ambac Assurance's surplus notes, with a par amount of $531 and $531 at December 31, 2020 and 2019, respectively, have a scheduled maturity of June 7, 2020. Surplus notes outstanding are recorded at their fair value at the date of issuance. The discount on surplus notes was accreted into income using the effective interest method based on projected cash flows at the date of issuance through June 7, 2020, using a weighted average imputed interest rate of 10.1%. Surplus note principal and interest payments require the approval of OCI. Since the issuance of the surplus notes in 2010, OCI has declined to approve regular payments of interest on surplus notes, although the OCI has permitted exceptional payments in connection with (a) increasing the percentage of deferred policy payments of the Segregated Account of Ambac Assurance from 25% to 45% in 2014 and (b) a one-time payment of approximately six months of interest on the surplus notes outstanding immediately after consummation of the Rehabilitation Exit Transactions in 2018 in the amount of $14, of which $3 was received by AFG for surplus notes that it owned and that are considered extinguished for accounting purposes. In April 2020, OCI declined the request of Ambac Assurance to pay the principal amount of the surplus notes, plus all accrued and unpaid interest thereon, on the scheduled maturity date of June 7, 2020. As a result, the scheduled payment date for interest, and the scheduled maturity date for payment of principal of the surplus notes, shall be extended until OCI grants approval to make the payment. Interest will accrue, compounded on each anniversary of the original scheduled payment date or scheduled maturity date, on any unpaid principal or interest through the actual date of payment, at 5.1% per annum. Refer to Note 1. Background and Business Description for further discussion of the Rehabilitation Exit Transactions, the AMPS Exchange and the 2021 Surplus Note Exchanges, each involving the issuance of surplus notes by AAC. The retirement of certain notes as part of the Rehabilitation Exit Transactions in 2018 resulted in gains of $3 for the year ended December 31, 2018, recognized in Net realized gains (losses) on extinguishment of debt on the Consolidated Statements of Total Comprehensive Income. Junior Surplus Notes The junior surplus notes have a par value of $365 and $365 at December 31, 2020 and 2019, respectively. Pursuant to the Second Amended Plan of Rehabilitation, Ambac Assurance became the obligor under the junior surplus notes (originally issued by the Segregated Account) as of February 12, 2018. • Par value at December 31, 2020 and 2019 includes $15 and $15, respectively, of junior surplus notes issued in connection with a settlement agreement (the “OSS Settlement Agreement”) entered into among Ambac, AAC, the Segregated Account and One State Street, LLC (“OSS”) with respect to the termination of Ambac’s office lease with OSS. A portion of the principal balance of the originally issued notes were reduced based on rents paid to OSS by AAC after December 31, 2015. Par value of these junior surplus notes was reduced by $0 and $2 during the years ended December 31, 2020 and 2019, respectively, as rent payments were made by Ambac Assurance. As of December 31, 2020, there was no remaining balance of the junior surplus notes that can be reduced on rents paid by AAC. These junior surplus notes were recorded at their fair value at the date of issuance. The discount on these notes was accreted into income from the date of issuance through June 7, 2020, using the effective interest method at an imputed interest rate of 19.5%. As further described in Note 1. Background and Business Description, on February 11, 2021, AAC completed the JSN Exchange, pursuant to which it acquired all remaining junior surplus notes originally issued in connection with the OSS Settlement Agreement. • Par value at December 31, 2020 and 2019 includes $350 of a junior surplus note originally issued to AFG pursuant to AFG's Chapter 11 Reorganization Plan in accordance with the Mediation Agreement dated September 21, 2011, among AFG, AAC, the Segregated Account, the Rehabilitator, the OCI and the Official Committee of Unsecured Creditors of AFG, and that AFG sold to the Corolla Trust on August 28, 2014. This junior surplus note was recorded at a discount to par based on its fair value on August 28, 2014. Ambac is accreting the discount on this junior surplus note into earnings using the effective interest method, based on an imputed interest rate of 8.4%. As further described in Note 1. Background and Business Description, on January 22, 2021, AAC completed the Corolla Note Exchange, pursuant to which it effectively acquired the junior surplus notes from the Corolla Trust. Ambac Note The Ambac Note, issued in connection with the Rehabilitation Exit Transactions on February 12, 2018, as more fully described in Note 1. Background and Business Description , has a par value of $1,641 and $1,763 at December 31, 2020 and 2019, respectively, and has a legal maturity of February 12, 2023. Interest on the Ambac Note is payable quarterly (on the last day of each quarter beginning with June 30, 2018) at an annual rate of 3-month U.S. Dollar LIBOR + 5.00%, subject to a 1.00% LIBOR floor. During the years ended December 31, 2020 and 2019, $121 and $178 par value of the Ambac Note was redeemed, respectively. The maturity date for the Ambac Note is the earlier of (x) February 12, 2023, and (y) if the Secured Notes are then outstanding, the date that is five business days prior to the date for which OCI has approved the repayment of the outstanding principal amount of the surplus notes issued by Ambac Assurance. Promptly, and in any event within four business days after the receipt (whether directly or indirectly) of any representation and warranty subrogation recoveries, Ambac Assurance shall (i) apply an amount (the “Mandatory Redemption Amount”) equal to the lesser of (a) the amount of representation and warranty subrogation recoveries up to $1,400 and (b) all outstanding principal and accrued and unpaid interest on the Ambac Note to redeem the Ambac Note, in whole or in part, as applicable; provided, that any non-cash representation and warranty subrogation recoveries shall be deemed to be received upon the receipt of the applicable appraisal. • The portion of the Ambac Note issued in connection with the exchange of surplus notes ("Ambac Note A") was accounted for as a debt modification since the creditors before and after the exchange remained the same and the change in terms was not considered substantial. A substantial change is considered to be a change in cash flows of equal to or greater than 10%, and because the change in cash flows was less than 10%, debt modification accounting is appropriate. Under debt modification accounting, Ambac Note A was recorded at a discount to par based on the carrying value of the surplus notes less the cash consideration paid. Furthermore, no gain or loss was recorded on the surplus note exchange and a new effective interest rate was established based on the cash flows of Ambac Note A. Any consideration paid directly related to the issuance of Ambac Note A was expensed as incurred. • The portion of the Ambac Note issued in connection with the exchange of Deferred Amounts ("Ambac Note B") was recorded at fair value. The Deferred Amount exchange was accounted for as an extinguishment of the Deferred Amounts with the gain reflected as a benefit to loss and loss expenses. Any consideration paid directly related to the issuance of Ambac Note B was capitalized and amortized as part of the effective yield calculation. The aggregate discount on the entire Ambac Note (portions A and B) was accreted into earnings from the date of issuance through September 30, 2018, using the effective interest method, based on an imputed interest rate of 7.6%. Refer to Note 1. Background and Business Description for further discussion of the Rehabilitation Exit Transactions in connection with which the Ambac Note was issued. Refer to the discussion under "Counterparty Collateral, Deposits with Regulators and Other Restrictions" in Note 10. Investments for further information on security and collateral related to the Ambac Note and the Secured Notes issued by Ambac LSNI. Tier 2 Notes The Tier 2 Notes, issued in connection with the Rehabilitation Exit Transactions on February 12, 2018, with a par value of $306 and $281 (including paid-in-kind interest of $66 and $41) at December 31, 2020 and 2019, respectively, have a legal maturity of February 12, 2055. Interest on the Tier 2 Notes is at an annual rate of 8.50%. Other than upon payment of principal at redemption or maturity, interest payments will not be made in cash on interest payment dates and shall be paid-in-kind and compounded on the last day of each calendar quarter. The Tier 2 Notes were recorded at a discount to par as any consideration paid that was directly related to the issuance of the Tier 2 Notes was capitalized and is part of the effective yield calculation. Ambac is accreting the discount on the Tier 2 Notes into earnings using the effective interest method, based on an imputed interest rate of 9.9%. The Tier 2 Notes are subject to mandatory redemption upon: (i) receipt of representation and warranty subrogation recoveries in excess of $1,600 ("Tier 2 Net Proceeds") and (ii) payment of principal or interest on AAC surplus notes. Promptly, and in any event within five business days after the receipt (whether directly or indirectly) of Tier 2 Net Proceeds, AAC shall deposit an amount equal to the Tier 2 Net Proceeds to a collateral account, provided, that any non-cash representation and warranty subrogation recoveries shall be deemed to be received upon the receipt of the applicable appraisal of the consideration received by AAC. Similarly, within five business dates after a surplus note payment (other than in connection with the Rehabilitation Exit Transactions), AAC shall deposit an amount based on the percentage of surplus notes paid applied to the outstanding balance of the Tier 2 Notes to a collateral account. In both cases, the amount deposited shall not be in excess of the amount required to redeem all outstanding Tier 2 Notes. Also, such amounts shall be used to initiate a redemption on the initial call date for the Tier 2 Notes or, if the initial call date has occurred, promptly following the receipt of the Tier 2 Net Proceeds or surplus note payment. The Tier 2 Notes may also be redeemed, in whole or in part, at the option of Ambac Assurance. Both mandatory and optional redemptions may be made at a price equal to 100% of the aggregate principal amount redeemed, plus accrued and unpaid interest, if any, plus a make-whole premium. Make-whole premiums are calculated based on future interest payments through the contractual call date ("Initial Call Date"). The Initial Call Date at issuance of December 17, 2020, extends ratably beginning the first anniversary of issuance to September 17, 2021 by the second anniversary, and to March 17, 2022 by the third anniversary of issuance. There are no extensions of the Initial Call Date beyond March 17, 2022. The Initial Call Date for redemptions is determined based on the date the applicable amounts are deposited to the collateral account. Ambac UK Debt The Ambac UK debt, issued in connection with the Ballantyne commutation on June 18, 2019, has a par value of $41 and $41 at December 31, 2020 and 2019, and a legal maturity of May 2, 2036. Interest on the Ambac UK debt is at an annual rate of 0.00%. The Ambac UK debt was recorded at its fair value at the date of issuance. The discount on the debt is currently being accreted into income using the effective interest method at an imputed interest rate of 7.4%. Variable Interest Entities, Long-term Debt The variable interest entity notes were issued by consolidated VIEs. Ambac is the primary beneficiary of the VIEs as a result of providing financial guarantees on certain of the VIEs obligations. Consequently, Ambac has consolidated these variable interest entity notes and all other assets and liabilities of the VIEs. Ambac is not primarily liable for the debt obligations of these entities. Ambac would only be required to make payments on these debt obligations in the event that the issuer defaults on any principal or interest due and to the extent such obligations are guaranteed by Ambac. The total unpaid principal amount of outstanding long-term debt associated with VIEs consolidated as a result of the financial guarantee provided by Ambac was $3,927 and $3,990 as of December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, the ranges of final maturity dates of the outstanding long-term debt associated with these VIEs were December 2025 to August 2054 and December 2025 to August 2054, respectively. As of December 31, 2020 and 2019, the interest rates on these VIEs’ long-term debt ranged from 0.00% to 7.93% in both years. respectively. Aggregated annual maturities of VIE long-term debt following December 31, 2020 are: 2021-$0; 2022-$0; 2023-$0; 2024-$0; 2025-$83; Thereafter-$3,844.

Income Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	14. INCOME TAXES AFG files a consolidated Federal income tax return with its subsidiaries. AFG and its subsidiaries also file separate or combined income tax returns in various states, local and foreign jurisdictions. The following are the major jurisdictions in which Ambac and its subsidiaries operate and the earliest tax years subject to examination: Jurisdiction Tax Year United States 2010 New York State 2013 New York City 2016 United Kingdom 2017 Italy 2016 Consolidated Pretax Income (Loss) U.S. and foreign components of pre-tax income (loss) were as follows: Year Ended December 31, 2020 2019 2018 U.S. $ (441) $ (174) $ 264 Foreign 1 (9) 8 Total $ (440) $ (183) $ 273 Provision (Benefit) for Income Taxes The components of the provision (benefit) for income taxes were as follows: Year Ended December 31, 2020 2019 2018 Current taxes U. S. federal $ — $ — $ (2) U.S. state and local — (3) 2 Foreign 8 37 (1) Total current taxes 8 34 — Deferred taxes Foreign (10) (1) 5 Total deferred taxes $ (10) $ (1) $ 5 Provision for income taxes $ (3) $ 32 $ 5 The total effect of income taxes on net income and stockholders’ equity for the years ended December 31, 2020, 2019 and 2018 is as follows: Year Ended December 31, 2020 2019 2018 Total income taxes charged to net income $ (3) $ 32 $ 5 Income taxes charged (credited) to stockholders’ equity: Unrealized gains (losses) on investment securities 3 14 12 Unrealized gains (losses) on foreign currency translations — — — Valuation allowance to equity (3) (23) (9) Total charged to stockholders’ equity: 1 (8) 3 Total effect of income taxes $ (1) $ 24 $ 8 Reconciliation of U.S. Federal Statutory Income Tax Rate to Actual Income Tax Rate The tax provisions in the accompanying Consolidated Statements of Total Comprehensive Loss reflect effective tax rates differing from prevailing Federal corporate income tax rates. The following is a reconciliation of these differences: 2020 2019 2018 Year Ended December 31, Amount % Amount % Amount % Tax on income (loss) at statutory rate $ (92) 21.0 % $ (38) 21.0 % $ 57 21.0 % Changes in expected tax resulting from: Tax-exempt interest (2) 0.4 % (3) 1.8 % (7) (2.5) % Foreign taxes 6 (1.4) % 40 (22.1) % 10 3.9 % Substantiation adjustment (29) 6.7 % 28 (15.3) % (60) (22.0) % Valuation allowance 113 (25.6) % 8 (4.4) % 5 1.9 % Change in Tax Law — — % — — % (2) (0.7) % Other, net — — % (2) 1.3 % 1 0.4 % Tax expense on income (loss) $ (3) 0.7 % $ 32 (17.7) % $ 5 2.0 % Unrecognized Tax Positions A reconciliation of the beginning and ending amounts of material unrecognized tax benefits for 2020, 2019 and 2018 is as follows: Year Ended December 31, 2020 2019 2018 Balance, beginning of period $ — $ — $ — Increases related to prior year tax positions — — — Decreases related to prior year tax positions — — — Balance, end of period $ — $ — $ — Deferred Income Taxes The tax effects of temporary differences that give rise to significant portions of the deferred tax liabilities and deferred tax assets at December 31, 2020 and 2019, are presented below: December 31, 2020 2019 Deferred tax liabilities: Insurance intangible $ 78 $ 90 Unearned premiums and credit fees 32 42 Investments 22 32 Variable interest entities 13 12 Other 7 8 Total deferred tax liabilities 152 183 Deferred tax assets: Net operating loss and capital carryforward 764 742 Loss reserves 218 148 Debentures 22 29 Compensation 9 7 Other 5 1 Subtotal deferred tax assets 1,019 927 Valuation allowance 891 777 Total deferred tax assets 128 151 Net deferred tax liability $ 24 $ 32 In accordance with the Income Tax Topic of the ASC, a valuation allowance is recognized if, based on the weight of available evidence, it is more-likely-than-not that some, or all, of the deferred tax asset will not be realized. As a result of the risks and uncertainties associated with future operating results, management believes it is more likely than not that the Company will not generate sufficient U.S. federal, state and/or local taxable income to recover the deferred tax operating assets and therefore maintains a full valuation allowance. The remaining net deferred tax liability of $24 is attributable to Ambac U.K. NOL Usage In December 2020, AFG and certain subsidiaries and affiliates amended their existing tax sharing agreement (the "Third TSA Amendment"). Under the Third TSA Amendment, AAC and AFG agreed to reallocate $210 of net operating loss carry-forwards (“NOLs”) from AAC to AFG and to eliminate AAC's requirement to make future payments based on its utilization of NOLs ("tolling payments") for any taxable year beginning on or after January 1, 2019. In connection with the Third TSA Amendment, AAC paid to AFG approximately $28 of accrued tolling payments based on NOLs used by AAC in 2017. The Third TSA Amendment did not affect the NOL tolling payments AAC would be required to make in connection with the 2013 Closing Agreement between Ambac and the United States Internal Revenue Service, which could amount to as much as $8. As of December 31, 2020, the Company has $3,639 of NOLs, which if not utilized will begin expiring in 2029, and will fully expire in 2041.

Employment Benefit Plans (Notes

Employment Benefit Plans (Notes)	12 Months Ended
Employment Benefit Plans (Notes)	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Compensation and Employee Benefit Plans [Text Block]	15. EMPLOYMENT BENEFIT PLANS Postretirement Health Care and Other Benefits Ambac provides postretirement and postemployment / severance benefits, including health and life benefits for certain employees who meet predefined age and service requirements. None of the plans are currently funded. Postretirement and postemployment benefits expense, including severance benefits paid, were $1, $3 and $1 for the years ended December 31, 2020, 2019 and 2018, respectively. Effective August 1, 2005, new employees were not eligible for postretirement benefits. The current postretirement benefit requires retirees to purchase their own medical insurance policy with a portion of their premium being reimbursed by Ambac. The unfunded accumulated postretirement benefit obligation was $10 as of December 31, 2020. The assumed health care cost trend rates range from 5.2% in 2021, decreasing ratably to 4.5% in 2030. The following table sets forth projected benefit payments from Ambac’s postretirement plan over the next ten years for current retirees: 2021 $ — 2022 — 2023 — 2024 — 2025 1 2025-2029 3 Total $ 5 The discount rate used in determining the projected benefit obligations for the postretirement plan is selected by reference to a pension liability index with similar duration to that of the benefit plan. The rates used for the projected plan benefit obligations at the measurement date for December 31, 2020 and 2019, were 2.25% and 3.00%, respectively. Savings Incentive Plan Substantially all employees of AAC are covered by a defined contribution plan (the “Savings Incentive Plan”). AAC makes employer matching contributions equal to 100% of the employees’ contributions, up to 3% of such participants’ compensation, as defined in the plan, plus 50% of contributions up to an additional 2% of compensation, subject to limits set by the Internal Revenue Code. The total cost of the Savings Incentive Plan was $1, $1 and $1 for the years December 31, 2020, 2019 and 2018, respectively. Incentive Compensation - Stock Units and Cash Incentive compensation is a key component of our compensation strategy. Our incentive compensation awards generally have two components: short term incentive compensation awards ("STIP") and long term incentive plan awards ("LTIP"). Annual decisions with regard to incentive compensation are generally made in the first quarter of each year and are based on Company performance and individual and business unit performance of the previous year. In addition to the stock based awards discussed below, Ambac's incentive compensation includes cash payments which may consist of annual awards under the STIP, deferred payments that vest over two years or other performance based cash awards. For all employees, an allocation of incentive compensation is made between STIP and LTIP awards. Employees, directors and consultants of Ambac are eligible to participate in Ambac’s 2020 Incentive Compensation Plan (“2020 Plan”), which is the successor plan to the 2013 Incentive Compensation Plan ("2013 Plan"), subject to the discretion of the compensation committee of Ambac’s Board of Directors. The 2020 Plan and 2013 Plan each provide for incentives and rewards that are valued or determined by reference to Ambac common stock as currently traded on the New York Stock Exchange. Beginning with the June 2, 2020, effective date (the "Effective Date") of the 2020 Plan, all new awards are granted under the 2020 Plan and may not be granted under the 2013 Plan. However, the terms and conditions of the 2013 Plan continue to govern outstanding awards granted under the 2013 Plan. There are 1,475,000 and 4,000,000 shares of Ambac's common stock authorized for issuance that can be awarded under the 2020 Plan and 2013 Plan, respectively. Awards may also be made under the 2020 Plan with respect to the shares that, as of the Effective Date, remained available for grant under the 2013 Plan. In addition, shares subject to outstanding awards granted under the 2013 Plan as of the Effective Date that subsequently terminate by expiration or forfeiture, cancellation, or otherwise without the issuance of such shares will become available for awards under the 2020 Plan. Of the total shares authorized for issuance pursuant to the 2020 Plan and 2013 Plan, 2,096,292 shares are available for future grant as of December 31, 2020. Shares available for future grant are reduced by the maximum number of shares that could be issued pursuant to outstanding performance awards. The number of shares available for future grant considering the target number of shares instead of the maximum number of shares related to performance awards is 3,058,603. The amount of stock-based compensation expense and corresponding after-tax expense are as follows: Year Ended December 31, 2020 2019 2018 Stock options $ — $ — $ — Restricted stock units 3 4 6 Performance awards 8 8 6 Total stock-based compensation $ 11 $ 12 $ 12 Total stock-based compensation (after-tax) $ 11 $ 12 $ 12 Stock Options Stock options were awarded in 2013 to directors that had an expiry term of seven years from the grant date, subject to earlier expiration upon the recipient's departure from the Company. A summary of stock option activity for 2020 is as follows: Shares Weighted Aggregate Weighted Outstanding at beginning of period 16,667 $ 20.63 Granted — — Exercised — — Forfeited or expired (16,667) 20.63 Outstanding at end of period — $ — $ — 0.00 Exercisable — $ — $ — 0.00 No stock options were exercised during the years ended December 31, 2020, 2019 and 2018, respectively. Restricted Stock Units (“RSUs”) RSUs have been awarded to certain employees for a portion of their STIP compensation, LTIP compensation, sign-on, special awards for exceptional performance. RSUs have also been awarded to consultants for meeting certain contractual performance goals. The previously issued STIP awards vested upon grant, but settlement was deferred (other than for employment tax withholdings) into two equal installments generally on the first and second anniversary date of the grant. The LTIP, sign-on and special awards generally vest in equal installments over a two to three year period. Awards granted to consultants vest on the second year anniversary of date of grant. Such vesting is expressly conditioned upon the respective employees or non-employees continued service with Ambac through the applicable vesting date, although vesting is accelerated for terminations due to death, disability, eligible retirement, or involuntary termination by the Ambac other than for cause. RSUs have been awarded annually to directors and vest on the last day of April of the following year. These RSUs will not settle until the respective director’s termination from the board of directors or, if earlier, upon a change in control. All RSUs provide for accelerated vesting upon a change in control, death or disability or involuntary removal other than for cause (not including removal pursuant to a shareholder vote at a regularly scheduled annual meeting of shareholders). Upon termination (other than for cause), the unvested RSUs shall partially vest as of the date of such termination in an amount equal to the number of then outstanding unvested RSUs multiplied by a fraction, the numerator of which shall be the number of calendar days which have lapsed since the grant date and the denominator of which shall be the total number of calendar days of the original vesting period. As of December 31, 2020, 773,657 RSUs remained outstanding, of which (i) 345,302 units required future service as a condition to the delivery of the underlying shares of common stock and (ii) 428,355 units do not require future service and are deferred for future settlement. As of December 31, 2019, 702,579 RSUs remained outstanding, of which (i) 248,942 units required future service as a condition to the delivery of the underlying shares of common stock, and (ii) 453,637 units did not require future service and were deferred for future settlement. A summary of RSU activity for 2020 is as follows: Shares Weighted Average Grant Date Fair Value Outstanding at beginning of period 702,579 $ 18.19 Granted 297,517 17.36 Delivered or returned to plan (1) (224,829) 17.59 Forfeited (1,610) 19.19 Outstanding at end of period 773,657 $ 18.04 (1) When restricted stock unit awards issued by Ambac become taxable compensation to employees, shares may be withheld to cover the employee’s withholding taxes. For the year ended December 31, 2020, Ambac purchased 85,654 of shares from employees that settled restricted stock units to meet the required tax withholdings. Ambac’s closing share price on the grant date was used to estimate the fair value of the service condition based RSU on the grant date. The weighted average grant date fair value of RSUs granted during 2020, 2019 and 2018 was $17.36, $19.75 and $16.35, respectively. As of December 31, 2020, there was $4 of total unrecognized compensation costs related to unvested RSUs granted. These costs are expected to be recognized over a weighted average period of 1.6 years. The fair value for RSUs vested and delivered during the year ended December 31, 2020, 2019 and 2018 was $4, $4 and $1, respectively. Performance Stock Awards ("PSUs") Performance awards granted vest in 3 years and awards have components relative to performance at AFG and AAC. Actual awards can payout 0% to 220% of the number of units granted. Under currently outstanding award agreements, performance will be evaluated as follows: • AFG performance will be evaluated relative to cumulative earnings before interest, taxes, depreciation and amortization over the vesting period (exclusive of AAC and its subsidiaries' earnings), which is intended to reward participants for generating pre-tax income. • AAC performance will be evaluated according to: (i) changes in AAC's assets relative to its insurance and financial obligations, which is intended to reward participants for increases in the relative value of AAC, and (ii) reductions in watch list and adversely classified credits, which is intended to reward participants for de-risking the financial guarantee insured portfolio. • In 2019, a relative Total Shareholder Return modifier was added as an additional metric with respect to the LTIP award payouts. The modifier will cause the payout at the end of the performance period to be increased or decreased by 10% if AFG's stock performance compared to a peer group is at or above the 75th percentile or at or below the 25th percentile, respectively . These performance metrics are subject to change by the Compensation Committee of the Board of Directors as Ambac's business evolves. Other than voluntary termination or involuntary termination for cause, and provided that the participant meets certain minimum service requirements, the performance awards are subject to either partial or accelerated vesting. The current performance awards shall be settled within 75 days after the end of the performance period, including those with partial or accelerated vesting. A summary of PSU activity for 2020 is as follows: Shares Weighted Average Grant Date Fair Value Outstanding at beginning of period 650,212 $ 17.98 Granted (1) 331,184 19.99 Delivered (2) (184,896) 22.35 Forfeited (6,071) 18.00 Performance adjustment (3) 54,085 22.35 Outstanding at end of period 844,514 $ 18.09 (1) Represents performance share units at 100% of units granted for LTIP Awards. (2) Reflects the number of performance shares attributable to the performance goals attained over the completed performance period and for which service conditions have been met. When performance stock unit awards issued by Ambac become taxable compensation to employees, shares may be withheld to cover the employee’s withholding taxes. For the year ended December 31, 2020, Ambac purchased 70,340 of shares from employees that settled performance based restricted stock units to meet the required tax withholdings. (3) Represents the increase (decrease) in shares issued for awards granted in 2017 based upon the attainment of performance metrics at the end of the performance period.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	17. COMMITMENTS AND CONTINGENCIES Litigation Against Ambac Monterey Bay Military Housing, LLC, et al. v. Ambac Assurance Corporation, et al. (United States District Court, Northern District of California, San Jose Division, Case No. 17-cv-04992-BLF, filed August 28, 2017). Plaintiffs, the corporate developers of various military housing projects, filed an amended complaint on October 27, 2017 against AAC, a former employee of AAC, and certain unaffiliated persons and entities, asserting claims for (i) violation of 18 U.S.C §§ 1962(c) and 1962(d) (civil Racketeer Influenced and Corrupt Organizations Act (“RICO”) and conspiracy to commit civil RICO), (ii) breach of fiduciary duty, (iii) aiding and abetting breach of fiduciary duty, (iv) fraudulent misrepresentation, (v) fraudulent concealment and (vi) conspiracy to commit fraud. The claims relate to bonds and debt certificates (insured by AAC) that were issued to finance the renovation and construction of housing at certain military bases. Plaintiffs allege that defendants secretly conspired to overcharge plaintiffs for the financing of the projects and directed the excess profits to themselves. Plaintiffs allege defendants generated these excess profits by supposedly charging inflated interest rates, manipulating “shadow ratings,” charging unnecessary fees, and hiding evidence of their alleged wrongdoing. Plaintiffs seek, among other things, compensatory damages, disgorgement of profits and fees, punitive damages, trebled damages and attorneys’ fees. Ambac and the other defendants filed motions to dismiss the amended complaint on November 13, 2017. On July 17, 2018, the court granted AAC’s and the other defendants’ motion to dismiss the first amended complaint without prejudice. On December 17, 2018, Plaintiffs filed a second amended complaint. On February 15, 2019, Ambac and the other defendants filed a motion to dismiss the second amended complaint. On September 26, 2019, the court issued a decision denying defendants’ motion to dismiss and sua sponte reconsidering its previous denial of defendants’ motion to transfer venue to the Southern District of New York (“SDNY”). On October 4, 2019, the case was transferred to the SDNY. On October 10, 2019, the defendants filed motions in the SDNY to vacate or reconsider the decision by the Northern District of California on the defendants’ motion to dismiss. On October 24, 2019, plaintiffs filed their brief in opposition to defendants' motions to vacate or reconsider, and on October 31, 2019, defendants filed their reply briefs in further support of their motions. On November 20, 2019, the court ordered that the defendants’ answers to the second amended complaint would be due seven days after the court issues a decision on their motions. Financial Oversight and Management Board for Puerto Rico, et al. v. Autonomy Master Fund Limited, et al. (United States District Court, District of Puerto Rico, No. 19-ap-00291, filed May 2, 2019). On May 2, 2019, the Financial Oversight and Management Board for Puerto Rico (the "Oversight Board"), together with the Official Committee of Unsecured Creditors for the Commonwealth (the "Committee") filed an adversary proceeding against certain parties that filed proofs of claim on account of general obligation bonds issued by the Commonwealth of Puerto Rico, including AAC. The complaint seeks declarations that the general obligation bonds are unsecured obligations and, in the alternative, seeks to avoid any security interests that holders of such bonds may have. On June 12, 2019, a group of general obligation bondholders moved to dismiss the complaint. On June 13, 2019, at the request of the Plaintiffs, the District Court stayed the case until September 1, 2019 as to all defendants; on July 24, 2019, the District Court referred this matter to mediation and ordered it stayed during the pendency of such mediation. AAC filed a statement of position and reservation of rights on February 5, 2020; certain other defendants filed motions to dismiss on this same date. On February 9, 2020, the Oversight Board announced that it intends to file, and to seek to confirm, an amended plan of adjustment (the “Amended POA”). On March 10, 2020, the District Court ordered that this case remain stayed while the Oversight Board attempts to confirm the Amended POA. Financial Oversight and Management Board for Puerto Rico, et al. v. Ambac Assurance Corporation, et al. (United States District Court, District of Puerto Rico, No. 19-ap-00363, filed May 20, 2019). On May 20, 2019, the Oversight Board, together with the Committee, as Plaintiffs, filed an adversary proceeding against certain parties that filed proofs of claim on account of bonds issued by the Puerto Rico Highways and Transportation Authority ("PRHTA"), including AAC. The complaint seeks declarations that the PRHTA bonds are only secured by revenues on deposit with the PRHTA Fiscal Agent and that PRHTA bondholders have no security interest in any other property of PRHTA or the Commonwealth, and in the alternative, to the extent such other security interests exist, the complaint seeks to avoid other security interests that holders of PRHTA bonds may have. On June 14, 2019, at the request of the Plaintiffs, the District Court stayed the case until September 1, 2019 as to all defendants; on July 24, 2019, the District Court referred this matter to mediation and ordered it stayed during the pendency of such mediation. On December 19, 2019, the District Court ordered that this matter will remain stayed pending further order of the District Court pursuant to the Oversight Board’s initiation of a separate adversary proceeding concerning PRHTA bonds (No. 20-ap-00005, discussed below). Financial Oversight and Management Board for Puerto Rico v. Ambac Assurance Corp., et al. (United States District Court, District of Puerto Rico, No. 20-ap-00003, filed Jan. 16, 2020). Pursuant to an order of the District Court setting out an agreed schedule for litigation submitted by the team of mediators designated in the Commonwealth’s restructuring cases (the “Mediation Team“), on January 16, 2020, the Oversight Board filed an adversary proceeding against monoline insurers insuring bonds issued by the Puerto Rico Infrastructure Financing Authority (“PRIFA”) and the PRIFA bond trustee, all of which Defendants filed proofs of claim against the Commonwealth relating to PRIFA bonds. The complaint seeks to disallow Defendants’ proofs of claim against the Commonwealth in their entirety, including for lack of secured status. On February 27, 2020, defendants filed motions to dismiss. On March 10, 2020, the District Court stayed the motions to dismiss and authorized the Oversight Board to move for summary judgment, which motion defendants opposed. Oral argument on the motion for summary judgment was held on September 23, 2020. On January 20, 2021, the District Court granted defendants’ request for deferral of the adjudication of the summary judgment motion until defendants have the opportunity to conduct certain discovery. Discovery is ongoing. Financial Oversight and Management Board for Puerto Rico v. Ambac Assurance Corp., et al. (United States District Court, District of Puerto Rico, No. 20-ap-00004, filed Jan. 16, 2020). Pursuant to an order of the District Court setting out an agreed schedule for litigation submitted by the Mediation Team, on January 16, 2020, the Oversight Board filed an adversary proceeding against monoline insurers insuring bonds issued by the Puerto Rico Convention Center District Authority (“PRCCDA”) and the PRCCDA bond trustee, all of which Defendants filed proofs of claim against the Commonwealth relating to PRCCDA bonds. The complaint seeks to disallow Defendants’ proofs of claim against the Commonwealth in their entirety, including for lack of secured status. On February 27, 2020, defendants filed motions to dismiss. On March 10, 2020, the District Court stayed the motions to dismiss and authorized the Oversight Board to move for summary judgment, which motion defendants opposed. Oral argument on the motion for summary judgment was held on September 23, 2020. On January 20, 2021, the District Court granted defendants’ request for deferral of the adjudication of the summary judgment motion until defendants have the opportunity to conduct certain discovery. Discovery is ongoing. Financial Oversight and Management Board for Puerto Rico v. Ambac Assurance Corp., et al. (United States District Court, District of Puerto Rico, No. 20-ap-00005, filed Jan. 16, 2020). Pursuant to an order of the District Court setting out an agreed schedule for litigation submitted by the Mediation Team, on January 16, 2020, the Oversight Board filed an adversary proceeding against monoline insurers insuring bonds issued by PRHTA, certain PRHTA bondholders, and the PRHTA fiscal agent for bondholders, all of which Defendants filed proofs of claim against the Commonwealth relating to PRHTA bonds. The complaint seeks to disallow Defendants’ proofs of claim against the Commonwealth in their entirety, including for lack of secured status. On February 27, 2020, defendants filed motions to dismiss. On March 10, 2020, the District Court stayed the motions to dismiss and authorized the Oversight Board to move for summary judgment, which motion defendants opposed. Oral argument on the motion for summary judgment was held on September 23, 2020. On January 20, 2021, the District Court granted defendants’ request for deferral of the adjudication of the summary judgment motion until defendants have the opportunity to conduct certain discovery. Discovery is ongoing. Financial Oversight and Management Board for Puerto Rico v. Ambac Assurance Corp., et al. (United States District Court, District of Puerto Rico, No. 20-ap-00007, filed Jan. 16, 2020). Pursuant to an order of the District Court setting out an agreed schedule for litigation submitted by the Mediation Team, on January 16, 2020, the Oversight Board and the Committee filed an adversary proceeding against monoline insurers insuring bonds issued by PRHTA, certain PRHTA bondholders, and the PRHTA fiscal agent for bondholders, all of which Defendants filed proofs of claim against PRHTA relating to PRHTA bonds. The complaint seeks to disallow portions of Defendants’ proofs of claim against the PRHTA, including for lack of secured status. On March 10, 2020, the District Court stayed this case. NC Residuals Owners Trust, et al. v. Wilmington Trust Co., et al. (Delaware Court of Chancery, C.A. No. 2019-0880, filed Nov. 1, 2019). On November 1, 2019, AAC became aware of a new declaratory judgment action filed by certain residual equity interest holders (“NC Owners” or “Plaintiffs”) in fourteen National Collegiate Student Loan Trusts (the “Trusts”) against Wilmington Trust Company, the Owner Trustee for the Trusts; U.S. Bank National Association, the Indenture Trustee; GSS Data Services, Inc., the Administrator; and AAC. Through this action, Plaintiffs seek a number of judicial determinations. On January 21, 2020, the presiding Vice Chancellor entered an order consolidating the action with previously filed litigation relating to the Trusts. On February 13, 2020, AAC, the Owner Trustee, the Indenture Trustee, and other parties filed declaratory judgment counterclaims. Several parties, including Plaintiffs and Ambac Assurance, filed motions for judgment on the pleadings in support of their requested judicial determinations. On August 27, 2020, the Vice Chancellor issued an opinion addressing all of the pending motions for judgment on the pleadings, which granted certain of the parties’ requested judicial determinations and denied others. He deferred judgment on still other declarations pending further factual development. Trial on the unresolved contractual interpretation issues has been scheduled for September 13–17, 2021. AAC’s estimates of projected losses for RMBS transactions consider, among other things, the RMBS transactions’ payment waterfall structure, including the application of interest and principal payments and recoveries, and depend in part on our interpretations of contracts and other bases of our legal rights. From time to time, bond trustees and other transaction participants have employed different contractual interpretations and have commenced, or threatened to commence, litigation to resolve these differences. It is not possible to predict whether additional disputes will arise, nor the outcomes of any potential litigation. It is possible that there could be unfavorable outcomes in this or other disputes or proceedings and that our interpretations may prove to be incorrect, which could lead to changes to our estimate of loss reserves. AAC has periodically received various regulatory inquiries and requests for information with respect to investigations and inquiries that such regulators are conducting. AAC has complied with all such inquiries and requests for information. The Company is involved from time to time in various routine legal proceedings, including proceedings related to litigation with present or former employees. Although the Company’s litigation with present or former employees is routine and incidental to the conduct of its business, such litigation can result in large monetary awards when a civil jury is allowed to determine compensatory and/or punitive damages for, among other things, termination of employment that is wrongful or in violation of implied contracts. From time to time, Ambac is subject to allegations concerning its corporate governance that may lead to litigation, including derivative litigation, and while the monetary impacts may not be material, the matters may distract management and the Board of Directors from their principal focus on Ambac's business, strategy and objectives. It is not reasonably possible to predict whether additional suits will be filed or whether additional inquiries or requests for information will be made, and it is also not possible to predict the outcome of litigation, inquiries or requests for information. It is possible that there could be unfavorable outcomes in these or other proceedings. Legal accruals for litigation against the Company which are probable and reasonably estimable, and management's estimated range of loss for such matters, are either not applicable or are not material to the operating results or financial position of the Company. For the litigation matters the Company is defending that do not meet the “probable and reasonably estimable” accrual threshold and where no loss estimates have been provided above, management is unable to make a meaningful estimate of the amount or range of loss that could result from unfavorable outcomes. Under some circumstances, adverse results in any such proceedings could be material to our business, operations, financial position, profitability or cash flows. The Company believes that it has substantial defenses to the claims above and, to the extent that these actions proceed, the Company intends to defend itself vigorously; however, the Company is not able to predict the outcomes of these actions. Litigation Filed or Joined by Ambac In the ordinary course of their businesses, certain of Ambac’s subsidiaries assert claims in legal proceedings against third parties to recover losses already paid and/or mitigate future losses. The amounts recovered and/or losses avoided which may result from these proceedings is uncertain, although recoveries and/or losses avoided in any one or more of these proceedings during any quarter or fiscal year could be material to Ambac’s results of operations in that quarter or fiscal year. Puerto Rico Assured Guaranty Corp., Assured Guaranty Municipal Corp., and Ambac Assurance Corporation v. Alejandro Garcia Padilla, et al. (United States District Court, District of Puerto Rico No. 3:16-cv-01037, filed January 7, 2016). AAC, along with co-plaintiffs Assured Guaranty Corp. and Assured Guaranty Municipal Corp., filed a complaint for declaratory and injunctive relief to protect its rights against the illegal clawback of certain revenue by the Commonwealth of Puerto Rico. Defendants moved to dismiss on January 29, 2016. On October 4, 2016, the court denied the Defendants’ motions to dismiss. On October 14, 2016, Defendants filed a Notice of Automatic Stay, asserting that Plaintiffs’ claims have been rendered moot and further asserting that the case was automatically stayed under section 405 of the Puerto Rico Oversight, Management and Economic Stability Act ("PROMESA"). On October 28, 2016, Plaintiffs informed the court that neither party was currently challenging the stay, and expressly reserved their right to seek to lift the stay at any time. Plaintiffs also objected to Defendants’ assertion that the case should be dismissed as moot. PROMESA’s litigation stay expired on May 2, 2017. On May 3, 2017, the Oversight Board filed a petition to adjust the Commonwealth’s debts under Title III of PROMESA, resulting in an automatic stay of litigation against the Commonwealth. On May 17, 2017, the court issued an order staying this case until further order of the court. Ambac Assurance Corporation v. Puerto Rico Highways and Transportation Authority (United States District Court, District of Puerto Rico, No. 16-cv-1893, filed May 10, 2016). AAC filed a complaint against the Puerto Rico Highways and Transportation Authority ("PRHTA") on May 10, 2016, alleging breach of fiduciary duty and breach of contract in connection with PRHTA’s extension of an existing toll road concession agreement. The complaint alleges that it was inappropriate for PRHTA to enter into the extension agreement in its current state of financial distress because PRHTA has no control over, and is unlikely to receive, the proceeds of the transaction. AAC also filed related motions seeking the appointment of a provisional receiver for PRHTA and expedited discovery. On May 21, 2017, the Oversight Board filed a petition to adjust PRHTA’s debts under Title III of PROMESA, resulting in an automatic stay of litigation against PRHTA. On May 24, 2017, the court issued an order staying this case until further order of the court. Lex Claims, LLC et al. v. Alejandro Garcia Padilla et al. (United States District Court, District of Puerto Rico, No. 16-2374, filed July 20, 2016). On October 7, 2016, certain General Obligation bondholder Plaintiffs in an action to which AAC was not then a party filed a motion for leave to amend an existing complaint, adding the Puerto Rico Sales Tax Financing Corporation ("COFINA"), COFINA’s executive director, and the trustee for the COFINA bonds as Defendants, and asserting numerous claims that challenged the legal validity of the COFINA structure and seek injunctive relief requiring the sales and use tax proceeds securing COFINA’s bonds to be transferred to the Puerto Rico Treasury. On February 17, 2017, the court permitted AAC to intervene. On May 3, 2017, a petition under Title III of PROMESA was filed on behalf of the Commonwealth of Puerto Rico, and on May 5, 2017, a petition under Title III of PROMESA was filed on behalf of COFINA, resulting in an automatic stay of litigation against the Commonwealth and COFINA (respectively). On May 17, 2017, the court issued an order staying this case until further order of the court. On October 19, 2018, the Oversight Board filed (i) a disclosure statement and a plan of adjustment for COFINA (the “COFINA Plan”) in the COFINA Title III case incorporating a resolution of the dispute between the Commonwealth and COFINA concerning entitlement to sales and use taxes (the “Commonwealth-COFINA Dispute”), and (ii) a motion under Bankruptcy Rule 9019 in the Commonwealth Title III case for approval of the settlement of the Commonwealth-COFINA Dispute (the “9019 Motion”). On February 4, 2019 the District Court granted the 9019 Motion and confirmed the COFINA Plan, which resolves the dispute in this case. The COFINA Plan became effective on February 12, 2019. Following confirmation of the COFINA Plan, several parties filed notices of appeal of the District Court’s confirmation order. On April 12, 2019, the Oversight Board and the Puerto Rico Fiscal Agency and Financial Advisory Authority ("AAFAF") moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. On February 8, 2021, the First Circuit dismissed the appeals of the confirmation order. Ambac Assurance Corporation v. Puerto Rico, et al. (United States District Court, District of Puerto Rico, No. 17-1567, filed May 2, 2017). On May 2, 2017, AAC filed a complaint seeking a declaration that the Commonwealth’s Fiscal and Economic Growth Plan (the "FEGP") and a recently enacted statute called the “Fiscal Plan Compliance Law” are unconstitutional and unlawful because they violate the Contracts, Takings, and Due Process Clauses of the U.S. Constitution, are preempted by PROMESA, and are unlawful transfers of property from COFINA to the Commonwealth in violation of PROMESA. On May 3, 2017, a petition under Title III of PROMESA was filed on behalf of the Commonwealth of Puerto Rico, and on May 5, 2017, a petition under Title III of PROMESA was filed on behalf of COFINA, resulting in an automatic stay of litigation against COFINA. On May 17, 2017, the court issued an order staying this case until further order of the court. On February 4, 2019, the District Court granted the 9019 Motion and confirmed the COFINA Plan. The COFINA Plan became effective on February 12, 2019. Following confirmation of the COFINA Plan, several parties filed notices of appeal of the District Court’s confirmation order. AAC anticipates that this case will be voluntarily dismissed given the effectiveness of the COFINA Plan. Ambac Assurance Corporation v. Puerto Rico, et al. (United States District Court, District of Puerto Rico, No. 17-1568, filed May 2, 2017). On May 2, 2017, AAC filed a complaint alleging that various moratorium laws and executive orders enacted by the Commonwealth to claw back funds from PRIFA, PRHTA, and PRCCDA bonds violate the Contracts, Takings, and Due Process Clauses of the U.S. Constitution, are preempted by PROMESA, and unlawfully transfer PRHTA, PRCCDA, and PRIFA property to the Commonwealth. On May 3, 2017, a petition under Title III of PROMESA was filed on behalf of the Commonwealth of Puerto Rico and on May 21, 2017, a petition under Title III of PROMESA was filed on behalf of PRHTA, resulting in an automatic stay of litigation against the Commonwealth and PRHTA (respectively). On May 17, 2017, the court issued an order staying this case until further order of the court. Ambac Assurance Corporation v. U.S. Department of Treasury et al. (United States District Court, District of Columbia, No. 17-809, filed May 2, 2017). On May 2, 2017, AAC filed a complaint against the U.S. Department of Treasury and Steven Mnuchin, in his official capacity as Secretary of the Treasury, alleging that Puerto Rico’s ongoing diversion of rum taxes from PRIFA violates the Contracts, Takings, and Due Process Clauses of the U.S. Constitution, and seeking an equitable lien on all rum taxes possessed by the U.S. Treasury, and an injunction preventing their transfer to the Commonwealth. On May 3, 2017, a petition under Title III of PROMESA was filed on behalf of the Commonwealth of Puerto Rico. On May 24, 2017, the Oversight Board filed a statement requesting that the court take notice of the stay resulting from the Commonwealth’s Title III filing. On May 25, 2017, the court issued an order staying this case as a result of the Title III proceedings. Ambac Assurance Corporation v. Bank of New York Mellon (United States District Court, Southern District of New York. No. 1:17-cv-03804, filed May 2, 2017). On May 2, 2017, AAC filed a complaint in New York State Supreme Court, New York County, against the trustee for the COFINA bonds, Bank of New York Mellon ("BNY"), alleging breach of fiduciary, contractual, and other duties for failing to adequately and appropriately protect the holders of certain AAC-insured senior COFINA bonds. On May 19, 2017, BNY filed a notice of removal of this action from New York state court to the United States District Court for the Southern District of New York. On May 30, 2017, the United States District Court for the District of Puerto Rico entered an order in an adversary proceeding brought by BNY (No. 1:17-ap-00133) staying this litigation pending further order of the court. The COFINA Plan became effective on February 12, 2019, and, pursuant to the District Court’s confirmation order, this litigation is permitted to continue, with Ambac’s claims against BNYM being limited to those for gross negligence, willful misconduct and intentional fraud. Following confirmation of the COFINA Plan, several parties filed notices of appeal of the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. On February 8, 2021, the First Circuit dismissed the appeals of the confirmation order. Bank of New York Mellon v. COFINA, et al. (United States District Court, District of Puerto Rico, No. 1:17-ap-00133, filed May 16, 2017). On May 16, 2017, BNY filed an interpleader action styled as an adversary proceeding against COFINA and certain creditors of COFINA, including AAC, that have made competing claims of entitlement to funds held by BNY in order to determine the parties’ respective entitlements to the funds. BNY also sought a release of liability in association with the COFINA funds in its possession.. On September 27, 2018, the court terminated competing motions for summary judgment without prejudice in light of the pending agreement in principle between the agent for COFINA and the agent for the Commonwealth in adversary proceeding no. 1:17-ap-00257 (the “Commonwealth-COFINA Dispute,” discussed below). On October 19, 2018, the Oversight Board filed (i) a disclosure statement and the COFINA Plan in the COFINA Title III case incorporating a resolution of the Commonwealth-COFINA Dispute, and (ii) the 9019 Motion in the Commonwealth Title III case for approval of the settlement of the Commonwealth-COFINA Dispute. On February 4, 2019 the District Court granted the 9019 Motion and confirmed the COFINA Plan, which resolves the dispute in this case. The COFINA Plan became effective on February 12, 2019. Following confirmation of the COFINA Plan, several parties filed notices of appeal of the District Court’s confirmation order to the First Circuit Court of Appeals. On February 20, 2019, on the joint motion of BNY and COFINA, the District Court dismissed this case with prejudice. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. On February 8, 2021, the First Circuit dismissed the appeals of the confirmation order. Official Committee of Unsecured Creditors v. Whyte (United States District Court, District of Puerto Rico, No. 1:17-ap-00257, filed September 8, 2017) (the Commonwealth-COFINA Dispute). On August 10, 2017, the court approved a stipulation between the Oversight Board, the Commonwealth, COFINA, and certain creditor parties, including AAC, to resolve the Commonwealth-COFINA Dispute regarding entitlement to sales and use taxes. The stipulation provided that separate agents for COFINA and the Commonwealth would litigate the dispute while preserving the ability of interested parties, to participate in the litigation. On September 8, 2017, the Commonwealth Agent filed an adversary proceeding against the COFINA Agent challenging the COFINA structure on various grounds. The Commonwealth Agent filed a revised complaint on October 25, 2017, making technical corrections to the original complaint. AAC made a motion to intervene in this action, which the court granted on November 21, 2017. The Commonwealth Agent filed an amended complaint on January 16, 2018, largely re-stating its original causes of action to fall within the parameters of the dispute set by the court. After extensive motion practice, on September 27, 2018, the court terminated competing summary judgment motions without prejudice in light of a pending agreement in principle between the Commonwealth Agent and COFINA Agent. On October 19, 2018, the Oversight Board filed (i) a disclosure statement and the COFINA Plan in the COFINA Title III case incorporating a resolution of the Commonwealth-COFINA Dispute, and (ii) the 9019 Motion in the Commonwealth Title III case for approval of the settlement of the Commonwealth-COFINA Dispute. On February 4, 2019, the District Court granted the 9019 Motion and confirmed the COFINA Plan, which resolves the dispute in this case. The COFINA Plan became effective on February 12, 2019. On February 21, 2019, on the joint motion of the agents for the Commonwealth and COFINA, the Oversight Board, AAFAF, and all participating interested parties, the District Court dismissed this case with prejudice. Following confirmation of the COFINA Plan, several parties filed notices of appeal of the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. On February 8, 2021, the First Circuit dismissed the appeals of the confirmation order. Financial Oversight and Management Board for Puerto Rico v. Public Buildings Authority (United States District Court, District of Puerto Rico, No. 1:18-ap-00149, filed December 21, 2018). On December 21, 2018, the Oversight Board, together with the Committee, as Plaintiffs, filed a complaint against the Puerto Rico Public Buildings Authority (“PBA”) seeking declaratory judgment that the leases between PBA and its lessees-many of whom are agencies and instrumentalities of the Commonwealth-are “disguised financings,” not true leases, and therefore should not be afforded administrative expense priority under the Bankruptcy Code. On March 12, 2019, AAC and other interested parties were permitted to intervene in order to argue that the PBA leases are valid leases, and are entitled to administrative expense treatment under the Bankruptcy Code. On June 16, 2019, the Oversight Board announced that it had entered into a plan support agreement ("PSA") with certain general obligation and PBA bondholders that includes a proposed resolution of claim objections to and issues surrounding both general obligation and PBA bonds, including a proposed settlement of this adversary proceeding. On July 24, 2019, the District Court referred this matter to mediation and ordered it stayed during the pendency of such mediation. On September 27, 2019, the Oversight Board filed a joint plan of adjustment and disclosure statement for the Commonwealth, PBA, and the Employees’ Retirement System for Puerto Rico. On February 9, 2020, the Oversight Board executed a new plan support agreement with additional creditors (the “Amended PSA”) and announced that it intends to file, and seek to confirm, the Amended POA. On March 10, 2020, the District Court ordered that this case be stayed while the Oversight Board attempts to confirm the Amended POA. In re Financial Oversight and Management Board for Puerto Rico (United States District Court, District of Puerto Rico, No. 1:17-bk-03283), Omnibus Objection of (I) Financial Oversight and Management Board, Acting Through its Special Claims Committee, and (II) Official Committee of Unsecured Creditors, Pursuant to Bankruptcy Code Section 502 and Bankruptcy Rule 3007, to Claims Filed or Asserted by Holders of Certain Commonwealth General Obligation Bonds (Dkt. No. 4784, filed January 14, 2019) (“GO Bond Claim Objection Procedures”). On January 14, 2019, the Oversight Board and the Committee filed an omnibus claim objection in the Commonwealth’s Title III case challenging claims arising from certain general obligation bonds issued by the Commonwealth in 2012 and 2014 totaling approximately $6 billion, none of which are held or insured by AAC. The court subsequently ordered certain consolidated procedures permitting parties in interest an opportunity to participate in litigation of the objection. On April 11, 2019, AAC filed a notice of participation in support of the objection, advancing the argument, among other things, that the PBA leases are true leases, but the associated debt nonetheless should be included in the Commonwealth’s debt ceiling calculation such that the 2012 and 2014 general obligation bon

Quarterly Information (unaudite

Quarterly Information (unaudited) (Notes)	12 Months Ended
Quarterly Information (unaudited) (Notes)	Dec. 31, 2020
Quarterly Financial Data [Abstract]
Quarterly Financial Information (unaudited)	2020 Quarters 2019 Quarters First Second Third Fourth First Second Third Fourth Gross premiums written $ 11 $ (1) $ (13) $ 1 $ 3 $ (21) $ (13) $ 2 Net premiums earned 10 11 15 18 28 8 10 20 Net investment income (21) 52 37 53 55 86 45 42 Net realized investment gains (losses) 8 10 2 2 17 36 18 9 Net gains (losses) on derivative contracts (70) 2 7 12 (16) (35) (10) 12 Other income (loss) — — 2 1 1 (9) 141 1 Income (loss) on Variable Interest Entities 3 — — 3 16 3 11 7 Losses and loss expenses (benefit) 117 16 83 9 12 (133) 37 97 Insurance intangible amortization 13 14 14 16 36 226 17 15 Operating expenses 24 21 23 26 25 29 26 23 Interest expense 63 58 50 50 68 67 67 66 Pre-tax income (loss) (287) (33) (108) (12) (41) (100) 69 (111) Net income (loss) attributable to Common Stockholders $ (280) $ (35) $ (108) $ (14) $ (43) $ (128) $ 66 $ (110) Net income (loss) per share: Basic $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.44 $ (2.40) Diluted $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.41 $ (2.40)

Basis of Presentation and Sig_9

Basis of Presentation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Operating Leased Assets [Line Items]
Consolidation	Consolidation The consolidated financial statements include the accounts of AFG and all other entities in which AFG (directly or through its subsidiaries) has a controlling financial interest, including variable interest entities (“VIEs”) for which AFG or an AFG subsidiary is deemed the primary beneficiary in accordance with the Consolidation Topic of the Accounting Standards Codification ("ASC"). All significant intercompany balances have been eliminated. The usual condition for a controlling financial interest is ownership of a majority of the voting interests of an entity. However, a controlling financial interest may also exist in entities, such as VIEs, through arrangements that do not involve controlling voting interests. A VIE is an entity: a) that lacks enough equity investment at risk to permit the entity to finance its activities without additional subordinated financial support from other parties; or b) where the group of equity holders does not have: (1) the power, through voting rights or similar rights, to direct the activities of an entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb the entity’s expected losses; or (3) the right to receive the entity’s expected residual returns. The determination of whether a variable interest holder is the primary beneficiary involves performing a qualitative analysis of the VIE that includes, among other factors, its capital structure, contractual terms including the rights of each variable interest holder, the activities of the VIE, whether the variable interest holder has the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance, whether the variable interest holder has the obligation to absorb losses of the VIE that could potentially be significant to the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE, related party relationships and the design of the VIE. An entity that is deemed the primary beneficiary of a VIE is required to consolidate the VIE. Refer to Note 4. Variable Interest Entities
Unconsolidated Financial Information	AFG Unconsolidated Financial InformationFinancial information of AFG is presented in Schedule II to this Form 10-K as of December 31, 2020 and 2019 and for the years ended December 31, 2020, 2019 and 2018. Investments in subsidiaries are accounted for using the equity method of accounting in Schedule II.
Financing Receivable, Allowance for Credit Losses, Policy for Uncollectible Amounts	Measurement of Credit Losses on Financial Instruments (CECL) On January 1, 2020 Ambac adopted ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) - Measurement of Credit Losses on Financial Instruments, subsequently amended by ASU 2018-19 , Codification Improvements to Topic 326, Financial Instruments - Credit Losses; ASU 2019-04 , Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments ; ASU 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief ; and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses (collectively the Current Expected Credit Loss standard or "CECL"). The new CECL standard affects how reporting entities measure credit losses for financial assets that are not accounted for at fair value through net income. For Ambac, these financial assets include available-for-sale debt securities and amortized cost assets, specifically premium receivables, reinsurance recoverables and loans. CECL does not apply to recoveries of previously paid losses on financial guarantee insurance contracts accounted for under ASC 944 nor does it apply to equity method investments accounted for under ASC 323. • For available-for-sale debt securities, credit losses under CECL are measured similarly to other-than-temporary impairments under prior GAAP. The updated guidance was applied prospectively. • For financial instruments measured at amortized cost, CECL replaces the "incurred loss" model, which generally delayed recognition of the full amount of credit losses until the loss was probable of occurring, with an "expected loss" model, which reflects an entity's current estimate of all expected lifetime credit losses. The estimate of expected lifetime credit losses should consider historical information, current information, as well as reasonable and supportable forecasts. Expected lifetime credit losses for amortized cost assets will be recorded as an allowance for credit losses, with subsequent increases or decreases in the allowance reflected in net income each period. The updated guidance was applied by a cumulative effect adjustment to the opening balance of retained earnings at January 1, 2020. This adjustment was not material to retained earnings or any individual balance sheet line item. Refer to the discussion below for each asset type.
Investments	Investments The Investments - Debt Securities Topic of the ASC requires that all debt instruments be classified in Ambac’s Consolidated Balance Sheets according to their purpose and, depending on that classification, be carried at either cost or fair market value. Ambac’s non-VIE debt investment portfolio is accounted for on a trade-date basis and consists primarily of investments in fixed maturity securities that are considered available-for-sale as defined by the Investments - Debt Securities Topic of the ASC. Available-for-sale debt securities are reported in the financial statements at fair value with unrealized gains and losses, net of deferred taxes, reflected in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity and computed using amortized cost as the basis. For purposes of computing amortized cost, premiums and discounts are accounted for using the effective interest method over a future term of the security. For structured debt securities with a large underlying pool of homogenous loans, such as mortgage-backed and asset-backed securities, premiums and discounts are adjusted for the effects of actual and anticipated prepayments. For other fixed maturity securities, such as corporate and municipal bonds, discounts were amortized or accreted over the remaining term of the securities. Ambac adopted ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities, on January 1, 2019. ASU 2017-08 shortened the amortization period for the premium on callable debt securities to the earliest call date. Under previous GAAP, Ambac generally amortized the premium over the contractual life (i.e. maturity) of the debt security and if that debt security was called, we would record a loss equal to the unamortized premium. Ambac’s non-VIE investment portfolio also includes equity interests in pooled investment funds which are accounted for in accordance with the Investments - Equity Securities Topic of the ASC and reported as Other investments on the Consolidated Balance Sheet with income reported through Net investment income on the Statement of Total Comprehensive Income (Loss). Equity interests in the form of common stock or in-substance common stock are classified as trading securities and reported at fair value while limited partner interests in such funds are reported using the equity method. Fair value is based primarily on quotes obtained from independent market sources. When quotes for fixed maturity securities are not available or cannot be reasonably corroborated, valuation models are used to estimate fair value. These models include estimates, made by management, which utilize current market information. When fair value is not readily determinable for pooled investment funds, the investments are valued using net asset value ("NAV") as a practical expedient as permitted under the Fair Value Measurement Topic of the ASC. Investment valuations could differ materially from amounts that would actually be realized in the market. Realized gains and losses on the sale of investments are determined on the basis of specific identification. VIE investments in fixed maturity securities are carried at fair value as they are either considered as available for sale securities or under the fair value option election. For additional information about VIE investments, including fair value by asset-type, see Note 4. Variable Interest Entities . Ambac has a formal credit impairment review process for fixed maturity available-for-sale securities in its investment portfolio. Ambac conducts a review each quarter to identify and evaluate investments that have indications of impairment in accordance with the Investments - Debt Securities Topic of the ASC. • Prior to the adoption of CECL, factors considered to identify and assess securities for other than temporary impairment include: (i) fair values that have declined by 20% or more below amortized cost; (ii) market values that have declined by 5% or more but less than 20% below amortized cost for a continuous period of at least six months; (iii) recent downgrades by rating agencies; (iv) the financial condition of the issuer and financial guarantor, as applicable, and an analysis of projected defaults on the underlying collateral; (v) whether scheduled interest payments are past due; (vi) whether Ambac has the intent to sell the security; and (vii) whether it is more likely than not that Ambac will be required to sell a security before the anticipated recovery of its amortized cost basis. If we believed a decline in the fair value of a particular investment is not credit-related, we recorded the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If it was determined that a credit impairment existed, the credit impairment loss was recognized in earnings, and the other-than-temporary amount related to all other factors was recognized in other comprehensive income. For fixed maturity securities that have credit impairments in a period, the previous amortized cost of the security less the amount of the credit impairment recorded through earnings becomes the investment’s new amortized cost basis. Ambac accretes the new amortized cost basis to par or to the estimated future cash flows to be recovered over the expected remaining life of the security. • Under CECL, credit losses are evaluated and measured similarly, however the recognition of credit impairment losses for available-for-sale debt securities are recorded as an allowance for credit losses with an offsetting charge to net income, rather than as a direct write-down of the security as was required under prior GAAP. As a result, improvements to estimated credit losses for available-for-sale debt securities are recognized immediately in net income rather than as interest income over time. Furthermore, as required under CECL, Ambac no longer considers the length of time a security has continuously been in an unrealized loss in the credit impairment process. If we believe a decline in the fair value of a particular investment is not credit impaired, we record the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If management either: (i) has the intent to sell its investment in a debt security or (ii) determines that the Company more likely than not will be required to sell the debt security before its anticipated recovery of the amortized cost basis less any current period credit impairment, then an impairment charge is recognized in earnings, with the amortized cost of the security being written-down to fair value. The evaluation of securities for credit impairment is a quantitative and qualitative process, which is subject to risks and uncertainties and is intended to determine whether, and to what extent, declines in the fair value of investments should be recognized in current period earnings. The risks and uncertainties include changes in general economic conditions, the issuer’s or guarantor’s financial condition and/or future prospects, the impact of regulatory actions on the investment portfolio, the performance of the underlying collateral, the effects of changes in interest rates or credit spreads and the expected recovery period. With respect to Ambac insured securities owned, future cash flows used to measure credit impairment represents the sum of (i) the bond’s intrinsic cash flows and (ii) the estimated AAC claim payments. Ambac’s assessment about whether a decline in value is considered a credit impairment reflects management’s current judgment regarding facts and circumstances specific to a security and the factors noted above. If that judgment changes, Ambac may ultimately record a charge for other-than-temporary impairment in future periods. Ambac has made certain accounting policy elections related to accrued interest receivable ("AIR") for available-for-sale investments under CECL, which are consistent with past practices under prior GAAP. Elections include: i) not measuring AIR for credit impairment, instead AIR is written off when it becomes 90 days past due; ii) writing off AIR by reversing interest income; iii) presenting AIR separately in Other Assets on the balance sheet and iv) excluding AIR from amortized cost balances in required CECL disclosures found in Note 11. Investments. AIR at December 31, 2020 was $10.
Net Premiums Earned	Net Premiums Gross premiums were received either upfront or in installments. For premiums received upfront, an unearned premium revenue (“UPR”) liability was established, which was initially recorded as the cash amount received. For installment premium transactions, a premium receivable asset and offsetting UPR liability was initially established in an amount equal to: (i) the present value of future contractual premiums due (the “contractual” method) or (ii) if the underlying insured obligation is a homogenous pool of assets which are contractually prepayable, the present value of premiums to be collected over the expected life of the transaction (the “expected” method). An appropriate risk-free rate corresponding to the weighted average life of each policy and currency is used to discount the future premiums contractually due or expected to be collected. For example, U.S. dollar exposures are discounted using U.S. Treasury rates while exposures denominated in a foreign currency are discounted using the appropriate risk-free rate for the respective currency. The weighted average risk-free rate at December 31, 2020 and 2019, was 2.2%. and 2.4%, respectively, and the weighted average period of future premiums used to estimate the premium receivable at December 31, 2020 and 2019, was 8.3 years and 8.5 years, respectively. Insured obligations consisting of homogeneous pools for which Ambac uses expected future premiums to estimate the premium receivable include residential mortgage-backed securities ("RMBS"). As prepayment assumptions change for homogenous pool transactions, or if there is an actual prepayment for a “contractual” method installment transaction, the related premium receivable and UPR are adjusted in equal and offsetting amounts with no immediate effect on earnings using new premium cash flows and the then current risk-free rate corresponding to the initial weighted average life of the related policy. For both upfront and installment premium policies, premium revenues are earned over the life of the financial guarantee contract in proportion to the insured principal amount outstanding at each reporting date (referred to as the level-yield method). For installment paying policies, the premium receivable discount, equating to the difference between the undiscounted future installment premiums and the present value of future installment premiums, is accreted as premiums earned in proportion to the premium receivable balance at each reporting date. • For financial guarantee contracts, the issuer's ability and willingness to pay its insured debt obligation impacts the payment of policy losses by Ambac as well as the receipt of premiums from the issuer. As such, management leverages its existing loss reserve estimation process to evaluate credit impairment for premium receivables. Key factors in assessing credit impairment include historical premium collection data, internal risk classifications, credit ratings and loss severities. For structured finance transactions involving special purpose entities, we further evaluate the priority of premiums paid to Ambac within the contractual waterfall, as required by bond indentures. Ambac has a formal quarterly credit impairment review process for premium receivables under financial guarantee insurance contracts. • Prior to the adoption of CECL, Ambac assessed collectability of premium receivables in accordance with ASC 944 and recorded an allowance for uncollectible premiums. • Under CECL, management utilizes either a discounted cash flow ("DCF") or probability of default/loss given default ("PD/LGD") approach to estimate credit impairment. The DCF approach utilizes expected cash flows developed by Ambac's Risk Management Group using the same (or similar) models used for estimating loss reserves where such models can identify shortfalls in premiums. Credit impairment using the DCF approach is equal to the difference between amortized cost and the present value of expected cash flows. Credit impairment under the PD/LGD approach is the product of (i) the premium receivable carrying value, (ii) internally developed default probability (considering internal ratings and average life), and (iii) internally developed loss severities. Refer to Note 8. Financial Guarantee Insurance Contracts for further credit impairment disclosures. AAC has reinsurance in place pursuant to surplus share treaty and facultative reinsurance agreements. Similar to gross premiums, premiums ceded to reinsurers were paid either upfront or in installments. For premiums paid upfront, a deferred ceded premium asset was established which is initially recorded as the cash amount paid. For installment premiums, a ceded premiums payable liability and offsetting deferred ceded premium asset were initially established in an amount equal to: i) the present value of future contractual premiums due or ii) if the underlying insured obligation is a homogenous pool of assets, the present value of expected premiums to be paid over the life of the transaction. An appropriate risk-free rate corresponding to the weighted average life of each policy and exposure currency is used to discount the future premiums contractually due or expected to be collected. Premiums ceded to reinsurers reduce the amount of premiums earned by Ambac from its financial guarantee insurance policies. For both upfront and installment premiums, ceded premiums written are primarily recognized in earnings in proportion to and at the same time as the related gross premium revenue is recognized. For premiums paid to reinsurers on an installment basis, Ambac records the present value of future ceding commissions as an offset to ceded premiums payable, using the same assumptions noted above for installment premiums. When a bond issue insured by Ambac has been retired early, typically due to an issuer call, any remaining UPR is recognized at that time to the extent the financial guarantee contract is legally extinguished, causing accelerated premium revenue. For installment premium paying transactions, we offset the recognition of any remaining UPR by the reduction of the related premium receivable to zero (as it will not be collected as a result of the retirement), which may cause negative accelerated
Loans	Loans Loans are reported at either their outstanding principal balance less unamortized discount or at fair value. • Loans not held by consolidated VIEs are reported at their outstanding principal balance less unamortized discount and are reported within Other assets on the Consolidated Balance Sheet. Interest income is earned using the effective interest method based upon interest accrued on the unpaid principal balance adjusted for accretion of discounts. A loan is considered impaired when, based on the financial condition of the borrower, it is probable that Ambac will be unable to collect all principal and interest due according to the contractual terms of the loan agreement. Under CECL, Ambac has a formal quarterly credit impairment review process for these loans. The key factors in assessing credit impairment are internal credit ratings and loss severities. Management utilizes a PD/LGD approach, similar to the one described above for premium receivables, which is applied to the loan carrying value. • Loans held by VIEs consolidated as required under the Consolidation Topic of the ASC are carried at fair value under the fair value option election with changes in fair value recorded in Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). Such loans are reported as Loans, at fair value within the Variable interest entity assets section of the Consolidated Balance Sheet.
Derivative Contracts	Derivative Contracts The Company has entered into derivative contracts to hedge certain economic risks inherent in its asset and liability portfolios. None of Ambac’s derivative contracts are designated as hedges under the Derivatives and Hedging Topic of the ASC. Ambac's derivatives consist primarily of interest rate swaps and futures contracts. • Ambac maintains a portfolio consisting primarily of interest rate swaps and futures contracts to economically hedge interest rate risk in the financial guarantee and investment portfolios. While this portfolio also includes certain legacy interest rate swaps executed in connection with financial guarantee client financings, the interest rate derivatives portfolio is managed on the basis of its net sensitivity to changes in interest rates. Changes in the fair value of these interest rate derivatives are recorded, along with changes in fair value of Ambac's remaining credit derivatives, within Net gains (losses) on derivative contracts on the Consolidated Statements of Total Comprehensive Income (Loss). • VIEs consolidated under the Consolidation Topic of the ASC entered into derivative contracts to meet specified purposes within their securitization structure. Changes in fair value of consolidated VIE derivatives are included within Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). All derivatives are recorded on the Consolidated Balance Sheets at fair value on a gross basis; assets and liabilities are netted by counterparty only when a legal right of offset exists. Variation payments on centrally cleared swaps and futures contracts are considered settlements of the associated derivative balances and are reflected as a reduction to derivative liabilities or assets on the Consolidated Balance Sheets. For other derivatives, Ambac has determined that the amounts recognized for the right to reclaim cash collateral or the obligation to return cash collateral may not be used to offset amounts due under the derivative instruments in the normal course of settlement. Therefore, such amounts are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement and are included in "Other assets" on the Consolidated Balance Sheets. Refer to Note 12. Derivative Instruments for further discussion of the Company’s use of derivative instruments and their impact of the consolidated financial statements. Refer to Note 10. Fair Value Measurements
Restricted Cash	Restricted Cash Cash that we do not have the right to use for general purposes is recorded as restricted cash in our consolidated balance sheets. Restricted cash includes (i) consolidated variable interest entity cash restricted to support the obligations of the consolidated VIEs, (ii) cash held by AAC received from its investment in Secured Notes and pledged for the benefit of holders of Secured Notes (other than AAC) and (iii) fiduciary cash held by Xchange described below. Fiduciary Assets and Liabilities : In Xchange's capacity as a managing general agent, generally it collects premiums from insureds and remits the premiums to the respective insurance
Loss Reserves	Loss and Loss Expenses The loss and loss expense reserve (“loss reserve”) policy relates only to Ambac’s non-derivative insurance business for insurance policies issued to beneficiaries, including VIEs, for which we do not consolidate the VIE. Losses and loss expenses are based upon estimates of the ultimate aggregate losses inherent in the non-derivative financial guarantee portfolio as of the reporting date. The policy for derivative contracts is discussed in the “Derivative Contracts” section above. A loss reserve is recorded on the balance sheet on a policy-by-policy basis based upon the present value ("PV") of expected net claim cash outflows or expected net recovery cash inflows, discounted at risk-free rates. The estimate for future net cash flows consider the likelihood of all possible outcomes that may occur from missed principal and/or interest payments on the insured obligation. This estimate also considers future recoveries related to breaches of contractual representations and warranties by RMBS transaction sponsors, remediation strategies, excess spread and other contractual or subrogation-related cash flows. Ambac’s approach to resolving disputes involving contractual breaches by transaction sponsors or other third parties has included negotiations and/or pursuing litigation. Ambac does not estimate recoveries for litigations where its sole claim is for fraudulent inducement, since any remedies under such claims would be non-contractual. • Net claim cash outflow policies represent contracts where the PV of expected cash outflows are greater than the PV of expected recovery cash inflows. For such policies, a “Loss and loss expense reserves” liability is recorded for the excess of the PV of expected net claim cash outflows over the unearned premium revenue. • Net recovery cash inflow policies represent contracts where the PV of expected recovery cash inflows are greater than the PV of expected claim cash outflows. For such policies, a “Subrogation recoverable” asset is recorded. The evaluation process for determining expected losses is subject to certain judgments based on our assumptions regarding the probability of default by the issuer of the insured security, probability of settlement outcomes (which may include commutation settlements, refinancing and/or other settlement outcomes) and expected severity of credits for each insurance contract. Ambac’s loss reserves are based on management’s ongoing review of the financial guarantee credit portfolio. Active surveillance of the insured portfolio enables Ambac’s Risk Management Group ("RMG") to track credit migration of insured obligations from period to period and update internal classifications and credit ratings for each transaction. Non- adversely classified credits are assigned a Class I rating while adversely classified credits are assigned a rating of Class IA through Class V. The criteria for an exposure to be assigned an adversely classified credit rating includes the deterioration of an issuer’s financial condition, underperformance of the underlying collateral (for collateral dependent transactions such as mortgage-backed or student loan securitizations), poor performance by the servicer of the underlying collateral and other adverse economic events or trends. The servicer of the underlying collateral of an insured securitization transaction is a consideration in assessing credit quality because the servicer’s performance can directly impact the performance of the related issue. For example, a servicer of a mortgage-backed securitization that does not remain current in its collection efforts could cause an increase in the delinquency and the potential for default of the underlying obligation. Similarly, loss severities increase when a servicer does not effectively handle loss mitigation activities such as (i) the advancing of delinquent principal and interest and of default related expenses which are deemed to be recoverable by the servicer, (ii) pursuit of loan charge-offs which maximize cash flows from the mortgage loan pool and (iii) foreclosure and real estate owned disposition strategies and timelines. All credits are assigned risk classifications by RMG using the following guidelines: CLASS I - “Fully Performing - Meets Ambac Criteria with Remote Probability of Claim” - Credits that demonstrate adequate security and structural protection with a strong capacity to pay interest, repay principal and perform as underwritten. Factors supporting debt service payment and performance are considered unlikely to change and any such change would not have a negative impact upon the fundamental credit quality. Through ongoing surveillance, Ambac may also designate Class I credits into one or more of the following categories: • Survey List - credits that may lack information or demonstrate a weakness but further deterioration is not expected. • Watch List - credits that demonstrate the potential for future material adverse development due to such factors as long-term uncertainty about a particular sector, a certain structural element or concern related to the issuer or transaction or the overall financial and economic sustainability. CLASS IA - “Potential Problem with Risks to be Dimensioned” - Credits that are fully current and monetary default or claims-payment are not anticipated. The payor’s or issuer’s financial condition may be deteriorating or the credits may lack adequate collateral. A structured financing may also evidence weakness in its fundamental credit quality as evidenced by its under-performance relative to its modeled projections at underwriting, issues related to the servicer’s ability to perform or questions about the structural integrity of the transaction. While certain of these credits may still retain an investment grade rating, they usually have experienced or are vulnerable to a ratings downgrade. Further investigation is required to dimension and correct any deficiencies. A complete legal review of documents may be required. An action plan should be developed with triggers for future classification changes upward or downward. CLASS II - “Substandard Requiring Intervention” - Credits whose fundamental credit quality has deteriorated to the point that timely payment of debt service may be jeopardized by adversely developing trends of a financial, economic, structural, managerial or political nature. No claim payment is currently foreseen but the probability of loss or claim payment over the life of the transaction is now existent (generally 10% or greater probability). Class II credits may be border-line or below investment grade (BBB- to B). Prompt and sustained action must be taken to execute a comprehensive loss mitigation plan and correct deficiencies. CLASS III - “Doubtful with Clear Potential for Loss” - Credits whose fundamental credit quality has deteriorated to the point that timely payment of debt service has been or will be jeopardized by adverse trends of a financial, economic, structural, managerial or political nature which, in the absence of positive change or corrective action, are likely to result in a loss. The probability of monetary default or claims paying over the life of the transaction is generally 50% or greater. Full exercise of all available remedial actions is required to avert or minimize losses. Class III credits will generally be rated below investment grade (B to CCC). CLASS IV - “Imminent Default or Defaulted” - Monetary default or claim payments have occurred or are expected imminently. Class IV credits are generally rated D. CLASS V - “Fully Reserved” - The credit has defaulted and payments have occurred. The claim payments are scheduled and known, reserves have been established to fully cover such claims, and no claim volatility is expected. The population of credits evaluated in Ambac’s loss reserve process are: (i) all adversely classified credits (Class IA through V) and ii) non-adversely classified credits which had an internal Ambac rating downgrade since the transaction’s inception. One of two approaches is then utilized to estimate losses to ultimately determine if a loss reserve should be established. The first approach is a statistical expected loss approach, which considers the likelihood of all possible outcomes. The “base case” statistical expected loss is the product of: (i) the par outstanding on the credit; (ii) internally developed default information (taking into consideration internal ratings and average life of an obligation); (iii) internally developed loss severities; and (iv) a discount factor. The loss severities and default information are based on rating agency information, are specific to each bond type and are established and approved by senior RMG officers. For certain credit exposures, Ambac’s additional monitoring, loss remediation efforts and probabilities of potential settlement outcomes may provide information relevant to adjust this estimate of “base case” statistical expected losses. Analysts may accept the “base case” statistical expected loss as the best estimate of expected loss or assign multiple probability weighted scenarios to determine an adjusted statistical expected loss that better reflects management’s view of a given transaction’s expected losses, as well as the potential for additional remediation activities (e.g., commutations). The second approach entails the use of cash-flow based models to estimate expected losses (future claims, net of potential recoveries, expected to be paid to the holder of the insured financial obligation). Ambac’s RMG group will consider the likelihood of all possible outcomes and develop appropriate cash flow scenarios. This approach can include the utilization of internal or third party models and tools to project future losses and resultant claim payment estimates. We utilize cash flow models for RMBS, student loan, Puerto Rico and other exposures. RMBS and student loan models use historical performance of the collateral pools in order to then derive future performance characteristics, such as default and voluntary prepayment rates, which in turn determine projected future claim payments. In other cases, such as many public finance exposures, including our Puerto Rico exposures, we do not specifically forecast resources available to pay debt service in the cash flow model itself. Rather, we consider the issuers’ overall ability and willingness to pay, including the fiscal, economic, legal and political framework. In this approach, a probability-weighted expected loss estimate is developed based on assigning probabilities to multiple claim payment scenarios and applying an appropriate discount factor. Additionally, we assign a probability to the issuer’s ability to refinance an insured issue, Ambac’s ability to execute a potential settlement (i.e., commutation) of the insurance policy, including the impact on future installment premiums, and/or other restructuring scenarios. The commutation scenarios and the related probabilities of occurrence vary by transaction, depending on our view of the likelihood of negotiating such a transaction with issuers and/or investors. The discount factor applied to the statistical expected loss approach is based on a risk-free discount rate corresponding to the remaining expected weighted-average life of the exposure and the exposure currency. For the cash flow scenario approach, discount factors are applied based on a risk-free discount rate term structure and correspond to the date of each respective cash flow payment or recovery and the exposure currency. Discount factors are updated for the current risk-free rate each reporting period. Ambac establishes loss expense reserves based on our estimate of expected net cash outflows for loss expenses, such as legal and consulting costs. Below we provide further details of our loss reserve models for both RMBS and student loan exposures: RMBS Expected Loss Estimate Ambac insures RMBS transactions collateralized by first-lien mortgages. Ambac has also insured RMBS transactions collateralized predominantly by second-lien mortgage loans such as closed-end seconds and home equity lines of credit. A second-lien mortgage loan is a type of loan in which the borrower uses the equity in their home as collateral and the second-lien loan is subordinate to the first-lien loan outstanding on the home. Borrowers are obligated to make monthly payments on both their first and second-lien loans. If the borrower defaults on the payments due under these loans and the property is subsequently liquidated, the liquidation proceeds are first utilized to pay off the first-lien loan (as well as other costs) and any remaining funds are applied to pay off the second-lien loan. As a result of this subordinate position to the first-lien loan, second-lien loans may carry a significantly higher severity in the event of a loss, approaching or exceeding 100%. Ambac primarily utilizes a cash flow model (“RMBS cash flow model”) to develop estimates of projected losses for both our first and second lien transactions. First, the RMBS cash flow model projects collateral performance utilizing: (i) the transaction’s underlying loans' characteristics and status, (ii) projected home price appreciation (“HPA”) and (iii) projected interest rates. Depending in the amount of collateral information available for each transaction, we project such performance either at the loan-level or the deal-level. In the absence of specific loan-level information, the deal-level approach evaluates a loan pool as if it were a single loan, selecting certain aggregated deal-level characteristics to then perform a series of statistical analyses. The deal-level approach projects performance using a roll-rate that evaluates the possible future state of a loan based on its current status and three variables: average FICO (credit score), average current consolidated loan to value ratio (“CLTV”) and an overall quality indicator. Observed servicer-level behavior may also have an impact on projected transaction performance. We source HPA projections from a market accepted vendor and interest rate projections are developed from market sources. We use three HPA projection scenarios to develop a base case as well as stress and upside cases. The highest probability is assigned to the base case, with lower probabilities to the stress and upside cases. For the liabilities of the transaction which we insure, we generally utilize waterfall projections generated from a tool provided by a market accepted vendor. This waterfall tool allows us to capture the impact of each transaction’s specific structure (e.g., the waterfall priority of payments, triggers, redemption priority) to generate our specific projected claims profile in the base, upside and downside scenarios. On a monthly basis, we compare monthly claims submitted against the trustees’ reports, waterfall projections and our understanding of the transactions’ structures to identify and resolve discrepancies. We also review the vendor’s published waterfall revisions to identify significant discrepancies. Resolving discrepancies is challenging and may take place over an extended period of time. Moreover, transaction documents are subject to interpretation, and our interpretation or that of the vendor and as reflected in our loss reserves may prove to be incorrect and/or not consistent with trustees directing cash flows in the future. In some cases, we may utilize an alternative waterfall structure when our legal and commercial analysis of the transaction’s payment structure differs from the vendor’s waterfall structure. In our experience, market performance and model characteristics change and therefore need to be updated and reflected in our models through time. As such, we conduct regular reviews of current models, alternative models and the overall approach to loss estimation. RMBS Representation and Warranty Subrogation Recoveries Ambac records, as a component of its loss reserve estimate, subrogation recoveries related to securitized loans in RMBS transactions that breached certain representations and warranties described herein. Generally, the sponsor of an RMBS transaction provided representations and warranties with respect to the securitized loans, including representations and warranties with respect to loan characteristics, the absence of borrower fraud in the underlying loan pools and other misconduct in the origination process and attesting to the compliance of loans with the prevailing underwriting policies. In such cases, the sponsor of the transaction is contractually obligated to repurchase, cure or substitute collateral for any loan that breaches the representations or warranties. Ambac or its counsel engaged consultants with significant mortgage underwriting experience to review the underwriting documentation for mortgage loans underlying certain insured RMBS transactions which exhibited exceptionally poor performance. Factors which Ambac believes to be indicative of poor performance include (i) high levels of early payment defaults, (ii) significant numbers of loan liquidations or charge-offs and resulting high levels of losses and (iii) rapid elimination of credit protections inherent in the transactions’ structures. With respect to item (ii), “loan liquidations” refers to loans for which the servicer has liquidated the related collateral and the securitization has realized losses on the loan; “charge-offs” refers to loans which have been written off as uncollectible by the servicer, generating no recoveries to the securitization, and may also refer to the unrecovered balance of liquidated loans. In either case, the servicer has taken actions to recover against the collateral, and the securitization has incurred losses to the extent such actions did not result in full repayment of the borrower’s obligations. Generally, subsequent to the forensic exercise of examining loan files to ascertain whether the loans conformed to the representations and warranties, we submitted nonconforming loans for repurchase to the contractual counterparty bearing the repurchase obligation, typically the transaction sponsor. In cases where loans are repurchased by a sponsor, the effect is typically to offset current period losses and then to increase the over-collateralization of the securitization, depending on the extent of loan repurchases and the structure of the securitization. Specifically, the repurchase price is paid by the sponsor to the securitization trust which holds the loan. The cash becomes an asset of the trust, replacing the loan that was repurchased by the sponsor. On a monthly basis, the cash received related to loan repurchases by the sponsor is aggregated with cash collections from the underlying mortgages and applied in accordance with the trust indenture payment waterfall. To the extent there continues to be insufficient cash in the waterfall in the current month to make scheduled principal and interest payments to the note holders, Ambac is required to make additional claim payments to cover this shortfall. Ambac may also receive payments directly from transaction sponsors in settlement of their repurchase obligations pursuant to negotiated settlement agreements or otherwise as a result of related litigation. While the obligation by sponsors to repurchase loans with material breaches is clear, generally the sponsors have not honored those obligations without actual or threatened litigation. Ambac has utilized the results of the above described loan file examinations to make demands for loan repurchases from sponsors or their successors and, in certain instances, as a part of the basis for litigation. Ambac’s approach to resolving these disputes has included negotiating with individual sponsors at the transaction level and in some cases at the individual loan level and has resulted in the repurchase of some loans. Ambac has initiated and will continue to pursue lawsuits seeking compliance with the repurchase obligations in the securitization documents. Ambac has performed the above-mentioned, detailed examinations on a variety of second-lien and first-lien transactions that have experienced exceptionally poor performance. However, the loan file examinations and related estimated recoveries we have reviewed and recorded to date have been limited to only those transactions whose sponsors (or their successors) are subsidiaries of large financial institutions, all of which carry an investment grade rating from at least one nationally recognized rating agency, or are otherwise deemed to have the financial wherewithal to live up to their repurchase obligations. While our contractual recourse is generally to the sponsor/subsidiary, rather than to the parent, each of these large institutions has significant financial resources and may have an ongoing interest in mortgage finance, and we therefore believe that the financial institution/parent would ultimately assume financial responsibility for these obligations if the sponsor/subsidiary is unable to honor its contractual obligations or pay a judgment that we may obtain in litigation. Additionally, in the case of successor institutions, we are not aware of any provisions that explicitly preclude or limit the successors’ ability to honor the obligations of the original sponsor. Certain successor financial institutions have made significant payments to certain claimants to settle breaches of representations and warranties perpetrated by sponsors that have been acquired by such financial institutions. For example, Ambac received a significant payment in 2016 from JP Morgan to settle RMBS-related litigation. As a result of these factors, we do not make significant adjustments to our estimated subrogation recoveries with respect to the credit risk of these sponsors or their successors. Our ability to realize RMBS R&W subrogation recoveries is subject to significant uncertainty, including risks inherent in litigation; collectability of such amounts from counterparties (and/or their respective parents and affiliates); timing of receipt of any such recoveries; intervention by OCI, which could impede our ability to take actions required to realize such recoveries; and uncertainty inherent in the assumptions used in estimating such recoveries. Failure to realize RMBS R&W subrogation recoveries for any reason or the realization of RMBS R&W subrogation recoveries materially below the amount recorded on Ambac's consolidated balance sheet would have a material adverse effect on our results of operations and financial condition and may result in adverse consequences such as impairing the ability of AAC to honor its financial obligations; the initiation of rehabilitation proceedings against AAC; decreased likelihood of AAC delivering value to Ambac, through dividends or otherwise; and a significant drop in the value of securities issued and/or insured by Ambac or AAC. The approach used to estimate RMBS R&W subrogation recoveries is based on obtaining loan files from the original pool and conducting loan file re-underwriting to derive a breach rate to be extrapolated to determine an estimated repurchase obligation. We limit the estimated repurchase obligation by ever-to-date incurred losses. Multiple probability-weighted scenarios are developed by applying various realization factors to the estimated repurchase obligation. The realization factors in these scenarios reflect Ambac’s own assumptions about the likelihood of outcomes based on all the information available to it including, but not limited to, (i) discussions with external legal counsel and their views on ultimate settlement and/or litigation outcomes, (ii) assessment of the strength of the specific case and (iii) experience in settling similar claims. The probability weightings are developed based on the unique facts and circumstances for each transaction. The sum of these probability-weighted scenarios represents the undiscounted RMBS R&W subrogation recovery, which is then discounted using a factor derived from a risk-free discount rate term structure that corresponds to the estimated date of each respective recovery. Student Loan Expected Loss Estimate The student loan insured portfolio consists of credits collateralized by private student loans. The calculation of loss reserves for our student loan portfolio involves evaluating numerous factors that can impact ultimate losses. The factor which contributes the greatest degree of uncertainty in ascertaining appropriate loss reserves is the long final legal maturity date of the insured bonds. Most of the student loan bonds which we insure were issued with original terms of 20 to 40 years until final maturity. Since our policy covers timely interest and ultimate principal payment, our loss projections must make assumptions for many factors covering a long time horizon. Key assumptions that will impact ultimate losses include, but are not limited to, the following: collateral performance (which is highly correlated to the economic environment); interest rates; operating risks associated with the issuer, servicers, special servicers, and administrators; investor appetite for tendering or commuting insured obligations and; as applicable, Ambac’s ability and willingness to commute policies. In addition, we consider in our student loan loss projections the potential impact, if any, of proposed or final regulatory actions or orders, including by the Consumer Financial Protection Bureau ("CFPB"), affecting our insured transactions. In evaluating our student loan portfolio, our losses are projected using a cash flow modeling approach. In order to project collateral performance under the cash flow approach, we use a default projection tool that constructs lifetime cohort default curves based on loan and deal-level historical performance data. To determine ultimate losses on the transactions, the cohort default curves are used to extrapolate future default behavior. Additionally, a regression-based model is used to estimate recoveries on defaulted loans. This regression-based recovery forecast is grounded in deal-level performance data. For the liabilities of the transaction which we insure, the transaction losses are then incorporated into a waterfall tool to develop loss
Reinsurance Accounting Policy	Ceded Reinsurance Loss and loss expense reserve reported on the balance sheet relates only to direct insurance policies. The corresponding reserve ceded to reinsurers is reported as reinsurance recoverable on paid and unpaid losses. AAC has reinsurance in place pursuant to surplus share treaty and facultative reinsurance agreements. The reinsurance of risk does not relieve AAC of its original liability to its policyholders. In the event that any of Ambac Assurance’s reinsurers are unable to meet their obligations under reinsurance contracts, AAC would, nonetheless, be liable to its policyholders for the full amount of its policy. Credit exposure exists with respect to reinsurance recoverables to the extent that any reinsurer may not be able to reimburse AAC under the terms of these reinsurance arrangements. To minimize its exposure to losses from reinsurers, AAC (i) monitors the financial condition of its reinsurers; (ii) is entitled to receive collateral from its reinsurance counterparties in certain reinsurance contracts; and (iii) has certain cancellation rights that can be exercised by AAC in the event of rating agency downgrades of a reinsurer (among other events and circumstances). Under CECL, Ambac has a formal quarterly credit impairment review process whereby Ambac has elected to use the practical expedient of considering the fair value of collateral posted by reinsurers when evaluating credit impairment. To determine the total unsecured recoverable to be evaluated for credit impairment, Ambac nets the reinsurance recoverable amount by ceded premiums payable and the fair value of collateral posted, if any. The key factors in assessing credit impairment for reinsurance recoverables are independent rating agency credit ratings and loss severities. Management utilizes a PD/LGD approach, similar to the one described above for premium receivables, which is applied to the net unsecured reinsurance recoverable amount.
Debt, Policy	Long-Term Debt Long-term debt issued by Ambac is carried at par value less unamortized discount. Accrued interest and discount accretion on long-term debt is reported as Interest expense on the Consolidated Statements of Total Comprehensive Income (Loss). To the extent Ambac repurchases or redeems its long- term debt, such repurchases or redemptions may be settled for an amount different than the carrying value of the obligation. Any difference between the payment and carrying value of the obligation is reported in Net realized gains (losses) on extinguishment of debt on the Consolidated Statements of Total Comprehensive Income (Loss). For long-term debt issued by consolidated VIEs in which Ambac's variable interest arises from financial guarantees written by Ambac's subsidiaries ("FG VIEs"), we may elect to use the fair value option on an instrument by instrument basis. When the fair value option is elected, changes in the fair value of the FG VIEs' long-term debt is reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss), except for the portion of the total change in fair value of financial liabilities caused by changes in the instrument-specific credit risk which is presented separately in Other comprehensive income (loss). In cases where the fair value option has not been elected, the FG VIEs' long-term debt is carried at par less unamortized discount, with interest expense reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss).
Noncontrolling Interest Policy [Policy Text Block]	Noncontrolling Interests Nonredeemable noncontrolling interests At December 31, 2020 and 2019, AAC had 5,501 shares of issued and outstanding AMPS with a liquidation preference of $138 (reported as nonredeemable noncontrolling interest of $60 on Ambac's balance sheet). The auction occurs every 28 days and the dividend rate has continuously been reset at the maximum rate of one-month LIBOR plus 200 basis points. Under the terms of the AMPS, dividends may not be paid on the common stock of AAC unless all accrued and unpaid dividends on the AMPS for the then current dividend period have been paid, provided, that dividends on the common stock may be made at all times for the purpose of, and only in such amounts as are necessary for, enabling AFG (i) to service its indebtedness for borrowed money as such payments become due or (ii) to pay its operating expenses. If dividends are paid on the common stock as provided in the prior sentence, dividends on the AMPS become cumulative until the date that all accumulated and unpaid dividends have been paid on the AMPS. AAC has not paid dividends on its AMPS since 2010. Redeemable noncontrolling interests
Postretirement and Postemployment Benefits	Postretirement and Postemployment BenefitsAmbac provides postretirement and postemployment benefits, including health and life benefits covering employees who meet certain age and service requirements. Ambac accounts for these benefits under the accrual method of accounting. Amounts related to the postretirement health benefits liability are established and charged to expense based on actuarial determinations.
Long Term Incentive and Stock Compensation Plan	Incentive Compensation Incentive compensation is a key component of our compensation strategy. Incentive compensation has two components: short term incentive compensation (consisting of an annual cash bonus and, prior to 2020, awards of deferred stock units for certain officers) and long term incentive plan awards (consisting of deferred cash and awards of restricted and performance stock units). Annual decisions with regard to incentive compensation are generally made in the first quarter of each year and are based on the prior year's performance for the Company, the employee and the employee's business unit. In 2020, the Ambac 2013 Incentive Compensation Plan (the “2013 Incentive Plan”) was superseded by the 2020 Incentive Compensation Plan ("2020 Incentive Plan"). Both plans allow for the granting of stock options, restricted stock, stock appreciation rights, restricted and performance units and other awards to employees, directors and consultants that are valued or determined by reference to Ambac's common stock. Under these plans, Ambac has issued both cash and equity awards to US employees. In connection with the adoption of the 2020 Incentive Plan, all shares reserved but unissued under the 2013 Incentive Plan were transferred to the the 2020 Incentive Plan in addition to any shares underlying outstanding awards under the 2013 Incentive Plan as of June 2, 2020 that subsequently terminate by expiration or forfeiture, cancellation, or otherwise are not issued. Under the 2013 and 2020 Incentive Compensation Plans. Ambac recognizes compensation costs for all equity classified awards granted at fair value, which is measured on the grant date, and records forfeitures for unvested shares only when they occur. For awards that only include service and performance conditions, the fair value is the market price of Ambac stock on the grant date. For awards that also contain a market condition, specifically a total shareholder return ("TSR") modifier, the fair value is estimated using a Monte Carlo simulation. The types of equity awards granted to employees are as follows: • Deferred stock units granted vest upon grant and will settle and convert to Ambac common stock annually over a two-year period (50% on the first anniversary of the grant date and 50% on the second anniversary of the grant date). The fair value of these grants is recognized as compensation expense on the date of grant since no future service is required. • Restricted stock units granted only require future service and accordingly the respective fair value is recognized as compensation expense over the relevant service period. • Performance stock units granted require both future service and achieving specified performance targets to vest. Certain performance stock unit grants also include a market condition TSR modifier that will cause the total payout at the end the performance period to increase or decrease depending on Ambac's stock performance relative to a peer group. Compensation costs for all performance stock units are only recognized when the achievement of the performance conditions are considered probable. Once deemed probable, such compensation costs are recognized as compensation expense over the relevant service period. Compensation costs are initially based on the probable outcome of the performance conditions and adjusted for subsequent changes in the estimated or actual outcome each reporting period as necessary. Changes in the estimated or actual outcome of a performance condition are recognized by reflecting a retrospective adjustment to compensation cost in the current period.
Lessee, Operating Lease, Disclosure [Table Text Block]	Operating Leases In 2019, Ambac adopted ASU 2016-02, Leases (Topic 842), amended by ASU 2018-01, Land Easement Practical Expedient; ASU 2018-10, Codification Improvements to Topic 842; ASU 2018-11, Targeted Improvements; ASU 2018-20, Narrow-Scope Improvements for Lessors; and ASU 2019-01, Leases (Topic 842): Codification Improvements (collectively the "New Lease Standard"). Ambac used a modified retrospective approach and applied the New Lease Standard on its effective date of January 1, 2019. Additionally, Ambac applied the New Lease Standard to its recently acquired affiliate, Xchange, on the acquisition date of December 31, 2020. Refer to Note 3. Business Combination for further discussion of the acquisition. A contract contains a lease if it conveys the right to control the use of identified property, plant, or equipment for a period of time in exchange for consideration. Ambac's evaluation of whether certain contracts contain leases requires judgment regarding what party controls the asset and whether the asset is physically distinct. Ambac is the lessee in leases which are classified as operating leases. In accordance with the New Lease Standard, Ambac recognizes a single lease cost, calculated so that the cost is allocated generally on a straight-line basis over the lease term within operating expenses in the Consolidated Statements of
Depreciation and Amortization	Depreciation and AmortizationDepreciation of furniture and fixtures, certain information technology development costs and electronic data processing equipment is charged over the estimated useful lives of the respective assets, ranging from three five
Foreign Currency	Foreign Currency Financial statement accounts expressed in foreign currencies are translated into U.S. dollars in accordance with the Foreign Currency Matters Topic of the ASC. The functional currencies of Ambac's subsidiaries are the local currencies of the country where the respective subsidiaries are based, which are also the primary operating environments in which the subsidiaries operate. Foreign currency translation : Functional currency assets and liabilities of Ambac’s foreign subsidiaries are translated into U.S. dollars using exchange rates in effect at the balance sheet dates and the related translation adjustments, net of deferred taxes, are included as a component of Accumulated Other Comprehensive Income (Loss) in Stockholders' Equity. Functional currency operating results of foreign subsidiaries are translated using average exchange rates. Foreign currency transactions : The impact of non-functional currency transactions and the remeasurement of non-functional currency assets and liabilities into the respective subsidiaries' functional currency (collectively "foreign currency transactions gains/(losses)") are $(1), $12 and $(7) for the years ended December 31, 2020, 2019 and 2018. Foreign currency transaction gains/(losses) are primarily the result of remeasuring Ambac UK's assets and liabilities denominated in currencies
Income Taxes	Income Taxes Ambac files a consolidated U.S. Federal income tax return with its subsidiaries. Ambac UK files tax returns in both the United Kingdom and Italy (for its Milan branch). Current tax assets and liabilities are recognized for taxes refundable or payable for the current year. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on current and deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. In July 2020, United Kingdom legislation increasing the tax rate from 17% to 19% was fully enacted. As such, we incorporated the effects of the tax rate increase in our current and deferred tax evaluation for the year ended December 31, 2020. The Income Taxes Topic of the ASC requires that companies assess whether valuation allowances should be established against their deferred tax assets based on the consideration of all available evidence using a ‘more likely than not” standard. In making such judgments, significant weight is given to evidence that can be objectively verified.
Net Income Per Share	Net Income Per ShareBasic net income per share is computed by dividing net income attributable to common stockholders by the weighted-average number of common shares outstanding and vested restricted stock units (together, "Basic Weighted Average Shares Outstanding"). Diluted net income per share is computed by dividing net income attributable to common stockholders by the Basic Weighted-Average Shares Outstanding plus all potential dilutive common shares outstanding during the period. All potential dilutive common shares outstanding consider common stock deliverable pursuant to warrants, vested and unvested options, unvested restricted stock units and performance stock units granted under existing compensation plans.
Reclassifications	Reclassifications and Rounding Reclassifications may have been made to prior years' amounts to conform to the current year's presentation. Certain amounts and tables in the consolidated financial statements and associated notes may not add due to rounding.
Adoption of New Accounting Standards	Adopted Accounting Standards Effective January 1, 2020, Ambac adopted the following accounting standards: CECL For further discussion of CECL, refer to the Measurement of Credit Losses (CECL), Investments, Net Premiums, Loans, and Loss and Loss Expenses sections in Note 2. Basis of Presentation and Significant Accounting Policies; Note 8. Financial Guarantee Insurance Contracts; and Note 11. Investments in the Notes to Consolidated Financial Statements included in this Annual Report on Form 10-K for the year ended December 31, 2020. For available-for-sale debt securities, the updated guidance was applied prospectively and for financial instruments measured at amortized cost (i.e. premiums receivable, loans and reinsurance recoverables), the updated guidance was applied by a cumulative effect adjustment to the opening balance of retained earnings at January 1, 2020. This adjustment was not material to retained earnings or any individual balance sheet line item. Fair Value Measurement Disclosures In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820) - Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. The ASU modified various disclosure requirements on fair value measurements. Relevant disclosures that were removed, modified and added are as follows: • Removals : (1) Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, (2) Policy for timing of transfers between levels, and (3) Valuation processes for Level 3 fair value measurements. • Modifications: (1) For investments in certain entities that calculate net asset value, disclosures are required for the timing of liquidation of an investee's assets and the date when restrictions from redemption might lapse, only if the investee has communicated the timing to the reporting entity or publicly announced it and (2) Clarification that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date and not possible future changes. • Additions : (1) Changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and (2) Range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Alternatively, an entity may disclose other quantitative information (such as the median or arithmetic average) if it determines that it is a more reasonable and rational method to reflect the distribution of unobservable inputs used. Disclosure amendments related to changes in unrealized gains and losses included in other comprehensive income (loss) for Level 3 instruments, the range and weighted average of significant unobservable inputs, and the narrative description of measurement uncertainty were applied prospectively only for the most recent interim or annual period presented. All other disclosure amendments were applied retrospectively to all periods presented. Refer to Note 10. Fair Value Measurements for further disclosures. VIE Related Party Guidance In October 2018, the FASB issued ASU 2018-17, Consolidation (Topic 810) - Targeted Improvements to Related Party Guidance for Variable Interest Entities. To determine whether a decision-making fee is a variable interest, under the new guidance a reporting entity must consider indirect interests held through related parties under common control on a proportional basis rather than as a direct interest in its entirety (as was previously required under prior GAAP). These amendments create alignment between determining whether a decision making fee is a variable interest and determining whether a reporting entity within a related party group is the primary beneficiary of a VIE. Adoption of this ASU did not impact Ambac's financial statements. Cloud Computing Arrangement Service Contracts In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other— Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The new guidance requires a customer in a cloud computing arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. The internal-use software guidance requires the capitalization of certain costs incurred only during the application development stage. That guidance also requires entities to expense costs during the preliminary project and post-implementation stages as they are incurred. Adoption of this ASU did not impact Ambac's financial statements. Effective December 31, 2020, Ambac adopted the following accounting standard: Defined Benefit and Other Postretirement Plans Disclosures In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) - Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans . The ASU modifies various disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans. Relevant disclosures that have been removed are the effects of a one percentage point change in assumed health care cost trend rates on the (a) aggregate of the service and interest cost components of the net periodic pension cost and (b) benefit obligation for postretirement healthcare benefits. Adoption of this ASU only affected disclosures and did not have an impact on Ambac's financial statements. Future Application of Accounting Standards: Reference Rate Reform In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848) - Facilitation of the Effects of Reference Rate Reform on Financial Reporting . The ASU provides companies with optional guidance to ease the potential accounting burden related to transitioning away from reference rates, such as LIBOR, that are expected to be discontinued as a result of initiatives undertaken by various jurisdictions around the world. For example, under current GAAP, contract modifications which change a reference rate are required to be evaluated in determining whether the modifications result in the establishment of new contracts or the continuation of existing contracts. The amendments in this ASU provide optional expedients and exceptions for applying GAAP to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The ASU can be applied prospectively as of the beginning of the interim period that includes March 12, 2020, (January 1, 2020 for calendar year

Variable Interest Entities (Tab

Variable Interest Entities (Tables)	12 Months Ended
Variable Interest Entities (Tables)	Dec. 31, 2020
Variable Interest Entity Not Primary Beneficiary Table [Line Items]
Schedule of Variable Interest Entities Assets and Liabilities [Table Text Block]	The following schedule details the components of Income (loss) on variable interest entities for the affected periods: Year ended December 31, 2020 2019 2018 Net change in fair value of VIE assets and liabilities reported under the fair value option $ (1) $ 13 $ 3 Less: Credit risk changes of fair value option long-term debt reported through other comprehensive income (loss) (1) — (1) Net change in fair value of VIE assets and liabilities reported in earnings (3) 14 2 Investment income on available-for-sale securities 7 10 — Net realized investment gains (losses) on available-for-sale securities 8 13 — Interest expense on long-term debt carried at par less unamortized cost (6) (11) — Other expenses — (1) — Gain (loss) from consolidating FG VIEs — 15 — Gain (loss) from de-consolidating FG VIEs — (2) 2 Income (loss) on variable interest entities $ 5 $ 38 $ 3
Variable Interest Entity Not Primary Beneficiary Table [Table Text Block]	The following table displays the carrying amount of the assets, liabilities and maximum exposure to loss of Ambac’s variable interests in non-consolidated VIEs resulting from financial guarantee and derivative contracts by major underlying asset classes, as of December 31, 2020 and 2019: Carrying Value of Assets and Liabilities Maximum (1) Insurance (2) Insurance (3) Net Derivative (Liabilities) (4) December 31, 2020: Global structured finance: Mortgage-backed—residential $ 4,308 $ 2,024 $ 580 $ — Other consumer asset-backed 1,050 24 239 — Other commercial asset-backed 24 3 1 — Other 970 — 13 8 Total global structured finance 6,352 2,051 834 8 Global public finance 21,646 263 287 — Total $ 27,998 $ 2,314 $ 1,122 $ 8 December 31, 2019: Global structured finance: Mortgage-backed—residential $ 5,373 $ 1,913 $ 523 $ — Other consumer asset-backed 1,373 31 216 — Other commercial asset-backed 314 9 6 — Other 1,107 7 18 8 Total global structured finance 8,165 1,961 762 8 Global public finance 23,341 287 321 — Total $ 31,506 $ 2,247 $ 1,083 $ 7 (1) Maximum exposure to loss represents the maximum future payments of principal and interest on insured obligations and derivative contracts. Ambac’s maximum exposure to loss does not include the benefit of any financial instruments (such as reinsurance or hedge contracts) that Ambac may utilize to mitigate the risks associated with these variable interests. (2) Insurance assets represent the amount included in “Premium receivables” and “Subrogation recoverable” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (3) Insurance liabilities represent the amount included in “Loss and loss expense reserves” and “Unearned premiums” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (4) Net derivative assets (liabilities) represent the fair value recognized on credit derivative contracts and interest rate swaps on Ambac’s Consolidated Balance Sheets.

Business Combinations (Tables)

Business Combinations (Tables)	3 Months Ended	12 Months Ended
Business Combinations (Tables)	Dec. 31, 2020	Dec. 31, 2020
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table summarizes the consideration paid for Xchange and the estimated fair values of the aggregate assets and liabilities acquired, as well as the fair value of the noncontrolling interest, at the acquisition date: Fair Value Cash $ 2 Restricted cash 4 Intangible assets 36 Goodwill 46 Other assets 8 Total assets acquired $ 96 Other liabilities 8 Total liabilities assumed 8 Less: Redeemable noncontrolling interest 7 Total consideration $ 81
Schedule of Indefinite-lived Intangible Assets Acquired as Part of Business Combination		The following table sets forth the estimated fair values of identifiable intangible assets acquired and their estimated useful lives as of the date of acquisition. Fair Weighted Distribution relationships $ 33 15.0 Non-compete agreements 1 5.0 Trade name 1 8.0 Total $ 36
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense		As of December 31, 2020, future annual amortization of finite-lived acquired intangible assets for the years 2021 through 2025 and thereafter is estimated to be: Year Estimated 2021 $ 3 2022 3 2023 3 2024 3 2025 3 Thereafter 22 Total $ 36

Net Income Per Share (Tables)

Net Income Per Share (Tables)	12 Months Ended
Net Income Per Share (Tables)	Dec. 31, 2020
Earnings Per Share [Abstract]
Reconciliation of Common Shares Used for Basic and Diluted Earnings Per Share	The following table provides a reconciliation of the common shares used for basic net income per share to the diluted shares used for diluted net income per share: Year Ended December 31, 2020 2019 2018 Basic weighted average shares outstanding 46,147,062 45,954,908 45,665,883 Effect of potential dilutive shares (1) : Warrants — — 441,104 Stock options — — — Restricted stock units — — 77,572 Performance stock units (2) — — 375,276 Diluted weighted average shares outstanding 46,147,062 45,954,908 46,559,835 Anti-dilutive shares excluded from the above reconciliation Stock options 16,121 16,667 16,667 Warrants 4,877,754 4,877,783 — Restricted stock units 302,145 249,263 — Performance stock units (2) 1,002,501 872,258 — (1) For the years ended December 31, 2020 and 2019 , Ambac had a net loss and accordingly excluded all potentially dilutive securities from the determination of diluted loss per share as their impact was anti-dilutive. (2) Performance stock units are reflected based on the performance metrics through the balance sheet date. Vesting of these units is

Financial Guarantees in Force_2

Financial Guarantees in Force (Tables)	12 Months Ended
Financial Guarantees in Force (Tables)	Dec. 31, 2020
Guarantees [Abstract]
Schedule of Financial Guarantee Insurance In Force By Bond Type	As of December 31, 2020 and 2019, the guarantee portfolio was diversified by type of guaranteed bond as shown in the following table: December 31, Net Par Outstanding December 31, 2020 2019 Public Finance: Housing revenue $ 5,855 $ 5,991 Lease and tax-backed revenue 4,179 5,102 General obligation 2,345 3,011 Transportation revenue 771 855 Higher education 747 885 Utility revenue 675 768 Other 925 1,041 Total Public Finance 15,497 17,653 Structured Finance: Mortgage-backed and home equity 3,635 4,423 Investor-owned utilities 1,617 1,675 Student loan 626 769 Structured Insurance 311 395 Asset-backed and other 148 246 Total Structured Finance 6,337 7,508 International Finance: Sovereign/sub-sovereign 5,270 5,264 Investor-owned and public utilities 3,899 4,436 Transportation 1,511 1,532 Asset-backed and other 1,374 1,625 Total International Finance 12,054 12,857 Total $ 33,888 $ 38,018 (1) Includes $5,575 and $5,654 of Military Housing net par at December 31, 2020 and 2019, respectively.
Schedule of International Financial Guarantee Insurance In Force Portfolio By Location Of Risk	As of December 31, 2020 and 2019, the International Finance guaranteed portfolio by location of risk was as outlined in the table below: Net Par Outstanding December 31, 2020 2019 United Kingdom $ 9,711 $ 10,593 Italy 803 767 Austria 707 674 Australia 420 382 France 277 303 Other international (1) 136 138 Total International Finance $ 12,054 $ 12,857

Financial Guarantee Insurance_4

Financial Guarantee Insurance Contracts (Tables)	12 Months Ended
Financial Guarantee Insurance Contracts (Tables)	Dec. 31, 2020
Insurance [Line Items]
Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts)	Below is the gross premium receivable roll-forward (direct and assumed contracts) for the affected periods: Year Ended December 31, 2020 2019 2018 Beginning premium receivable $ 416 $ 495 $ 586 Adjustment to initially apply ASU 2016-13 (3) — — Premium receipts (46) (48) (56) Adjustments for changes in expected and contractual cash flows (1) (6) (38) (42) Accretion of premium receivable discount 9 11 15 Deconsolidation of certain VIEs — 3 — Changes to allowance for credit losses (4) (2) 2 Other adjustments (including foreign exchange) 5 (6) (10) Ending premium receivable (2) $ 370 $ 416 $ 495 (1) Adjustments for changes in expected and contractual cash flows primarily due to reductions in insured exposure as a result of early policy terminations and unscheduled principal paydowns.
Effect of Reinsurance on Premiums Written and Earned	The effect of reinsurance on premiums written and earned was as follows: Year Ended December 31, Direct Assumed Ceded (1) Net Premiums 2020: Written $ (1) $ — $ (1) $ — Earned 65 1 12 54 2019: Written $ (28) $ — $ 31 $ (60) Earned 75 — 10 66 2018: Written $ (24) $ — $ 17 $ (41) Earned 119 — 8 111 (1) Includes ceded premium activity related to the execution of new reinsurance transactions during 2020, 2019 and 2018.
Schedule of Revenue from External Customers Attributed to Foreign Countries by Geographic Area [Table Text Block]	The following table summarizes net premiums earned by location of risk: Year Ended December 31, 2020 2019 2018 United States $ 32 $ 55 $ 88 United Kingdom 24 17 19 Other international (2) (6) 5 Total $ 54 $ 66 $ 111
Schedule Of Reinsurance Recoverable And Provision By Category Of Reinsurer	The following table represents the percentage ceded to reinsurers and unsecured reinsurance recoverable at December 31, 2020. Reinsurers Percentage Ceded Par Net Unsecured Reinsurance Recoverable (1) Assured Guaranty Re Ltd 46% $ — Build America Mutual Assurance Company (2) 43 30 Assured Guaranty Corporation 8 4 Sompo Japan Nipponkoa Insurance, Inc. 3 — Total 100% $ 34 (1) Represents reinsurance recoverables on paid and unpaid losses and deferred ceded premiums, net of ceded premium payables due to reinsurers, letters of credit, and collateral posted for the benefit of AAC. (2) Build America Mutual Assurance Company has an S&P rating of AA.
Summarized Future Gross Undiscounted Premiums Expected to be Collected and Future Expected Premiums Earned, Net of Reinsurance	The table below summarizes the future gross undiscounted premiums to be collected and future premiums earned, net of reinsurance at December 31, 2020: Future Premiums Collected (1) Future to be Earned Net of (2) Three months ended: March 31, 2021 $ 12 $ 9 June 30, 2021 8 9 September 30, 2021 9 9 December 31, 2021 8 9 Twelve months ended: December 31, 2022 36 33 December 31, 2023 34 31 December 31, 2024 33 29 December 31, 2025 31 27 Five years ended: December 31, 2030 132 113 December 31, 2035 91 72 December 31, 2040 42 30 December 31, 2045 19 12 December 31, 2050 7 4 December 31, 2055 1 — Total $ 462 $ 386 (1) Future premiums to be collected are undiscounted and are used to derive the discounted premium receivable asset recorded on Ambac's balance sheet. (2) Future premiums to be earned, net of reinsurance relate to the unearned premiums liability and deferred ceded premium asset recorded on Ambac’s balance sheet. The use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral is required in the calculation of the premium receivable as further described in Note 2. Basis of Presentation and Significant Accounting Policies . This results in a different premium receivable balance than if expected lives were considered. If installment paying policies are retired or prepay early, premiums reflected in the premium receivable asset and amounts reported in the above table for such policies may not be collected. Future premiums to be earned also considers the use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral, which may result in different unearned premium than if expected lives were considered. If those bonds types are retired early, premium earnings may be negative in the period of call or refinancing.
Schedule of Loss And Loss Expense Reserves And Subrogation Recoverable Table	Below are the components of the Loss and loss expense reserves liability and the Subrogation recoverable asset at December 31, 2020 and 2019: Present Value of Expected Unearned Gross Loss and Balance Sheet Line Item Claims and Recoveries December 31, 2020: Loss and loss expense reserves $ 2,060 $ (229) $ (72) $ 1,759 Subrogation recoverable 100 (2,256) — (2,156) Totals $ 2,160 $ (2,486) $ (72) $ (397) December 31, 2019: Loss and loss expense reserves $ 1,835 $ (233) $ (54) $ 1,548 Subrogation recoverable 131 (2,160) — (2,029) Totals $ 1,966 $ (2,394) $ (54) $ (482)
Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance	Below is the loss and loss expense reserve roll-forward, net of subrogation recoverable and reinsurance, for the affected periods. Year Ended 2020 2019 2018 Beginning gross loss and loss expense reserves $ (482) $ (107) $ 4,114 Reinsurance recoverable 26 23 41 Beginning balance of net loss and loss expense reserves (508) (130) 4,073 Losses and loss expenses (benefit) incurred: Current year 15 1 5 Prior years (1) 210 12 (228) Total (2)(3) 225 13 (224) Loss and loss expenses (recovered) paid: Current year 1 — — Prior years (1) 148 318 3,963 Total 149 318 3,964 Foreign exchange effect 2 (1) (15) Ending net loss and loss expense reserves (430) (436) (130) Impact of VIE consolidation — (72) — Reinsurance recoverable (4) 33 26 23 Ending gross loss and loss expense reserves (397) (482) (107) (1) 2018 loss and loss expenses (recovered) paid includes the settlement of Deferred Amounts and Interest Accrued on Deferred Amounts in the amount of $3,000 and $857, respectively in connection with the Rehabilitation Exit Transactions through a combination of cash, surplus notes and secured notes. 2018 loss and loss expenses incurred includes a $288 loss and loss expense benefit on these settled Deferred Amounts. (2) Total losses and loss expenses (benefit) includes $(11), $(7) and $(2) for the years ended December 31, 2020, 2019 and 2018, respectively, related to ceded reinsurance. (3) Ambac records the impact of estimated recoveries related to securitized loans in RMBS transactions that breached certain representations and warranties within losses and loss expenses (benefit). The losses and loss expense (benefit) incurred associated with changes in estimated representation and warranty recoveries for the year ended December 31, 2020, 2019 and 2018 was $(23), $42 and $62, respectively. (4) Represents reinsurance recoverable on future loss and loss expenses. Additionally, the Balance Sheet line "Reinsurance recoverable on paid and unpaid losses (net of allowance for credit losses of $0 at December 31, 2020)" includes reinsurance recoverables (payables) of $1, $0 and $1 as of December 31, 2020, 2019 and 2018, respectively, related to previously presented loss and loss expenses and subrogation.
Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable	The tables below summarize information related to policies currently included in Ambac’s loss and loss expense reserves or subrogation recoverable at December 31, 2020 and 2019. Gross par exposures include capital appreciation bonds which are reported at the par amount at the time of issuance of the insurance policy as opposed to the current accreted value of the bond. The weighted average risk-free rate used to discount loss reserves at December 31, 2020 and 2019 was 1.1% and 2.1%, respectively. Surveillance Categories as of December 31, 2020 I IA II III IV V Total Number of policies 40 25 15 15 132 5 232 Remaining weighted-average contract period (in years) (1) 10 18 8 16 14 7 14 Gross insured contractual payments outstanding: Principal $ 842 $ 1,375 $ 595 $ 1,469 $ 3,246 $ 47 $ 7,573 Interest 279 1,011 484 215 1,427 26 3,443 Total $ 1,121 $ 2,386 $ 1,079 $ 1,685 $ 4,673 $ 72 $ 11,016 Gross undiscounted claim liability $ 3 $ 49 $ 40 $ 541 $ 1,690 $ 72 $ 2,395 Discount, gross claim liability — (2) (1) (85) (213) (3) (303) Gross claim liability before all subrogation and before reinsurance $ 3 $ 47 $ 40 $ 456 $ 1,477 $ 69 $ 2,092 Less: Gross RMBS subrogation (2) $ — $ — $ — $ — $ (1,753) $ — $ (1,753) Discount, RMBS subrogation — — — — 3 — 3 Discounted RMBS subrogation, before reinsurance — — — — (1,751) — (1,751) Less: Gross other subrogation (3) — — — (36) (706) (12) (755) Discount, other subrogation — — — 1 18 1 20 Discounted other subrogation, before reinsurance — — — (35) (689) (11) (735) Gross claim liability, net of all subrogation and discounts, before reinsurance $ 3 $ 47 $ 39 $ 421 $ (963) $ 58 $ (394) Less: Unearned premium revenue $ (2) $ (16) $ (5) $ (17) $ (30) $ (1) $ (72) Plus: Loss expense reserves 1 2 1 5 59 — 68 Gross loss and loss expense reserves $ 2 $ 32 $ 35 $ 409 $ (933) $ 57 $ (397) Reinsurance recoverable reported on Balance Sheet (4) $ — $ 6 $ 9 $ 24 $ (6) $ — $ 33 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac’s estimate of subrogation recoveries from RMBS transaction sponsors for representation and warranty ("R&W") breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions including RMBS. (4) Reinsurance recoverable reported on Balance Sheet includes reinsurance recoverables of $33 related to future loss and loss expenses and $1 related to presented loss and loss expenses and subrogation. Surveillance Categories as of December 31, 2019 I IA II III IV V Total Number of policies 34 18 11 16 139 3 221 Remaining weighted-average contract period (in years) (1) 8 21 9 17 14 3 15 Gross insured contractual payments outstanding: Principal $ 668 $ 510 $ 277 $ 857 $ 3,819 $ 37 $ 6,168 Interest 340 507 128 366 1,678 11 3,029 Total $ 1,007 $ 1,016 $ 404 $ 1,223 $ 5,498 $ 48 $ 9,197 Gross undiscounted claim liability $ 2 $ 44 $ 21 $ 541 $ 1,778 $ 48 $ 2,434 Discount, gross claim liability — (5) (1) (152) (381) (2) (541) Gross claim liability before all subrogation and before reinsurance $ 2 $ 39 $ 20 $ 389 $ 1,397 $ 46 $ 1,893 Less: Gross RMBS subrogation (2) $ — $ — $ — $ — $ (1,777) $ — $ (1,777) Discount, RMBS subrogation — — — — 49 — 49 Discounted RMBS subrogation, before reinsurance — — — — (1,727) — (1,727) Less: Gross other subrogation (3) — — — (41) (666) (13) (720) Discount, other subrogation — — — 4 47 3 53 Discounted other subrogation, before reinsurance — — — (37) (620) (10) (666) Gross claim liability, net of all subrogation and discounts, before reinsurance $ 2 $ 39 $ 20 $ 353 $ (950) $ 36 $ (501) Less: Unearned premium revenue $ (1) $ (9) $ (1) $ (7) $ (35) $ — $ (54) Plus: Loss expense reserves 1 1 1 4 67 — 73 Gross loss and loss expense reserves $ 1 $ 30 $ 20 $ 349 $ (918) $ 36 $ (482) Reinsurance recoverable reported on Balance Sheet (4) $ — $ 6 $ 7 $ 24 $ (10) $ — $ 26 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac's estimate of subrogation recoveries from RMBS transaction sponsors for R&W breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions, including RMBS. (4) Reinsurance recoverable reported on Balance Sheet includes reinsurance recoverables of $26 related to future loss and loss expenses and $0 related to presented loss and loss expenses and subrogation.
Summary of Rollforward of RMBS Subrogation, by Estimation Approach	Below is the rollforward of RMBS R&W subrogation for the affected periods: Year ended December 31, 2020 2019 2018 Discounted RMBS subrogation recovery (gross of reinsurance) at beginning of year $ 1,727 $ 1,771 $ 1,834 All other changes (1) 23 (43) (64) Discounted RMBS subrogation recovery (gross of reinsurance) at end of year $ 1,751 $ 1,727 $ 1,771
Premium Receivable, Allowance for Credit Loss	Below is a rollforward of the premium receivable allowance for credit losses as of December 31, 2020: Year Ended December 31, 2020 Beginning balance (1) $ 9 Current period provision (2) 9 Write-offs of the allowance (2) Recoveries of previously written-off amounts — Ending balance $ 17 (1) At December 31, 2019, $9 of premiums receivable were deemed uncollectible as determined under prior GAAP rules. (2) The year ended December 31, 2020, includes $3 from the adoption of CECL.
Schedule of Financial Guarantee Insurance In Force By Bond Type	As of December 31, 2020 and 2019, the guarantee portfolio was diversified by type of guaranteed bond as shown in the following table: December 31, Net Par Outstanding December 31, 2020 2019 Public Finance: Housing revenue $ 5,855 $ 5,991 Lease and tax-backed revenue 4,179 5,102 General obligation 2,345 3,011 Transportation revenue 771 855 Higher education 747 885 Utility revenue 675 768 Other 925 1,041 Total Public Finance 15,497 17,653 Structured Finance: Mortgage-backed and home equity 3,635 4,423 Investor-owned utilities 1,617 1,675 Student loan 626 769 Structured Insurance 311 395 Asset-backed and other 148 246 Total Structured Finance 6,337 7,508 International Finance: Sovereign/sub-sovereign 5,270 5,264 Investor-owned and public utilities 3,899 4,436 Transportation 1,511 1,532 Asset-backed and other 1,374 1,625 Total International Finance 12,054 12,857 Total $ 33,888 $ 38,018 (1) Includes $5,575 and $5,654 of Military Housing net par at December 31, 2020 and 2019, respectively.
Schedule of Intangible Assets and Goodwill	The insurance intangible amortization expense is included in the Consolidated Statements of Total Comprehensive Income (Loss) as shown below. Year Ended December 31, 2020 2019 2018 Insurance amortization expense $ 57 $ 295 $ 107 The insurance intangible asset and accumulated amortization are included in the Consolidated Balance Sheets, as shown below. December 31, 2020 2019 Gross carrying value of insurance intangible asset $ 1,281 $ 1,273 Accumulated amortization of insurance intangible asset 908 847 Net insurance intangible asset $ 373 $ 427
Insurance Intangible Asset [Member]
Insurance [Line Items]
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	The estimated future amortization expense for the net insurance intangible asset is as follows: Amortization expense (1) (2) 2021 $ 39 2022 35 2023 32 2024 29 2025 26 Thereafter $ 213 (1) The insurance intangible asset will be amortized using a level-yield method based on par exposure of the related financial guarantee insurance or reinsurance contracts as described in Note 2. Basis of Presentation and Significant Accounting Policies. Future amortization considers the use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral. Actual maturities will differ from contractual maturities because borrowers may have the right to call or prepay certain obligations. If those bonds types are retired early, amortization expense may differ in the period of call or refinancing from the amounts provided in the table above. (2) The weighted-average amortizations period is 7.5 years.

Investments (Tables)

Investments (Tables)	12 Months Ended
Investments (Tables)	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments	The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at December 31, 2020 and 2019 were as follows: Amortized Allowance for Credit Losses Gross Gross Estimated December 31, 2020 Fixed maturity securities: Municipal obligations $ 321 $ — $ 37 $ — $ 358 Corporate obligations (1) 1,059 — 24 6 1,077 Foreign obligations 97 — 1 — 98 U.S. government obligations 105 — 2 1 106 Residential mortgage-backed securities 256 — 46 — 302 Collateralized debt obligations 74 — — — 74 Other asset-backed securities (2) 263 — 40 — 303 2,175 — 149 8 2,317 Short-term 492 — — — 492 2,667 — 149 8 2,809 Fixed maturity securities pledged as collateral: U.S. government obligations 15 — — — 15 Short-term 125 — — — 125 140 — — — 140 Total available-for-sale investments $ 2,807 $ — $ 149 $ 8 $ 2,949 Amortized Gross Gross Estimated Non-credit Other-than temporary Impairments (3) December 31, 2019 Fixed maturity securities: Municipal obligations $ 194 $ 22 $ — $ 215 $ — Corporate obligations (1) 1,396 36 2 1,430 — Foreign obligations 44 1 — 44 — U.S. government obligations 157 2 2 156 — Residential mortgage-backed securities 200 47 — 248 — Commercial mortgage-backed securities 49 1 — 50 — Collateralized debt obligations 147 — 1 146 — Other asset-backed securities (2) 263 24 — 287 — 2,450 132 5 2,577 — Short-term 653 — — 653 — 3,103 132 5 3,230 — Fixed maturity securities pledged as collateral: Short-term 85 — — 85 — 85 — — 85 — Total available-for-sale investments 3,187 132 5 3,314 — (1) Includes Ambac's holdings of the secured notes issued by Ambac LSNI in connection with the Rehabilitation Exit Transactions. (2) Consists primarily of Ambac's holdings of the military housing securitization bonds. (3) At December 31, 2019, represents the amount of non-credit other-than-temporary impairment losses remaining in accumulated other comprehensive income on securities that also had a credit impairment. These losses included in gross unrealized losses at December 31, 2019.
Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity	The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at December 31, 2020, by contractual maturity, were as follows: Amortized Estimated Due in one year or less $ 718 $ 719 Due after one year through five years 873 880 Due after five years through ten years 439 459 Due after ten years 184 213 2,214 2,271 Residential mortgage-backed securities 256 302 Collateralized debt obligations 74 74 Other asset-backed securities 263 303 Total $ 2,807 $ 2,949
Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments	The following table shows gross unrealized losses and fair values of Ambac’s available-for-sale investments, excluding VIE investments, which at December 31, 2020, did not have an allowance for credit losses under the CECL standard and at December 2019, did not have other-than-temporary impairments recorded in earnings under prior GAAP. This information is aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at December 31, 2020 and 2019: Less Than 12 Months 12 Months or More Total Fair Value Gross Fair Value Gross Fair Value Gross December 31, 2020 Fixed maturity securities: Municipal obligations $ 25 $ — $ 6 $ — $ 31 $ — Corporate obligations 543 6 — — 543 6 Foreign obligations 3 — — — 3 — U.S. government obligations 17 1 — — 17 1 Residential mortgage-backed securities 14 — — — 14 — Commercial mortgage-backed securities — — — — — — Collateralized debt obligations 27 — 15 — 42 — Other asset-backed securities — — 4 — 4 — 629 7 25 — 654 8 Short-term 187 — — — 187 — Total temporarily impaired securities $ 816 $ 7 $ 25 $ — $ 841 $ 8 December 31, 2019 Fixed maturity securities: Municipal obligations $ 13 $ — $ 10 $ — $ 23 $ — Corporate obligations 63 2 5 — 68 2 Foreign obligations 20 — — — 20 — U.S. government obligations 36 2 2 — 38 2 Residential mortgage-backed securities 5 — — — 5 — Commercial mortgage-backed securities 7 — — — 7 — Collateralized debt obligations 53 — 63 1 116 1 Other asset-backed securities 2 — 7 — 10 — 200 4 88 1 288 5 Short-term 201 — — — 201 — Total securities $ 401 $ 4 $ 88 $ 1 $ 489 $ 5
Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments	The following table details amounts included in net realized gains (losses) and impairments included in earnings for the affected periods: Year Ended December 31, 2020 2019 2018 Gross realized gains on securities $ 38 $ 64 $ 111 Gross realized losses on securities (12) (5) (7) Foreign exchange (losses) gains (4) 22 7 Credit impairments $ — $ — $ — Intent / requirement to sell impairments $ — $ — $ (3) Net realized gains (losses) $ 22 $ 81 $ 108
Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income	Year Ended December 31, 2019 2018 Balance, beginning of period 12 67 Additions for credit impairments recognized on: Securities not previously impaired — 1 Reductions for credit impairments previously recognized on: Securities that matured or were sold during the period (1) (56) Balance, end of period 12 12
Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities	The following table represents the fair value and weighted-average underlying rating of insured securities in Ambac's investment portfolio at December 31, 2020 and 2019, respectively: Municipal Corporate (2) Mortgage Total Weighted (1) December 31, 2020: Ambac Assurance Corporation $ 320 $ 465 $ 481 $ 1,266 CCC+ National Public Finance Guarantee Corporation 6 — — 6 BBB- Assured Guaranty Municipal Corporation 1 — — 1 C Total $ 327 $ 465 $ 481 $ 1,273 CCC+ December 31, 2019: Ambac Assurance Corporation $ 176 $ 535 $ 442 $ 1,153 B- National Public Finance Guarantee Corporation 11 — — 11 BBB- Total $ 186 $ 535 $ 442 $ 1,164 B- (1) Ratings are based on the lower of Standard & Poor’s or Moody’s rating. If unavailable, Ambac’s internal rating is used. (2) Represents Ambac's holdings of secured notes issued by Ambac LSNI in connection with the Rehabilitation Exit Transactions. These secured notes are insured by AAC.
Summary of Net Investment Income	Net investment income was comprised of the following for the affected periods: Year Ended December 31, 2020 2019 2018 Fixed maturity securities $ 103 $ 183 $ 265 Short-term investments 5 17 11 Loans 1 1 1 Investment expense (6) (6) (7) Securities available-for-sale and short-term 103 196 271 Other investments 19 32 2 Total net investment income (loss) $ 122 $ 227 $ 273 Net investment income from Other investments primarily represents changes in fair value on securities classified as trading or accounted for under the fair value option, income from investment limited partnerships accounted for under the equity method and the above noted equity interest in an unconsolidated trust accounted for under the equity method. The portion of net unrealized gains (losses) related to trading securities still held at the end of each period is as follows: Year Ended December 31, 2020 2019 2018 Net gains (losses) recognized during the period on trading securities $ — $ 24 $ (3) Less: net gains (losses) recognized during the reporting period on trading securities sold during the period (18) 7 1 Unrealized gains (losses) recognized during the reporting period on trading securities still held at the reporting date $ 18 $ 17 $ (4)

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
Derivative Instruments (Tables)	Dec. 31, 2020
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The following tables summarize the gross fair values of individual derivative instruments and the impact of legal rights of offset as reported in the Consolidated Balance Sheets as of December 31, 2020 and 2019. Gross Gross Net Amounts Gross Amount Net Amount December 31, 2020: Derivative Assets: Interest rate swaps $ 93 $ — $ 93 $ — $ 93 Total non-VIE derivative assets $ 93 $ — $ 93 $ — $ 93 Derivative Liabilities: Credit derivatives $ — $ — $ — $ — $ — Interest rate swaps 114 — 114 113 1 Total non-VIE derivative liabilities $ 114 $ — $ 114 $ 113 $ 1 Variable Interest Entities Derivative Assets: Currency swaps $ 41 $ — $ 41 $ — $ 41 Total VIE derivative assets $ 41 $ — $ 41 $ — $ 41 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,835 $ — $ 1,835 $ — $ 1,835 Total VIE derivative liabilities $ 1,835 $ — $ 1,835 $ — $ 1,835 Gross Gross Net Amounts Gross Amount Net Amount December 31, 2019: Derivative Assets: Interest rate swaps $ 75 $ — $ 75 $ — $ 75 Total non-VIE derivative assets $ 75 $ — $ 75 $ — $ 75 Derivative Liabilities: Credit derivatives $ — $ — $ — $ — $ — Interest rate swaps 89 — 90 89 1 Total non-VIE derivative liabilities $ 90 $ — $ 90 $ 89 $ 1 Variable Interest Entities Derivative Assets: Currency swaps $ 52 $ — $ 52 $ — $ 52 Total VIE derivative assets $ 52 $ — $ 52 $ — $ 52 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,657 $ — $ 1,657 $ — $ 1,657 Total VIE derivative liabilities $ 1,657 $ — $ 1,657 $ — $ 1,657
Summary of Location and Amount of Gains and Losses of Derivative Contracts	The following tables summarize the location and amount of gains and losses of derivative contracts in the Consolidated Statements of Total Comprehensive Income (Loss) for the years ended December 31, 2020, 2019 and 2018: Location of Gain (Loss) Recognized Amount of Gain (Loss) Recognized in Consolidated Statement of Total Comprehensive Income (Loss) – 2020 2019 2018 Non-VIE derivatives: Credit derivatives Net gains (losses) on derivative contracts $ — $ 2 $ (1) Interest rate swaps Net gains (losses) on derivative contracts (9) (6) 1 Futures contracts Net gains (losses) on derivative contracts (41) (45) 7 Total non-VIE derivatives (50) (50) 7 Variable Interest Entities: Currency swaps Income (loss) on variable interest entities (6) (12) 11 Interest rate swaps Income (loss) on variable interest entities (138) (20) 493 Total Variable Interest Entities (144) (32) 505 Total derivative contracts $ (193) $ (82) $ 512
Summary of Notional Amounts of AFS's Trading Derivative Products	As of December 31, 2020 and 2019, the notional amounts of AFS's derivatives are as follows: Notional - December 31, Type of Derivative 2020 2019 Interest rate swaps—pay-fixed/receive-variable $ 726 $ 1,261 US Treasury futures contracts—short 240 755 Interest rate swaps—receive-fixed/pay-variable 195 332
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member]
Summary of Notional Amounts of AFS's Trading Derivative Products	The notional for VIE derivatives outstanding as of December 31, 2020 and 2019, were as follows: Notional - December 31, Type of VIE Derivative 2020 2019 Interest rate swaps—receive-fixed/pay-variable $ 1,233 $ 1,194 Interest rate swaps—pay-fixed/receive-variable 1,151 1,176 Currency swaps 308 329 Credit derivatives — 9

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
Long-Term Debt (Tables)	Dec. 31, 2020
Debt Instrument [Line Items]
Schedule of Maturities of Long-term Debt [Table Text Block]	Aggregated annual maturities of non-VIE long-term debt obligations (based on scheduled maturity dates as further discussed below) are as follows: 2021 $ 531 (1) 2022 — 2023 1,641 2024 — 2025 — Thereafter 712 (2) Total $ 2,884
Schedule of Long-term Debt Instruments	Long-term debt outstanding, excluding VIE long-term debt, was as follows: December 31, 2020 2019 Par Value Unamortized Discount Carrying Value Par Value Unamortized Discount Carrying Value Ambac Assurance: 5.1% surplus notes $ 531 $ — $ 531 $ 531 $ (14) $ 517 5.1% junior surplus notes 365 (118) 247 365 (113) 252 Ambac note 1,641 — 1,641 1,763 — 1,763 Tier 2 notes 306 — 306 281 (4) 278 Ambac UK debt 41 (27) 14 41 (28) 13 Long-term debt $ 2,884 $ (145) $ 2,739 $ 2,980 $ (159) $ 2,822

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Major Jurisdictions	The following are the major jurisdictions in which Ambac and its subsidiaries operate and the earliest tax years subject to examination: Jurisdiction Tax Year United States 2010 New York State 2013 New York City 2016 United Kingdom 2017 Italy 2016
Significant Portions of Deferred Tax Liabilities and Deferred Tax Assets	The tax effects of temporary differences that give rise to significant portions of the deferred tax liabilities and deferred tax assets at December 31, 2020 and 2019, are presented below: December 31, 2020 2019 Deferred tax liabilities: Insurance intangible $ 78 $ 90 Unearned premiums and credit fees 32 42 Investments 22 32 Variable interest entities 13 12 Other 7 8 Total deferred tax liabilities 152 183 Deferred tax assets: Net operating loss and capital carryforward 764 742 Loss reserves 218 148 Debentures 22 29 Compensation 9 7 Other 5 1 Subtotal deferred tax assets 1,019 927 Valuation allowance 891 777 Total deferred tax assets 128 151 Net deferred tax liability $ 24 $ 32
Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block]	U.S. and foreign components of pre-tax income (loss) were as follows: Year Ended December 31, 2020 2019 2018 U.S. $ (441) $ (174) $ 264 Foreign 1 (9) 8 Total $ (440) $ (183) $ 273
Schedule of Components of Income Tax Expense (Benefit)	he components of the provision (benefit) for income taxes were as follows: Year Ended December 31, 2020 2019 2018 Current taxes U. S. federal $ — $ — $ (2) U.S. state and local — (3) 2 Foreign 8 37 (1) Total current taxes 8 34 — Deferred taxes Foreign (10) (1) 5 Total deferred taxes $ (10) $ (1) $ 5 Provision for income taxes $ (3) $ 32 $ 5
Schedule Of Income Taxes Charged Credited Directly To Equity	The total effect of income taxes on net income and stockholders’ equity for the years ended December 31, 2020, 2019 and 2018 is as follows: Year Ended December 31, 2020 2019 2018 Total income taxes charged to net income $ (3) $ 32 $ 5 Income taxes charged (credited) to stockholders’ equity: Unrealized gains (losses) on investment securities 3 14 12 Unrealized gains (losses) on foreign currency translations — — — Valuation allowance to equity (3) (23) (9) Total charged to stockholders’ equity: 1 (8) 3 Total effect of income taxes $ (1) $ 24 $ 8
Schedule of Effective Income Tax Rate Reconciliation	The tax provisions in the accompanying Consolidated Statements of Total Comprehensive Loss reflect effective tax rates differing from prevailing Federal corporate income tax rates. The following is a reconciliation of these differences: 2020 2019 2018 Year Ended December 31, Amount % Amount % Amount % Tax on income (loss) at statutory rate $ (92) 21.0 % $ (38) 21.0 % $ 57 21.0 % Changes in expected tax resulting from: Tax-exempt interest (2) 0.4 % (3) 1.8 % (7) (2.5) % Foreign taxes 6 (1.4) % 40 (22.1) % 10 3.9 % Substantiation adjustment (29) 6.7 % 28 (15.3) % (60) (22.0) % Valuation allowance 113 (25.6) % 8 (4.4) % 5 1.9 % Change in Tax Law — — % — — % (2) (0.7) % Other, net — — % (2) 1.3 % 1 0.4 % Tax expense on income (loss) $ (3) 0.7 % $ 32 (17.7) % $ 5 2.0 %
Schedule of Unrecognized Tax Benefits Roll Forward	A reconciliation of the beginning and ending amounts of material unrecognized tax benefits for 2020, 2019 and 2018 is as follows: Year Ended December 31, 2020 2019 2018 Balance, beginning of period $ — $ — $ — Increases related to prior year tax positions — — — Decreases related to prior year tax positions — — — Balance, end of period $ — $ — $ —
Schedule of Net Operating Loss And Tax Credit Carryovers	In December 2020, AFG and certain subsidiaries and affiliates amended their existing tax sharing agreement (the "Third TSA Amendment"). Under the Third TSA Amendment, AAC and AFG agreed to reallocate $210 of net operating loss carry-forwards (“NOLs”) from AAC to AFG and to eliminate AAC's requirement to make future payments based on its utilization of NOLs ("tolling payments") for any taxable year beginning on or after January 1, 2019. In connection with the Third TSA Amendment, AAC paid to AFG approximately $28 of accrued tolling payments based on NOLs used by AAC in 2017. The Third TSA Amendment did not affect the NOL tolling payments AAC would be required to make in connection with the 2013 Closing Agreement between Ambac and the United States Internal Revenue Service, which could amount to as much as $8.

Employment Benefit Plans (Table

Employment Benefit Plans (Tables)	12 Months Ended
Employment Benefit Plans (Tables)	Dec. 31, 2020
Postemployment Benefits [Abstract]
Schedule of Expected Benefit Payments [Table Text Block]	The following table sets forth projected benefit payments from Ambac’s postretirement plan over the next ten years for current retirees: 2021 $ — 2022 — 2023 — 2024 — 2025 1 2025-2029 3 Total $ 5
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable [Table Text Block]	The amount of stock-based compensation expense and corresponding after-tax expense are as follows: Year Ended December 31, 2020 2019 2018 Stock options $ — $ — $ — Restricted stock units 3 4 6 Performance awards 8 8 6 Total stock-based compensation $ 11 $ 12 $ 12 Total stock-based compensation (after-tax) $ 11 $ 12 $ 12
Share-based Payment Arrangement, Option, Activity [Table Text Block]	A summary of stock option activity for 2020 is as follows: Shares Weighted Aggregate Weighted Outstanding at beginning of period 16,667 $ 20.63 Granted — — Exercised — — Forfeited or expired (16,667) 20.63 Outstanding at end of period — $ — $ — 0.00 Exercisable — $ — $ — 0.00
Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]	A summary of RSU activity for 2020 is as follows: Shares Weighted Average Grant Date Fair Value Outstanding at beginning of period 702,579 $ 18.19 Granted 297,517 17.36 Delivered or returned to plan (1) (224,829) 17.59 Forfeited (1,610) 19.19 Outstanding at end of period 773,657 $ 18.04 (1) When restricted stock unit awards issued by Ambac become taxable compensation to employees, shares may be withheld to
Schedule of Nonvested Performance-based Units Activity [Table Text Block]	A summary of PSU activity for 2020 is as follows: Shares Weighted Average Grant Date Fair Value Outstanding at beginning of period 650,212 $ 17.98 Granted (1) 331,184 19.99 Delivered (2) (184,896) 22.35 Forfeited (6,071) 18.00 Performance adjustment (3) 54,085 22.35 Outstanding at end of period 844,514 $ 18.09 (1) Represents performance share units at 100% of units granted for LTIP Awards. (2) Reflects the number of performance shares attributable to the performance goals attained over the completed performance period and for which service conditions have been met. When performance stock unit awards issued by Ambac become taxable compensation to employees, shares may be withheld to cover the employee’s withholding taxes. For the year ended December 31, 2020, Ambac purchased 70,340 of shares from employees that settled performance based restricted stock units to meet the required tax withholdings.

Quarterly Information (unaudi_2

Quarterly Information (unaudited) (Tables)	12 Months Ended
Quarterly Information (unaudited) (Tables)	Dec. 31, 2020
Quarterly Financial Data [Abstract]
Quarterly Financial Information (unaudited)	2020 Quarters 2019 Quarters First Second Third Fourth First Second Third Fourth Gross premiums written $ 11 $ (1) $ (13) $ 1 $ 3 $ (21) $ (13) $ 2 Net premiums earned 10 11 15 18 28 8 10 20 Net investment income (21) 52 37 53 55 86 45 42 Net realized investment gains (losses) 8 10 2 2 17 36 18 9 Net gains (losses) on derivative contracts (70) 2 7 12 (16) (35) (10) 12 Other income (loss) — — 2 1 1 (9) 141 1 Income (loss) on Variable Interest Entities 3 — — 3 16 3 11 7 Losses and loss expenses (benefit) 117 16 83 9 12 (133) 37 97 Insurance intangible amortization 13 14 14 16 36 226 17 15 Operating expenses 24 21 23 26 25 29 26 23 Interest expense 63 58 50 50 68 67 67 66 Pre-tax income (loss) (287) (33) (108) (12) (41) (100) 69 (111) Net income (loss) attributable to Common Stockholders $ (280) $ (35) $ (108) $ (14) $ (43) $ (128) $ 66 $ (110) Net income (loss) per share: Basic $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.44 $ (2.40) Diluted $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.41 $ (2.40)
Schedule of Quarterly Financial Information	18. QUARTERLY INFORMATION (Unaudited) 2020 Quarters 2019 Quarters First Second Third Fourth First Second Third Fourth Gross premiums written $ 11 $ (1) $ (13) $ 1 $ 3 $ (21) $ (13) $ 2 Net premiums earned 10 11 15 18 28 8 10 20 Net investment income (21) 52 37 53 55 86 45 42 Net realized investment gains (losses) 8 10 2 2 17 36 18 9 Net gains (losses) on derivative contracts (70) 2 7 12 (16) (35) (10) 12 Other income (loss) — — 2 1 1 (9) 141 1 Income (loss) on Variable Interest Entities 3 — — 3 16 3 11 7 Losses and loss expenses (benefit) 117 16 83 9 12 (133) 37 97 Insurance intangible amortization 13 14 14 16 36 226 17 15 Operating expenses 24 21 23 26 25 29 26 23 Interest expense 63 58 50 50 68 67 67 66 Pre-tax income (loss) (287) (33) (108) (12) (41) (100) 69 (111) Net income (loss) attributable to Common Stockholders $ (280) $ (35) $ (108) $ (14) $ (43) $ (128) $ 66 $ (110) Net income (loss) per share: Basic $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.44 $ (2.40) Diluted $ (6.07) $ (0.77) $ (2.33) $ (0.31) $ (0.94) $ (2.79) $ 1.41 $ (2.40)

Background and Business Descr_5

Background and Business Description - Additional Information (Detail) - USD ($)	Apr. 29, 1991	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2014	Feb. 12, 2018	May 01, 2013	Apr. 30, 2013
Background And Basis Of Presentation [Line Items]
Entity Incorporation, Date of Incorporation	Apr. 29, 1991
Tier 2 Notes						$ 240,000,000
Tier 2 Note Security in RMBS Litigation Settlement Proceeds						1,600,000,000
Common stock, shares authorized		130,000,000	130,000,000
Common Stock, Par or Stated Value Per Share		$ 0.01	$ 0.01					$ 0.01
Preferred stock, shares authorized		20,000,000	20,000,000
Preferred Stock, Par or Stated Value Per Share		$ 0.01	$ 0.01
Common stock, shares issued		45,865,081	45,571,743
Class of Warrant or Right, Exercise Price of Warrants or Rights							$ 16.67
Junior surplus note of Ambac Assurance Segregated Account							$ 350,000,000
Common Stock Voting Restrictions	10.00%
Common Stock Voting Restriction Less One Vote	10.00%
Common Stock Transfer Restrictions	5.00%
Common Stock Additional Transfer Restrictions					5.00%
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount		$ 12,000,000	$ 10,000,000	$ 32,000,000
Business Acquisition, Percentage of Voting Interests Acquired		80.00%
Ambac [Member] \| Deferred Payment Obligation [Member]
Background And Basis Of Presentation [Line Items]
Discount Amount						0.09
One Dollar Outstanding						1
Payment Amount in New Secured Notes						$ 0.91

Background and Basis of Present

Background and Basis of Presentation - Surplus Note Exchange - Ambac Assurance Corporation [Member]	3 Months Ended
	Dec. 31, 2020USD ($)	Feb. 11, 2021USD ($)	Jan. 19, 2021USD ($)	Dec. 31, 2019USD ($)
Insurance [Line Items]
Long-term Debt, Gross	$ 2,884,000,000			$ 2,980,000,000
5.1 % Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.10%			5.10%
5.1% Junior Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.10%			5.10%
Corolla Notes \| 5.1 % Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			5.10%
Lease Reducing Notes \| 5.1% Junior Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%
5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Insurance [Line Items]
Long-term Debt, Gross	$ 365,000,000			$ 365,000,000
5.1% Junior Surplus Notes, General Account Due 2020 [Member] \| Corolla Notes
Insurance [Line Items]
One Dollar Outstanding			$ 1
Payment Amount in Exchange			0.9125
Note Holders	1
5.1% Junior Surplus Notes, General Account Due 2020 [Member] \| Corolla Certificate
Insurance [Line Items]
One Dollar Outstanding			1
Payment Amount in Exchange			0.64
5.1% Junior Surplus Notes, General Account Due 2020 [Member] \| Lease Reducing Notes
Insurance [Line Items]
One Dollar Outstanding		$ 1
Payment Amount in Exchange		0.8581
5.1% Junior Surplus Notes, General Account Due 2020 [Member] \| Lease Reducing Notes \| Lease Reducing Notes
Insurance [Line Items]
Long-term Debt Reduced in Exchange		15,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member]
Insurance [Line Items]
Long-term Debt, Gross	$ 531,000,000			$ 531,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] \| Corolla Notes
Insurance [Line Items]
Long-term Debt Issued in Exchange Transaction			267,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] \| Lease Reducing Notes
Insurance [Line Items]
Long-term Debt Issued in Exchange Transaction			$ 13,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] \| Corolla Notes & Junior Surplus Notes
Insurance [Line Items]
Long-term Debt, Gross		853,000,000
Long-term Debt, Gross and Accrued and Unpaid Interest		1,414,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] \| Corolla Notes & Junior Surplus Notes \| Parent Company [Member]
Insurance [Line Items]
Long-term Debt, Gross		$ 83,000,000

Basis of Presentation and Si_10

Basis of Presentation and Significant Accounting Policies FX gain (loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Foreign Currency Transaction Gain (Loss), before Tax	$ (1)	$ 12	$ (7)
Gain (Loss) on Investments [Member]
Foreign Currency Transaction Gain (Loss), before Tax	$ (4)	$ 22	$ 7

Variable Interest Entities - Ad

Variable Interest Entities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2020USD ($)Entity	Dec. 31, 2020USD ($)Entity	Dec. 31, 2019USD ($)Entity	Dec. 31, 2018USD ($)	Feb. 12, 2018USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 2,807,000,000	$ 2,807,000,000	$ 3,187,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	149,000,000	149,000,000	132,000,000
satisfaction and discharge of segregated account deferred amounts					$ 3,000,000,000
satisfaction and discharge of segregated account deferred interest amounts					857,000,000
Available-for-sale Securities	2,949,000,000	2,949,000,000	3,314,000,000
Restricted Cash	13,000,000	13,000,000	55,000,000	$ 19,000,000
Fair value of special purpose entities	1,000,000	1,000,000	3,000,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets	292,000,000	292,000,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	364,000,000	364,000,000
Equity Method Investments	51,000,000	51,000,000	46,000,000
Other Investments	595,000,000	595,000,000	478,000,000
Deconsolidated VIE Assets	0	0
Deconsolidated VIE Liabilities	0	0
Long-term debt	2,739,000,000	2,739,000,000	2,822,000,000
Derivative assets	93,000,000	93,000,000	75,000,000
Other assets	114,000,000	114,000,000	95,000,000
Assets	13,220,000,000	13,220,000,000	13,320,000,000
Accrued interest payable	517,000,000	517,000,000	441,000,000
Derivative liabilities	114,000,000	114,000,000	90,000,000
Other liabilities	106,000,000	106,000,000	93,000,000
Liabilities	12,074,000,000	$ 12,074,000,000	11,783,000,000
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life		2 months 12 days
Average rating of assets held by sponsored special purpose entities		BBB+
Sponsored Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Total principal amount of debt outstanding	$ 410,000,000	$ 410,000,000	$ 403,000,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities		1	1	4
Number of consolidated Variable Interest Entities \| Entity	6	6	7
Ambac UK [Member] \| Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities \| Entity	5	5	6
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Restricted Cash	$ 9,000,000	$ 9,000,000	$ 55,000,000
Secured Notes Received from Ambac LSNI					644,000,000
Ambac Assurance [Member] \| Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities \| Entity	1	1	1
AFG [Member]
Variable Interest Entities [Line Items]
Secured Notes Received from Ambac LSNI					$ 125,000,000
Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 2,739,000,000	$ 2,739,000,000	$ 2,822,000,000
Ambac Assurance Corporation [Member] \| 5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 247,000,000	$ 247,000,000	$ 252,000,000
Ambac Assurance Corporation [Member] \| 5.1% Junior Surplus Notes, General Account Due 2020 [Member] \| Corolla Notes
Variable Interest Entities [Line Items]
Note Holders	1

Variable Interest Entities - Su

Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entities [Line Items]
Variable Interest Entity Nonconsolidated Carrying Amount Insurance Assets	$ 2,314	$ 2,247
Variable Interest Entity Nonconsolidated Carrying Amount Insurance Liabilities	1,122	1,083
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	27,998	31,506
Investments:
Variable Interest Entity Nonconsolidated Carrying Amount Derivative Net Assets Or Liabilities	8
Fixed income securities, amortized cost	2,807	3,187
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	149	132
Variable Interest Entity [Member]
Investments:
Variable Interest Entity Nonconsolidated Carrying Amount Derivative Net Assets Or Liabilities		7
Municipal Bonds [Member] \| Variable Interest Entity [Member]
Investments:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	$ 27	$ 25

Variable Interest Entities - _2

Variable Interest Entities - Supplemental Information about Loans Held as Assets and Long-Term Debt Associated with Consolidated Variable Interest Entities (Detail) $ in Millions	Dec. 31, 2020USD ($)
Variable Interest Entities [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	$ 364

Variable Interest Entities - _3

Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 6,352	$ 8,165
Insurance Assets	2,051	1,961
Insurance Liabilities	834	762
Derivative Liabilities	8	8
Global Structured Finance [Member] \| Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,308	5,373
Insurance Assets	2,024	1,913
Insurance Liabilities	580	523
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,050	1,373
Insurance Assets	24	31
Insurance Liabilities	239	216
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	24	314
Insurance Assets	3	9
Insurance Liabilities	1	6
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	970	1,107
Insurance Assets	0	7
Insurance Liabilities	13	18
Derivative Liabilities	8	8
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	21,646	23,341
Insurance Assets	263	287
Insurance Liabilities	287	321
Derivative Liabilities	0	0
Maximum Exposure To Loss	27,998	31,506
Insurance Assets	2,314	2,247
Insurance Liabilities	1,122	1,083
Derivative Liabilities	$ 8
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		$ 7

Variable Interest Entities Spec

Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Millions	Dec. 31, 2020USD ($)Entity	Dec. 31, 2019USD ($)Entity	Dec. 31, 2018USD ($)
Available-for-sale Securities, Amortized Cost Basis	$ 2,807
Available-for-sale Securities	2,949	$ 3,314
Restricted Cash	13	55	$ 19
Derivative assets	93	75
Other assets	114	95
Assets	13,220	13,320
Accrued interest payable	517	441
Long-term debt	2,739	2,822
Derivative liabilities	114	90
Liabilities	12,074	11,783
Ambac Assurance [Member]
Restricted Cash	9	55
Variable Interest Entity [Member]
Available-for-sale Securities	3,354	3,121
Restricted Cash	2	2	$ 1
Loans, at fair value	2,998	3,108
Derivative assets	41	52
Other assets	2	3
Assets	6,398	6,286
Accrued interest payable	0	1
Long-term Debt	4,324	4,351
Long-term Debt, at par less amortized discount	169	203
Long-term debt	4,493	4,554
Derivative liabilities	1,835	1,657
Liabilities	6,328	6,212
Variable Interest Entity [Member] \| Ambac UK [Member]
Available-for-sale Securities	3,215	2,957
Restricted Cash	1	1
Loans, at fair value	2,998	3,108
Derivative assets	41	52
Other assets	0	1
Assets	6,255	6,119
Accrued interest payable	0	1
Long-term Debt	4,324	4,351
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,324	4,351
Derivative liabilities	1,835	1,657
Liabilities	6,159	6,009
Variable Interest Entity [Member] \| Ambac Assurance [Member]
Available-for-sale Securities	139	164
Restricted Cash	1	1
Loans, at fair value	0	0
Derivative assets	0	0
Other assets	2	2
Assets	143	167
Accrued interest payable	0	0
Long-term Debt	0	0
Long-term Debt, at par less amortized discount	169	203
Long-term debt	169	203
Derivative liabilities	0	0
Liabilities	169	203
Variable Interest Entity [Member] \| Corporate Debt Securities [Member]
Available-for-sale Securities	3,215	2,957
Variable Interest Entity [Member] \| Corporate Debt Securities [Member] \| Ambac UK [Member]
Available-for-sale Securities	3,215	2,957
Variable Interest Entity [Member] \| Corporate Debt Securities [Member] \| Ambac Assurance [Member]
Available-for-sale Securities	0	0
Variable Interest Entity [Member] \| Municipal Bonds [Member]
Available-for-sale Securities, Amortized Cost Basis	113	139
Available-for-sale Securities	139	164
Variable Interest Entity [Member] \| Municipal Bonds [Member] \| Ambac UK [Member]
Available-for-sale Securities	0	0
Variable Interest Entity [Member] \| Municipal Bonds [Member] \| Ambac Assurance [Member]
Available-for-sale Securities	$ 139	$ 164
Consolidated Entities [Member]
Number of consolidated Variable Interest Entities \| Entity	6	7
Consolidated Entities [Member] \| Ambac UK [Member]
Number of consolidated Variable Interest Entities \| Entity	5	6
Consolidated Entities [Member] \| Ambac Assurance [Member]
Number of consolidated Variable Interest Entities \| Entity	1	1

Business Combinations (Details)

Business Combinations (Details) $ in Millions	3 Months Ended	12 Months Ended
Business Combinations (Details) $ in Millions	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 81
Distribution Relationships
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	15 years
Non-compete Agreements
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Trade Names
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	8 years
Distribution Relationships
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	$ 33	33
Non-compete Agreements
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	1	1
Trade Names
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross	1	1
Xchange
Business Acquisition [Line Items]
Business Combination, Consideration Transferred	81
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	2	2
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedrResstrictedCash	4	4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	36	36
Goodwill, Acquired During Period	46
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	8	8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	96	96
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	8	8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	8	8
Redeemable Noncontrolling Interest, Equity, Carrying Amount	7	7
Business Acquisition, Goodwill, Expected Tax Deductible Amount	36	36
2018	3	3
2019	3	3
2020	3	3
2021	3	3
2022	3	3
Thereafter	$ 22	$ 22

Net Income Per Share - Addition

Net Income Per Share - Additional Information (Detail) $ / shares in Units, $ in Millions	12 Months Ended			66 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019$ / sharesshares	Dec. 31, 2018shares	Dec. 31, 2020USD ($)$ / sharesshares	Aug. 03, 2018$ / shares	Nov. 02, 2016USD ($)	Jun. 30, 2015USD ($)	May 01, 2013$ / shares	Apr. 30, 2013$ / shares
Schedule Of Earnings Per Share [Line Items]
Common Stock, Shares, Outstanding	45,809,139			45,809,139
Class of Warrant or Right, Outstanding	4,877,749			4,877,749
Number of new common stock issued	45,865,081	45,571,743		45,865,081
Common Stock, Par or Stated Value Per Share \| $ / shares	$ 0.01	$ 0.01		$ 0.01					$ 0.01
Exercise price of common stock \| $ / shares								$ 16.67
Warrant exercised	34	0	194
Stock Issued During Period, Shares, New Issues	8	0	194
First Warrant Buyback Authorization Dollars \| $							$ 10
Payments for Repurchase of Warrants \| $				$ 8
Warrants Repurchased Average Cost per Warrant \| $ / shares				$ 8.21
SecondWarrantBuyBackAuthorizedDollars \| $						$ 10
Warrants Purchased Shares				985,331
RemainingWarrantBuyBackAuthorizedDollars \| $	$ 12			$ 12
AMPS Exchange Total Warrants Issued					824,307
AMPS Exchange Total Warrants Issued Cost Per Share \| $ / shares					$ 9.72

Net Income Per Share - Reconcil

Net Income Per Share - Reconciliation of Common Shares Used for Basic and Diluted Earnings Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of common shares and potential dilutive shares used for diluted earnings per share	46,147,062	45,954,908	46,559,835
Weighted Average Number of Shares Outstanding, Basic	46,147,062	45,954,908	45,665,883
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	4,877,754	4,877,783	0
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	16,121	16,667	16,667
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	302,145	249,263	0
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,002,501	872,258	0
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Warrant [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares			0
Share-based Payment Arrangement, Option [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares			441,104
Performance Shares [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares			375,276
Restricted Stock Units (RSUs) [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares			77,572

Financial Guarantees in Force -

Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 12	$ 10	$ 32
Gross par amount of financial guarantees outstanding	39,070	43,908
Net par amount of financial guarantees	33,888	38,018
Gross financial guarantees in force	61,895	69,826
Net financial guarantees in force	$ 51,603	58,245
Highest single insured risk of aggregate net par amount guaranteed	2.90%
Ceded Principal Outstanding Major Reinsurer Percentage	6.10%
California [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	6.20%
NEW JERSEY
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	7.00%
NEW YORK
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.40%
No other state [Member] \| Maximum [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.00%
Public Finance [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 15,497	17,653
Public Finance [Member] \| Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	5,855	5,991
Military Housing [Member] \| Public Finance [Member] \| Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 5,575	$ 5,654

Financial Guarantees in Force_3

Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 33,888	$ 38,018
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	15,497	17,653
Public Finance [Member] \| Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,179	5,102
Public Finance [Member] \| General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,345	3,011
Public Finance [Member] \| Utility Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	675	768
Public Finance [Member] \| Higher Education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	747	885
Public Finance [Member] \| Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,855	5,991
Public Finance [Member] \| Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	925	1,041
Public Finance [Member] \| Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	771	855
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,337	7,508
Structured Finance [Member] \| Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,635	4,423
Structured Finance [Member] \| Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,617	1,675
Structured Finance [Member] \| Structured Insurance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	311	395
Structured Finance [Member] \| Student Loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	626	769
Structured Finance [Member] \| Asset Backed and Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	148	246
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,054	12,857
International Finance [Member] \| Asset Backed and Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,374	1,625
International Finance [Member] \| Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,899	4,436
International Finance [Member] \| Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,270	5,264
International Finance [Member] \| Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 1,511	$ 1,532

Financial Guarantees in Force_4

Financial Guarantees in Force - Summary of International Finance Guaranteed Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 33,888	$ 38,018
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,054	12,857
International Finance [Member] \| United Kingdom [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	9,711	10,593
International Finance [Member] \| Australia [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	420	382
International Finance [Member] \| Italy [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	803	767
International Finance [Member] \| Austria [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	707	674
International Finance [Member] \| FRANCE
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	277	303
International Finance [Member] \| Other International [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 136	$ 138

Financial Guarantee Insurance_5

Financial Guarantee Insurance Contracts - Additional Information (Detail) £ in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2020GBP (£)	Jan. 01, 2020USD ($)	Dec. 31, 2019GBP (£)	Feb. 12, 2019	Dec. 31, 2018GBP (£)	Feb. 12, 2018USD ($)	Dec. 31, 2017USD ($)
Insurance [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1				$ 0				$ 1	$ 0	$ 1
Net par amount of financial guarantees	33,888				38,018				33,888	38,018
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,140				$ 1,536				1,140	$ 1,536	1,633							$ 1,645
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	(60)								(60)
Possible Increase in Domestic Public Finance Loss Reserves	$ 1,200								$ 1,200
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%				2.40%				2.20%	2.40%		2.20%		2.40%
Financial Guarantee Insurance Contracts, Premium Receivable	$ 370				$ 416				$ 370	$ 416	495							586
Amount Of Insured Par Outstanding Ceded To Reinsurer	2,398								$ 2,398
Ceded Principal Outstanding Major Reinsurer Percentage									6.10%
Deferred Ceded Premiums And Reinsurance Recoverables	103								$ 103
Reinsurance Payable	27				29				$ 27	$ 29
Weighted average period of future premiums									8 years 3 months 18 days	8 years 6 months
Uncollectable premium receivables	17								$ 17
Accelerated premium revenue for retired obligations									12	$ 10	32
Reinsurance recoveries of losses included in losses and loss expenses									(11)	(7)	(2)
Losses and loss expense reserves ceded to reinsurers	33				26				33	26	23							$ 41
Subrogation recoveries, net of reinsurance	(1,725)				(1,702)				(1,725)	(1,702)
Amortization of insurance intangible assets	16	$ 14	$ 14	$ 13	15	$ 17	$ 226	$ 36	57	295	107
Intangible Assets, Gross (Excluding Goodwill)	1,281				1,273				1,281	1,273
Finite-Lived Intangible Assets, Accumulated Amortization	908				847				908	847
Intangible assets	409				427				409	427
Letters of Credit Outstanding, Amount	117								117
Financial Guarantee Outstanding Principal Ceded To Third Parties	5,182								5,182
Public Finance Puerto Rico Net Par Outstanding	1,070								1,070
Percent Cash & New COFINA Bonds Allocated Under POA															93.00%
Ambac Assurance Remaining Policy Obligations	80								80
Premiums Receivable, Gross	387								387
Allowance for Doubtful Accounts, Premiums and Other Receivables	17				9				17	9
Premium Receivable, Credit Loss Expense (Reversal)									9
Premium Receivable, Allowance for Credit Loss, Writeoff									(2)
Premium Receivable, Allowance for Credit Loss, Recovery									0
Financing Receivable, Past Due	0								0
Financing Receivable, 90 Days or More Past Due, Still Accruing	0								0
Reinsurance Recoverable, Allowance for Credit Loss	0								0
satisfaction and discharge of segregated account deferred amounts																	$ 3,000
satisfaction and discharge of segregated account deferred interest amounts																	857
Loss and Loss Expense Benefit on Settled Deferred Amounts																	$ 288
Net Incurred RMBS Subrogation Recoveries									(23)	$ 42	62
Reinsurance Recoverable Credit Exposure	1								1
Accounting Standards Update 2016-13 [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4)								(4)
Accounting Standards Update 2016-13 [Member] \| Premiums Receivable [Member]
Insurance [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption													$ (3)
Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	302								302
IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	40								40
II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								$ 15
Maximum [Member] \| Furniture and Fixtures [Member]
Insurance [Line Items]
Finite-Lived Intangible Asset, Useful Life									5 years
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves									1.10%	2.10%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	117				129				$ 117	$ 129	$ 131	£ 86		£ 97		£ 103
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses									256
Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	15,497				17,653				15,497	17,653
Premiums Receivable, Gross	185								185
Public Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	157								157
Public Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
Public Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Public Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Public Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	6,337				7,508				6,337	7,508
Premiums Receivable, Gross	59								59
Structured Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	27								27
Structured Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Structured Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14								14
Structured Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	12,054				12,857				12,054	12,857
Premiums Receivable, Gross	144								144
International Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	118								118
International Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
International Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
International Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
International Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] \| Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,855				5,991				5,855	5,991
Premiums Receivable, Gross	168								168
Housing Revenue [Member] \| Public Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	155								155
Housing Revenue [Member] \| Public Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Housing Revenue [Member] \| Public Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] \| Public Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] \| Public Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Housing Revenue [Member] \| Military Housing [Member] \| Public Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,575				5,654				5,575	5,654
Other Public Finance Sectors [Member] \| Public Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	17								17
Other Public Finance Sectors [Member] \| Public Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2								2
Other Public Finance Sectors [Member] \| Public Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
Other Public Finance Sectors [Member] \| Public Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Public Finance Sectors [Member] \| Public Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Public Finance Sectors [Member] \| Public Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	3,635				4,423				3,635	4,423
Premiums Receivable, Gross	22								22
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	1								1
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Mortgage-backed and Home Equity [Member] \| Structured Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	15								15
Structured Insurance [Member] \| Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	311				395				311	395
Premiums Receivable, Gross	14								14
Structured Insurance [Member] \| Structured Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	14								14
Structured Insurance [Member] \| Structured Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] \| Structured Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] \| Structured Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Structured Insurance [Member] \| Structured Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Student Loan [Member] \| Structured Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	626				769				626	769
Premiums Receivable, Gross	16								16
Student Loan [Member] \| Structured Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	3								3
Student Loan [Member] \| Structured Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Student Loan [Member] \| Structured Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	2								2
Student Loan [Member] \| Structured Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	11								11
Student Loan [Member] \| Structured Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] \| Structured Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	7								7
Other Structured Finance [Member] \| Structured Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	7								7
Other Structured Finance [Member] \| Structured Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] \| Structured Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] \| Structured Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other Structured Finance [Member] \| Structured Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Sovereign/Sub-sovereign [Member] \| International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	5,270				5,264				5,270	5,264
Premiums Receivable, Gross	108								108
Sovereign/Sub-sovereign [Member] \| International Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	82								82
Sovereign/Sub-sovereign [Member] \| International Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Sovereign/Sub-sovereign [Member] \| International Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Sovereign/Sub-sovereign [Member] \| International Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	13								13
Sovereign/Sub-sovereign [Member] \| International Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] \| International Finance [Member]
Insurance [Line Items]
Net par amount of financial guarantees	3,899				$ 4,436				3,899	$ 4,436
Premiums Receivable, Gross	31								31
Investor-owned and Public Utilities [Member] \| International Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	31								31
Investor-owned and Public Utilities [Member] \| International Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] \| International Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] \| International Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Investor-owned and Public Utilities [Member] \| International Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] \| International Finance [Member]
Insurance [Line Items]
Premiums Receivable, Gross	5								5
Other International [Member] \| International Finance [Member] \| Surveillance Category One [Member]
Insurance [Line Items]
Premiums Receivable, Gross	5								5
Other International [Member] \| International Finance [Member] \| IA [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] \| International Finance [Member] \| II [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] \| International Finance [Member] \| III [Member]
Insurance [Line Items]
Premiums Receivable, Gross	0								0
Other International [Member] \| International Finance [Member] \| IV [Member]
Insurance [Line Items]
Premiums Receivable, Gross	$ 0								$ 0

Financial Guarantee Insurance_6

Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Detail) € in Millions, £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2020EUR (€)	Dec. 31, 2019USD ($)	Dec. 31, 2019GBP (£)	Dec. 31, 2019EUR (€)	Dec. 31, 2018USD ($)	Jan. 01, 2020USD ($)
Insurance [Line Items]
Premium Receivable, Allowance for Credit Loss	$ 17
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	416			$ 495			$ 586
Premium receipts	46			48			(56)
Adjustments for changes in expected and contractual cash flows	(6)			(38)			(42)
Accretion of premium receivable discount	9			11			15
Financial Guarantee Insurance Contracts, Premium Receivable, Deconsolidation of VIE	0			3
Uncollectable premiums	(4)			(2)			2
Other adjustments (including foreign exchange)	5			(6)			(10)
Ending premium receivable	370			416			495
Allowance for Doubtful Accounts, Premiums and Other Receivables	17			9
Premium Receivable, Credit Loss Expense (Reversal)	9
Premium Receivable, Allowance for Credit Loss, Writeoff	(2)
Premium Receivable, Allowance for Credit Loss, Recovery	0
Financing Receivable, Past Due	0
Financing Receivable, 90 Days or More Past Due, Still Accruing	0
Accounting Standards Update 2016-13 [Member]
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4)
Accounting Standards Update 2016-13 [Member] \| Premiums Receivable [Member]
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption								$ (3)
United Kingdom, Pounds
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	129	£ 97		131	£ 103
Ending premium receivable	117	£ 86		129	£ 97		131
Euro Member Countries, Euro
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	26		€ 23	31		€ 27
Ending premium receivable	$ 19		€ 16	$ 26		€ 23	$ 31

Financial Guarantee Insurance_7

Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written									$ 1	$ 28	$ 24
Assumed Reinsurance Premiums Written									0	0	0
Ceded Reinsurance Premiums Written									1	(31)	(17)
Premiums written, net of reinsurance									0	60	41
Direct Premiums Earned									65	75	119
Premiums Earned, Net									54	66	111
Assumed Reinsurance Premiums Earned									1	0	0
Ceded Reinsurance Premiums Earned									12	10	8
Reinsurance on premiums earned, Net	$ 18	$ 15	$ 11	$ 10	$ 20	$ 10	$ 8	$ 28	$ 54	$ 66	$ 111

Financial Guarantee Insurance_8

Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Detail) $ in Millions	Dec. 31, 2020USD ($)
Future premiums expected to be collected, March 31, 2018	$ 12
Future premiums expected to be collected, June 30. 2018	8
Future premiums expected to be collected, September 30, 2018	9
Future premiums expected to be collected, December 31, 2018	8
Future premiums expected to be collected, December 31, 2019	36
Future premiums expected to be collected, December 31, 2020	34
Future premiums expected to be collected, December 31, 2021	33
Future premiums expected to be collected, December 31, 2022	31
Future premiums expected to be collected, December 31, 2027	132
Future premiums expected to be collected, December 31, 2032	91
Future premiums expected to be collected, December 31, 2037	42
Future premiums expected to be collected, December 31, 2042	19
Future premiums expected to be collected, December 31, 2047	7
Future premiums expected to be collected, December 31, 2052	1
Future premiums expected to be collected, Total	462
Future expected premium to be earned, net of reinsurance, March 31, 2018	9
Future expected premiums to be earned, net of reinsurance, June 30, 2018	9
Future expected premiums to be earned, net of reinsurance, September 30, 2018	9
Future expected premiums to be earned, net of reinsurance, December 31, 2018	9
Future expected premiums to be earned, net of reinsurance, December 31, 2019	33
Future expected premiums to be earned, net of reinsurance, December 31, 2020	31
Future expected premiums to be earned, net of reinsurance, December 31, 2021	29
Future expected premiums to be earned, net of reinsurance, December 31, 2022	27
Future expected premiums to be earned, net of reinsurance, December 31, 2027	113
Future expected premiums to be earned, net of reinsurance, December 31, 2032	72
Future expected premiums to be earned, net of reinsurance, December 31, 2037	30
Future expected premiums to be earned, net of reinsurance, December 31, 2042	12
Future expected premiums to be earned, net of reinsurance, December 31, 2047	4
Future expected premiums to be earned, net of reinsurance, December 31, 2052	0
Future expected premiums to be earned, net of reinsurance, Total	$ 386

Financial Guarantee Insurance_9

Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 12, 2018
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves				$ 0
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	$ 2	$ (1)	$ (15)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	225	13	(224)
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation				(1,751)	$ (1,727)
Policyholder Benefits and Claims Incurred, Ceded	11	7	2
Net Incurred RMBS Subrogation Recoveries	23	(42)	(62)
satisfaction and discharge of segregated account deferred amounts							$ 3,000
satisfaction and discharge of segregated account deferred interest amounts							857
Loss and Loss Expense Benefit on Settled Deferred Amounts							$ 288
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	(482)	(107)	4,114
Less reinsurance on loss and loss expense reserves	26	23	41
Beginning balance of net loss and loss expense reserves	(436)	(130)	4,073
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	(1)	0	0
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(148)	(318)	(3,963)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(436)	(130)	4,073	(430)	(436)	$ (130)
Add reinsurance on loss and loss expense reserves	33	26	23
us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet					(508)
Ending gross loss and loss expense reserves	(397)	(482)	(107)
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance				(1,725)	(1,702)
Current Year Claims and Claims Adjustment Expense	15	1	5
Prior Year Claims and Claims Adjustment Expense	210	12	(228)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 149	$ 318	$ 3,964
Ceded Loss And Loss Expenses Paid Not Yet Recovered				$ 1	$ 0	$ 1

Financial Guarantee Insuranc_10

Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)Policies	Dec. 31, 2019USD ($)Policies	Dec. 31, 2018USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	232	221
Remaining weighted-average contract period (in years)	14 years	15 years
Gross insured contractual payments outstanding:
Principal	$ 7,573	$ 6,168
Interest	3,443	3,029
Total	11,016	9,197
Gross undiscounted claim liability	2,395	2,434
Discount, gross claim liability	303	541
Gross claim liability before all subrogation and before reinsurance	2,092	1,893
Less:
Gross RMBS subrogation	(1,753)	(1,777)
Discount, RMBS subrogation	3	49
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(1,751)	(1,727)
Less:
Gross other subrogation	(755)	(720)
Discount, other subrogation	20	53
Discounted other subrogation, before reinsurance	(735)	(666)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(394)	(501)
Less: Unearned premium reserves	(72)	(54)
Plus: Loss adjustment expenses reserves	68	73
Claim liability reported on Balance Sheet, before reinsurance	(397)	(482)
Reinsurance recoverable reported on Balance Sheet	33	26
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 1
Loss Reserves [Member]
Less:
Weighted average risk-free rate used to discount loss reserves	1.10%	2.10%
Surveillance Category One [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	40	34
Remaining weighted-average contract period (in years)	10 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 842	$ 668
Interest	279	340
Total	1,121	1,007
Gross undiscounted claim liability	3	2
Discount, gross claim liability	0	0
Gross claim liability before all subrogation and before reinsurance	3	2
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	3	2
Less: Unearned premium reserves	(2)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	2	1
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	25	18
Remaining weighted-average contract period (in years)	18 years	21 years
Gross insured contractual payments outstanding:
Principal	$ 1,375	$ 510
Interest	1,011	507
Total	2,386	1,016
Gross undiscounted claim liability	49	44
Discount, gross claim liability	2	5
Gross claim liability before all subrogation and before reinsurance	47	39
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	47	39
Less: Unearned premium reserves	(16)	(9)
Plus: Loss adjustment expenses reserves	2	1
Claim liability reported on Balance Sheet, before reinsurance	32	30
Reinsurance recoverable reported on Balance Sheet	$ 6	$ 6
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	15	11
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 595	$ 277
Interest	484	128
Total	1,079	404
Gross undiscounted claim liability	40	21
Discount, gross claim liability	1	1
Gross claim liability before all subrogation and before reinsurance	40	20
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	39	20
Less: Unearned premium reserves	(5)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	35	20
Reinsurance recoverable reported on Balance Sheet	$ 9	$ 7
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	15	16
Remaining weighted-average contract period (in years)	16 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,469	$ 857
Interest	215	366
Total	1,685	1,223
Gross undiscounted claim liability	541	541
Discount, gross claim liability	85	152
Gross claim liability before all subrogation and before reinsurance	456	389
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(36)	(41)
Discount, other subrogation	1	4
Discounted other subrogation, before reinsurance	(35)	(37)
Gross claim liability, net of all subrogation and discounts, before reinsurance	421	353
Less: Unearned premium reserves	(17)	(7)
Plus: Loss adjustment expenses reserves	5	4
Claim liability reported on Balance Sheet, before reinsurance	409	349
Reinsurance recoverable reported on Balance Sheet	$ 24	$ 24
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	132	139
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 3,246	$ 3,819
Interest	1,427	1,678
Total	4,673	5,498
Gross undiscounted claim liability	1,690	1,778
Discount, gross claim liability	213	381
Gross claim liability before all subrogation and before reinsurance	1,477	1,397
Less:
Gross RMBS subrogation	(1,753)	(1,777)
Discount, RMBS subrogation	3	49
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(1,751)	(1,727)
Less:
Gross other subrogation	(706)	(666)
Discount, other subrogation	18	47
Discounted other subrogation, before reinsurance	(689)	(620)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(963)	(950)
Less: Unearned premium reserves	(30)	(35)
Plus: Loss adjustment expenses reserves	59	67
Claim liability reported on Balance Sheet, before reinsurance	(933)	(918)
Reinsurance recoverable reported on Balance Sheet	$ (6)	$ (10)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies \| Policies	5	3
Remaining weighted-average contract period (in years)	7 years	3 years
Gross insured contractual payments outstanding:
Principal	$ 47	$ 37
Interest	26	11
Total	72	48
Gross undiscounted claim liability	72	48
Discount, gross claim liability	3	2
Gross claim liability before all subrogation and before reinsurance	69	46
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(12)	(13)
Discount, other subrogation	1	3
Discounted other subrogation, before reinsurance	(11)	(10)
Gross claim liability, net of all subrogation and discounts, before reinsurance	58	36
Less: Unearned premium reserves	(1)	0
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	57	36
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0

Financial Guarantee Insuranc_11

Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	$ 1,751	$ 1,727
Random Samples [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,751	1,727	$ 1,771	$ 1,834
Other Changes Rmbs Subrogation	$ 23	$ (43)	$ (64)

Financial Guarantee Insuranc_12

Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,727
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,751	$ 1,727
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,727	1,771	$ 1,834
Changes recognized
Other Changes Rmbs Subrogation	23	(43)	(64)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,751	$ 1,727	$ 1,771

Financial Guarantee Insuranc_13

Financial Guarantee Insurance Contracts - Estimated Future Amortization Expense for Insurance Intangible Asset (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortization Of Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									7 years 6 months
Amortization of insurance intangible assets	$ 16	$ 14	$ 14	$ 13	$ 15	$ 17	$ 226	$ 36	$ 57	$ 295	$ 107
Intangible Assets, Gross (Excluding Goodwill)	1,281				1,273				1,281	1,273
Finite-Lived Intangible Assets, Accumulated Amortization	908				847				908	847
Intangible assets	409				$ 427				409	$ 427
Insurance Intangible Asset [Member]
Amortization Of Intangible Assets [Line Items]
2018	39								39
2019	35								35
2020	32								32
2021	29								29
2022	26								26
Thereafter	$ 213								$ 213

Insurance Regulatory Restrict_2

Insurance Regulatory Restrictions Additional Details (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Average Annual Rate Of Return	4.56%	5.43%
Prescribed Discount Rate Percentage	5.10%
Quarterly Dividend Increase Limitation Percentage	15.00%
WISCONSIN
Percentage Of Policy Holders Surplus	10.00%
ARIZONA
Percentage Of Policy Holders Surplus	10.00%
Minimum [Member]
Settlement Agreement Annual Restricted Payment Amount	$ 5
Maximum [Member]
Settlement Agreement Annual Restricted Payment Amount	8
Ambac Assurance [Member]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	865	$ 1,088
Ambac Assurance [Member] \| Prescribed Or Permitted Additional Accounting Practices [Member]
Statutory Accounting Practices, Prescribed Practice, Amount	40	$ (12)
Everspan Indemnity
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	$ 26
5.1 % Surplus Notes Due 2020 [Member] \| Ambac Assurance Corporation [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.10%	5.10%

Investments - Summary of Amorti

Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,807	$ 3,187
Gross Unrealized Gains	149	132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
Available-for-sale Securities, Current	2,949
Estimated Fair Value	2,949	3,314
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	2,807
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	492	653
Gross Unrealized Gains	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	492
Available-for-sale Securities, Amortized Cost Basis	492
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,175	2,450
Gross Unrealized Gains	149	132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
Estimated Fair Value	2,317	2,577
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	2,175
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8
Fixed Income Securities [Member] \| Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		194
Gross Unrealized Gains	37	22
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	358	215
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	321
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member] \| Corporate Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,396
Gross Unrealized Gains	24	36
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	2
Estimated Fair Value	1,077	1,430
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	1,059
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6
Fixed Income Securities [Member] \| Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		44
Gross Unrealized Gains	1	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	98	44
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	97
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member] \| U.S. Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		157
Gross Unrealized Gains	2	2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	2
Estimated Fair Value	106	156
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	105
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	256	200
Gross Unrealized Gains	46	47
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	302	248
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	256
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	74	147
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	1
Estimated Fair Value	74	146
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	74
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	263	263
Gross Unrealized Gains	40	24
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	303	287
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	263
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		653
Gross Unrealized Gains		0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Estimated Fair Value		653
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		49
Gross Unrealized Gains		1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Estimated Fair Value		50
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		3,103
Gross Unrealized Gains	149	132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		5
Estimated Fair Value	2,809	3,230
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		0
Available-for-sale Securities, Amortized Cost Basis	2,667
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8
Collateral Pledged [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	140
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15	0
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Estimated Fair Value	15	85
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Estimated Fair Value	125	85
Available-for-sale Securities, Amortized Cost Basis	125
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| US Treasury and Government
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0
Estimated Fair Value	15
Available-for-sale Securities, Amortized Cost Basis	15
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Asset Pledged as Collateral [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		85
Available-for-sale Securities, Amortized Cost Basis	140
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0
Asset Pledged as Collateral [Member] \| Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 85

Investments - Summary of Amor_2

Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 718
Amortized Cost, Due after one year through five years	873
Amortized Cost, Due after five years through ten years	439
Amortized Cost, Due after ten years	184
Amortized Cost, Total	2,214
Fixed income securities, amortized cost	2,807	$ 3,187
Estimated Fair Value, Due in one year or less	719
Estimated Fair Value, Due after one year through five years	880
Estimated Fair Value, Due after five years through ten years	459
Estimated Fair Value, Due after ten years	213
Estimated Fair Value due, Total	2,271
Debt Securities, Available-for-sale	2,949	3,314
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	2,175	2,450
Debt Securities, Available-for-sale	2,317	2,577
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	256	200
Debt Securities, Available-for-sale	302	248
Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost		49
Debt Securities, Available-for-sale		50
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	74	147
Debt Securities, Available-for-sale	74	146
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	263	263
Debt Securities, Available-for-sale	$ 303	$ 287

Investments - Summary of Gross

Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 816	$ 401
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25	88
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	841	489
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	187	201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	187	201
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		5
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	629	200
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25	88
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	654	288
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
Fixed Income Securities [Member] \| Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	25	13
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	6	10
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	31	23
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	17	36
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	2
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	17	38
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	2
Fixed Income Securities [Member] \| Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	543	63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	5
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	543	68
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	2
Fixed Income Securities [Member] \| Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3	20
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	3	20
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	14	5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	14	5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	27	53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	15	63
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	42	116
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	1
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4	7
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4	10
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 0	7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		$ 0

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Reported Value Measurement [Member]
Investment [Line Items]
Other Investments	$ 89	$ 136
Reported Value Measurement [Member] \| Interest Rate Contract [Member]
Investment [Line Items]
Other Investments	3	81
Reported Value Measurement [Member] \| Equity [Member]
Investment [Line Items]
Other Investments	60	55
Reported Value Measurement [Member] \| Emerging Market Debt [Member]
Investment [Line Items]
Other Investments	25	0
Other Investments	595	478
Securities fair value	8	6
Market Value of Secured Note Collateral	178	197
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Real Estate [Member]
Investment [Line Items]
Other Investments	16	16
Hedge Funds, Multi-strategy [Member]
Investment [Line Items]
Other Investments	196	65
Interest Rate Contract [Member]
Investment [Line Items]
Other Investments	78	176
Illiquid Investments [Member]
Investment [Line Items]
Other Investments	65	51
Insurance Linked [Member]
Investment [Line Items]
Other Investments	3	3
Equity [Member]
Investment [Line Items]
Other Investments	73	55
Credit Index Product [Member]
Investment [Line Items]
Other Investments	73	66
Equity investments in pooled funds [Member]
Investment [Line Items]
Other Investments	543	432
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	140	$ 85
Commitments [Member] \| Partnership Interest [Member]
Investment [Line Items]
Other Investments	$ 81

Investments - Summary of Amount

Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Gross realized gains on securities	$ 38	$ 64	$ 111
Gross realized losses on securities	(12)	(5)	(7)
Foreign exchange (losses) gains	(1)	12	(7)
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal)	0	0	0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery	0	0	(3)
Net realized (losses) gains	22	81	108
Gain (Loss) on Investments [Member]
Investment [Line Items]
Foreign exchange (losses) gains	$ (4)	$ 22	$ 7

Investments - Summary of Roll-F

Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 12	$ 67
Additions for credit impairments recognized on:
Securities not previously impaired	0	1
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(1)	(56)
Balance at end	$ 12	$ 12
Debt Securities, Available-for-sale, Allowance for Credit Loss			$ 0

Investments - Summary of Source

Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 8	$ 6
Market Value of Secured Note Collateral	178	197
Restricted Cash	13	55	$ 19
Ambac Assurance [Member]
Investment [Line Items]
Restricted Cash	$ 9	$ 55

Investments - Summary of Fair V

Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 1,164
Standard & Poor's, B- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		186
Standard & Poor's, B- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		535
Standard & Poor's, B- Rating [Member] \| Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		442
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 6	11
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	6	11
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 0	0
Ambac Assurance Corporation [Member] \| Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,153
Ambac Assurance Corporation [Member] \| Standard & Poor's, B- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		176
Ambac Assurance Corporation [Member] \| Standard & Poor's, B- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		535
Ambac Assurance Corporation [Member] \| Standard & Poor's, B- Rating [Member] \| Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 442

Investments - Summary of Fair_2

Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail2) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 1,164
Standard & Poor's, B- Rating [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,153
Standard & Poor's, B- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		186
Standard & Poor's, B- Rating [Member] \| Municipal Bonds [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		176
Standard & Poor's, B- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		535
Standard & Poor's, B- Rating [Member] \| Corporate Debt Securities [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		535
Standard & Poor's, B- Rating [Member] \| Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		442
Standard & Poor's, B- Rating [Member] \| Mortgage and Asset-Backed Securities [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		442
Standard & Poor's, BBB- Rating [Member] \| National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 6	11
Standard & Poor's, BBB- Rating [Member] \| Municipal Bonds [Member] \| National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	6	11
Standard & Poor's, BBB- Rating [Member] \| Corporate Debt Securities [Member] \| National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0	$ 0
Standard & Poor's, C Rating \| Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Standard & Poor's, C Rating \| Municipal Bonds [Member] \| Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Standard & Poor's, C Rating \| Corporate Debt Securities [Member] \| Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0
Standard & Poor's, CCC+ Rating
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,273
Standard & Poor's, CCC+ Rating \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,266
Standard & Poor's, CCC+ Rating \| Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	327
Standard & Poor's, CCC+ Rating \| Municipal Bonds [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	320
Standard & Poor's, CCC+ Rating \| Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	465
Standard & Poor's, CCC+ Rating \| Corporate Debt Securities [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	465
Standard & Poor's, CCC+ Rating \| Mortgage and Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	481
Standard & Poor's, CCC+ Rating \| Mortgage and Asset-Backed Securities [Member] \| Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 481

Investments - Summary of Net In

Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities									$ 0	$ 24	$ (3)
Trading Securities, Realized Gain (Loss)									(18)	7	1
Fixed income securities										183	265
Investment expense									(6)	(6)	(7)
Securities available-for-sale and short-term									103	196	271
Net investment income	$ 53	$ 37	$ 52	$ (21)	$ 42	$ 45	$ 86	$ 55	122	227	273
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income									18	17	(4)
Available-for-sale Securities [Member]
Investment [Line Items]
Fixed income securities									103
Short-term Investments [Member]
Investment [Line Items]
Fixed income securities									5	17	11
Financing Receivable [Member]
Investment [Line Items]
Fixed income securities									1	1	1
Other Investments [Member]
Investment [Line Items]
Fixed income securities									$ 19	$ 32	$ 2

Derivative Instruments - Summar

Derivative Instruments - Summary of Gross Fair Values of Individual Derivative Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 93	$ 75
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	93	75
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	114	90
Derivative Liabilities Gross Amount Of Collateral Cash And Securities Pledged Not Offset		89
Fair value of derivative liabilities	113
Net Amount, Derivative Assets	93	75
Net Amount, Derivative Liabilities	1	1
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative Liability, Fair Value, Gross Liability	114	90
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Variable Interest Entity [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	41	52
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	41	52
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,835	1,657
Derivative Liabilities Gross Amount Of Collateral Cash And Securities Pledged Not Offset		0
Fair value of derivative liabilities	0
Net Amount, Derivative Assets	41	52
Net Amount, Derivative Liabilities	1,835	1,657
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative Liability, Fair Value, Gross Liability	1,835	1,657
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Other Credit Derivatives [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	0	0
Net Amount, Derivative Liabilities	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	93	75
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	93	75
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	114	90
Fair value of derivative liabilities	113	89
Net Amount, Derivative Assets	93	75
Net Amount, Derivative Liabilities	1	1
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0
Derivative Liability, Fair Value, Gross Liability	114	89
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,835	1,657
Net Amount, Derivative Liabilities	1,835	1,657
Derivative Liability, Fair Value, Gross Liability	1,835	1,657
Currency Swaps [Member] \| Variable Interest Entity [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	41	52
Derivative Asset, Fair Value, Amount Offset Against Collateral	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	41	52
Net Amount, Derivative Assets	$ 41	52
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ 0

Derivative Instruments - Additi

Derivative Instruments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	$ 113	$ 89
Derivative, Collateral, Right to Reclaim Cash	1	36
Collateral Already Posted, Aggregate Fair Value	130	109
AFS [Member] \| Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	195	332
AFS [Member] \| Interest Rate Swap One [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 726	$ 1,261

Derivative Instruments - Summ_2

Derivative Instruments - Summary of Location and Amount of Gains and Losses of Derivative Contracts (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 12	$ 7	$ 2	$ (70)	$ 12	$ (10)	$ (35)	$ (16)	$ (50)	$ (50)	$ 7
Derivative, Gain (Loss) on Derivatives, Total Net									(193)	(82)	512
Net gains (losses) on derivative contracts [Domain] \| Credit Derivatives [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									0	2	(1)
Net gains (losses) on derivative contracts [Domain] \| Interest Rate Swaps [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									(9)	(6)	1
Net gains (losses) on derivative contracts [Domain] \| Futures Contracts [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									(41)	(45)	7
Income Loss On Variable Interest Entities [Member] \| Variable Interest Entity [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									(144)	(32)	505
Income Loss On Variable Interest Entities [Member] \| Interest Rate Swaps [Member] \| Variable Interest Entity [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									(138)	(20)	493
Income Loss On Variable Interest Entities [Member] \| Currency Swaps [Member] \| Variable Interest Entity [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net									$ (6)	$ (12)	$ 11

Derivative Instruments - Summ_3

Derivative Instruments - Summary of Gross Principal Notional Outstanding for CDS Contracts (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Member] \| Interest Rate Swap [Member]
Ambac Rating
Derivative, Notional Amount	$ 1,233	$ 1,194
Variable Interest Entity [Member] \| Interest Rate Swap One [Member]
Ambac Rating
Derivative, Notional Amount	1,151	1,176
Variable Interest Entity [Member] \| Currency Swaps [Member]
Ambac Rating
Derivative, Notional Amount	308	329
Variable Interest Entity [Member] \| Other Credit Derivatives [Member]
Ambac Rating
Derivative, Notional Amount	0	9
Other Credit Derivatives [Member]
Ambac Rating
Derivative, Notional Amount	$ 257	$ 280

Derivative Instruments - Summ_4

Derivative Instruments - Summarize Information by Major Category of CDS Contracts (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Summary of information by major category of CDS contracts
Net derivative liabilities at fair value	$ 113
Other Credit Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative, Notional Amount	$ 257	$ 280

Derivative Instruments - Summ_5

Derivative Instruments - Summary of Notional Amounts of AFS's Trading Derivative Products (Detail) - AFS [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative, Notional Amount	$ 195	$ 332
Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative, Notional Amount	726	1,261
US Treasury Securities
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative, Notional Amount	$ 240	$ 755

Financial Guarantee Insuranc_14

Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Details) $ in Millions	Dec. 31, 2020USD ($)
Premiums Receivable, Gross	$ 387
Public Finance [Member]
Premiums Receivable, Gross	185
Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	168
Surveillance Category One [Member]
Premiums Receivable, Gross	302
Surveillance Category One [Member] \| Public Finance [Member]
Premiums Receivable, Gross	157
Surveillance Category One [Member] \| Housing Revenue [Member] \| Public Finance [Member]
Premiums Receivable, Gross	$ 155

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 19, 2021	Aug. 03, 2018	Feb. 12, 2018	May 01, 2013
Debt Instrument [Line Items]
Tier 2 Note Security in RMBS Litigation Settlement Proceeds							$ 1,600
Long-term debt		$ 2,739	$ 2,822
One Time Interest Payment on Surplus Notes							14
Junior surplus note of Ambac Assurance Segregated Account								$ 350
AMPS Exchange Total Surplus Notes Issued						$ 213
Net realized gains (losses) on extinguishment of debt		0	0	$ 3
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months		531
Long-term Debt, Maturities, Repayments of Principal in Year Two		0
Long-term Debt, Maturities, Repayments of Principal in Year Three		1,641
Long-term Debt, Maturities, Repayments of Principal in Year Four		0
Long-term Debt, Maturities, Repayments of Principal in Year Five		0
Paydown of Ambac Note		121	178	$ 214
Long-term Debt, Maturities, Repayments of Principal after Year Five		$ 712
Ambac Note Post Exit From Rehabilitation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		7.60%
Paydown of Ambac Note		$ (121)	(178)
Tier 2 Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		8.50%
Debt Instrument, Interest Rate, Effective Percentage		9.90%
Debt Instrument, Face Amount		$ 306
Paid-in-Kind Interest		$ 66	41
5.1% Surplus Notes, General Account, Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		10.10%
5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		19.50%
One State Street [Member] \| 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 15	15
Debt Instrument Principal Amount Subject To Repurchase		0
Extinguishment of Debt, Amount	$ 2	$ 0
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member] \| 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		8.40%
Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt		$ 2,739	2,822
Long-term Debt, Gross		2,884	2,980
Debt Instrument, Unamortized Discount (Premium), Net		(145)	(159)
Ambac Assurance Corporation [Member] \| 5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt		247	252
Long-term Debt, Gross		365	365
Debt Instrument, Unamortized Discount (Premium), Net		(118)	(113)
Ambac Assurance Corporation [Member] \| Ambac Note Post Exit From Rehabilitation [Member]
Debt Instrument [Line Items]
Long-term debt		1,641	1,763
Long-term Debt, Gross		1,641	1,763
Debt Instrument, Unamortized Discount (Premium), Net		0	0
Ambac Assurance Corporation [Member] \| Tier 2 Notes [Member]
Debt Instrument [Line Items]
Long-term debt		306	278
Long-term Debt, Gross		306	281
Debt Instrument, Unamortized Discount (Premium), Net		0	(4)
Ambac Assurance Corporation [Member] \| 5.1% Surplus Notes, General Account, Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt		531	517
Long-term Debt, Gross		531	531
Debt Instrument, Unamortized Discount (Premium), Net		$ 0	(14)
Ambac Assurance Corporation [Member] \| 5.1% Surplus Notes, General Account, Due 2020 [Member] \| Corolla Notes
Debt Instrument [Line Items]
Long-term Debt Issued in Exchange Transaction					$ 267
Ambac Assurance Corporation [Member] \| 5.1% Surplus Notes, General Account, Due 2020 [Member] \| Corolla Notes and Lease Reducing
Debt Instrument [Line Items]
Long-term Debt Issued in Exchange Transaction					$ 279
Ambac UK [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		0.00%
Long-term debt		$ 14	13
Long-term Debt, Gross		41	41
Debt Instrument, Unamortized Discount (Premium), Net		$ (27)	$ (28)
Debt Instrument, Interest Rate, Effective Percentage		7.40%
5.1% Junior Surplus Notes Due 2020 [Member] \| Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%	5.10%
5.1 % Surplus Notes Due 2020 [Member] \| Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%	5.10%
5.1 % Surplus Notes Due 2020 [Member] \| Ambac Assurance Corporation [Member] \| Corolla Notes
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					5.10%
Consolidated Variable Interest Entities [Member]
Debt Instrument [Line Items]
Unpaid Principal Amount Of Fixed Rate Debt Accounted For Under Fair Value Option		$ 3,927	$ 3,990
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months		0
Long-term Debt, Maturities, Repayments of Principal in Year Two		0
Long-term Debt, Maturities, Repayments of Principal in Year Three		0
Long-term Debt, Maturities, Repayments of Principal in Year Four		0
Long-term Debt, Maturities, Repayments of Principal in Year Five		83
Long-term Debt, Maturities, Repayments of Principal after Year Five		$ 3,844
Consolidated Variable Interest Entities [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		0.00%	0.00%
Consolidated Variable Interest Entities [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		7.93%	7.93%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Interest Rate on Ambac Note Post Exit From Rehabilitation		5.00%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Interest Rate on Ambac Note Post Exit From Rehabilitation		1.00%
Ambac [Member]
Debt Instrument [Line Items]
One Time Interest Payment on Surplus Notes							$ 3
Long-term Debt [Member]
Debt Instrument [Line Items]
Gain Contingency, Unrecorded Amount		$ 1,400

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2011
Tax Credit Carryforward [Line Items]
Deferred Tax Liabilities, Net		$ 24,000	$ 32,000
Impact of AMT Repeal on Current Taxes	$ 29,581
Net Impact of New Tax Law	$ 1,886
NOL allocated amount		3,639,000		$ 3,440,000
AFG [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount		1,457,000
U. S. Federal Net Operating Tax [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount		$ 3,639,000
Minimum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
Minimum [Member] \| AFG [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
Maximum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2040
Maximum [Member] \| AFG [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date		Jan. 1, 2034

Income Taxes - Significant Port

Income Taxes - Significant Portions of Deferred Tax Liabilities and Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Deferred Income Tax Assets and Liabilities [Line Items]
Deferred Tax Assets, Net of Valuation Allowance	$ 128	$ 151
Deferred tax assets:
Net operating loss and capital carryforward	764	742
Loss reserves	218	148
Deferred Tax Assets, Debentures	22	29
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	9	7
Other	5	1
Sub total deferred tax assets	1,019	927
Valuation allowance	891	777
Insurance intangible	78	90
Variable interest entities	13	12
Investments	22	32
Unearned premiums and credit fees	32	42
Other	7	8
Deferred Tax Liabilities, Gross, Noncurrent	152	183
Deferred Tax Liabilities, Net	$ 24	$ 32

Income Taxes Income Taxes - Pro

Income Taxes Income Taxes - Provision for Income Taxes Charged To Income From Continuing Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Provision for Income Taxes [Line Items]
Current Federal Tax Expense (Benefit)	$ 0	$ 0	$ (2)
Current Income Tax Expense (Benefit)	8	34	0
Deferred Income Tax Expense (Benefit)	(10)	(1)	5
Income tax expense (benefit)	(3)	32	5
Deferred Foreign Income Tax Expense (Benefit)	(10)	(1)	5
Current State and Local Tax Expense (Benefit)	0	(3)	2
Current Foreign Tax Expense (Benefit)	$ 8	$ 37	$ (1)

Income Taxes Income Taxes - Eff

Income Taxes Income Taxes - Effect of Income Taxes on Net Income and Stockholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense (Benefit) [Roll Forward]
Income tax expense (benefit)	$ (3)	$ 32	$ 5
Net Unrealized Losses On Securities Tax Effect	3	14	12
Valuation Allowances And Reserves Charged To Equity	(3)	(23)	9
Income Tax Effects Allocated Directly to Equity	1	(8)	3
Unrealized Loss on FX Tax Effect	0	0	0
Income Tax Effects Allocated Directly to Equity, Other	$ (1)	$ 24	$ 8

Income Taxes Income Taxes - E_2

Income Taxes Income Taxes - Effective Tax Rates Differing From Prevailing Federal Corporate Income Tax Rates (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Effective Tax Rates [Line Items]
Tax on income from continuing operations at statutory rate									$ (92)	$ (38)	$ 57
Tax on income from continuing operations at statutory rate,tax rate									21.00%	21.00%	21.00%
Tax-exempt interest									$ (2)	$ (3)	$ (7)
Tax-exempt interest, tax rate									0.40%	1.80%	(2.50%)
Derivative, Gain (Loss) on Derivative, Net	$ 12	$ 7	$ 2	$ (70)	$ 12	$ (10)	$ (35)	$ (16)	$ (50)	$ (50)	$ 7
Valuation allowance									$ 113	$ 8	$ 5
Valuation allowance, tax rate									(25.60%)	(4.40%)	1.90%
Foreign taxes									$ 6	$ 40	$ 10
Foreign taxes, tax rate									(1.40%)	(22.10%)	3.90%
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount									$ (29)	$ 28	$ (60)
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent									6.70%	(15.30%)	(22.00%)
Other, net									$ 0	$ (2)	$ 1
Other, net, tax rate									0.00%	1.30%	0.40%
Income tax expense (benefit)									$ (3)	$ 32	$ 5
Effective Income Tax Rate Reconciliation, Percent									0.70%	(17.70%)	2.00%
Income Tax Reconciliation Tax Bankruptcy Adjustments									$ 0	$ 0	$ (2)
Effective income Tax Reconciliation Tax Bankruptcy Adjustments									0.00%	0.00%	(0.70%)
Variable Interest Entity, Primary Beneficiary [Member] \| Income Loss On Variable Interest Entities [Member]
Reconciliation of Effective Tax Rates [Line Items]
Derivative, Gain (Loss) on Derivative, Net									$ (144)	$ (32)	$ 505
Variable Interest Entity, Primary Beneficiary [Member] \| Currency Swaps [Member] \| Income Loss On Variable Interest Entities [Member]
Reconciliation of Effective Tax Rates [Line Items]
Derivative, Gain (Loss) on Derivative, Net									(6)	(12)	11
Variable Interest Entity, Primary Beneficiary [Member] \| Interest Rate Swaps [Member] \| Income Loss On Variable Interest Entities [Member]
Reconciliation of Effective Tax Rates [Line Items]
Derivative, Gain (Loss) on Derivative, Net									$ (138)	$ (20)	$ 493

Income Taxes Income Taxes - Rec

Income Taxes Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized Tax Benefits	$ 0	$ 0	$ 0
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	0	0	0
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	0	0	0
Unrecognized Tax Benefits	$ 0	$ 0	$ 0

Income Taxes Income Taxes - NOL

Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2011
Net Income Loss Reconciliation [Line Items]
Impact of AMT Repeal on Current Taxes		$ 29,581
Change in Impact of AMT Repeal on Current Taxes	$ 1,902
AMT Related to Filing of 2017 Tax Return		5,536
AMT Receivable Resulting from TCJA				$ 37,019
Net Impact of New Tax Law		$ 1,886
NOL allocated amount			$ 3,639,000		$ 3,440,000
Amount of NOL Reallocated from AAC to AFG			210,000
Tolling Payments Paid by AAC to AFG			28,000
AFG [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount			$ 1,457,000
Minimum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2029
Minimum [Member] \| Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2029
Minimum [Member] \| AFG [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2029
Maximum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2040
Potential Tolling Payment From AAC to IRS			$ 8,000
Maximum [Member] \| Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2040
Maximum [Member] \| AFG [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Jan. 1, 2034

Income Taxes Schedule of Income

Income Taxes Schedule of Income Before Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Income Before Income Tax, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic									$ (441)	$ (174)	$ 264
Income (Loss) from Continuing Operations before Income Taxes, Foreign									1	(9)	8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ (12)	$ (108)	$ (33)	$ (287)	$ (111)	$ 69	$ (100)	$ (41)	$ (440)	$ (183)	$ 273

Employment Benefit Plans Postre

Employment Benefit Plans Postretirement Health Care and Other Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postemployment Benefits, Period Expense	$ 1	$ 3	$ 1
Unfunded Accumulated Postretirement Benefit Obligations Determined By Company	$ 10
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.25%	3.00%
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	$ 0
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	0
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	0
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	0
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	1
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	3
Defined Benefit Plan, Benefit Obligation	$ 5
Maximum [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year	5.20%
Minimum [Member]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year	4.50%

Employment Benefit Plans Saving

Employment Benefit Plans Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	3.00%
Defined Contribution Plan, Cost	$ 1	$ 1	$ 1
Minimum [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	100.00%
Maximum [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	50.00%
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	2.00%

Employment Benefit Plans Stock

Employment Benefit Plans Stock Compensation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based compensation expense [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2,096,292
Share Based Compensation Arrangement Shares Available for Future Issuance at Target	3,058,603
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ 0
Share-based Payment Arrangement, Expense	$ 11	$ 12	12
Total stock-based compensation (after-tax)	$ 11	$ 12	12
Common stock, shares authorized	130,000,000	130,000,000
2013 Plan
Share-based compensation expense [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	4,000,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	4,000,000
2020 plan
Share-based compensation expense [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,475,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,475,000
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 0	$ 0	0
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	3	4	6
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	8	8	6
Retained Earnings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 0	$ 0	$ 0

Employment Benefit Plans Stoc_2

Employment Benefit Plans Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	7 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	0	16,667
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	(16,667)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 0	$ 20.63
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	0
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 20.63
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	0 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	0 years

Employment Benefit Plans Restri

Employment Benefit Plans Restricted Stock Units (Details)	12 Months Ended
Employment Benefit Plans Restricted Stock Units (Details)	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment, Tax Withholding, Share-based Payment Arrangement \| $	$ 3,000,000	$ 3,000,000	$ 1,000,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Restricted Stock Units Vested Number	428,355	453,637
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	248,942
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	345,302	248,942
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment, Tax Withholding, Share-based Payment Arrangement \| $	$ 85,654
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	702,579
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	297,517
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	224,829
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(1,610)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	773,657	702,579
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value \| $ / shares	$ 18.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value \| $ / shares	17.36	$ 19.75	$ 16.35
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value \| $ / shares	17.59
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value \| $ / shares	19.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value \| $ / shares	$ 18.04	$ 18.19
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized \| $	$ 4,000,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value \| $	$ 4,000,000	$ 4,000,000	$ 1,000,000
Annual Bonus and Special Awards [Domain] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU Award Vesting Period	2
Long-term Incentive Compensation [Domain] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU Award Vesting Period	3

Employment Benefit Plans Perfor

Employment Benefit Plans Performance Units (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification			$ 0
Total Shareholder Return Modifier	10.00%
Payment, Tax Withholding, Share-based Payment Arrangement	$ 3,000,000	$ 3,000,000	$ 1,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	248,942
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	345,302	248,942
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Shareholder Return Modifier	2500.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Shareholder Return Modifier	7500.00%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
PSUUnrecognizedCompensationYearstoGo	1 year 8 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
PerformanceAwardsSettlementPeriod	75 days
Target Units	100.00%
Payment, Tax Withholding, Share-based Payment Arrangement	$ 70,340
PerformanceStockUnitsUnrecognizedCompensationExpense	$ 6,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	650,212
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	331,184
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	(184,896)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(6,071)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	844,514	650,212
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 17.98
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	19.99
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value	22.35
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value	18
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 18.09	$ 17.98
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Period Increase (Decrease)	54,085
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIncrease(Decrease)WeightedAverageGrantDateFairValue	$ 22.35
Performance Shares [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	0.00%
Performance Shares [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	220.00%
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
PSUUnrecognizedCompensationYearstoGo	1 year 7 months 6 days
Payment, Tax Withholding, Share-based Payment Arrangement	$ 85,654
PerformanceStockUnitsUnrecognizedCompensationExpense	$ 4,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	702,579
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	297,517
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	224,829
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(1,610)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	773,657	702,579
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 18.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	17.36	$ 19.75	$ 16.35
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value	17.59
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value	19.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 18.04	$ 18.19

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	Dec. 31, 2020USD ($)
Loss Contingencies [Line Items]
Operating Leases, Future Minimum Payments, Due Thereafter	$ 5

Quarterly Information (unaudi_3

Quarterly Information (unaudited) Quarterly Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gross Premiums Written	$ 1,000	$ 13,000	$ 1,000	$ 11,000	$ (2,000)	$ 13,000	$ 21,000	$ 3,000
Premiums Earned, Net, Financial Guarantee Insurance Contracts	18,000	15,000	11,000	10,000	20,000	10,000	8,000	28,000	$ 54,000	$ 66,000	$ 111,000
Net Investment Income	53,000	37,000	52,000	(21,000)	42,000	45,000	86,000	55,000	122,000	227,000	273,000
Realized Investment Gains (Losses)	2,000	2,000	10,000	8,000	9,000	18,000	36,000	17,000	22,000	81,000	108,000
Derivative, Gain (Loss) on Derivative, Net	12,000	7,000	2,000	(70,000)	12,000	(10,000)	(35,000)	(16,000)	(50,000)	(50,000)	7,000
Net realized gains (losses) on extinguishment of debt									0	0	3,000
Other income	1,000	2,000	0	0	1,000	141,000	(9,000)	1,000	3,000	134,000	5,000
Variable Interest Entity, Measure of Activity, Operating Income or Loss	3,000	0	0	3,000	7,000	11,000	3,000	16,000	5,000	38,000	3,000
Policyholder Benefits and Claims Incurred, Net, Financial Guarantee	(9,000)	(83,000)	(16,000)	(117,000)	(97,000)	(37,000)	133,000	12,000	(225,000)	(13,000)	224,000
Amortization of insurance intangible assets	16,000	14,000	14,000	13,000	15,000	17,000	226,000	36,000	57,000	295,000	107,000
Other Underwriting Expense	26,000	23,000	21,000	24,000	23,000	26,000	29,000	25,000	92,000	103,000	112,000
Interest Expense, Debt	50,000	50,000	58,000	63,000	66,000	67,000	67,000	68,000	222,000	269,000	242,000
Pre-tax (loss) income from continuing operations	(12,000)	(108,000)	(33,000)	(287,000)	(111,000)	69,000	(100,000)	(41,000)	(440,000)	(183,000)	273,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									(437,000)	(216,000)	267,000
Net Income (Loss) Attributable to Parent	$ (14,000)	$ (108,000)	$ (35,000)	$ (280,000)	$ (110,000)	$ 66,000	$ (128,000)	$ (43,000)	$ (437,000)	$ (216,000)	$ 186,000
Earnings Per Share, Basic	$ (0.31)	$ (2.33)	$ (0.77)	$ (6.07)	$ (2.40)	$ 1.44	$ (2.79)	$ (0.94)	$ (9.47)	$ (4.69)	$ 4.07
Earnings Per Share, Diluted	$ (0.31)	$ (2.33)	$ (0.77)	$ (6.07)	$ (2.40)	$ 1.41	$ (2.79)	$ (0.94)	$ (9.47)	$ (4.69)	$ 3.99

Schedule I - Summary Of Investm

Schedule I - Summary Of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Cost	$ 2,847
Investments	3,028
Available-for-sale Securities, Amortized Cost Basis	2,807
Other Investments	595	$ 478
Available-for-sale Securities	2,949	3,314
Investments	3,544	3,792
Short-term Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	492
Available-for-sale Securities	492
ShortTermInvestmentsMember, including Pledged
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	617
Available-for-sale Securities	617
Fixed Income Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,175
Available-for-sale Securities	2,317	2,577
Fixed Income Securities [Member] \| Municipal Bonds [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	321
Available-for-sale Securities	358	215
Fixed Income Securities [Member] \| Corporate Debt Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,059
Available-for-sale Securities	1,077	1,430
Fixed Income Securities [Member] \| Debt Security, Government, Non-US [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	97
Available-for-sale Securities	98	44
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	256
Available-for-sale Securities	302	248
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	74
Available-for-sale Securities	74	146
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	263
Available-for-sale Securities	303	287
Fixed Income Securities [Member] \| Short-term Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities		653
Fixed Income Securities [Member] \| U S Government Obligations [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	120
Available-for-sale Securities	121
Fixed Income Securities [Member] \| US Government Debt Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	105
Available-for-sale Securities	106	$ 156
Fixed Income Securities [Member] \| Corporate Obligations, Excluding Related Party Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Available-for-sale Securities, Amortized Cost Basis	592
Available-for-sale Securities	612
Fair Value Amount, Excluding Related Party Investments [Domain]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Investments	$ 3,028

Schedule IV - Reinsurance (Deta

Schedule IV - Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
Schedule IV - Reinsurance (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Direct Premiums Written	$ (1)	$ (28)	$ (24)
Ceded Premiums Written	(1)	31	17
Assumed Premiums Written	0	0	0
Premiums Written, Net	$ 0	$ (60)	$ (41)
Percentage of Amount Assumed to Net	0.00%	0.00%	0.00%