The Hanover Insurance Group, Inc. (THG) 10-Q2 Nov 232023 Q3 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2023	Oct. 31, 2023
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	0000944695
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		35,762,432
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	1-13754
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	04-3263626
Entity Address, Address Line One	440 Lincoln Street
Entity Address, City or Town	Worcester
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01653
City Area Code	508
Local Phone Number	855-1000
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Common Stock, $.01 Par Value [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 par value
Trading Symbol	THG
Security Exchange Name	NYSE
7 5/8% Senior Debentures Due 2025 [Member]
Document Information [Line Items]
Title of 12(b) Security	7 5/8% Senior Debentures due 2025
Trading Symbol	THG
Security Exchange Name	NYSE

Consolidated Statements of Inco

Consolidated Statements of Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues
Premiums	$ 1,431.1	$ 1,331.2	$ 4,222.8	$ 3,888.8
Net investment income	84.2	73	250.5	220.4
Net realized and unrealized investment gains (losses):
Net realized losses from sales and other	(0.9)	(0.1)	(1.9)	(16.3)
Net change in fair value of equity securities	(5.2)	(29.1)	(13.4)	(106.1)
Impairments on investments:
Credit related impairments		(1.4)	(6.2)	(1.5)
Losses on intent to sell securities		(14.3)	(10.3)	(14.8)
Impairments on investments		(15.7)	(16.5)	(16.3)
Total net realized and unrealized investment losses	(6.1)	(44.9)	(31.8)	(138.7)
Fees and other income	7.4	7	23.2	19.4
Total revenues	1,516.6	1,366.3	4,464.7	3,989.9
Losses and expenses
Losses and loss adjustment expenses	1,061.5	939.6	3,218.8	2,572.6
Amortization of deferred acquisition costs	296.6	277.1	878.1	809.3
Interest expense	8.5	8.5	25.6	25.5
Other operating expenses	150.9	140.6	451.3	423.8
Total losses and expenses	1,517.5	1,365.8	4,573.8	3,831.2
Income (loss) from continuing operations before income taxes	(0.9)	0.5	(109.1)	158.7
Income tax expense (benefit):
Current	(1.1)	12.9	(13.7)	72.5
Deferred	(8)	(13)	(21.6)	(42.5)
Total income tax expense (benefit)	(9.1)	(0.1)	(35.3)	30
Income (loss) from continuing operations	8.2	0.6	(73.8)	128.7
Discontinued operations (net of taxes):
Net income (loss)	$ 8.6	$ 0.5	$ (72.6)	$ 128.1
Basic:
Income (loss) from continuing operations	$ 0.23	$ 0.02	$ (2.07)	$ 3.62
Net income (loss) per share	$ 0.24	$ 0.01	$ (2.03)	$ 3.6
Weighted average shares outstanding	35.8	35.6	35.7	35.6
Diluted:
Income (loss) from continuing operations	$ 0.23	$ 0.02	$ (2.07)	$ 3.56
Net income (loss) per share	$ 0.24	$ 0.01	$ (2.03)	$ 3.54
Weighted average shares outstanding	36.1	36.1	35.7	36.1
Life [Member]
Discontinued operations (net of taxes):
Income (loss) from discontinued businesses		$ (0.1)		$ (0.6)
Basic:
Income (loss) from discontinued businesses, Basic		$ (0.01)		$ (0.02)
Diluted:
Income (loss) from discontinued businesses, Diluted		$ (0.01)		$ (0.02)
Chaucer [Member]
Discontinued operations (net of taxes):
Income (loss) from discontinued businesses	$ 0.4		$ 1.2
Basic:
Income (loss) from discontinued businesses, Basic	$ 0.01		$ 0.04
Diluted:
Income (loss) from discontinued businesses, Diluted	$ 0.01		$ 0.04

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 8.6	$ 0.5	$ (72.6)	$ 128.1
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income (Loss)	(113.7)	(245.1)	(87)	(885.9)
Having credit losses recognized in the Consolidated Statements of Income (Loss)	(0.5)	(2.2)	0.9	(5.7)
Total available-for-sale securities	(114.2)	(247.3)	(86.1)	(891.6)
Pension and postretirement benefits:
Net change in net actuarial loss	1.5	1.1	4.7	3.3
Long-duration insurance contracts:
Net change in market risk	3.3	4.3	2.6	17.4
Other comprehensive loss, net of tax	(109.4)	(241.9)	(78.8)	(870.9)
Comprehensive loss	$ (100.8)	$ (241.4)	$ (151.4)	$ (742.8)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Assets
Fixed maturities, at fair value (amortized cost of $8,535.5 and $8,294.5)	$ 7,618.3	$ 7,481.8
Equity securities, at fair value	123.5	241.9
Other investments	817.9	786.1
Total investments	8,559.7	8,509.8
Cash and cash equivalents	294.5	305
Accrued investment income	56.4	54.5
Premiums and accounts receivable, net	1,771.4	1,601.4
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,992.2	1,964.5
Deferred acquisition costs	631.4	604.8
Deferred income tax asset	240.4	199.2
Goodwill	178.8	178.8
Other assets	509.6	493
Assets of discontinued businesses	80.6	84.1
Total assets	14,315	13,995.1
Liabilities
Loss and loss adjustment expense reserves	7,329.8	7,012.6
Unearned premiums	3,188.1	2,954.2
Expenses and taxes payable	727.6	731.7
Reinsurance premiums payable	63.8	70.3
Debt	783	782.4
Liabilities of discontinued businesses	106.4	110.2
Total liabilities	12,198.7	11,661.4
Commitments and contingencies
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,931.4	1,913.1
Accumulated other comprehensive loss	(780.3)	(701.5)
Retained earnings	2,832.3	2,992.9
Treasury stock at cost (24.7 million and 24.9 million shares)	(1,867.7)	(1,871.4)
Total shareholders’ equity	2,116.3	2,333.7
Total liabilities and shareholders’ equity	$ 14,315	$ 13,995.1

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 8,535.5	$ 8,294.5
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	24,700,000	24,900,000

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Long Duration Insurance Contracts [Member]	Retained Earnings [Member]	Retained Earnings [Member] Adjustment [Member]	Retained Earnings [Member] Adjusted Balance [Member]	Accumulated Other Comprehensive Income (Loss), Net of Tax [Member]	Treasury Stock [Member]
Balance at beginning of period at Dec. 31, 2021		$ 0.6	$ 1,887.2	$ 184.9	$ (62.7)		$ 2,983.2		$ 2,987.3
Balance at beginning of period (ASU 2018-12) at Dec. 31, 2021						$ (13)		$ 4.1
Balance at beginning of period at Dec. 31, 2021											$ (1,848.3)
Net income (loss)	$ 128.1						128.1
Shares purchased at cost											(30.8)
Employee and director stock-based awards and other			17.3
Net depreciation on available-for-sale securities	(891.6)			(891.6)
Net amount recognized as net periodic benefit					3.3
Net change in market risk						17.4
Dividends to shareholders							(81.2)
Net shares reissued at cost under employee stock-based compensation plans											6.4
Balance at end of period at Sep. 30, 2022	2,304.9		1,904.5	(706.7)	(59.4)	4.4	3,034.2			$ (761.7)
Balance at end of period at Sep. 30, 2022											(1,872.7)
Balance at beginning of period at Jun. 30, 2022		0.6	1,896.8	(459.4)	(60.5)	(0.1)	3,061		3,061
Balance at beginning of period at Jun. 30, 2022											(1,862.4)
Net income (loss)	0.5						0.5
Shares purchased at cost											(10.4)
Employee and director stock-based awards and other			7.7
Net depreciation on available-for-sale securities	(247.3)			(247.3)
Net amount recognized as net periodic benefit					1.1
Net change in market risk						4.3
Dividends to shareholders							(27.3)
Net shares reissued at cost under employee stock-based compensation plans											0.1
Balance at end of period at Sep. 30, 2022	2,304.9		1,904.5	(706.7)	(59.4)	4.4	3,034.2			(761.7)
Balance at end of period at Sep. 30, 2022											(1,872.7)
Balance at beginning of period at Dec. 31, 2022	2,333.7	0.6	1,913.1	(641.4)	(64.1)	$ 4	2,992.9		2,992.9
Balance at beginning of period at Dec. 31, 2022	(1,871.4)										(1,871.4)
Net income (loss)	(72.6)						$ (72.6)
Shares purchased at cost											0
Employee and director stock-based awards and other			18.3
Net depreciation on available-for-sale securities	$ (86.1)			86.1
Net amount recognized as net periodic benefit					4.7
Accounting Standards Update [Extensible Enumeration]	ASU 2018-12					ASU 2018-12	ASU 2018-12
Net change in market risk						$ (2.6)
Dividends to shareholders							$ (88)
Net shares reissued at cost under employee stock-based compensation plans											3.7
Balance at end of period at Sep. 30, 2023	$ 2,116.3		1,931.4	(727.5)	(59.4)	6.6	2,832.3			(780.3)
Balance at end of period at Sep. 30, 2023	(1,867.7)										(1,867.7)
Balance at beginning of period at Jun. 30, 2023		$ 0.6	1,923	(613.3)	(60.9)	3.3	2,853		$ 2,853
Balance at beginning of period at Jun. 30, 2023											(1,867.8)
Net income (loss)	8.6						8.6
Employee and director stock-based awards and other			8.4
Net depreciation on available-for-sale securities	(114.2)			(114.2)
Net amount recognized as net periodic benefit					1.5
Net change in market risk						3.3
Dividends to shareholders							(29.3)
Net shares reissued at cost under employee stock-based compensation plans											0.1
Balance at end of period at Sep. 30, 2023	2,116.3		$ 1,931.4	$ (727.5)	$ (59.4)	$ 6.6	$ 2,832.3			$ (780.3)
Balance at end of period at Sep. 30, 2023	$ (1,867.7)										$ (1,867.7)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash Flows From Operating Activities
Net income (loss)	$ (72.6)	$ 128.1
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Net realized and unrealized investment losses	31.6	138.9
Net amortization and depreciation	5.5	10
Stock-based compensation expense	23.9	21.8
Amortization of defined benefit plan costs	5.9	4.1
Deferred income tax benefit	(21.5)	(42.4)
Change in deferred acquisition costs	(26.6)	(48.2)
Change in premiums receivable, net of reinsurance premiums payable	(176.6)	(131)
Change in loss, loss adjustment expense and unearned premium reserves	550.5	562.9
Change in reinsurance recoverable	(27.7)	(38.5)
Change in expenses and taxes payable	(9.6)	(29.3)
Other, net	(13.8)	(52.6)
Net cash provided by operating activities	269	523.8
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	503.4	799
Proceeds from disposals of equity securities and other investments	121.5	260.1
Purchase of fixed maturities	(744)	(1,429.2)
Purchase of equity securities and other investments	(57.6)	(92.2)
Capital expenditures	(9.7)	(13.9)
Net cash used in investing activities	(186.4)	(476.2)
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	4.3	10.6
Dividends paid to shareholders	(86.9)	(80.1)
Repurchases of common stock	0	(30.8)
Other financing activities	(10.5)	(13.4)
Net cash used in financing activities	(93.1)	(113.7)
Net change in cash and cash equivalents	(10.5)	(66.1)
Cash and cash equivalents, beginning of period	305	230.9
Cash and cash equivalents, end of period	$ 294.5	$ 164.8

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pay vs Performance Disclosure
Net Income (Loss)	$ 8.6	$ 0.5	$ (72.6)	$ 128.1

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Sep. 30, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Basis of Presentation and Princ

Basis of Presentation and Principles of Consolidation	9 Months Ended
Basis of Presentation and Principles of Consolidation	Sep. 30, 2023
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	1. Basis of Presentation and Principles of Consolidation The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and its subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted. The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America (“Citizens”), THG’s principal property and casualty companies; and other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 8 – “Segment Information.” The interim consolidated financial statements also include the Company’s discontinued operations, consisting of the Company’s former accident and health insurance business, as well as its former Chaucer business (U.K.-based Lloyd's entities). All intercompany accounts and transactions have been eliminated. The preparation of financial statements in conformity with U.S. GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of the Company’s management, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the nine months ended September 30, 2023, are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s 2022 Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 23, 2023.

New Accounting Pronouncements

New Accounting Pronouncements	9 Months Ended
New Accounting Pronouncements	Sep. 30, 2023
Accounting Standards Update and Change in Accounting Principle [Abstract]
New Accounting Pronouncements	2. New Accounting Pronouncements Recently Implemented Standards In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update 2018-12 (“ASU 2018-12”), Financial Services - Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts. For the Company's discounted accident and health reserves, ASU 2018-12 requires that the discount rate assumption be updated each quarter and be based on an upper-medium grade (low-credit-risk) fixed-income investment yield with a similar duration to the reserves. Previously, any changes in the reserve discount rate assumption was reflected in earnings and these changes will now be recognized in other comprehensive income (loss). This Update is effective for both interim and annual reporting periods beginning after December 15, 2022, and is applicable to the long-term care portion of the Company’s discontinued accident and health insurance reserves. The Company implemented this guidance effective January 1, 2023 , on a modified retrospective basis, and it did not have a material impact on its financial position or results of operations.

Investments

Investments	9 Months Ended
Investments	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	3. Investments A. Fixed maturities The amortized cost and fair value of available-for-sale fixed maturities were as follows: September 30, 2023 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 510.2 $ — $ 510.2 $ — $ 71.2 $ 439.0 Foreign government 2.2 — 2.2 — 0.1 2.1 Municipal 1,285.0 — 1,285.0 0.2 176.7 1,108.5 Corporate 4,115.2 ( 2.5 ) 4,112.7 3.6 361.3 3,755.0 Residential mortgage-backed 1,342.0 — 1,342.0 — 189.8 1,152.2 Commercial mortgage-backed 914.8 — 914.8 — 97.2 817.6 Other asset-backed 368.6 — 368.6 — 24.7 343.9 Total fixed maturities $ 8,538.0 $ ( 2.5 ) $ 8,535.5 $ 3.8 $ 921.0 $ 7,618.3 December 31, 2022 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 478.9 $ — $ 478.9 $ 0.2 $ 58.9 $ 420.2 Foreign government 2.3 — 2.3 — 0.1 2.2 Municipal 1,239.5 — 1,239.5 0.8 164.2 1,076.1 Corporate 4,066.4 ( 2.1 ) 4,064.3 3.3 337.7 3,729.9 Residential mortgage-backed 1,215.3 — 1,215.3 0.6 142.1 1,073.8 Commercial mortgage-backed 924.1 — 924.1 — 87.7 836.4 Other asset-backed 370.1 — 370.1 0.1 27.0 343.2 Total fixed maturities $ 8,296.6 $ ( 2.1 ) $ 8,294.5 $ 5.0 $ 817.7 $ 7,481.8 The Company deposits funds with various state and governmental authorities. For a discussion of the Company’s deposits with state and governmental authorities, see also Note 2 – “Investments” in the Notes to Consolidated Financial Statements in the Company’s 2022 Annual Report on Form 10-K. The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2023 Amortized Cost, Net of Allowance for Fair (in millions) Credit Losses Value Due in one year or less $ 459.2 $ 454.0 Due after one year through five years 2,641.5 2,488.1 Due after five years through ten years 2,521.2 2,123.7 Due after ten years 288.2 238.8 5,910.1 5,304.6 Mortgage-backed and other asset-backed securities 2,625.4 2,313.7 Total fixed maturities $ 8,535.5 $ 7,618.3 B. Fixed maturity securities in an unrealized loss position The following tables provide information about the Company’s available-for-sale fixed maturity securities that were in an unrealized loss position at September 30, 2023 and December 31, 2022, including the length of time the securities have been in an unrealized loss position: September 30, 2023 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 2.7 $ 85.0 $ 68.5 $ 339.7 $ 71.2 $ 424.7 Foreign governments — — 0.1 1.9 0.1 1.9 Municipal 9.6 174.5 167.1 902.6 176.7 1,077.1 Corporate 12.8 453.3 332.1 2,865.0 344.9 3,318.3 Residential mortgage-backed 10.4 312.8 179.4 839.4 189.8 1,152.2 Commercial mortgage-backed 0.7 42.4 96.5 770.2 97.2 812.6 Other asset-backed 0.3 69.7 24.4 269.6 24.7 339.3 Total investment grade 36.5 1,137.7 868.1 5,988.4 904.6 7,126.1 Below investment grade: Corporate 6.2 180.1 10.2 69.5 16.4 249.6 Total fixed maturities $ 42.7 $ 1,317.8 $ 878.3 $ 6,057.9 $ 921.0 $ 7,375.7 December 31, 2022 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 15.8 $ 237.3 $ 43.1 $ 160.7 $ 58.9 $ 398.0 Foreign governments 0.1 1.9 — — 0.1 1.9 Municipal 64.9 595.5 99.3 386.6 164.2 982.1 Corporate 202.2 2,786.2 118.2 489.8 320.4 3,276.0 Residential mortgage-backed 51.6 642.0 90.5 387.6 142.1 1,029.6 Commercial mortgage-backed 48.7 663.2 39.0 164.4 87.7 827.6 Other asset-backed 11.0 234.8 16.0 75.1 27.0 309.9 Total investment grade 394.3 5,160.9 406.1 1,664.2 800.4 6,825.1 Below investment grade: Corporate 13.5 253.2 3.8 17.3 17.3 270.5 Total fixed maturities $ 407.8 $ 5,414.1 $ 409.9 $ 1,681.5 $ 817.7 $ 7,095.6 The Company views gross unrealized losses on fixed maturities as non-credit related and through its assessment of unrealized losses has determined that these securities will recover, allowing the Company to realize the anticipated long-term economic value. The Company currently does not intend to sell, nor does it expect to be required to sell these securities before recovery of their amortized cost. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities. In determining impairments, the Company evaluates several factors and circumstances, including the issuer’s overall financial condition; the issuer’s credit and financial strength ratings; the issuer’s financial performance, including earnings trends and asset quality; any specific events which may influence the operations of the issuer; the general outlook for market conditions in the industry or geographic region in which the issuer operates; and the degree to which the fair value of an issuer’s securities is below the Company’s amortized cost. The Company also considers any factors that might raise doubt about the issuer’s ability to make contractual payments as they come due and whether the Company expects to recover the entire amortized cost basis of the security. C. Proceeds from sales The proceeds from sales of available-for-sale fixed maturities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 (in millions) Proceeds from sales $ 16.9 $ 35.2 $ 103.5 $ 308.9 Gross gains — — 0.9 1.8 Gross losses 1.3 0.5 3.3 21.3 D. Impairments For the three months ended September 30, 2023, impairments were not material. For the nine months ended September 30, 2023, the Company recognized net impairments of $ 16.5 million, primarily consisting of $ 10.3 million of intent to sell fixed maturity securities and $ 5.4 million of estimated credit losses on mortgage loans. For the three and nine months ended September 30, 2022, the Company recognized net impairments of $ 15.7 million and $ 16.3 million, respectively, consisting primarily of intent to sell fixed maturity securities. At September 30, 2023 and December 31, 2022, the allowance for credit losses on mortgage loans was $ 8.5 million and $ 3.2 million respectively, and the allowance for credit losses on available-for-sale debt securities was $ 2.5 million and $ 2.1 million, respectively. The following table provides a rollforward of the allowance for credit losses on mortgage loans: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Allowance for credit losses as of the beginning $ 8.5 $ 5.2 $ 3.2 $ 7.1 Additional credit losses on investments for which an — 0.1 5.4 0.1 Reductions for disposals — — — ( 0.1 ) Recoveries — ( 0.1 ) ( 0.1 ) ( 1.9 ) Allowance for credit losses as of the end of the period $ 8.5 $ 5.2 $ 8.5 $ 5.2 The methodology and significant inputs used to measure the amount of credit losses were as follows: Mortgage loans – the Company estimated losses by applying expected loss rates, which are based on historical data. Embedded in expected loss rates are mortgage risk ratings and risk factors associated with property type such as office, retail, lodging, multi-family and industrial. Risk ratings, based on property characteristics and metrics including the geographic market, are predominantly driven by estimates of loan-to-value and debt service coverage ratios. Ratings may be adjusted to reflect current conditions and to incorporate reasonable and supportable forecasts, such as volatility of cash flows and valuation. Fixed maturities, Corporate bonds – the Company utilized a financial model that derives expected cash flows based on probability-of-default factors by credit rating and asset duration, and loss-given-default factors based on security type. These factors are based on historical data provided by an independent third-party rating agency. In addition, other qualitative market data relevant to the realizability of contractual cash flows may be considered, including current conditions and reasonable and supportable forecasts. E. Equity securities The following table provides pre-tax net realized and unrealized gains (losses) on equity securities: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Net losses recognized during the period $ ( 5.2 ) $ ( 29.1 ) $ ( 13.4 ) $ ( 106.1 ) Less: net losses recognized on equity securities sold during ( 11.9 ) ( 0.1 ) ( 11.9 ) ( 8.8 ) Net unrealized gains (losses) recognized during the period on equity $ 6.7 $ ( 29.0 ) $ ( 1.5 ) $ ( 97.3 )

Fair Value

Fair Value	9 Months Ended
Fair Value	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value	4. Fair Value Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability, i.e., exit price, in an orderly transaction between market participants. The Company emphasizes the use of observable market data whenever available in determining fair value. Fair values presented for certain financial instruments are estimates which, in many cases, may differ significantly from the amounts that could be realized upon immediate liquidation. A hierarchy of the three broad levels of fair value is as follows, with the highest priority given to Level 1 as these are the most observable, and the lowest priority given to Level 3: Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 – Quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data, including model-derived valuations. Level 3 – Unobservable inputs that are supported by little or no market activity. When more than one level of input is used to determine fair value, the financial instrument is classified as Level 2 or Level 3 according to the lowest level input that has a significant impact on the fair value measurement. The following methods and assumptions were used to estimate the fair value of each class of financial instruments and have not changed since last year. Fixed Maturities Level 1 securities generally include U.S. Treasury issues and other securities that are highly liquid, and for which quoted market prices are available. Level 2 securities are valued using pricing for similar securities and pricing models that incorporate observable inputs including, but not limited to, yield curves and issuer spreads. Level 3 securities include issues for which little observable data can be obtained, primarily due to the illiquid nature of the securities, and for which significant inputs used to determine fair value are based on the Company’s own assumptions. The Company utilizes third-party pricing services for the valuation of the majority of its fixed maturity securities and receives one quote per security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. Since fixed maturities other than U.S. Treasury securities generally do not trade on a daily basis, the pricing services prepare estimates of fair value for those securities using pricing techniques based on a market approach. Inputs into the fair value pricing common to all asset classes include: benchmark U.S. Treasury security yield curves; reported trades of identical or similar fixed maturity securities; broker/dealer quotes of identical or similar fixed maturity securities and structural characteristics such as maturity date, coupon, mandatory principal payment dates, frequency of interest and principal payments, and optional redemption features. Inputs into the fair value applications that are unique by asset class include, but are not limited to: • U.S. government agencies – determination of direct versus indirect government support and whether any contingencies exist with respect to the timely payment of principal and interest. • Foreign government – estimates of appropriate market spread versus underlying related sovereign treasury curve(s) dependent on liquidity and direct or contingent support. • Municipals – overall credit quality, including assessments of the level and variability of: sources of payment such as income, sales or property taxes, levies or user fees; credit support such as insurance; state or local economic and political base; natural resource availability; and susceptibility to natural or man-made catastrophic events such as hurricanes, earthquakes or acts of terrorism. • Corporate fixed maturities – overall credit quality, including assessments of the level and variability of: economic sensitivity; liquidity; corporate financial policies; management quality; regulatory environment; competitive position; ownership; restrictive covenants; and security or collateral. • Residential mortgage-backed securities – estimates of prepayment speeds based upon: historical prepayment rate trends; underlying collateral interest rates; geographic concentration; vintage year; borrower credit quality characteristics; interest rate and yield curve forecasts; government or monetary authority support programs; tax policies; and delinquency/default trends; and in the case of non-agency collateralized mortgage obligations, severity of loss upon default and length of time to recover proceeds following default. • Commercial mortgage-backed securities – overall credit quality, including assessments of the value and supply/demand characteristics of: collateral type such as office, retail, residential, lodging, or other; geographic concentration by region, state, metropolitan statistical area and locale; vintage year; historical collateral performance including defeasance, delinquency, default and special servicer trends; and capital structure support features. • Other asset-backed securities – overall credit quality, including assessments of the underlying collateral type such as credit card receivables, automobile loan receivables and equipment lease receivables; geographic diversification; vintage year; historical collateral performance including delinquency, default and casualty trends; economic conditions influencing use rates and resale values; and contract structural support features. Generally, all prices provided by the pricing services, except actively traded securities with quoted market prices, are reported as Level 2. The Company holds privately placed fixed maturity securities and certain other fixed maturity securities that do not have an active market and for which the pricing services cannot provide fair values. The Company determines fair values for these securities using either matrix pricing, which utilizes the market approach, or broker quotes. The Company will use observable market data as inputs into the fair value techniques, as discussed in the determination of Level 2 fair values, to the extent it is available, but is also required to use a certain amount of unobservable judgment due to the illiquid nature of the securities involved. Unobservable judgment reflected in the Company’s matrix model accounts for estimates of additional spread required by market participants for factors such as issue size, credit stress, structural complexity, high bond coupon, or other unique features. These matrix-priced securities are reported as Level 2 or Level 3, depending on the significance of the impact of unobservable judgment on the security’s value. Additionally, the Company may obtain non-binding broker quotes, which are reported as Level 3. Equity Securities Level 1 consists of publicly traded securities, including exchange-traded funds, valued at quoted market prices. Level 2 includes securities that are valued using pricing for similar securities and pricing models that incorporate observable inputs. Level 3 consists of common or preferred stock of private companies for which observable inputs are not available. The Company utilizes a third-party pricing service for the valuation of the majority of its equity securities and receives one quote for each equity security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. The Company holds certain equity securities that have been issued by privately-held entities that do not have an active market and for which the pricing service cannot provide fair values. The Company estimates fair value for these securities based on prices from recent financing rounds, which may be adjusted for liquidity and other factors, or based on the issuer’s book value and market multiples, and reports them as Level 3. Additionally, the Company may obtain non-binding broker quotes, which are reported as Level 3. Other Investments Other investments primarily include mortgage participations and limited partnerships not subject to the equity method of accounting. The fair values of limited partnerships not subject to the equity method of accounting are based on the net asset value (“NAV”) provided by the general partner, adjusted for recent financial information, and are excluded from the fair value hierarchy. The estimated fair values of the financial instruments were as follows: September 30, 2023 December 31, 2022 Carrying Market Carrying Market (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 7,618.3 $ 7,618.3 $ 7,481.8 $ 7,481.8 Fair Value through Net Income: Equity securities 123.5 123.5 241.9 241.9 Other investments 125.0 125.0 131.8 131.8 Amortized Cost/Cost: Other investments 424.1 397.1 432.3 401.0 Cash and cash equivalents 294.5 294.5 305.0 305.0 Total financial instruments $ 8,585.4 $ 8,558.4 $ 8,592.8 $ 8,561.5 Financial Liabilities carried at: Amortized Cost: Debt $ 783.0 $ 703.4 $ 782.4 $ 713.9 The Company has processes designed to ensure that the values received from its third-party pricing services are accurately recorded, that the data inputs and valuation approaches and techniques utilized are appropriate and consistently applied, and that the assumptions are reasonable and consistent with the objective of determining fair value. The Company reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. For assets carried at fair value, the Company performs a review of the fair value hierarchy classifications and of prices received from its pricing services on a quarterly basis. Also, the Company reviews the portfolio pricing, including a process for which securities with changes in prices that exceed a defined threshold are verified to independent sources, if available. If upon review, the Company is not satisfied with the validity of a given price, a pricing challenge would be submitted to the applicable pricing service along with supporting documentation for its review. The Company does not adjust quotes or prices obtained from the pricing services unless the pricing service agrees with the Company’s challenge. During the first nine months of 2023 and 2022, the Company did not adjust any prices received from its pricing services. Changes in the observability of valuation inputs may result in a reclassification of certain financial assets or liabilities within the fair value hierarchy. As previously discussed, the Company utilizes third-party pricing services for the valuation of the majority of its fixed maturities and equity securities. The pricing services have indicated that they will only produce an estimate of fair value if there is objectively verifiable information to produce a valuation. If a pricing service discontinues pricing an investment, the Company will use observable market data to the extent it is available, but may also be required to make assumptions for market-based inputs that are unavailable due to market conditions. The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2023 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 439.0 $ 309.7 $ 129.3 $ — Foreign government 2.1 — 2.1 — Municipal 1,108.5 — 1,100.0 8.5 Corporate 3,755.0 — 3,754.9 0.1 Residential mortgage-backed, U.S. agency backed 1,017.7 — 1,017.7 — Residential mortgage-backed, non-agency 134.5 — 134.5 — Commercial mortgage-backed 817.6 — 810.7 6.9 Other asset-backed 343.9 — 343.9 — Total fixed maturities 7,618.3 309.7 7,293.1 15.5 Equity securities 123.5 115.9 — 7.6 Other investments 3.6 — — 3.6 Total investment assets at fair value $ 7,745.4 $ 425.6 $ 7,293.1 $ 26.7 December 31, 2022 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 420.2 $ 282.7 $ 137.5 $ — Foreign government 2.2 — 2.2 — Municipal 1,076.1 — 1,066.8 9.3 Corporate 3,729.9 — 3,729.8 0.1 Residential mortgage-backed, U.S. agency backed 974.0 — 974.0 — Residential mortgage-backed, non-agency 99.8 — 99.8 — Commercial mortgage-backed 836.4 — 828.9 7.5 Other asset-backed 343.2 — 343.2 — Total fixed maturities 7,481.8 282.7 7,182.2 16.9 Equity securities 241.9 234.4 — 7.5 Other investments 3.6 — — 3.6 Total investment assets at fair value $ 7,727.3 $ 517.1 $ 7,182.2 $ 28.0 Limited partnerships measured at fair value using the NAV based on an ownership interest in partners’ capital have not been included in the hierarchy tables. At September 30, 2023 and December 31, 2022, the fair values of these investments were $ 121.4 million and $ 128.2 million, respectively, less than 2 % of total investment assets. The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2023 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 294.5 $ 294.5 $ — $ — Other investments 397.1 — 6.2 390.9 Total financial instruments $ 691.6 $ 294.5 $ 6.2 $ 390.9 Liabilities: Debt $ 703.4 $ — $ 703.4 $ — December 31, 2022 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 305.0 $ 305.0 $ — $ — Other investments 401.0 — 6.0 395.0 Total financial instruments $ 706.0 $ 305.0 $ 6.0 $ 395.0 Liabilities: Debt $ 713.9 $ — $ 713.9 $ — The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Fixed Maturities (in millions) Municipal Corporate Commercial Total Equity Total Three Months Ended September 30, 2023 Balance July 1, 2023 $ 8.8 $ 0.1 $ 7.2 $ 16.1 $ 11.2 $ 27.3 Total losses: Included in other comprehensive loss - changes in ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Settlements ( 0.1 ) — ( 0.2 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.2 ) $ — $ ( 0.1 ) $ ( 0.3 ) $ — $ ( 0.3 ) Three Months Ended September 30, 2022 Balance July 1, 2022 $ 11.0 $ 0.1 $ 8.1 $ 19.2 $ 14.4 $ 33.6 Total losses: Included in net realized and unrealized investment gains (losses) — — — — ( 0.1 ) ( 0.1 ) Included in other comprehensive loss - changes in ( 0.3 ) — ( 0.4 ) ( 0.7 ) — ( 0.7 ) Settlements ( 0.7 ) — ( 0.1 ) ( 0.8 ) — ( 0.8 ) Balance September 30, 2022 $ 10.0 $ 0.1 $ 7.6 $ 17.7 $ 14.3 $ 32.0 Changes in net unrealized gains (losses) for the period $ ( 0.4 ) $ — $ ( 0.3 ) $ ( 0.7 ) $ — $ ( 0.7 ) Fixed Maturities (in millions) Municipal Corporate Commercial Total Equity Total Nine Months Ended September 30, 2023 Balance January 1, 2023 $ 9.3 $ 0.1 $ 7.5 $ 16.9 $ 11.1 $ 28.0 Total gains (losses): Included in net realized and unrealized investment — — — — 0.1 0.1 Included in other comprehensive loss - changes ( 0.1 ) — ( 0.1 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.7 ) — ( 0.5 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.1 ) $ — $ ( 0.1 ) $ ( 0.2 ) $ — $ ( 0.2 ) Nine Months Ended September 30, 2022 Balance January 1, 2022 $ 14.0 $ 0.1 $ 11.3 $ 25.4 $ 14.4 $ 39.8 Total losses: Included in net realized and unrealized investment — — — — ( 0.1 ) ( 0.1 ) Included in other comprehensive loss - changes ( 1.1 ) — ( 1.2 ) ( 2.3 ) — ( 2.3 ) Settlements ( 2.9 ) — ( 2.5 ) ( 5.4 ) — ( 5.4 ) Balance September 30, 2022 $ 10.0 $ 0.1 $ 7.6 $ 17.7 $ 14.3 $ 32.0 Changes in net unrealized gains (losses) for the period $ ( 1.2 ) $ — $ ( 1.1 ) $ ( 2.3 ) $ — $ ( 2.3 ) There were no transfers between Level 2 and Level 3, and there were no Level 3 liabilities held by the Company for the three or nine months ended September 30, 2023 and 2022. The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. Where discounted cash flows were used in the valuation of fixed maturities, the internally-developed discount rate was adjusted by the significant unobservable inputs shown in the table. September 30, 2023 December 31, 2022 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 8.5 6.1 - 6.8 % ( 6.6 %) $ 9.3 4.5 - 6.8 % ( 6.6 %) Corporate Discounted Discount for: 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) Commercial Discounted Discount for: 6.9 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) 7.5 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) Equity securities Market Net tangible asset 1.2 N/A 1.1 N/A Internal price based on financing round Discount for: 6.4 27.0 % ( 27.0 %) 6.4 27.0 % ( 27.0 %) Other Discounted Discount rate 3.6 17.2 % ( 17.2 %) 3.6 17.2 % ( 17.2 %) The weighted average of the unobservable inputs was weighted by the relative fair value of the securities to which the inputs were applied. Each unobservable input is based on the Company’s subjective opinion and therefore inherently contains a degree of uncertainty. Significant changes in any of the above inputs in isolation would result in a significantly lower or higher fair value measurement. There were no interrelationships between these inputs that might magnify or mitigate the effect of changes in unobservable inputs on the fair value measurement.

Income Taxes

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	5. Income Taxes Income tax expense (benefit) for the nine months ended September 30, 2023 and 2022 has been computed using estimated annual effective tax rates. These rates are revised, if necessary, at the end of each successive interim period to reflect current estimates of the annual effective tax rates. The tax provision was comprised of a U.S. federal income tax benefit of $ 35.3 million and an expense of $ 30.0 million for the nine months ended September 30, 2023 and 2022, respectively. The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction and various state jurisdictions, and have previously filed in foreign jurisdictions. The Company and its subsidiaries are subject to U.S. federal and state income tax examinations and foreign examinations for years after 2018. The audit of the Company’s 2019 Illinois business income tax was settled in June 2023.

Pension Plans

Pension Plans	9 Months Ended
Pension Plans	Sep. 30, 2023
Retirement Benefits [Abstract]
Pension Plans	6. Pension Plans The components of net periodic pension cost for the defined benefit pension plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2023 2022 (in millions) Pension Plans Interest cost $ 5.5 $ 3.9 Expected return on plan assets ( 5.5 ) ( 4.3 ) Recognized net actuarial loss 1.9 1.3 Net periodic pension cost $ 1.9 $ 0.9 Nine Months Ended September 30, 2023 2022 (in millions) Pension Plans Interest cost $ 16.4 $ 11.5 Expected return on plan assets ( 16.5 ) ( 12.9 ) Recognized net actuarial loss 5.8 3.9 Net periodic pension cost $ 5.7 $ 2.5

Other Comprehensive Loss

Other Comprehensive Loss	9 Months Ended
Other Comprehensive Loss	Sep. 30, 2023
Equity [Abstract]
Other Comprehensive Loss	7. Other Comprehensive Loss The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2023 2022 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized losses arising during period $ ( 144.3 ) $ 30.3 $ ( 114.0 ) $ ( 324.5 ) $ 68.1 $ ( 256.4 ) Net unrealized losses arising during period ( 0.7 ) 0.2 ( 0.5 ) ( 4.2 ) 0.9 ( 3.3 ) Amount of losses realized from sales and 0.9 ( 0.6 ) 0.3 0.3 ( 0.3 ) — Amount of credit-related impairments recognized — — — 1.4 ( 0.3 ) 1.1 Amount of additional impairment losses — — — 14.3 ( 3.0 ) 11.3 Net unrealized losses ( 144.1 ) 29.9 ( 114.2 ) ( 312.7 ) 65.4 ( 247.3 ) Pension and postretirement benefits: Amortization of net actuarial losses 1.9 ( 0.4 ) 1.5 1.4 ( 0.3 ) 1.1 Long-duration insurance contracts: Net change in market risk 4.2 ( 0.9 ) 3.3 5.4 ( 1.1 ) 4.3 Other comprehensive loss $ ( 138.0 ) $ 28.6 $ ( 109.4 ) $ ( 305.9 ) $ 64.0 $ ( 241.9 ) Nine Months Ended September 30, 2023 2022 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized losses arising during period for $ ( 120.8 ) $ 25.4 $ ( 95.4 ) $ ( 1,151.7 ) $ 241.8 $ ( 909.9 ) Net unrealized gains (losses) arising during period for 0.3 — 0.3 ( 10.5 ) 2.2 ( 8.3 ) Amount of losses realized from sales and 1.9 ( 1.6 ) 0.3 16.9 ( 4.6 ) 12.3 Amount of credit-related impairments 0.8 ( 0.2 ) 0.6 3.3 ( 0.7 ) 2.6 Amount of additional impairment losses 10.3 ( 2.2 ) 8.1 14.8 ( 3.1 ) 11.7 Net unrealized losses ( 107.5 ) 21.4 ( 86.1 ) ( 1,127.2 ) 235.6 ( 891.6 ) Pension and postretirement benefits: Amortization of net actuarial losses 5.9 ( 1.2 ) 4.7 4.1 ( 0.8 ) 3.3 Long-duration insurance contracts: Net change in market risk 3.3 ( 0.7 ) 2.6 22.0 ( 4.6 ) 17.4 Other comprehensive loss $ ( 98.3 ) $ 19.5 $ ( 78.8 ) $ ( 1,101.1 ) $ 230.2 $ ( 870.9 ) Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2023 2022 2023 2022 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Loss Components Comprehensive Loss Where Net Income (Loss) is Presented Net unrealized losses on $ ( 0.9 ) $ — $ ( 1.9 ) $ ( 16.7 ) Net realized losses from sales — ( 15.7 ) ( 11.1 ) ( 18.1 ) Impairments on investments ( 0.9 ) ( 15.7 ) ( 13.0 ) ( 34.8 ) Total before tax 0.6 3.6 4.0 8.4 Income tax benefit ( 0.3 ) ( 12.1 ) ( 9.0 ) ( 26.4 ) Continuing operations; net of tax — ( 0.3 ) — ( 0.2 ) Discontinued operations - loss from ( 0.3 ) ( 12.4 ) ( 9.0 ) ( 26.6 ) Net of tax Amortization of defined benefit pension ( 1.9 ) ( 1.4 ) ( 5.9 ) ( 4.1 ) Loss adjustment expenses and other (1) 0.4 0.3 1.2 0.8 Income tax benefit ( 1.5 ) ( 1.1 ) ( 4.7 ) ( 3.3 ) Continuing operations; net of tax Net change in market risk — 0.3 — 0.7 Loss from discontinued life businesses — — — ( 0.2 ) Income tax benefit (expense) — 0.3 — 0.5 Discontinuing operations; net of tax Total reclassifications for the period $ ( 1.8 ) $ ( 13.2 ) $ ( 13.7 ) $ ( 29.4 ) Expense reflected in income (1) The amount reclassified from accumulated other comprehensive income (loss) for the pension and postretirement benefits was allocated approximately 40 % to loss adjustment expenses and 60 % to other operating expenses for the three and nine months ended September 30, 2023 and 2022.

Segment Information

Segment Information	9 Months Ended
Segment Information	Sep. 30, 2023
Segment Reporting [Abstract]
Segment Information	8. Segment Information The Company’s primary business operations include insurance products and services provided through four operating segments: Core Commercial, Specialty, Personal Lines and Other. Core Commercial includes commercial multiple peril, commercial automobile, workers’ compensation, and other commercial coverages provided to small and mid-sized businesses. Specialty includes four divisions of business: Professional and Executive Lines, Specialty Property and Casualty (“Specialty P&C”), Marine, and Surety and Other. Specialty P&C includes coverages such as program business, which provides commercial insurance to markets with specialized coverage or risk management needs related to groups of similar businesses, specialty industrial and commercial property, excess and surplus lines and specialty general liability coverage. Personal Lines includes personal automobile, homeowners and other personal coverages. Included in the Other segment are Opus Investment Management, Inc., which markets investment management services to institutions, pension funds and other organizations; earnings on holding company assets; holding company and other expenses, including certain costs associated with retirement benefits due to the Company’s former life insurance employees and agents; and run-off voluntary assumed property and casualty pools and run-off direct asbestos and environmental businesses. The Company reports interest expense related to debt separately from the earnings of its operating segments. This consists primarily of interest on the Company’s senior and subordinated debentures. Management evaluates the results of its segments based on operating income (loss) before income taxes, excluding interest expense on debt. Operating income (loss) before income taxes excludes certain items which are included in net income (loss), such as net realized and unrealized investment gains and losses. Such gains and losses are excluded since they are determined by interest rates, financial markets and the timing of sales. Also, operating income (loss) before income taxes excludes net gains and losses on disposals of businesses, gains and losses related to the repayment of debt, discontinued operations, costs to acquire businesses, restructuring costs, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income (loss) before interest expense and income taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income (loss) before interest expense and income taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income (loss) attributable to the core operations of the business. However, operating income (loss) before income taxes should not be construed as a substitute for income (loss) before income taxes or income (loss) from continuing operations, and operating income (loss) should not be construed as a substitute for net income (loss). Summarized below is financial information with respect to the Company’s business segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2023 2022 2023 2022 Operating revenues: Core Commercial $ 557.1 $ 527.1 $ 1,658.2 $ 1,551.8 Specialty 340.9 319.9 1,011.6 931.0 Personal Lines 620.5 560.5 1,815.1 1,635.4 Other 4.2 3.7 11.6 10.4 Total 1,522.7 1,411.2 4,496.5 4,128.6 Net realized and unrealized investment losses ( 6.1 ) ( 44.9 ) ( 31.8 ) ( 138.7 ) Total revenues $ 1,516.6 $ 1,366.3 $ 4,464.7 $ 3,989.9 Operating income (loss) before interest expense Core Commercial: Underwriting income (loss) $ 5.3 $ ( 7.7 ) $ 1.0 $ 58.8 Net investment income 38.4 33.4 114.2 101.4 Other expense ( 0.6 ) ( 0.5 ) ( 0.8 ) ( 0.6 ) Core Commercial operating income 43.1 25.2 114.4 159.6 Specialty: Underwriting income 52.7 32.0 119.5 96.2 Net investment income 18.0 15.2 53.6 46.2 Other expense ( 0.4 ) ( 0.3 ) ( 0.1 ) ( 0.3 ) Specialty operating income 70.3 46.9 173.0 142.1 Personal Lines: Underwriting loss ( 127.3 ) ( 42.7 ) ( 422.4 ) ( 50.3 ) Net investment income 24.3 21.4 73.3 64.6 Other income 2.6 2.5 8.0 6.0 Personal Lines operating income (loss) ( 100.4 ) ( 18.8 ) ( 341.1 ) 20.3 Other: Underwriting income — — — — Net investment income 3.5 3.0 9.4 8.2 Other expense ( 2.8 ) ( 2.4 ) ( 8.2 ) ( 6.9 ) Other operating income 0.7 0.6 1.2 1.3 Operating income (loss) before interest expense 13.7 53.9 ( 52.5 ) 323.3 Interest on debt ( 8.5 ) ( 8.5 ) ( 25.6 ) ( 25.5 ) Operating income (loss) before income taxes 5.2 45.4 ( 78.1 ) 297.8 Non-operating items: Net realized and unrealized investment losses ( 6.1 ) ( 44.9 ) ( 31.8 ) ( 138.7 ) Other non-operating — — 0.8 ( 0.4 ) Income (loss) from continuing operations before income taxes $ ( 0.9 ) $ 0.5 $ ( 109.1 ) $ 158.7 The following table provides identifiable assets for the Company’s segments and discontinued operations: (in millions) September 30, 2023 December 31, 2022 Property and Casualty $ 14,234.4 $ 13,911.0 Assets of discontinued businesses 80.6 84.1 Total $ 14,315.0 $ 13,995.1 The Company reviews the assets of its insurance subsidiaries collectively and does not allocate them among the Core Commercial, Specialty, Personal Lines and Other segments.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
Stock-based Compensation	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Stock-based Compensation	9. Stock-based Compensation As of September 30, 2023, there were 2,940,189 shares available for grant under The Hanover Insurance Group 2022 Long-Term Incentive Plan. On May 9, 2023, the shareholders approved The Hanover Insurance Group 2023 Employee Stock Purchase plan (the “ESPP Plan”), authorizing the issuance of 1,250,000 shares under the plan, all of which were available for issuance as of September 30, 2023. This plan replaces the 2014 Employee Stock Purchase Plan. Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Stock-based compensation expense $ 8.6 $ 7.7 $ 23.9 $ 21.8 Tax benefit ( 1.8 ) ( 1.7 ) ( 5.0 ) ( 4.6 ) Stock-based compensation expense, net of taxes $ 6.8 $ 6.0 $ 18.9 $ 17.2 Stock Options Information on the Company’s stock option activity for the nine months ended September 30, 2023 and 2022 is summarized below. Nine Months Ended September 30, 2023 2022 (in whole shares and dollars) Shares Weighted Average Shares Weighted Average Outstanding, beginning of period 1,080,852 $ 107.07 1,230,211 $ 99.14 Granted 135,019 140.01 140,339 139.51 Exercised ( 40,544 ) 92.39 ( 262,900 ) 86.82 Forfeited or cancelled ( 22,184 ) 125.15 ( 6,075 ) 116.44 Outstanding, end of period 1,153,143 111.09 1,101,575 107.13 Restricted Stock Units The Company currently issues time-based, market-based and performance-based restricted stock units to eligible employees, all of which generally vest after three years of continued employment. The following tables summarize activity information about employee restricted stock units: Nine Months Ended September 30, 2023 2022 (in whole shares and dollars) Shares Weighted Shares Weighted Time-based restricted stock units: Outstanding, beginning of period 378,256 $ 124.88 380,100 $ 117.60 Granted 149,509 138.35 147,438 138.46 Vested ( 132,086 ) 118.21 ( 120,715 ) 119.29 Forfeited ( 19,132 ) 131.14 ( 23,148 ) 122.94 Outstanding, end of period 376,547 132.25 383,675 124.77 Performance-based and market-based restricted stock units: Outstanding, beginning of period 119,163 $ 122.81 113,848 $ 115.92 Granted 50,115 137.90 47,954 140.36 Vested ( 44,781 ) 114.14 ( 39,338 ) 124.42 Forfeited ( 3,993 ) 109.19 ( 3,301 ) 120.66 Outstanding, end of period 120,504 132.76 119,163 122.81 In the first nine months of 2023 and 2022 , the Company granted market-based awards totaling 21,789 shares and 19,057 shares, respectively, to certain members of senior management, which are included in the table above as performance and market-based restricted stock activity. The vesting of these stock units is based on the relative total shareholder return (“TSR”) of the Company. This metric is generally based on relative TSR for a three-year period as compared to a pre-selected group of property and casualty companies. The fair value of market-based awards was estimated at the date of grant using a valuation model. These units have the potential to range from 0 % to 150 % of the shares disclosed. Included in the amount forfeited above in 2023 and 2022 are 2,836 shares and 1,282 shares, respectively, related to market-based awards that achieved a payout below 100 %. These awards were forfeited in the first quarter of 2023 and 2022, respectively. The Company also granted performance-based restricted stock units in 2023 and 2022 , totaling 28,326 shares and 28,897 shares, respectively, which are based upon the Company’s achievement of return on equity objectives. These units have the potential to range from 0 % to 150 % of the shares disclosed. Increases above the 100% target level are reflected as granted in the period after which performance-based stock unit goals are achieved. Decreases below the 100% target level are reflected as forfeited. Included in the amount granted above in 2023 and 2022 are 5,961 shares and 7,988 shares, respectively, related to performance-based awards that achieved a payout in excess of 100 %. These awards vested in the first quarter of 2023 and 2022, respectively.

Earnings Per Share and Sharehol

Earnings Per Share and Shareholders' Equity Transactions	9 Months Ended
Earnings Per Share and Shareholders' Equity Transactions	Sep. 30, 2023
Earnings Per Share [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	10. Earnings Per Share and Shareholders’ Equity Transactions The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2023 2022 2023 2022 Basic shares used in the calculation of earnings per share 35.8 35.6 35.7 35.6 Dilutive effect of securities: Employee stock options 0.1 0.2 — 0.3 Non-vested stock grants 0.2 0.3 — 0.2 Diluted shares used in the calculation of earnings per share 36.1 36.1 35.7 36.1 Per share effect of dilutive securities on income (loss) from $ — $ — $ — $ ( 0.06 ) Per share effect of dilutive securities on net income (loss) $ — $ — $ — $ ( 0.06 ) Diluted earnings per share for the three and nine months ended September 30, 2023 excludes 0.7 million and 0.6 million shares, respectively, issuable under the Company’s stock compensation plans because their effect would be antidilutive. Diluted earnings per share for the three and nine months ended September 30, 2022 excludes 0.1 million shares issuable under the Company’s stock compensation plans because their effect would be antidilutive. T he Board of Directors authorized a stock repurchase program which provides for aggregate repurchases of the Company’s common stock of up to $ 1.3 billion. Under this program, the Company had approximately $ 330 million available at September 30, 2023 . Under the repurchase authorization, the Company may repurchase, from time to time, common stock in amounts, at prices and at such times as the Company deems appropriate, subject to market conditions and other considerations. Repurchases may be executed using open market purchases, privately negotiated transactions, accelerated repurchase programs or other transactions. The Company is not required to purchase any specific number of shares or to make purchases by any certain date under this program.

Liabilities for Outstanding Cla

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2023
Insurance [Abstract]
Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	11. Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses Reserve Rollforward and Prior Year Development The Company regularly updates its reserve estimates as new information becomes available and further events occur which may impact the resolution of unsettled claims. Reserve adjustments are reflected in results of operations as adjustments to losses and loss adjustment expenses (“LAE”). Often these adjustments are recognized in periods subsequent to the period in which the underlying policy was written, and loss event occurred. These types of subsequent adjustments are described as “prior years’ loss reserves.” Such development can be either favorable or unfavorable to the Company’s financial results and may vary by line of business. In this section, all amounts presented include catastrophe losses and LAE, unless otherwise indicated. The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2023 2022 Gross reserve for losses and LAE, beginning of period $ 7,012.6 $ 6,447.6 Reinsurance recoverable on unpaid losses 1,748.6 1,693.8 Net reserve for losses and LAE, beginning of period 5,264.0 4,753.8 Net incurred losses and LAE in respect of losses occurring in: Current year 3,225.6 2,603.8 Prior years ( 6.8 ) ( 31.2 ) Total incurred losses and LAE 3,218.8 2,572.6 Net payments of losses and LAE in respect of losses occurring in: Current year 1,375.7 1,041.3 Prior years 1,523.8 1,226.2 Total payments 2,899.5 2,267.5 Net reserve for losses and LAE, end of period 5,583.3 5,058.9 Reinsurance recoverable on unpaid losses 1,746.5 1,715.1 Gross reserve for losses and LAE, end of period $ 7,329.8 $ 6,774.0 As a result of continuing trends in the Company’s business, reserves, including catastrophes, have been re-estimated for all prior accident years and were decreased by $ 6.8 million and $ 31.2 million in 2023 and 2022, respectively. 2023 For the nine months ended September 30, 2023 , net favorable loss and LAE development was $ 6.8 million, primarily as a result of favorable development of $ 43.1 million in Specialty, partially offset by net unfavorable development of $ 35.1 million in Personal Lines and, to a lesser extent, net unfavorable development of $ 1.2 million in Core Commercial. The favorable development in Specialty was primarily due to lower than expected losses and LAE of $ 28.6 million in the Professional and Executive division, primarily in accident years 2019 through 2022 and, to a lesser extent, lower than expected losses and LAE of $ 9.2 million in the surety line. The unfavorable development in Personal Lines was primarily due to higher than expected losses of $ 16.4 million within the personal automobile line, higher than expected losses of $ 11.4 million in the homeowners line, primarily in accident year 2022 due to Winter Storm Elliott and, to a lesser extent, higher than expected losses in personal umbrella. The higher than expected losses in the personal automobile line were primarily within bodily injury coverages in accident years 2018, 2019 and 2022, and within property damage coverages in accident years 2021 and 2022, partially offset by lower than expected losses within personal injury protection coverages in accident years 2021 and 2022. The net unfavorable development in Core Commercial was primarily due to higher than expected losses of $ 13.5 million within the commercial automobile line driven by higher bodily injury and personal injury protection losses in accident years 2014 through 2019 and, to a lesser extent, higher than expected losses in miscellaneous property and commercial umbrella coverages. The unfavorable development in Core Commercial was partially offset by lower than expected losses of $ 17.7 million within the workers’ compensation line, primarily in accident years 2013 through 2021, and within the commercial multiple peril line, primarily in accident year 2022, due to Winter Storm Elliott. 2022 For the nine months ended September 30, 2022 , net favorable loss and LAE development was $ 31.2 million, primarily as a result of favorable development of $ 24.2 million in Specialty and favorable development of $ 20.2 million in Core Commercial, partially offset by unfavorable development of $ 13.2 million in Personal Lines. The favorable development in Specialty was primarily due to lower than expected losses and LAE of $ 13.2 million in the Professional and Executive division, lower than expected losses of $ 11.5 million in the surety line, and lower than expected losses of $ 10.2 million in the marine line, partially offset by higher than expected losses of $ 10.5 million in the Specialty P&C division. The favorable development in Core Commercial was primarily due to lower than expected losses of $ 21.0 million in the workers’ compensation line primarily in accident years 2013 through 2018 and 2020, and lower than expected losses of $ 9.9 million in the commercial multiple peril line, partially offset by higher than expected losses and LAE of $ 11.5 million in the commercial automobile line. The unfavorable development in Personal Lines was due to higher than expected losses of $ 19.4 million in the homeowners line, primarily due to higher severity and longer cycle times in repair activity, primarily related to claims incurred in the fourth quarter of 2021.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Legal Proceedings The Company has been named a defendant in various legal proceedings arising in the normal course of business. In addition, the Company is involved, from time to time, in examinations, investigations and proceedings by governmental and self-regulatory agencies. The potential outcome of any such action or regulatory proceedings in which the Company has been named a defendant or the subject of an inquiry, examination or investigation, and its ultimate liability, if any, from such action or regulatory proceedings, is difficult to predict at this time. The ultimate resolutions of such proceedings are not expected to have a material effect on the Company's financial position, although they could have a material effect on the results of operations for a particular quarterly or annual period. Residual Markets The Company is required to participate in residual markets in various states, which generally pertain to high risk insureds, disrupted markets or lines of business or geographic areas where rates are regarded as excessive. The results of the residual markets are difficult to predict and may not be correlated with the Company’s own managed business, and are significant to both the personal and commercial automobile lines of business.

Subsequent Events

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2023
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events There were no subsequent events requiring adjustment to the financial statements and no additional disclosure required in the notes to the consolidated financial statements.

New Accounting Pronouncements (

New Accounting Pronouncements (Policies)	9 Months Ended
New Accounting Pronouncements (Policies)	Sep. 30, 2023
Accounting Standards Update and Change in Accounting Principle [Abstract]
New Accounting Pronouncements	New Accounting Pronouncements Recently Implemented Standards In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update 2018-12 (“ASU 2018-12”), Financial Services - Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts. For the Company's discounted accident and health reserves, ASU 2018-12 requires that the discount rate assumption be updated each quarter and be based on an upper-medium grade (low-credit-risk) fixed-income investment yield with a similar duration to the reserves. Previously, any changes in the reserve discount rate assumption was reflected in earnings and these changes will now be recognized in other comprehensive income (loss). This Update is effective for both interim and annual reporting periods beginning after December 15, 2022, and is applicable to the long-term care portion of the Company’s discontinued accident and health insurance reserves. The Company implemented this guidance effective January 1, 2023 , on a modified retrospective basis, and it did not have a material impact on its financial position or results of operations.

Investments (Tables)

Investments (Tables)	9 Months Ended
Investments (Tables)	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-sale Fixed Maturities Reconciliation	The amortized cost and fair value of available-for-sale fixed maturities were as follows: September 30, 2023 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 510.2 $ — $ 510.2 $ — $ 71.2 $ 439.0 Foreign government 2.2 — 2.2 — 0.1 2.1 Municipal 1,285.0 — 1,285.0 0.2 176.7 1,108.5 Corporate 4,115.2 ( 2.5 ) 4,112.7 3.6 361.3 3,755.0 Residential mortgage-backed 1,342.0 — 1,342.0 — 189.8 1,152.2 Commercial mortgage-backed 914.8 — 914.8 — 97.2 817.6 Other asset-backed 368.6 — 368.6 — 24.7 343.9 Total fixed maturities $ 8,538.0 $ ( 2.5 ) $ 8,535.5 $ 3.8 $ 921.0 $ 7,618.3 December 31, 2022 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 478.9 $ — $ 478.9 $ 0.2 $ 58.9 $ 420.2 Foreign government 2.3 — 2.3 — 0.1 2.2 Municipal 1,239.5 — 1,239.5 0.8 164.2 1,076.1 Corporate 4,066.4 ( 2.1 ) 4,064.3 3.3 337.7 3,729.9 Residential mortgage-backed 1,215.3 — 1,215.3 0.6 142.1 1,073.8 Commercial mortgage-backed 924.1 — 924.1 — 87.7 836.4 Other asset-backed 370.1 — 370.1 0.1 27.0 343.2 Total fixed maturities $ 8,296.6 $ ( 2.1 ) $ 8,294.5 $ 5.0 $ 817.7 $ 7,481.8
Investments Classified by Contractual Maturity Date	The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2023 Amortized Cost, Net of Allowance for Fair (in millions) Credit Losses Value Due in one year or less $ 459.2 $ 454.0 Due after one year through five years 2,641.5 2,488.1 Due after five years through ten years 2,521.2 2,123.7 Due after ten years 288.2 238.8 5,910.1 5,304.6 Mortgage-backed and other asset-backed securities 2,625.4 2,313.7 Total fixed maturities $ 8,535.5 $ 7,618.3
Schedule of Unrealized Loss on Investments	The following tables provide information about the Company’s available-for-sale fixed maturity securities that were in an unrealized loss position at September 30, 2023 and December 31, 2022, including the length of time the securities have been in an unrealized loss position: September 30, 2023 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 2.7 $ 85.0 $ 68.5 $ 339.7 $ 71.2 $ 424.7 Foreign governments — — 0.1 1.9 0.1 1.9 Municipal 9.6 174.5 167.1 902.6 176.7 1,077.1 Corporate 12.8 453.3 332.1 2,865.0 344.9 3,318.3 Residential mortgage-backed 10.4 312.8 179.4 839.4 189.8 1,152.2 Commercial mortgage-backed 0.7 42.4 96.5 770.2 97.2 812.6 Other asset-backed 0.3 69.7 24.4 269.6 24.7 339.3 Total investment grade 36.5 1,137.7 868.1 5,988.4 904.6 7,126.1 Below investment grade: Corporate 6.2 180.1 10.2 69.5 16.4 249.6 Total fixed maturities $ 42.7 $ 1,317.8 $ 878.3 $ 6,057.9 $ 921.0 $ 7,375.7 December 31, 2022 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 15.8 $ 237.3 $ 43.1 $ 160.7 $ 58.9 $ 398.0 Foreign governments 0.1 1.9 — — 0.1 1.9 Municipal 64.9 595.5 99.3 386.6 164.2 982.1 Corporate 202.2 2,786.2 118.2 489.8 320.4 3,276.0 Residential mortgage-backed 51.6 642.0 90.5 387.6 142.1 1,029.6 Commercial mortgage-backed 48.7 663.2 39.0 164.4 87.7 827.6 Other asset-backed 11.0 234.8 16.0 75.1 27.0 309.9 Total investment grade 394.3 5,160.9 406.1 1,664.2 800.4 6,825.1 Below investment grade: Corporate 13.5 253.2 3.8 17.3 17.3 270.5 Total fixed maturities $ 407.8 $ 5,414.1 $ 409.9 $ 1,681.5 $ 817.7 $ 7,095.6
Schedule of Realized Gain (Loss)	The proceeds from sales of available-for-sale fixed maturities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 (in millions) Proceeds from sales $ 16.9 $ 35.2 $ 103.5 $ 308.9 Gross gains — — 0.9 1.8 Gross losses 1.3 0.5 3.3 21.3
Schedule of Allowance for Credit Losses	The following table provides a rollforward of the allowance for credit losses on mortgage loans: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Allowance for credit losses as of the beginning $ 8.5 $ 5.2 $ 3.2 $ 7.1 Additional credit losses on investments for which an — 0.1 5.4 0.1 Reductions for disposals — — — ( 0.1 ) Recoveries — ( 0.1 ) ( 0.1 ) ( 1.9 ) Allowance for credit losses as of the end of the period $ 8.5 $ 5.2 $ 8.5 $ 5.2
Schedule of Pre-tax Net Realized and Unrealized Losses on Equity Securities	The following table provides pre-tax net realized and unrealized gains (losses) on equity securities: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Net losses recognized during the period $ ( 5.2 ) $ ( 29.1 ) $ ( 13.4 ) $ ( 106.1 ) Less: net losses recognized on equity securities sold during ( 11.9 ) ( 0.1 ) ( 11.9 ) ( 8.8 ) Net unrealized gains (losses) recognized during the period on equity $ 6.7 $ ( 29.0 ) $ ( 1.5 ) $ ( 97.3 )

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
Fair Value (Tables)	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	The estimated fair values of the financial instruments were as follows: September 30, 2023 December 31, 2022 Carrying Market Carrying Market (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 7,618.3 $ 7,618.3 $ 7,481.8 $ 7,481.8 Fair Value through Net Income: Equity securities 123.5 123.5 241.9 241.9 Other investments 125.0 125.0 131.8 131.8 Amortized Cost/Cost: Other investments 424.1 397.1 432.3 401.0 Cash and cash equivalents 294.5 294.5 305.0 305.0 Total financial instruments $ 8,585.4 $ 8,558.4 $ 8,592.8 $ 8,561.5 Financial Liabilities carried at: Amortized Cost: Debt $ 783.0 $ 703.4 $ 782.4 $ 713.9
Fair Value, Investment Assets Measured on Recurring Basis	The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2023 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 439.0 $ 309.7 $ 129.3 $ — Foreign government 2.1 — 2.1 — Municipal 1,108.5 — 1,100.0 8.5 Corporate 3,755.0 — 3,754.9 0.1 Residential mortgage-backed, U.S. agency backed 1,017.7 — 1,017.7 — Residential mortgage-backed, non-agency 134.5 — 134.5 — Commercial mortgage-backed 817.6 — 810.7 6.9 Other asset-backed 343.9 — 343.9 — Total fixed maturities 7,618.3 309.7 7,293.1 15.5 Equity securities 123.5 115.9 — 7.6 Other investments 3.6 — — 3.6 Total investment assets at fair value $ 7,745.4 $ 425.6 $ 7,293.1 $ 26.7 December 31, 2022 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 420.2 $ 282.7 $ 137.5 $ — Foreign government 2.2 — 2.2 — Municipal 1,076.1 — 1,066.8 9.3 Corporate 3,729.9 — 3,729.8 0.1 Residential mortgage-backed, U.S. agency backed 974.0 — 974.0 — Residential mortgage-backed, non-agency 99.8 — 99.8 — Commercial mortgage-backed 836.4 — 828.9 7.5 Other asset-backed 343.2 — 343.2 — Total fixed maturities 7,481.8 282.7 7,182.2 16.9 Equity securities 241.9 234.4 — 7.5 Other investments 3.6 — — 3.6 Total investment assets at fair value $ 7,727.3 $ 517.1 $ 7,182.2 $ 28.0
Estimated Fair Values of Financial Instruments Not Carried at Fair Value	The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2023 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 294.5 $ 294.5 $ — $ — Other investments 397.1 — 6.2 390.9 Total financial instruments $ 691.6 $ 294.5 $ 6.2 $ 390.9 Liabilities: Debt $ 703.4 $ — $ 703.4 $ — December 31, 2022 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 305.0 $ 305.0 $ — $ — Other investments 401.0 — 6.0 395.0 Total financial instruments $ 706.0 $ 305.0 $ 6.0 $ 395.0 Liabilities: Debt $ 713.9 $ — $ 713.9 $ —
Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Fixed Maturities (in millions) Municipal Corporate Commercial Total Equity Total Three Months Ended September 30, 2023 Balance July 1, 2023 $ 8.8 $ 0.1 $ 7.2 $ 16.1 $ 11.2 $ 27.3 Total losses: Included in other comprehensive loss - changes in ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Settlements ( 0.1 ) — ( 0.2 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.2 ) $ — $ ( 0.1 ) $ ( 0.3 ) $ — $ ( 0.3 ) Three Months Ended September 30, 2022 Balance July 1, 2022 $ 11.0 $ 0.1 $ 8.1 $ 19.2 $ 14.4 $ 33.6 Total losses: Included in net realized and unrealized investment gains (losses) — — — — ( 0.1 ) ( 0.1 ) Included in other comprehensive loss - changes in ( 0.3 ) — ( 0.4 ) ( 0.7 ) — ( 0.7 ) Settlements ( 0.7 ) — ( 0.1 ) ( 0.8 ) — ( 0.8 ) Balance September 30, 2022 $ 10.0 $ 0.1 $ 7.6 $ 17.7 $ 14.3 $ 32.0 Changes in net unrealized gains (losses) for the period $ ( 0.4 ) $ — $ ( 0.3 ) $ ( 0.7 ) $ — $ ( 0.7 ) Fixed Maturities (in millions) Municipal Corporate Commercial Total Equity Total Nine Months Ended September 30, 2023 Balance January 1, 2023 $ 9.3 $ 0.1 $ 7.5 $ 16.9 $ 11.1 $ 28.0 Total gains (losses): Included in net realized and unrealized investment — — — — 0.1 0.1 Included in other comprehensive loss - changes ( 0.1 ) — ( 0.1 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.7 ) — ( 0.5 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.1 ) $ — $ ( 0.1 ) $ ( 0.2 ) $ — $ ( 0.2 ) Nine Months Ended September 30, 2022 Balance January 1, 2022 $ 14.0 $ 0.1 $ 11.3 $ 25.4 $ 14.4 $ 39.8 Total losses: Included in net realized and unrealized investment — — — — ( 0.1 ) ( 0.1 ) Included in other comprehensive loss - changes ( 1.1 ) — ( 1.2 ) ( 2.3 ) — ( 2.3 ) Settlements ( 2.9 ) — ( 2.5 ) ( 5.4 ) — ( 5.4 ) Balance September 30, 2022 $ 10.0 $ 0.1 $ 7.6 $ 17.7 $ 14.3 $ 32.0 Changes in net unrealized gains (losses) for the period $ ( 1.2 ) $ — $ ( 1.1 ) $ ( 2.3 ) $ — $ ( 2.3 )
Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3	The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. Where discounted cash flows were used in the valuation of fixed maturities, the internally-developed discount rate was adjusted by the significant unobservable inputs shown in the table. September 30, 2023 December 31, 2022 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 8.5 6.1 - 6.8 % ( 6.6 %) $ 9.3 4.5 - 6.8 % ( 6.6 %) Corporate Discounted Discount for: 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) Commercial Discounted Discount for: 6.9 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) 7.5 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) Equity securities Market Net tangible asset 1.2 N/A 1.1 N/A Internal price based on financing round Discount for: 6.4 27.0 % ( 27.0 %) 6.4 27.0 % ( 27.0 %) Other Discounted Discount rate 3.6 17.2 % ( 17.2 %) 3.6 17.2 % ( 17.2 %)

Pension Plans (Tables)

Pension Plans (Tables)	9 Months Ended
Pension Plans (Tables)	Sep. 30, 2023
Retirement Benefits [Abstract]
Components of Net Periodic Pension Cost	The components of net periodic pension cost for the defined benefit pension plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2023 2022 (in millions) Pension Plans Interest cost $ 5.5 $ 3.9 Expected return on plan assets ( 5.5 ) ( 4.3 ) Recognized net actuarial loss 1.9 1.3 Net periodic pension cost $ 1.9 $ 0.9 Nine Months Ended September 30, 2023 2022 (in millions) Pension Plans Interest cost $ 16.4 $ 11.5 Expected return on plan assets ( 16.5 ) ( 12.9 ) Recognized net actuarial loss 5.8 3.9 Net periodic pension cost $ 5.7 $ 2.5

Other Comprehensive Loss (Table

Other Comprehensive Loss (Tables)	9 Months Ended
Other Comprehensive Loss (Tables)	Sep. 30, 2023
Equity [Abstract]
Changes in Other Comprehensive Income (Loss)	The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2023 2022 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized losses arising during period $ ( 144.3 ) $ 30.3 $ ( 114.0 ) $ ( 324.5 ) $ 68.1 $ ( 256.4 ) Net unrealized losses arising during period ( 0.7 ) 0.2 ( 0.5 ) ( 4.2 ) 0.9 ( 3.3 ) Amount of losses realized from sales and 0.9 ( 0.6 ) 0.3 0.3 ( 0.3 ) — Amount of credit-related impairments recognized — — — 1.4 ( 0.3 ) 1.1 Amount of additional impairment losses — — — 14.3 ( 3.0 ) 11.3 Net unrealized losses ( 144.1 ) 29.9 ( 114.2 ) ( 312.7 ) 65.4 ( 247.3 ) Pension and postretirement benefits: Amortization of net actuarial losses 1.9 ( 0.4 ) 1.5 1.4 ( 0.3 ) 1.1 Long-duration insurance contracts: Net change in market risk 4.2 ( 0.9 ) 3.3 5.4 ( 1.1 ) 4.3 Other comprehensive loss $ ( 138.0 ) $ 28.6 $ ( 109.4 ) $ ( 305.9 ) $ 64.0 $ ( 241.9 ) Nine Months Ended September 30, 2023 2022 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized losses arising during period for $ ( 120.8 ) $ 25.4 $ ( 95.4 ) $ ( 1,151.7 ) $ 241.8 $ ( 909.9 ) Net unrealized gains (losses) arising during period for 0.3 — 0.3 ( 10.5 ) 2.2 ( 8.3 ) Amount of losses realized from sales and 1.9 ( 1.6 ) 0.3 16.9 ( 4.6 ) 12.3 Amount of credit-related impairments 0.8 ( 0.2 ) 0.6 3.3 ( 0.7 ) 2.6 Amount of additional impairment losses 10.3 ( 2.2 ) 8.1 14.8 ( 3.1 ) 11.7 Net unrealized losses ( 107.5 ) 21.4 ( 86.1 ) ( 1,127.2 ) 235.6 ( 891.6 ) Pension and postretirement benefits: Amortization of net actuarial losses 5.9 ( 1.2 ) 4.7 4.1 ( 0.8 ) 3.3 Long-duration insurance contracts: Net change in market risk 3.3 ( 0.7 ) 2.6 22.0 ( 4.6 ) 17.4 Other comprehensive loss $ ( 98.3 ) $ 19.5 $ ( 78.8 ) $ ( 1,101.1 ) $ 230.2 $ ( 870.9 )
Reclassifications Out of Accumulated Other Comprehensive Loss	Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2023 2022 2023 2022 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Loss Components Comprehensive Loss Where Net Income (Loss) is Presented Net unrealized losses on $ ( 0.9 ) $ — $ ( 1.9 ) $ ( 16.7 ) Net realized losses from sales — ( 15.7 ) ( 11.1 ) ( 18.1 ) Impairments on investments ( 0.9 ) ( 15.7 ) ( 13.0 ) ( 34.8 ) Total before tax 0.6 3.6 4.0 8.4 Income tax benefit ( 0.3 ) ( 12.1 ) ( 9.0 ) ( 26.4 ) Continuing operations; net of tax — ( 0.3 ) — ( 0.2 ) Discontinued operations - loss from ( 0.3 ) ( 12.4 ) ( 9.0 ) ( 26.6 ) Net of tax Amortization of defined benefit pension ( 1.9 ) ( 1.4 ) ( 5.9 ) ( 4.1 ) Loss adjustment expenses and other (1) 0.4 0.3 1.2 0.8 Income tax benefit ( 1.5 ) ( 1.1 ) ( 4.7 ) ( 3.3 ) Continuing operations; net of tax Net change in market risk — 0.3 — 0.7 Loss from discontinued life businesses — — — ( 0.2 ) Income tax benefit (expense) — 0.3 — 0.5 Discontinuing operations; net of tax Total reclassifications for the period $ ( 1.8 ) $ ( 13.2 ) $ ( 13.7 ) $ ( 29.4 ) Expense reflected in income (1) The amount reclassified from accumulated other comprehensive income (loss) for the pension and postretirement benefits was allocated approximately 40 % to loss adjustment expenses and 60 % to other operating expenses for the three and nine months ended September 30, 2023 and 2022.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
Segment Information (Tables)	Sep. 30, 2023
Segment Reporting [Abstract]
Financial Information with Respect to Business Segments	Summarized below is financial information with respect to the Company’s business segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2023 2022 2023 2022 Operating revenues: Core Commercial $ 557.1 $ 527.1 $ 1,658.2 $ 1,551.8 Specialty 340.9 319.9 1,011.6 931.0 Personal Lines 620.5 560.5 1,815.1 1,635.4 Other 4.2 3.7 11.6 10.4 Total 1,522.7 1,411.2 4,496.5 4,128.6 Net realized and unrealized investment losses ( 6.1 ) ( 44.9 ) ( 31.8 ) ( 138.7 ) Total revenues $ 1,516.6 $ 1,366.3 $ 4,464.7 $ 3,989.9 Operating income (loss) before interest expense Core Commercial: Underwriting income (loss) $ 5.3 $ ( 7.7 ) $ 1.0 $ 58.8 Net investment income 38.4 33.4 114.2 101.4 Other expense ( 0.6 ) ( 0.5 ) ( 0.8 ) ( 0.6 ) Core Commercial operating income 43.1 25.2 114.4 159.6 Specialty: Underwriting income 52.7 32.0 119.5 96.2 Net investment income 18.0 15.2 53.6 46.2 Other expense ( 0.4 ) ( 0.3 ) ( 0.1 ) ( 0.3 ) Specialty operating income 70.3 46.9 173.0 142.1 Personal Lines: Underwriting loss ( 127.3 ) ( 42.7 ) ( 422.4 ) ( 50.3 ) Net investment income 24.3 21.4 73.3 64.6 Other income 2.6 2.5 8.0 6.0 Personal Lines operating income (loss) ( 100.4 ) ( 18.8 ) ( 341.1 ) 20.3 Other: Underwriting income — — — — Net investment income 3.5 3.0 9.4 8.2 Other expense ( 2.8 ) ( 2.4 ) ( 8.2 ) ( 6.9 ) Other operating income 0.7 0.6 1.2 1.3 Operating income (loss) before interest expense 13.7 53.9 ( 52.5 ) 323.3 Interest on debt ( 8.5 ) ( 8.5 ) ( 25.6 ) ( 25.5 ) Operating income (loss) before income taxes 5.2 45.4 ( 78.1 ) 297.8 Non-operating items: Net realized and unrealized investment losses ( 6.1 ) ( 44.9 ) ( 31.8 ) ( 138.7 ) Other non-operating — — 0.8 ( 0.4 ) Income (loss) from continuing operations before income taxes $ ( 0.9 ) $ 0.5 $ ( 109.1 ) $ 158.7
Identifiable Assets by Business Segment	The following table provides identifiable assets for the Company’s segments and discontinued operations: (in millions) September 30, 2023 December 31, 2022 Property and Casualty $ 14,234.4 $ 13,911.0 Assets of discontinued businesses 80.6 84.1 Total $ 14,315.0 $ 13,995.1

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
Stock-based Compensation (Tables)	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Compensation Cost and Related Tax Benefits	Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2023 2022 2023 2022 Stock-based compensation expense $ 8.6 $ 7.7 $ 23.9 $ 21.8 Tax benefit ( 1.8 ) ( 1.7 ) ( 5.0 ) ( 4.6 ) Stock-based compensation expense, net of taxes $ 6.8 $ 6.0 $ 18.9 $ 17.2
Summary of Stock Option Activity	Information on the Company’s stock option activity for the nine months ended September 30, 2023 and 2022 is summarized below. Nine Months Ended September 30, 2023 2022 (in whole shares and dollars) Shares Weighted Average Shares Weighted Average Outstanding, beginning of period 1,080,852 $ 107.07 1,230,211 $ 99.14 Granted 135,019 140.01 140,339 139.51 Exercised ( 40,544 ) 92.39 ( 262,900 ) 86.82 Forfeited or cancelled ( 22,184 ) 125.15 ( 6,075 ) 116.44 Outstanding, end of period 1,153,143 111.09 1,101,575 107.13
Summary of Activity Information about Employee Restricted Stock Units	The following tables summarize activity information about employee restricted stock units: Nine Months Ended September 30, 2023 2022 (in whole shares and dollars) Shares Weighted Shares Weighted Time-based restricted stock units: Outstanding, beginning of period 378,256 $ 124.88 380,100 $ 117.60 Granted 149,509 138.35 147,438 138.46 Vested ( 132,086 ) 118.21 ( 120,715 ) 119.29 Forfeited ( 19,132 ) 131.14 ( 23,148 ) 122.94 Outstanding, end of period 376,547 132.25 383,675 124.77 Performance-based and market-based restricted stock units: Outstanding, beginning of period 119,163 $ 122.81 113,848 $ 115.92 Granted 50,115 137.90 47,954 140.36 Vested ( 44,781 ) 114.14 ( 39,338 ) 124.42 Forfeited ( 3,993 ) 109.19 ( 3,301 ) 120.66 Outstanding, end of period 120,504 132.76 119,163 122.81

Earnings Per Share and Shareh_2

Earnings Per Share and Shareholders' Equity Transactions (Tables)	9 Months Ended
	Sep. 30, 2023
Earnings Per Share [Abstract]
Information Regarding Basic and Diluted Earnings Per Share	The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2023 2022 2023 2022 Basic shares used in the calculation of earnings per share 35.8 35.6 35.7 35.6 Dilutive effect of securities: Employee stock options 0.1 0.2 — 0.3 Non-vested stock grants 0.2 0.3 — 0.2 Diluted shares used in the calculation of earnings per share 36.1 36.1 35.7 36.1 Per share effect of dilutive securities on income (loss) from $ — $ — $ — $ ( 0.06 ) Per share effect of dilutive securities on net income (loss) $ — $ — $ — $ ( 0.06 )

Liabilities for Outstanding C_2

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2023
Insurance [Abstract]
Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses	The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2023 2022 Gross reserve for losses and LAE, beginning of period $ 7,012.6 $ 6,447.6 Reinsurance recoverable on unpaid losses 1,748.6 1,693.8 Net reserve for losses and LAE, beginning of period 5,264.0 4,753.8 Net incurred losses and LAE in respect of losses occurring in: Current year 3,225.6 2,603.8 Prior years ( 6.8 ) ( 31.2 ) Total incurred losses and LAE 3,218.8 2,572.6 Net payments of losses and LAE in respect of losses occurring in: Current year 1,375.7 1,041.3 Prior years 1,523.8 1,226.2 Total payments 2,899.5 2,267.5 Net reserve for losses and LAE, end of period 5,583.3 5,058.9 Reinsurance recoverable on unpaid losses 1,746.5 1,715.1 Gross reserve for losses and LAE, end of period $ 7,329.8 $ 6,774.0

New Accounting Pronouncements_2

New Accounting Pronouncements (Narrative) (Details)	9 Months Ended
New Accounting Pronouncements (Narrative) (Details)	Sep. 30, 2023
Accounting Standards Update and Change in Accounting Principle [Abstract]
Accounting Standards Update [Extensible Enumeration]	ASU 2018-12
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2023
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true

Investments (Schedule of Availa

Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (8.5)	$ (8.5)	$ (3.2)	$ (5.2)	$ (5.2)	$ (7.1)
Amortized Cost, Net of Allowance for Credit Losses	8,535.5		8,294.5
Fair Value	7,618.3		7,481.8
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	510.2		478.9
Amortized Cost, Net of Allowance for Credit Losses	510.2		478.9
Gross Unrealized Gains			0.2
Gross Unrealized Loss	71.2		58.9
Fair Value	439		420.2
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2.2		2.3
Amortized Cost, Net of Allowance for Credit Losses	2.2		2.3
Gross Unrealized Loss	0.1		0.1
Fair Value	2.1		2.2
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,285		1,239.5
Amortized Cost, Net of Allowance for Credit Losses	1,285		1,239.5
Gross Unrealized Gains	0.2		0.8
Gross Unrealized Loss	176.7		164.2
Fair Value	1,108.5		1,076.1
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,115.2		4,066.4
Allowance for Credit Losses	(2.5)		(2.1)
Amortized Cost, Net of Allowance for Credit Losses	4,112.7		4,064.3
Gross Unrealized Gains	3.6		3.3
Gross Unrealized Loss	361.3		337.7
Fair Value	3,755		3,729.9
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,342		1,215.3
Amortized Cost, Net of Allowance for Credit Losses	1,342		1,215.3
Gross Unrealized Gains			0.6
Gross Unrealized Loss	189.8		142.1
Fair Value	1,152.2		1,073.8
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	914.8		924.1
Amortized Cost, Net of Allowance for Credit Losses	914.8		924.1
Gross Unrealized Loss	97.2		87.7
Fair Value	817.6		836.4
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	368.6		370.1
Amortized Cost, Net of Allowance for Credit Losses	368.6		370.1
Gross Unrealized Gains			0.1
Gross Unrealized Loss	24.7		27
Fair Value	343.9		343.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,538		8,296.6
Allowance for Credit Losses	(2.5)		(2.1)
Amortized Cost, Net of Allowance for Credit Losses	8,535.5		8,294.5
Gross Unrealized Gains	3.8		5
Gross Unrealized Loss	921		817.7
Fair Value	$ 7,618.3		$ 7,481.8

Investments (Investments Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost, Net of Allowance for Credit Losses	$ 459.2
Due after one year through five years, Amortized Cost, Net of Allowance for Credit Losses	2,641.5
Due after five years through ten years, Amortized Cost, Net of Allowance for Credit Losses	2,521.2
Due after ten years, Amortized Cost, Net of Allowance for Credit Losses	288.2
Gross fixed maturities, Amortized Cost, Net of Allowance for Credit Losses	5,910.1
Mortgage-backed and other asset-backed securities, Amortized Cost, Net of Allowance for Credit Losses	2,625.4
Amortized Cost, Net of Allowance for Credit Losses	8,535.5
Due in one year or less, Fair Value	454
Due after one year through five years, Fair Value	2,488.1
Due after five years through ten years, Fair Value	2,123.7
Due after ten years, Fair Value	238.8
Gross fixed maturities, Fair Value	5,304.6
Mortgage-backed and other asset-backed securities, Fair Value	2,313.7
Fixed maturities, Fair Value	$ 7,618.3	$ 7,481.8

Investments (Schedule of Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 42.7	$ 407.8
12 months or less, Fair Value	1,317.8	5,414.1
Greater than 12 months, Gross Unrealized Losses	878.3	409.9
Greater than 12 months, Fair Value	6,057.9	1,681.5
Total, Gross Unrealized Losses	921	817.7
Total, Fair Value	7,375.7	7,095.6
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	36.5	394.3
12 months or less, Fair Value	1,137.7	5,160.9
Greater than 12 months, Gross Unrealized Losses	868.1	406.1
Greater than 12 months, Fair Value	5,988.4	1,664.2
Total, Gross Unrealized Losses	904.6	800.4
Total, Fair Value	7,126.1	6,825.1
U.S. Treasury and Government Agencies [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.7	15.8
12 months or less, Fair Value	85	237.3
Greater than 12 months, Gross Unrealized Losses	68.5	43.1
Greater than 12 months, Fair Value	339.7	160.7
Total, Gross Unrealized Losses	71.2	58.9
Total, Fair Value	424.7	398
Foreign Government [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		1.9
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	1.9
Total, Gross Unrealized Losses	0.1	0.1
Total, Fair Value	1.9	1.9
Municipal [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	9.6	64.9
12 months or less, Fair Value	174.5	595.5
Greater than 12 months, Gross Unrealized Losses	167.1	99.3
Greater than 12 months, Fair Value	902.6	386.6
Total, Gross Unrealized Losses	176.7	164.2
Total, Fair Value	1,077.1	982.1
Corporate [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	12.8	202.2
12 months or less, Fair Value	453.3	2,786.2
Greater than 12 months, Gross Unrealized Losses	332.1	118.2
Greater than 12 months, Fair Value	2,865	489.8
Total, Gross Unrealized Losses	344.9	320.4
Total, Fair Value	3,318.3	3,276
Corporate [Member] \| Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	6.2	13.5
12 months or less, Fair Value	180.1	253.2
Greater than 12 months, Gross Unrealized Losses	10.2	3.8
Greater than 12 months, Fair Value	69.5	17.3
Total, Gross Unrealized Losses	16.4	17.3
Total, Fair Value	249.6	270.5
Residential Mortgage-Backed [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.4	51.6
12 months or less, Fair Value	312.8	642
Greater than 12 months, Gross Unrealized Losses	179.4	90.5
Greater than 12 months, Fair Value	839.4	387.6
Total, Gross Unrealized Losses	189.8	142.1
Total, Fair Value	1,152.2	1,029.6
Commercial Mortgage-Backed [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.7	48.7
12 months or less, Fair Value	42.4	663.2
Greater than 12 months, Gross Unrealized Losses	96.5	39
Greater than 12 months, Fair Value	770.2	164.4
Total, Gross Unrealized Losses	97.2	87.7
Total, Fair Value	812.6	827.6
Other Asset-Backed [Member] \| Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	11
12 months or less, Fair Value	69.7	234.8
Greater than 12 months, Gross Unrealized Losses	24.4	16
Greater than 12 months, Fair Value	269.6	75.1
Total, Gross Unrealized Losses	24.7	27
Total, Fair Value	$ 339.3	$ 309.9

Investment Income and Gains and

Investment Income and Gains and Losses (Proceeds from Sales of Available for Sale Fixed Maturities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 16.9	$ 35.2	$ 103.5	$ 308.9
Gross gains			0.9	1.8
Gross losses	$ 1.3	$ 0.5	$ 3.3	$ 21.3

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Investments (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Impairment (recoveries) losses on investments	$ 15.7	$ 16.5	$ 16.3
Allowance for credit losses	$ 5.2	8.5	$ 5.2	$ 8.5	$ 3.2	$ 5.2	$ 7.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Impairment (recoveries) losses on investments		10.3
Fixed Maturities Including Held For Sale [Member]
Schedule of Investments [Line Items]
Allowance for credit losses		2.5			2.1
Mortgage Loans [Member]
Schedule of Investments [Line Items]
Impairment (recoveries) losses on investments		5.4
Allowance for credit losses		$ 8.5			$ 3.2

Investments - (Schedule of Allo

Investments - (Schedule of Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Allowance for credit losses as of the beginning of the period	$ 5.2	$ 3.2	$ 7.1
Additional credit losses on investments for which an allowance was previously recognized	0.1	5.4	0.1
Reductions for disposals			(0.1)
Recoveries	(0.1)	(0.1)	(1.9)
Allowance for credit losses as of the end of the period	$ 5.2	$ 8.5	$ 5.2

Investments (Schedule of Pre-ta

Investments (Schedule of Pre-tax Net Realized and Unrealized Losses on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Net losses recognized during the period	$ (5.2)	$ (29.1)	$ (13.4)	$ (106.1)
Less: net losses recognized on equity securities sold during the period	(11.9)	(0.1)	(11.9)	(8.8)
Net unrealized losses recognized during the period on equity securities still held	$ (6.7)	$ (29)	$ (1.5)	$ (97.3)

Fair Value (Fair Values of Fina

Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 294.5	$ 305
Total financial instruments	8,585.4	8,592.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,618.3	7,481.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	125	131.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	123.5	241.9
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Debt	783	782.4
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	424.1	432.3
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Total financial instruments	8,558.4	8,561.5
Debt	703.4	713.9
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,618.3	7,481.8
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	125	131.8
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	123.5	241.9
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Debt	703.4	713.9
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	$ 397.1	$ 401

Fair Value (Fair Value, Investm

Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,618.3	$ 7,481.8
Equity securities	123.5	241.9
Other investments	817.9	786.1
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	439	420.2
Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.1	2.2
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,108.5	1,076.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,755	3,729.9
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,152.2	1,073.8
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	817.6	836.4
Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	343.9	343.2
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,618.3	7,481.8
Equity securities	123.5	241.9
Other investments	3.6	3.6
Total investment assets at fair value	7,745.4	7,727.3
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	439	420.2
Fair Value, Measurements, Recurring [Member] \| Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.1	2.2
Fair Value, Measurements, Recurring [Member] \| Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,108.5	1,076.1
Fair Value, Measurements, Recurring [Member] \| Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,755	3,729.9
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed, U.S. Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,017.7	974
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	134.5	99.8
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	817.6	836.4
Fair Value, Measurements, Recurring [Member] \| Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	343.9	343.2
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	309.7	282.7
Equity securities	115.9	234.4
Total investment assets at fair value	425.6	517.1
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	309.7	282.7
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,293.1	7,182.2
Total investment assets at fair value	7,293.1	7,182.2
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	129.3	137.5
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.1	2.2
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,100	1,066.8
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,754.9	3,729.8
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Residential Mortgage-Backed, U.S. Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,017.7	974
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Residential Mortgage-Backed, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	134.5	99.8
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	810.7	828.9
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	343.9	343.2
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	15.5	16.9
Equity securities	7.6	7.5
Other investments	3.6	3.6
Total investment assets at fair value	26.7	28
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	8.5	9.3
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.1	0.1
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6.9	$ 7.5

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
Fair Value (Narrative) (Details) - USD ($)	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value Measurements [Line Items]
Transfer between Level 2 and Level 3	$ 0	$ 0	$ 0	$ 0
Maximum [Member]
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	2%		2%		2%
Fair Value Measured Using NAV [Member] \| Limited Partnerships Interest [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	$ 121,400,000		$ 121,400,000		$ 128,200,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	3,600,000		3,600,000		$ 3,600,000
Liabilities held	$ 0	$ 0	$ 0	$ 0

Fair Value (Estimated Fair Valu

Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	$ 3.6	$ 3.6
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Other investments	397.1	401
Total financial instruments	691.6	706
Debt	703.4	713.9
Estimated Fair Value [Member] \| Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	294.5	305
Total financial instruments	294.5	305
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	6.2	6
Total financial instruments	6.2	6
Debt	703.4	713.9
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	390.9	395
Total financial instruments	$ 390.9	$ 395

Fair Value (Fair Value on Recur

Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 27.3	$ 33.6	$ 28	$ 39.8
Included in net realized and unrealized investment gains (losses)		(0.1)	0.1	(0.1)
Included in other comprehensive loss - changes in net unrealized gains (losses) on investment securities	(0.3)	(0.7)	(0.2)	(2.3)
Settlements	(0.3)	(0.8)	(1.2)	(5.4)
Ending Balance	26.7	32	26.7	32
Changes in net unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.3)	(0.7)	(0.2)	(2.3)
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	8.8	11	9.3	14
Included in other comprehensive loss - changes in net unrealized gains (losses) on investment securities	(0.2)	(0.3)	(0.1)	(1.1)
Settlements	(0.1)	(0.7)	(0.7)	(2.9)
Ending Balance	8.5	10	8.5	10
Changes in net unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.2)	(0.4)	(0.1)	(1.2)
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.1	0.1	0.1	0.1
Ending Balance	0.1	0.1	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	7.2	8.1	7.5	11.3
Included in other comprehensive loss - changes in net unrealized gains (losses) on investment securities	(0.1)	(0.4)	(0.1)	(1.2)
Settlements	(0.2)	(0.1)	(0.5)	(2.5)
Ending Balance	6.9	7.6	6.9	7.6
Changes in net unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.1)	(0.3)	(0.1)	(1.1)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	16.1	19.2	16.9	25.4
Included in other comprehensive loss - changes in net unrealized gains (losses) on investment securities	(0.3)	(0.7)	(0.2)	(2.3)
Settlements	(0.3)	(0.8)	(1.2)	(5.4)
Ending Balance	15.5	17.7	15.5	17.7
Changes in net unrealized gains (losses) for the period included in other comprehensive loss for assets held at the end of the period	(0.3)	(0.7)	(0.2)	(2.3)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	11.2	14.4	11.1	14.4
Included in net realized and unrealized investment gains (losses)		(0.1)	0.1	(0.1)
Ending Balance	$ 11.2	$ 14.3	$ 11.2	$ 14.3

Fair Value (Schedule of Additio

Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value [Line Items]
Fair Value	$ 7,618.3	$ 7,481.8
Equity securities, at fair value	123.5	241.9
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.6	$ 3.6
Alternative Investment, Valuation Technique [Extensible Enumeration]	Discounted cash flow [Member]	Discounted cash flow [Member]
Alternative Investment, Measurement Input [Extensible Enumeration]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Range	0.172	0.172
Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Range	0.172	0.172
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,108.5	$ 1,076.1
Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	8.5	9.3
Corporate [Member]
Fair Value [Line Items]
Fair Value	3,755	3,729.9
Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	817.6	836.4
Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Fair Value	6.9	7.5
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 6.4	$ 6.4
Fair value measurement market multiples	27	27
Equity Securities [Member] \| Internal Model Pricing Approach Valuation Technique \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	27	27
Net Tangible Asset [Member] \| Equity Securities [Member] \| Market comparables [Member] \| Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.2	$ 1.1
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.10%	4.50%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.80%	6.80%
Measurement Input, Discount for Small Issue Size [Member] \| Municipal [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.60%	6.60%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	2.50%	2.50%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3%	3%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3.10%	3.10%
Measurement Input, Discount for Small Issue Size [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3%	3%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Corporate [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Above-market coupon [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Lease Structure [Member] \| Commercial Mortgage-Backed [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
Income Taxes (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Federal income tax expense (benefit)	$ (35.3)	$ 30

Pension Plans (Components of Ne

Pension Plans (Components of Net Periodic Pension Cost) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Retirement Benefits [Abstract]
Defined Benefit Plan, Type [Extensible Enumeration]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Interest cost	$ 5.5	$ 3.9	$ 16.4	$ 11.5
Expected return on plan assets	(5.5)	(4.3)	(16.5)	(12.9)
Recognized net actuarial loss	1.9	1.3	5.8	3.9
Net periodic pension cost	$ 1.9	$ 0.9	$ 5.7	$ 2.5

Other Comprehensive Loss (Chang

Other Comprehensive Loss (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Equity [Abstract]
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income (Loss), Pre-Tax	$ (144.3)	$ (324.5)	$ (120.8)	$ (1,151.7)
Net unrealized losses arising during period for those having credit losses in Consolidated Statement of Income (Loss), Pre-Tax	(0.7)	(4.2)	0.3	(10.5)
Amount of losses realized from sales and other recognized in Consolidated Statement of Income (Loss), Pre-Tax	0.9	0.3	1.9	16.9
Amount of credit-related impairments recognized in the Consolidated Statement of Income (Loss), Pre-Tax		1.4	0.8	3.3
Amount of additional impairment losses recognized in the Consolidated Statement of Income (Loss), Pre-Tax		14.3	10.3	14.8
Net unrealized losses, Pre-Tax	(144.1)	(312.7)	(107.5)	(1,127.2)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	1.9	1.4	5.9	4.1
Net change in market risk, Pre-tax	4.2	5.4	3.3	22
Other comprehensive loss, Pre-Tax	(138)	(305.9)	(98.3)	(1,101.1)
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income (Loss), Tax Benefit (Expense)	30.3	68.1	25.4	241.8
Net unrealized losses arising during period for those having credit losses in Consolidated Statement of Income (Loss), Tax Benefit (Expense)	0.2	0.9		2.2
Amount of losses realized from sales and other recognized in Consolidated Statement of Income (Loss), Tax Benefit (Expense)	(0.6)	(0.3)	(1.6)	(4.6)
Amount of credit-related impairments recognized in the Consolidated Statement of Income (Loss), Tax Benefit (Expense)		(0.3)	(0.2)	(0.7)
Amount of additional impairment losses recognized in the Consolidated Statement of Income (Loss), Tax Benefit (Expense)		(3)	(2.2)	(3.1)
Net unrealized losses, Tax Benefit (Expense)	29.9	65.4	21.4	235.6
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.4)	(0.3)	(1.2)	(0.8)
Net change in market risk, Tax Benefit (Expense)	(0.9)	(1.1)	(0.7)	(4.6)
Other comprehensive loss, Tax Benefit (Expense)	28.6	64	19.5	230.2
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income (Loss), Net of Tax	(114)	(256.4)	(95.4)	(909.9)
Net unrealized losses arising during period for those having credit losses in Consolidated Statement of Income (Loss), Net of Tax	(0.5)	(3.3)	0.3	(8.3)
Amount of losses realized from sales and other recognized in Consolidated Statement of Income (Loss), Net of Tax	0.3		0.3	12.3
Amount of credit-related impairments recognized in the Consolidated Statement of Income (Loss), Net of Tax		1.1	0.6	2.6
Amount of additional impairment losses recognized in the Consolidated Statement of Income (Loss), Net of Tax		11.3	8.1	11.7
Total available-for-sale securities	(114.2)	(247.3)	(86.1)	(891.6)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	1.5	1.1	4.7	3.3
Net change in market risk, Net of Tax	3.3	4.3	2.6	17.4
Other comprehensive loss, net of tax	$ (109.4)	$ (241.9)	$ (78.8)	$ (870.9)

Other Comprehensive Loss (Recla

Other Comprehensive Loss (Reclassifications out of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	$ (0.9)	$ (0.1)	$ (1.9)	$ (16.3)
Income (loss) from continuing operations before income taxes	(0.9)	0.5	(109.1)	158.7
Loss adjustment expenses and other operating expenses	(1,061.5)	(939.6)	(3,218.8)	(2,572.6)
Income tax benefit	9.1	0.1	35.3	(30)
Income (loss) from continuing operations	8.2	0.6	(73.8)	128.7
Net income (loss)	8.6	0.5	(72.6)	128.1
Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from discontinued operations		(0.1)		(0.6)
Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	(0.9)		(1.9)	(16.7)
Impairments on investments		(15.7)	(11.1)	(18.1)
Income tax benefit	0.6	3.6	4	8.4
Net income (loss)	(1.8)	(13.2)	(13.7)	(29.4)
Reclassification out of Accumulated Other Comprehensive Loss [Member] \| Defined Benefit Pension and Postretirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses	(1.9)	(1.4)	(5.9)	(4.1)
Income tax benefit	0.4	0.3	1.2	0.8
Income (loss) from continuing operations	(1.5)	(1.1)	(4.7)	(3.3)
Reclassification out of Accumulated Other Comprehensive Loss [Member] \| Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations before income taxes	(0.9)	(15.7)	(13)	(34.8)
Income (loss) from continuing operations	(0.3)	(12.1)	(9)	(26.4)
Net income (loss)	$ (0.3)	(12.4)	$ (9)	(26.6)
Reclassification out of Accumulated Other Comprehensive Loss [Member] \| Net Unrealized Appreciation (Depreciation) on Investments [Member] \| Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from discontinued operations		(0.3)		(0.2)
Reclassification out of Accumulated Other Comprehensive Loss [Member] \| Net change in market risk
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit				(0.2)
Loss from discontinued operations		0.3		0.5
Reclassification out of Accumulated Other Comprehensive Loss [Member] \| Net change in market risk \| Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from discontinued operations		$ 0.3		$ 0.7

Other Comprehensive Loss (Rec_2

Other Comprehensive Loss (Reclassifications out of Accumulated Other Comprehensive Loss) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Comprehensive Income (Loss) [Line Items]
Loss adjustment expenses	40%	40%	40%	40%
Percentage of other operating expenses	60%	60%	60%	60%

Segment Information (Narrative)

Segment Information (Narrative) (Details)	9 Months Ended
Segment Information (Narrative) (Details)	Sep. 30, 2023 Segment
Segment Reporting [Abstract]
Operating segments	4

Segment Information (Financial

Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Operating revenues	$ 1,522.7	$ 1,411.2	$ 4,496.5	$ 4,128.6
Net realized and unrealized investment gains (losses)	(6.1)	(44.9)	(31.8)	(138.7)
Total revenues	1,516.6	1,366.3	4,464.7	3,989.9
Net investment income	84.2	73	250.5	220.4
Operating income (loss) before interest expense and income taxes	13.7	53.9	(52.5)	323.3
Interest on debt	(8.5)	(8.5)	(25.6)	(25.5)
Operating income (loss) before income taxes	5.2	45.4	(78.1)	297.8
Other non-operating			0.8	(0.4)
Income (loss) from continuing operations before income taxes	(0.9)	0.5	(109.1)	158.7
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	557.1	527.1	1,658.2	1,551.8
Underwriting (loss) income	5.3	(7.7)	1	58.8
Net investment income	38.4	33.4	114.2	101.4
Other income (expense)	(0.6)	(0.5)	(0.8)	(0.6)
Operating income (loss) before interest expense and income taxes	43.1	25.2	114.4	159.6
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	340.9	319.9	1,011.6	931
Underwriting (loss) income	52.7	32	119.5	96.2
Net investment income	18	15.2	53.6	46.2
Other income (expense)	(0.4)	(0.3)	(0.1)	(0.3)
Operating income (loss) before interest expense and income taxes	70.3	46.9	173	142.1
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	620.5	560.5	1,815.1	1,635.4
Underwriting (loss) income	(127.3)	(42.7)	(422.4)	(50.3)
Net investment income	24.3	21.4	73.3	64.6
Other income (expense)	2.6	2.5	8	6
Operating income (loss) before interest expense and income taxes	(100.4)	(18.8)	(341.1)	20.3
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	4.2	3.7	11.6	10.4
Net investment income	3.5	3	9.4	8.2
Other income (expense)	(2.8)	(2.4)	(8.2)	(6.9)
Operating income (loss) before interest expense and income taxes	$ 0.7	$ 0.6	$ 1.2	$ 1.3

Segment Information (Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Identifiable assets	$ 14,315	$ 13,995.1
Identifiable assets	80.6	84.1
Property and Casualty [Member]
Segment Reporting Information [Line Items]
Identifiable assets	14,234.4	13,911
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 80.6	$ 84.1

Stock-based Compensation (Narra

Stock-based Compensation (Narrative) (Details) - shares	9 Months Ended
Stock-based Compensation (Narrative) (Details) - shares	Sep. 30, 2023	Sep. 30, 2022	May 09, 2023
Time-based, Market-based and Performance-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Granted, Shares	21,789	19,057
Forfeited, shares	2,836	1,282
Decrease as grant in period, percentage	100%
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	50,115	47,954
Forfeited, shares	3,993	3,301
Performance-Based and Market-Based Restricted Stock Units [Member] \| Minimum [Member] \| Senior Management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	0%
Performance-Based and Market-Based Restricted Stock Units [Member] \| Maximum [Member] \| Senior Management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	150%
Performance-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	28,326	28,897
Performance-Based Restricted Stock Units [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	0%
Performance-Based Restricted Stock Units [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	150%
Performance-Based Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	5,961	7,988
Increase as grant in period, percentage	100%
Hanover Insurance Group 2022 Long-Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	2,940,189
ESPP Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	1,250,000
Number of shares authorized under plan			1,250,000

Stock-based Compensation (Compe

Stock-based Compensation (Compensation Cost and Related Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share-Based Payment Arrangement [Abstract]
Stock-based compensation expense	$ 8.6	$ 7.7	$ 23.9	$ 21.8
Tax benefit	(1.8)	(1.7)	(5)	(4.6)
Stock-based compensation expense, net of taxes	$ 6.8	$ 6	$ 18.9	$ 17.2

Stock-based Compensation (Summa

Stock-based Compensation (Summary of Stock Option Activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Shares
Outstanding, beginning of period, Shares	1,080,852	1,230,211
Granted, Shares	135,019	140,339
Exercised, Shares	(40,544)	(262,900)
Forfeited or cancelled, Shares	(22,184)	(6,075)
Outstanding, end of period, Shares	1,153,143	1,101,575
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price	$ 107.07	$ 99.14
Granted, Weighted Average Exercise Price	140.01	139.51
Exercised, Weighted Average Exercise Price	92.39	86.82
Forfeited or cancelled, Weighted Average Exercise Price	125.15	116.44
Outstanding, end of period, Weighted Average Exercise Price	$ 111.09	$ 107.13

Stock-based Compensation (Sum_2

Stock-based Compensation (Summary of Activity Information about Employee Restricted Stock Units) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Time-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	378,256	380,100
Granted, Shares	149,509	147,438
Vested, Shares	(132,086)	(120,715)
Forfeited, Shares	(19,132)	(23,148)
Outstanding, end of period, Shares	376,547	383,675
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 124.88	$ 117.6
Granted, Weighted Average Grant Date Fair Value	138.35	138.46
Vested, Weighted Average Grant Date Fair Value	118.21	119.29
Forfeited, Weighted Average Grant Date Fair Value	131.14	122.94
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 132.25	$ 124.77
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	119,163	113,848
Granted, Shares	50,115	47,954
Vested, Shares	(44,781)	(39,338)
Forfeited, Shares	(3,993)	(3,301)
Outstanding, end of period, Shares	120,504	119,163
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 122.81	$ 115.92
Granted, Weighted Average Grant Date Fair Value	137.9	140.36
Vested, Weighted Average Grant Date Fair Value	114.14	124.42
Forfeited, Weighted Average Grant Date Fair Value	109.19	120.66
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 132.76	$ 122.81

Earnings Per Share and Shareh_3

Earnings Per Share and Shareholders' Equity Transactions (Information Regarding Basic and Diluted Earnings Per Share) (Details) - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	35.8	35.6	35.7	35.6
Diluted shares used in the calculation of earnings per share	36.1	36.1	35.7	36.1
Per share effect of dilutive securities on income (loss) from continuing operations				$ (0.06)
Per share effect of dilutive securities on net income (loss)				$ (0.06)
Employee Stock Option
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.1	0.2		0.3
Non-Vested Stock Grants [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.2	0.3		0.2

Earnings Per Share and Shareh_4

Earnings Per Share and Shareholders' Equity Transactions (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Stock repurchase program, available authorized repurchase amount	$ 330,000,000		$ 330,000,000
Maximum [Member] \| Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Repurchases common stock, authorized	$ 1,300,000,000		$ 1,300,000,000
Stock Compensation Plans [Member]
Class Of Stock [Line Items]
Antidilutive securities excluded from calculation of earnings per share	0.7	0.1	0.6	0.1

Liabilities for Outstanding C_3

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of period	$ 7,012.6	$ 6,447.6
Reinsurance recoverable on unpaid losses, beginning of period	1,748.6	1,693.8
Net reserve for losses and LAE, beginning of period	5,264	4,753.8
Net incurred losses and LAE in respect of losses occurring in current year	3,225.6	2,603.8
Net incurred losses and LAE in respect of losses occurring in prior years	(6.8)	(31.2)
Total incurred losses and LAE	3,218.8	2,572.6
Net payments of losses and LAE in respect of losses occurring in current year	1,375.7	1,041.3
Net payments of losses and LAE in respect of losses occurring in prior years	1,523.8	1,226.2
Total payments	2,899.5	2,267.5
Net reserve for losses and LAE, end of period	5,583.3	5,058.9
Reinsurance recoverable on unpaid losses, end of period	1,746.5	1,715.1
Gross reserve for losses and LAE, end of period	$ 7,329.8	$ 6,774

Liabilities for Outstanding C_4

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (6.8)	$ (31.2)
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(43.1)	(24.2)
Specialty [Member] \| Professional and Executive Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(28.6)	(13.2)
Specialty [Member] \| Surety Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(9.2)	(11.5)
Specialty [Member] \| Marine Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(10.2)
Specialty [Member] \| Specialty P&C Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(10.5)
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(1.2)	(20.2)
Core Commercial [Member] \| Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(9.9)
Core Commercial [Member] \| Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(17.7)	(21)
Core Commercial [Member] \| Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(13.5)	(11.5)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	35.1	13.2
Personal Lines [Member] \| Homeowners [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	11.4	$ 19.4
Personal Lines [Member] \| Personal Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 16.4