NorthWestern (NWE) 10-Q24 Jul 232023 Q2 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information Document - shares	6 Months Ended
Document and Entity Information Document - shares	Jun. 30, 2023	Jul. 21, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2023
Document Transition Report	false
Entity File Number	1-10499
Entity Registrant Name	NORTHWESTERN CORP
Entity Central Index Key	0000073088
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-0172280
Entity Address, Address Line One	3010 W. 69th Street
Entity Address, City or Town	Sioux Falls
Entity Address, State or Province	SD
Entity Address, Postal Zip Code	57108
City Area Code	605
Local Phone Number	978-2900
Title of 12(b) Security	Common stock
Trading Symbol	NWE
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		60,041,809

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues
Electric	$ 229,266	$ 243,418	$ 524,574	$ 515,145
Gas	61,236	79,586	220,470	202,341
Total Revenues	290,502	323,004	745,044	717,486
Operating expenses
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	67,578	95,001	233,070	230,074
Operating and maintenance	54,840	53,337	110,701	106,131
Administrative and general	29,955	27,220	64,703	58,864
Property and other taxes	40,129	46,893	89,280	93,743
Depreciation and depletion	52,380	48,212	105,628	97,117
Total Operating Expenses	244,882	270,663	603,382	585,929
Operating income	45,620	52,341	141,662	131,557
Interest expense, net	(28,411)	(24,033)	(56,419)	(47,749)
Other income, net	4,062	2,913	8,799	7,634
Income before income taxes	21,271	31,221	94,042	91,442
Income tax expense	(2,147)	(1,435)	(12,388)	(2,546)
Net Income	$ 19,124	$ 29,786	$ 81,654	$ 88,896
Average Common Shares Outstanding	59,804,283	54,271,862	59,790,316	54,184,798
Basic Earnings per Average Common Share	$ 0.32	$ 0.55	$ 1.37	$ 1.64
Diluted Earnings per Average Common Share	0.32	0.54	1.37	1.62
Dividends Declared per Common Share	$ 0.64	$ 0.63	$ 1.280	$ 1.260

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Comprehensive Income (Loss)
Net Income	$ 19,124	$ 29,786	$ 81,654	$ 88,896
Other comprehensive income, net of tax:
Foreign currency translation adjustment	(1)	1	(3)	(1)
Postretirement medical liability adjustment	(167)	(158)	(334)	(316)
Reclassification of net losses on derivative instruments	113	113	226	226
Total Other Comprehensive Loss	(55)	(44)	(111)	(91)
Comprehensive Income	$ 19,069	$ 29,742	$ 81,543	$ 88,805

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEET - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Current Assets:
Cash and cash equivalents	$ 7,757	$ 8,489
Restricted cash	16,263	13,974
Accounts receivable, net	147,173	244,952
Inventories	107,577	107,359
Regulatory assets	52,869	136,009
Prepaid expenses and other	25,567	28,041
Total current assets	357,206	538,824
Property, plant, and equipment, net	5,802,526	5,657,480
Goodwill	357,586	357,586
Regulatory assets	726,129	716,570
Other noncurrent assets	50,795	47,323
Total Assets	7,294,242	7,317,783
Current Liabilities:
Current maturities of finance leases	3,213	3,098
Current portion of long-term debt	99,850	144,525
Accounts payable	94,552	201,498
Accrued expenses and other	247,287	250,579
Regulatory liabilities	28,444	21,145
Total current liabilities	473,346	620,845
Long-term finance leases	7,192	8,799
Long-term debt	2,558,192	2,474,357
Deferred income taxes	546,066	538,983
Noncurrent regulatory liabilities	666,060	654,213
Other noncurrent liabilities	356,662	355,403
Total Liabilities	4,607,518	4,652,600
Commitments and Contingencies (Note 10)
Shareholders' Equity:
Common Stock, Value, Issued	635	633
Treasury stock at cost	(98,302)	(98,392)
Paid-in capital	2,015,367	1,999,376
Retained earnings	776,983	771,414
Accumulated other comprehensive loss	(7,959)	(7,848)
Total Shareholders' Equity	2,686,724	2,665,183
Total Liabilities and Shareholders' Equity	$ 7,294,242	$ 7,317,783
Preferred Stock, Shares Issued	0
Common Stock, Par or Stated Value Per Share	$ 0.01
Common Stock, Shares Authorized	200,000,000
Common Stock, Shares, Issued	63,517,861
Common Stock, Shares, Outstanding	59,991,283
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Shares Authorized	50,000,000
Preferred Stock, Shares Outstanding	0

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 81,654	$ 88,896
Items not affecting cash:
Depreciation and depletion	105,628	97,117
Amortization of debt issuance costs, discount and deferred hedge gain	2,636	2,546
Stock-based compensation costs	4,868	4,002
Equity portion of allowance for funds used during construction	(7,812)	(6,653)
Loss on disposition of assets	(20)	(1)
Deferred income taxes	(10,005)	(3,394)
Changes in current assets and liabilities:
Accounts receivable	97,779	52,629
Inventories	(218)	(18,405)
Other current assets	2,474	(1,474)
Accounts payable	(63,127)	10,877
Accrued expenses and other	(3,029)	10,072
Regulatory assets	83,139	9,035
Regulatory liabilities	7,299	(9,904)
Other noncurrent assets	1,454	7,517
Other noncurrent liabilities	(8,655)	(9,967)
Cash Provided by Operating Activities	294,065	232,893
Net Cash Provided by (Used in) Investing Activities [Abstract]
Property, plant, and equipment additions	(263,362)	(234,438)
Investment in equity securities	(2,426)	(914)
Cash Used in Investing Activities	(265,788)	(235,352)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net	10,802	99,903
Dividends on common stock	(76,085)	(67,806)
Issuance of long-term debt	300,000	0
Line of credit repayments, net	(259,000)	(21,000)
Other financing activities, net	(2,437)	(1,320)
Cash (Used in) Provided by Financing Activities	(26,720)	9,777
Increase in Cash, Cash Equivalents, and Restricted Cash	1,557	7,318
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance	22,463	18,762
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance	24,020	26,080
Cash Paid [Abstract]
Income Taxes Paid, Net	3,204	1,634
Interest Paid, Excluding Capitalized Interest, Operating Activities	51,047	44,537
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital expenditures included in accounts payable	20,938	24,116
Refinancing of Pollution Control Revenue Refunding Bonds	$ 144,660	$ 0

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock, Common	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, shares at Dec. 31, 2021		57,606	3,546
Balance, beginning of period at Dec. 31, 2021	$ 2,339,713	$ 576	$ (98,248)	$ 1,716,227	$ 728,468	$ (7,310)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	88,896	0	0	0	88,896	0
Foreign currency translation adjustment	1	0	0	0	0	(1)
Reclassification of net losses on derivative instruments	226	0	0	0	0	226
Postretirement medical liability adjustment	(316)	$ 0	$ 0	0	0	(316)
Stock based compensation, shares		87	16
Stock based compensation, value	3,066	$ 1	$ (911)	3,976	0	0
Issuance of shares		2,004
Issuance of shares, treasury stock			(14)
Issuance of shares, value	100,742	$ 20	$ 394	100,328	0	0
Dividends on common stock	$ (67,806)	$ 0	$ 0	0	(67,806)	0
Dividends per share	$ 1.260
Balance, shares at Jun. 30, 2022		59,697	3,548
Balance, end of period at Jun. 30, 2022	$ 2,464,520	$ 597	$ (98,765)	1,820,531	749,558	(7,401)
Balance, shares at Mar. 31, 2022		57,693	3,556
Balance, beginning of period at Mar. 31, 2022	2,366,981	$ 577	$ (98,986)	1,719,070	753,677	(7,357)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	29,786	0	0	0	29,786	0
Foreign currency translation adjustment	(1)	0	0	0	0	(1)
Reclassification of net losses on derivative instruments	113	0	0	0	0	113
Postretirement medical liability adjustment	(158)	$ 0	$ 0	0	0	(158)
Stock based compensation, shares		0	0
Stock based compensation, value	1,230	$ 0	$ 0	1,230	0	0
Issuance of shares		2,004
Issuance of shares, treasury stock			(8)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 221
Issuance of shares, value	100,472	$ 20		100,231	0	0
Dividends on common stock	$ (33,905)	$ 0	$ 0	0	(33,905)	0
Dividends per share	$ 0.63
Balance, shares at Jun. 30, 2022		59,697	3,548
Balance, end of period at Jun. 30, 2022	$ 2,464,520	$ 597	$ (98,765)	1,820,531	749,558	(7,401)
Balance, shares at Dec. 31, 2022		63,278	3,534
Balance, beginning of period at Dec. 31, 2022	2,665,183	$ 633	$ (98,392)	1,999,376	771,414	(7,848)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	81,654	0	0	0	81,654	0
Foreign currency translation adjustment	3	0	0	0	0	3
Reclassification of net losses on derivative instruments	226	0	0	0	0	226
Postretirement medical liability adjustment	(334)	$ 0	$ 0	0	0	(334)
Stock based compensation, shares		51	0
Stock based compensation, value	4,672	$ 0	$ 0	4,672	0	0
Issuance of shares		189
Issuance of shares, treasury stock			(7)
Issuance of shares, value	11,411	$ 2	$ 90	11,319	0	0
Dividends on common stock	$ (76,085)	$ 0	$ 0	0	(76,085)	0
Dividends per share	$ 1.280
Balance, shares at Jun. 30, 2023		63,518	3,527
Balance, end of period at Jun. 30, 2023	$ 2,686,724	$ 635	$ (98,302)	2,015,367	776,983	(7,959)
Balance, shares at Mar. 31, 2023		63,326	3,533
Balance, beginning of period at Mar. 31, 2023	2,693,000	$ 633	$ (98,471)	2,002,839	795,903	(7,904)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	19,124	0	0	0	19,124	0
Foreign currency translation adjustment	1	0	0	0	0	1
Reclassification of net losses on derivative instruments	113	0	0	0	0	113
Postretirement medical liability adjustment	(167)	$ 0	$ 0	0	0	(167)
Stock based compensation, shares		3	0
Stock based compensation, value	1,378	$ 0	$ 0	1,378	0	0
Issuance of shares		189
Issuance of shares, treasury stock			(6)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 169
Issuance of shares, value	11,321	$ 2		11,150	0	0
Dividends on common stock	$ (38,044)	$ 0	$ 0	0	(38,044)	0
Dividends per share	$ 0.64
Balance, shares at Jun. 30, 2023		63,518	3,527
Balance, end of period at Jun. 30, 2023	$ 2,686,724	$ 635	$ (98,302)	$ 2,015,367	$ 776,983	$ (7,959)

Nature of Operations and Basis

Nature of Operations and Basis of Consolidation	6 Months Ended
Nature of Operations and Basis of Consolidation	Jun. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure	Nature of Operations and Basis of Consolidation NorthWestern Corporation, doing business as NorthWestern Energy, provides electricity and/or natural gas to approximately 764,200 customers in Montana, South Dakota, Nebraska and Yellowstone National Park. We have generated and distributed electricity in South Dakota and distributed natural gas in South Dakota and Nebraska since 1923 and have generated and distributed electricity and distributed natural gas in Montana since 2002. The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America (GAAP) requires us to make estimates and assumptions that may affect the reported amounts of assets, liabilities, revenues and expenses during the reporting period. Actual results could differ from those estimates. The unaudited Condensed Consolidated Financial Statements (Financial Statements) reflect all adjustments (which unless otherwise noted are normal and recurring in nature) that are, in our opinion, necessary to fairly present our financial position, results of operations and cash flows. The actual results for the interim periods are not necessarily indicative of the operating results to be expected for a full year or for other interim periods. Events occurring subsequent to June 30, 2023 have been evaluated as to their potential impact to the Financial Statements through the date of issuance. The Financial Statements included herein have been prepared by NorthWestern, without audit, pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations; however, we believe that the condensed disclosures provided are adequate to make the information presented not misleading. We recommend that these Financial Statements be read in conjunction with the audited financial statements and related footnotes included in our Annual Report on Form 10-K for the year ended December 31, 2022 . Supplemental Cash Flow Information The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the Condensed Consolidated Balance Sheets that sum to the total of the same such amounts shown in the Condensed Consolidated Statements of Cash Flows (in thousands): June 30, December 31, June 30, December 31, 2023 2022 2022 2021 Cash and cash equivalents $ 7,757 $ 8,489 $ 8,117 $ 2,820 Restricted cash 16,263 13,974 17,963 15,942 Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows $ 24,020 $ 22,463 $ 26,080 $ 18,762 Goodwill

Regulatory Matters

Regulatory Matters	6 Months Ended
Regulatory Matters	Jun. 30, 2023
Regulated Operations [Abstract]
Public Utilities Disclosure [Text Block]	Regulatory Matters Except as set forth below, the circumstances set forth in Note 3 - Regulatory Matters to the financial statements included in our Annual Report on the Form 10-K for the year ended December 31, 2022 appropriately represent, in all material respects, the current status of our regulatory matters. Montana Rate Review On August 8, 2022, we filed a Montana electric and natural gas rate review with the Montana Public Service Commission (MPSC) requesting an annual increase to electric and natural gas utility rates. On September 28, 2022, the MPSC approved interim rates effective October 1, 2022, subject to refund. Subsequently, we modified our request through rebuttal testimony. On April 3, 2023, we filed a settlement with certain parties, which is subject to approval by the MPSC. The details of our rebuttal request, interim rates granted, and settlement agreement are set forth below: Requested Revenue Increase Through Rebuttal Testimony (in millions) Electric Natural Gas Base Rates $90.6 $22.4 Power Cost & Credit Mechanism (PCCAM) (1) $69.7 n/a Property Tax (tracker base adjustment) (1) $14.5 $4.2 Total Revenue Increase Requested through Rebuttal Testimony $174.8 $26.6 Interim Revenue Increase Granted (in millions) Electric Natural Gas Base Rates $29.4 $1.7 PCCAM (1) $61.1 n/a Property Tax (tracker base adjustment) (1)(2) $10.8 $2.9 Total Interim Revenue Granted $101.3 $4.6 Requested Revenue Increase Through Settlement Agreement (in millions) Electric Natural Gas Base Rates $67.4 $14.1 PCCAM (1) $69.7 n/a Property Tax (tracker base adjustment) (1) $14.5 $4.2 Total Revenue Increase Requested Through Settlement Agreement $151.6 $18.3 (1) These items are flow-through costs. PCCAM reflects our fuel and purchased power costs. (2) Our requested interim property tax base increases went into effect on January 1, 2023, as part of our 2023 property tax tracker filing. The settlement includes, among other things, agreement on electric and natural gas base revenue increases, allocated cost of service, rate design, updates to the base amount of revenues associated with property taxes and electric supply costs, and regulatory policy issues related to requested changes in regulatory mechanisms. The settlement is based on a 48.02 percent equity component of our capital structure and an authorized return on equity (ROE) of 9.65 percent for electric operations and 9.55 percent for natural gas operations, which are consistent with current authorized ROE. The settlement agreement provides for an update to the PCCAM by adjusting the base costs from $138.7 million to $208.4 million and providing for more timely quarterly recovery of deferred balances instead of annual recovery. It also addresses the potential for future recovery of certain operating costs associated with the Yellowstone County Generating Station and provides for the deferral of incremental operating costs related to our Enhanced Wildfire Mitigation Plan. The settling parties agreed to terminate the pilot decoupling program (Fixed Cost Recovery Mechanism) and that the proposed business technology rider will not be implemented. A hearing on the settlement agreement was held in April 2023, post-hearing briefing concluded in June 2023, and we expect a decision from the MPSC during the third quarter of 2023. Interim rates remain in effect on a refundable basis until the MPSC issues a final order. South Dakota Electric Rate Review On June 15, 2023, we filed a South Dakota electric rate review filing (2022 test year) under Docket EL23-016 for an annual increase to electric rates totaling approximately $30.9 million. Our request was based on a ROE of 10.7 percent, a capital structure including 50.5 percent equity, and rate base of $787.3 million . Holding Company Filings On June 1, 2022, we filed a legal corporate restructuring application (Restructuring Plan) with the state commissions in Montana, South Dakota and Nebraska and the Federal Energy Regulatory Commission (FERC). Currently, our utility businesses are held in the same legal entity. Under the Restructuring Plan, we proposed to legally separate our Montana public utility business from our South Dakota and Nebraska public utility business and establish a holding company to hold the ownership interests of all of the subsidiaries. The purpose of the reorganization is to segregate our organizational structure to be more transparent and in line with the public utility industry. The Restructuring Plan does not include substantive changes in how the state public utility commissions regulate those services. We have received all necessary regulatory approvals and we expect to effectuate the Restructuring Plan by early 2024.

Income Taxes

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2023
Income Tax Disclosure [Abstract]
Income Tax Disclosure [Text Block]	Income Taxes We compute income tax expense for each quarter based on the estimated annual effective tax rate for the year, adjusted for certain discrete items. Our effective tax rate typically differs from the federal statutory tax rate due to the regulatory impact of flowing through the federal and state tax benefit of repairs deductions, state tax benefit of accelerated tax depreciation deductions (including bonus depreciation when applicable) and production tax credits. The regulatory accounting treatment of these deductions requires immediate income recognition for temporary tax differences of this type, which is referred to as the flow-through method. When the flow-through method of accounting for temporary differences is reflected in regulated revenues, we record deferred income taxes and establish related regulatory assets and liabilities. The following table summarizes the differences between our effective tax rate and the federal statutory rate (in thousands): Three Months Ended June 30, 2023 2022 Income before income taxes $ 21,271 $ 31,221 Income tax calculated at federal statutory rate 4,467 21.0 % 6,554 21.0 % Permanent or flow-through adjustments: State income tax, net of federal provisions 273 1.3 431 1.4 Flow-through repairs deductions (1,708) (8.0) (3,313) (10.6) Production tax credits (1,147) (5.4) (2,558) (8.2) Amortization of excess deferred income tax (233) (1.1) (162) (0.5) Plant and depreciation flow-through items 201 0.9 398 1.3 Other, net 294 1.4 85 0.2 (2,320) (10.9) (5,119) (16.4) Income tax expense $ 2,147 10.1 % $ 1,435 4.6 % Six Months Ended June 30, 2023 2022 Income before income taxes $ 94,042 $ 91,442 Income tax calculated at federal statutory rate 19,749 21.0 % 19,200 21.0 % Permanent or flow through adjustments: State income, net of federal provisions 1,232 1.3 831 0.9 Flow-through repairs deductions (7,553) (8.0) (10,114) (11.1) Production tax credits (4,346) (4.6) (6,382) (7.0) Reduction to previously claimed alternative minimum tax credit 3,186 3.4 — — Amortization of excess deferred income tax (1,032) (1.1) (573) (0.6) Share-based compensation 388 0.4 (253) (0.3) Plant and depreciation flow through items 889 0.9 143 0.2 Other, net (125) (0.1) (306) (0.3) (7,361) (7.8) (16,654) (18.2) Income tax expense $ 12,388 13.2 % $ 2,546 2.8 % Uncertain Tax Positions We recognize tax positions that meet the more-likely-than-not threshold as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. We had unrecognized tax benefits of approximately $29.5 million as of June 30, 2023, including approximately $27.8 million that, if recognized, would impact our effective tax rate. We do not anticipate that total unrecognized tax benefits will significantly change due to the settlement of audits or the expiration of statutes of limitation within the next twelve months. Our policy is to recognize interest and penalties related to uncertain tax positions in income tax expense. As of June 30, 2023, we have accrued $2.1 million for the payment of interest and penalties on the Condensed Consolidated Balance Sheets. As of December 31, 2022, we had accrued $1.4 million for the payment of interest and penalties on the Condensed Consolidated Balance Sheets. Tax years 2019 and forward remain subject to examination by the Internal Revenue Service (IRS) and state taxing authorities. During the first quarter of 2023 the IRS commenced and concluded a limited scope examination of our 2019 amended federal income tax return. This examination resulted in a reduction to our previously claimed alternative minimum tax credit refund that is reflected in the table above.

Comprehensive Income (Loss)

Comprehensive Income (Loss)	6 Months Ended
Comprehensive Income (Loss)	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Comprehensive Income (Loss) Note [Text Block]	Comprehensive (Loss) Income The following tables display the components of Other Comprehensive (Loss) Income, after-tax, and the related tax effects (in thousands): Three Months Ended June 30, 2023 June 30, 2022 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (1) $ — $ (1) $ 1 $ — $ 1 Reclassification of net income on derivative instruments 153 (40) 113 153 (40) 113 Postretirement medical liability adjustment (212) 45 (167) (212) 54 (158) Other comprehensive (loss) income $ (60) $ 5 $ (55) $ (58) $ 14 $ (44) Six Months Ended June 30, 2023 June 30, 2022 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (3) $ — $ (3) $ (1) $ — $ (1) Reclassification of net income on derivative instruments 306 (80) 226 306 (80) 226 Postretirement medical liability adjustment (424) 90 (334) (424) 108 (316) Other comprehensive (loss) income $ (121) $ 10 $ (111) $ (119) $ 28 $ (91) Balances by classification included within accumulated other comprehensive loss (AOCL) on the Condensed Consolidated Balance Sheets are as follows, net of tax (in thousands): June 30, 2023 December 31, 2022 Foreign currency translation $ 1,432 $ 1,435 Derivative instruments designated as cash flow hedges (9,599) (9,825) Postretirement medical plans 208 542 Accumulated other comprehensive loss $ (7,959) $ (7,848) The following tables display the changes in AOCL by component, net of tax (in thousands): Three Months Ended June 30, 2023 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,712) $ 375 $ 1,433 $ (7,904) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (167) — (167) Net current-period other comprehensive income (loss) 113 (167) (1) (55) Ending balance $ (9,599) $ 208 $ 1,432 $ (7,959) Three Months Ended June 30, 2022 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (10,164) $ 1,366 $ 1,441 $ (7,357) Other comprehensive loss before reclassifications — — 1 1 Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (158) — (158) Net current-period other comprehensive income (loss) 113 (158) 1 (44) Ending balance $ (10,051) $ 1,208 $ 1,442 $ (7,401) Six Months Ended June 30, 2023 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Pension and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,825) $ 542 $ 1,435 $ (7,848) Other comprehensive loss before reclassifications — — (3) (3) Amounts reclassified from AOCL Interest Expense 226 — — 226 Amounts reclassified from AOCL — (334) — (334) Net current-period other comprehensive income (loss) 226 (334) (3) (111) Ending balance $ (9,599) $ 208 $ 1,432 $ (7,959) Six Months Ended June 30, 2022 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Pension and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (10,277) $ 1,524 $ 1,443 $ (7,310) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 226 — — 226 Amounts reclassified from AOCL — (316) — (316) Net current-period other comprehensive income (loss) 226 (316) (1) (91) Ending balance $ (10,051) $ 1,208 $ 1,442 $ (7,401)

Financing Activities

Financing Activities	6 Months Ended
Financing Activities	Jun. 30, 2023
Financing Activities [Abstract]
Financing Activities	Financing Activities On March 30, 2023, we issued and sold $239.0 million aggregate principal amount of Montana First Mortgage Bonds at a fixed interest rate of 5.57 percent maturing on March 30, 2033. On this same day, we issued and sold $31.0 million aggregate principal amount of South Dakota First Mortgage Bonds at a fixed interest rate of 5.57 percent maturing on March 30, 2033. On May 1, 2023, we issued and sold an additional $30.0 million aggregate principal amount of South Dakota First Mortgage Bonds at a fixed interest rate of 5.42 percent maturing on May 1, 2033. These bonds were issued in transactions exempt from the registration requirements of the Securities Act of 1933. Proceeds were used to repay a portion of our outstanding borrowings under our revolving credit facilities and for other general corporate purposes. The bonds are secured by our electric and natural gas assets in Montana and South Dakota. In June 2023, we amended our Equity Distribution Agreement to replace one of the sales agents. Pursuant to the Equity Distribution Agreement we may offer and sell shares of our common stock from time to time, having an aggregate gross sales price of up to $200.0 million, through an At-the-Market (ATM) offering program, including an equity forward sales component. This is a three-year agreement, expiring on February 11, 2024. During the three months ended June 30, 2023, we issued 188,682 shares of common stock under the ATM program at an average price of $57.83 per share, for net proceeds of $10.8 million which is net of sales commissions and other fees paid of approximately $0.1 million.

Segment Information

Segment Information	6 Months Ended
Segment Information	Jun. 30, 2023
Segment Reporting Information [Line Items]
Segment Reporting Disclosure [Text Block]	Segment Information Our reportable business segments are primarily engaged in the electric and natural gas business. The remainder of our operations are presented as other, which primarily consists of unallocated corporate costs and unregulated activity. We evaluate the performance of these segments based on utility margin. The accounting policies of the operating segments are the same as the parent except that the parent allocates some of its operating expenses to the operating segments according to a methodology designed by us for internal reporting purposes and involves estimates and assumptions. Financial data for the business segments are as follows (in thousands): Three Months Ended June 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 229,266 $ 61,236 $ — $ — $ 290,502 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 42,363 25,215 — — 67,578 Utility margin 186,903 36,021 — — 222,924 Operating and maintenance 41,368 13,472 — — 54,840 Administrative and general 21,635 8,321 (1) — 29,955 Property and other taxes 31,022 9,104 3 — 40,129 Depreciation and depletion 43,319 9,061 — — 52,380 Operating income (loss) 49,559 (3,937) (2) — 45,620 Interest expense, net (21,724) (4,490) (2,197) — (28,411) Other income (expense), net 2,954 1,144 (36) — 4,062 Income tax (expense) benefit (3,515) (373) 1,741 — (2,147) Net income (loss) $ 27,274 $ (7,656) $ (494) $ — $ 19,124 Total assets $ 5,878,433 $ 1,406,068 $ 9,741 $ — $ 7,294,242 Capital expenditures $ 94,690 $ 32,068 $ — $ — $ 126,758 Three Months Ended June 30, 2022 Electric Gas Other Eliminations Total Operating revenues $ 243,418 $ 79,586 $ — $ — $ 323,004 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 57,696 37,305 — — 95,001 Utility margin 185,722 42,281 — — 228,003 Operating and maintenance 40,822 12,515 — — 53,337 Administrative and general 20,115 7,171 (66) — 27,220 Property and other taxes 36,426 10,465 2 — 46,893 Depreciation and depletion 40,185 8,027 — — 48,212 Operating income 48,174 4,103 64 — 52,341 Interest expense, net (18,837) (3,323) (1,873) — (24,033) Other income, net 1,319 1,412 182 — 2,913 Income tax (expense) benefit (790) (1,000) 355 — (1,435) Net income (loss) $ 29,866 $ 1,192 $ (1,272) $ — $ 29,786 Total assets $ 5,593,989 $ 1,319,829 $ 6,479 $ — $ 6,920,297 Capital expenditures $ 91,673 $ 27,263 $ — $ — $ 118,936 Six Months Ended June 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 524,574 $ 220,470 $ — $ — $ 745,044 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 120,497 112,573 — — 233,070 Utility margin 404,077 107,897 — — 511,974 Operating and maintenance 83,781 26,920 — — 110,701 Administrative and general 46,603 18,087 13 — 64,703 Property and other taxes 69,273 20,002 5 — 89,280 Depreciation and depletion 87,217 18,411 — — 105,628 Operating income 117,203 24,477 (18) — 141,662 Interest expense, net (40,284) (7,741) (8,394) — (56,419) Other income (expense), net 6,320 2,559 (80) — 8,799 Income tax expense (10,143) (139) (2,106) — (12,388) Net income (loss) $ 73,096 $ 19,156 $ (10,598) $ — $ 81,654 Total assets $ 5,878,433 $ 1,406,068 $ 9,741 $ — $ 7,294,242 Capital expenditures $ 215,509 $ 47,853 $ — $ — $ 263,362 Six Months Ended June 30, 2022 Electric Gas Other Eliminations Total Operating revenues $ 515,145 $ 202,341 $ — $ — $ 717,486 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 135,319 94,755 — — 230,074 Utility margin 379,826 107,586 — — 487,412 Operating and maintenance 80,323 25,808 — — 106,131 Administrative and general 42,852 15,823 189 — 58,864 Property and other taxes 72,851 20,888 4 — 93,743 Depreciation and depletion 80,609 16,508 — — 97,117 Operating income (loss) 103,191 28,559 (193) — 131,557 Interest expense, net (37,806) (6,713) (3,230) — (47,749) Other income, net 4,301 2,942 391 — 7,634 Income tax (expense) benefit (1,784) (2,382) 1,620 — (2,546) Net income (loss) $ 67,902 $ 22,406 $ (1,412) $ — $ 88,896 Total assets $ 5,593,989 $ 1,319,829 $ 6,479 $ — $ 6,920,297 Capital expenditures $ 190,282 $ 44,156 $ — $ — $ 234,438

Revenue from Contracts with Cus

Revenue from Contracts with Customers	6 Months Ended
Revenue from Contracts with Customers	Jun. 30, 2023
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue from Contracts with Customers Nature of Goods and Services We provide retail electric and natural gas services to three primary customer classes. Our largest customer class consists of residential customers, which includes single private dwellings and individual apartments. Our commercial customers consist primarily of main street businesses, and our industrial customers consist primarily of manufacturing and processing businesses that turn raw materials into products. Electric Segment - Our regulated electric utility business primarily provides generation, transmission, and distribution services to customers in our Montana and South Dakota jurisdictions. We recognize revenue when electricity is delivered to the customer. Payments on our tariff-based sales are generally due 20-30 days after the billing date. Natural Gas Segment - Our regulated natural gas utility business primarily provides production, storage, transmission, and distribution services to customers in our Montana, South Dakota, and Nebraska jurisdictions. We recognize revenue when natural gas is delivered to the customer. Payments on our tariff-based sales are generally due 20-30 days after the billing date. Disaggregation of Revenue The following tables disaggregate our revenue by major source and customer class (in millions): Three Months Ended June 30, 2023 June 30, 2022 Electric Natural Gas Total Electric Natural Gas Total Montana $ 83.8 $ 17.6 $ 101.4 $ 70.7 $ 28.6 $ 99.3 South Dakota 15.7 8.4 24.1 15.6 9.4 25.0 Nebraska — 7.4 7.4 — 7.4 7.4 Residential 99.5 33.4 132.9 86.3 45.4 131.7 Montana 101.9 9.9 111.8 84.3 14.7 99.0 South Dakota 25.1 5.5 30.6 26.5 6.4 32.9 Nebraska — 4.7 4.7 — 4.5 4.5 Commercial 127.0 20.1 147.1 110.8 25.6 136.4 Industrial 10.8 0.2 11.0 9.0 0.2 9.2 Lighting, governmental, irrigation, and interdepartmental 8.7 0.3 9.0 8.3 0.5 8.8 Total Customer Revenues 246.0 54.0 300.0 214.4 71.7 286.1 Other tariff and contract based revenues 20.0 10.6 30.6 21.6 9.2 30.8 Total Revenue from Contracts with Customers 266.0 64.6 330.6 236.0 80.9 316.9 Regulatory amortization and other (36.7) (3.4) (40.1) 7.4 (1.3) 6.1 Total Revenues $ 229.3 $ 61.2 $ 290.5 $ 243.4 $ 79.6 $ 323.0 Six Months Ended June 30, 2023 June 30, 2022 Electric Natural Gas Total Electric Natural Gas Total Montana $ 209.3 $ 84.5 $ 293.8 $ 167.7 $ 80.9 $ 248.6 South Dakota 35.5 28.3 63.8 36.0 29.3 65.3 Nebraska — 28.0 28.0 — 22.8 22.8 Residential 244.8 140.8 385.6 203.7 133.0 336.7 Montana 214.5 46.3 260.8 170.8 41.8 212.6 South Dakota 50.3 19.8 70.1 54.1 20.9 75.0 Nebraska — 17.8 17.8 — 13.7 13.7 Commercial 264.8 83.9 348.7 224.9 76.4 301.3 Industrial 22.6 0.9 23.5 18.7 0.8 19.5 Lighting, governmental, irrigation, and interdepartmental 13.9 1.0 14.9 12.8 1.2 14.0 Total Customer Revenues 546.1 226.6 772.7 460.1 211.4 671.5 Other tariff and contract based revenues 41.3 22.9 64.2 41.7 19.2 60.9 Total Revenue from Contracts with Customers 587.4 249.5 836.9 501.8 230.6 732.4 Regulatory amortization and other (62.8) (29.1) (91.9) 13.3 (28.2) (14.9) Total Revenues $ 524.6 $ 220.4 $ 745.0 $ 515.1 $ 202.4 $ 717.5

Earnings Per Share

Earnings Per Share	3 Months Ended
Earnings Per Share	Jun. 30, 2023
Earnings Per Share [Abstract]
Earnings Per Share [Text Block]	Earnings Per Share Basic earnings per share are computed by dividing earnings applicable to common stock by the weighted average number of common shares outstanding for the period. Diluted earnings per share reflect the potential dilution of common stock equivalent shares that could occur if unvested shares were to vest. Common stock equivalent shares are calculated using the treasury stock method, as applicable. The dilutive effect is computed by dividing earnings applicable to common stock by the weighted average number of common shares outstanding plus the effect of the outstanding unvested restricted stock and performance share awards and forward equity sale. Average shares used in computing the basic and diluted earnings per share are as follows: Three Months Ended June 30, 2023 June 30, 2022 Basic computation 59,804,283 54,271,862 Dilutive effect of: Performance share awards (1) 45,391 34,900 Forward equity sale (2) — 834,126 Diluted computation 59,849,674 55,140,888 Six Months Ended June 30, 2023 June 30, 2022 Basic computation 59,790,316 54,184,798 Dilutive effect of: Performance share awards (1) 29,200 23,072 Forward equity sale (2) — 772,755 Diluted computation 59,819,516 54,980,625 (1) Performance share awards are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the award. (2) Forward equity shares are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the forward sale agreement.

Employee Benefit Plans

Employee Benefit Plans	6 Months Ended
Employee Benefit Plans	Jun. 30, 2023
Retirement Benefits [Abstract]
Pension and Other Postretirement Benefits Disclosure [Text Block]	Employee Benefit Plans We sponsor and/or contribute to pension and postretirement health care and life insurance benefit plans for eligible employees. Net periodic benefit cost (credit) for our pension and other postretirement plans consists of the following (in thousands): Pension Benefits Other Postretirement Benefits Three Months Ended June 30, Three Months Ended June 30, 2023 2022 2023 2022 Components of Net Periodic Benefit Cost (Credit) Service cost $ 1,422 $ 2,228 $ 79 $ 84 Interest cost 6,482 4,725 161 88 Expected return on plan assets (6,671) (6,034) (273) (261) Amortization of prior service credit — — 29 (473) Recognized actuarial (gain) loss (3) 191 5 (10) Net periodic benefit cost (credit) $ 1,230 $ 1,110 $ 1 $ (572) Pension Benefits Other Postretirement Benefits Six Months Ended June 30, Six Months Ended June 30, 2023 2022 2023 2022 Components of Net Periodic Benefit Cost (Credit) Service cost $ 2,916 $ 5,112 $ 166 $ 175 Interest cost 13,047 9,393 337 179 Expected return on plan assets (13,357) (12,086) (548) (523) Amortization of prior service credit — — 58 (946) Recognized actuarial loss (gain) 137 191 36 (24) Net periodic benefit cost (credit) $ 2,743 $ 2,610 $ 49 $ (1,139)

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and Contingencies Except as set forth below and in Note 2 - Regulatory Matters above, the circumstances set forth in Note 18 - Commitments and Contingencies to the financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2022 appropriately represent, in all material respects, the current status of our material commitments and contingent liabilities. ENVIRONMENTAL LIABILITIES AND REGULATION Environmental Matters The operation of electric generating, transmission and distribution facilities, and gas gathering, storage, transportation and distribution facilities, along with the development (involving site selection, environmental assessments, and permitting) and construction of these assets, are subject to extensive federal, state, and local environmental and land use laws and regulations. Our activities involve compliance with diverse laws and regulations that address emissions and impacts to the environment, including air and water, protection of natural resources, avian and wildlife. We monitor federal, state, and local environmental initiatives to determine potential impacts on our financial results. As new laws or regulations are implemented, our policy is to assess their applicability and implement the necessary modifications to our facilities or their operation to maintain ongoing compliance. Our environmental exposure includes a number of components, including remediation expenses related to the cleanup of current or former properties, and costs to comply with changing environmental regulations related to our operations. At present, our environmental reserve, which relates primarily to the remediation of former manufactured gas plant sites owned by us or for which we are responsible, is estimated to range between $20.9 million to $32.0 million. As of June 30, 2023, we had a reserve of approximately $25.7 million, which has not been discounted. Environmental costs are recorded when it is probable we are liable for the remediation and we can reasonably estimate the liability. We use a combination of site investigations and monitoring to formulate an estimate of environmental remediation costs for specific sites. Our monitoring procedures and development of actual remediation plans depend not only on site specific information but also on coordination with the different environmental regulatory agencies in our respective jurisdictions; therefore, while remediation exposure exists, it may be many years before costs are incurred. Over time, as costs become determinable, we may seek authorization to recover such costs in rates or seek insurance reimbursement as available and applicable; therefore, although we cannot guarantee regulatory recovery, we do not expect these costs to have a material effect on our consolidated financial position or results of operations. Global Climate Change - National and international actions have been initiated to address global climate change and the contribution of greenhouse gas (GHG) including, most significantly, carbon dioxide (CO 2 ) and methane emissions from natural gas. These actions include legislative proposals, Executive, Congressional and Environmental Protection Agency (EPA) actions at the federal level, state level activity, investor activism and private party litigation relating to emissions. Coal-fired plants have come under particular scrutiny due to their level of emissions. We have joint ownership interests in four coal-fired electric generating plants, all of which are operated by other companies. We are responsible for our proportionate share of the capital and operating costs while being entitled to our proportionate share of the power generated. Proposed EPA Rules - Congress has not passed any federal climate change legislation regarding GHG emissions from coal fired plants, and we cannot predict the timing or form of any potential legislation. Section 111(d) of the Clean Air Act (CAA) confers authority on EPA and the states to regulate emissions, including GHGs, from existing stationary sources. In May 2023, EPA proposed new GHG emissions standards for coal and natural gas-fired plants. In particular, the proposed rules would (i) strengthen the current New Source Performance Standards for newly built fossil fuel-fired stationary combustion turbines (generally natural gas-fired); (ii) establish emission guidelines for states to follow in limiting carbon pollution from existing fossil fuel-fired steam generating electric generating units (including coal, oil and natural gas-fired units); and (iii) establish emission guidelines for large, frequently used existing fossil fuel-fired stationary combustion turbines (generally natural gas-fired). In addition, in April 2023, EPA proposed to amend the Mercury Air Toxics Standard (MATS). Among other things, MATS currently sets stringent emission limits for acid gases, mercury, and other hazardous air pollutants from new and existing electric generating units. We are in compliance with existing MATS requirements. The proposed amendment of the MATS would strengthen the MATS requirements, and if adopted as written, both the GHG and MATS proposed rules could have a material negative impact on our coal-fired plants, including requiring potentially expensive upgrades or the early retirement of Colstrip Unit's 3 and 4 due to the rules making the facility uneconomic. Previous efforts by the EPA were met with extensive litigation and we anticipate a similar response if the proposed rules are adopted. As MATS and GHG regulations are implemented, it could result in additional material compliance costs. We will continue working with federal and state regulatory authorities, other utilities, and stakeholders to seek relief from any MATS or GHG regulations that, in our view, disproportionately impact customers in our region. Future additional environmental requirements - federal or state - could cause us to incur material costs of compliance, increase our costs of procuring electricity, decrease transmission revenue and impact cost recovery. Technology to efficiently capture, remove and/or sequester such GHG emissions or hazardous air pollutants may not be available within a timeframe consistent with the implementation of any such requirements. Physical impacts of climate change also may present potential risks for severe weather, such as droughts, fires, floods, wind, ice storms and tornadoes, in the locations where we operate or have interests. These potential risks may impact costs for electric and natural gas supply and maintenance of generation, distribution, and transmission facilities. LEGAL PROCEEDINGS State of Montana - Riverbed Rents On April 1, 2016, the State of Montana (State) filed a complaint on remand (the State’s Complaint) with the Montana First Judicial District Court (State District Court), naming us, along with Talen Montana, LLC (Talen) as defendants. The State claimed it owns the riverbeds underlying 10 of our, and formerly Talen’s, hydroelectric facilities (dams, along with reservoirs and tailraces) on the Missouri, Madison and Clark Fork Rivers, and seeks rents for Talen’s and our use and occupancy of such lands. The facilities at issue include the Hebgen, Madison, Hauser, Holter, Black Eagle, Rainbow, Cochrane, Ryan, and Morony facilities on the Missouri and Madison Rivers and the Thompson Falls facility on the Clark Fork River. We acquired these facilities from Talen in November 2014. The litigation has a long prior history in state and federal court, including before the United States Supreme Court, as detailed in Note 18 - Commitments and Contingencies to the financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2022 . On April 20, 2016, we removed the case from State District Court to the United States District Court for the District of Montana (Federal District Court). On August 1, 2018, the Federal District Court granted our and Talen’s motions to dismiss the State’s Complaint as it pertains to the navigability of the riverbeds associated with four of our hydroelectric facilities near Great Falls. A bench trial before the Federal District Court commenced January 4, 2022, and concluded on January 18, 2022, which addressed the issue of navigability concerning our other six facilities. Damages were bifurcated by agreement and will be tried separately should the Federal District Court find any segments navigable. While we await the Federal District Court decision on navigability, the damages phase of the case remains stayed. We dispute the State’s claims and intend to continue to vigorously defend the lawsuit. At this time, we cannot predict an outcome. If the Federal District Court determines the riverbeds are navigable under the remaining six facilities that were not dismissed and if it calculates damages as the State District Court did in 2008, we estimate the annual rents could be approximately $3.8 million commencing when we acquired the facilities in November 2014. We anticipate that any obligation to pay the State rent for use and occupancy of the riverbeds would be recoverable in rates from customers, although there can be no assurances that the MPSC would approve any such recovery. Colstrip Arbitration The remaining depreciable life of our investment in Colstrip Unit 4 is through 2042. The six owners of Colstrip Units 3 and 4 currently share the operating costs pursuant to the terms of an Ownership and Operation Agreement (O&O Agreement). However, several of the owners are mandated by Washington and Oregon law to eliminate coal-fired resources in 2025 and 2029, respectively. As a result of the mandate, the owners have disagreed on various operational funding decisions, including whether closure requires each owner’s consent under the O&O Agreement. On March 12, 2021, we initiated an arbitration under the O&O Agreement (the “Arbitration”), to resolve the issues of whether closure requires each owner's consent and to clarify each owner's obligations to continue to fund operations until all joint owners agree on closure. The owners previously initiated efforts to identify arbitrators and have agreed to stay the Arbitration through September 29, 2023, while they explore a potential resolution to their disagreements. Colstrip Coal Dust Litigation On December 14, 2020, a claim was filed against Talen in the Montana Sixteenth Judicial District Court, Rosebud County, Cause No. CV-20-58. Talen is one of the co-owners of Colstrip Unit 3, and the operator of Units 3 and 4. The plaintiffs allege they have suffered adverse effects from coal dust generated during operations associated with Colstrip. On August 26, 2021, the claim was amended to add in excess of 100 plaintiffs. It also added NorthWestern, the other owners of Colstrip, and Westmoreland Rosebud Mining LLC, as defendants. Plaintiffs are seeking economic damages, costs and disbursements, punitive damages, attorneys’ fees, and an injunction prohibiting defendants from allowing coal dust to blow onto plaintiffs’ properties. Since this lawsuit remains in its early discovery stages, we are unable to predict outcomes or estimate a range of reasonably possible losses. BNSF Demands for Indemnity and Remediation Costs NorthWestern has received a demand for indemnity from BNSF Railway Company (BNSF) for past and future environmental investigation and remediation costs incurred by BNSF at one of the three operable units at the Anaconda Copper Mining (ACM) Smelter and Refinery Superfund Site, located near Great Falls, Montana. Smelter and refining operations at the site commenced in 1893 and continued until 1980. According to EPA, the smelter and refining operations have contaminated soil, groundwater and surface water resources around the site with lead, arsenic and other metal wastes. ARCO (Atlantic Richfield Company) initiated reclamation and maintenance activities in the 1980s and 1990s. Between 2002 and 2008, the EPA conducted several site investigations. In March 2011, the EPA placed the ACM Smelter and Refinery Site on the Superfund program’s National Priority List. The Superfund Site is 427 acres and contains three operable units: Operable Unit 1 (consisting of five subsections including the Railroad Corridor and four other “areas of interest”), Operable Unit 2 (the former smelter and refinery site), and Operable Unit 3 (the Missouri River that flows along the south sides of Operable Units 1 and 2). NorthWestern owns property in the Railroad Corridor sub-section of Operable Unit 1. BNSF claims it is entitled to indemnity and contribution from NorthWestern for the costs it has and will incur to investigate and remediate contamination in Operable Unit 1. BNSF reports it has incurred in excess of $4.4 million, pending final resolution, of response and oversight costs incurred by government agencies (EPA and Montana DEQ), in investigative and other response costs associated with Operable Unit 1, and that in the future it will incur additional costs to implement the final remedy for Operable Unit 1. In the Record of Decision (ROD) for Operable Unit 1 issued on August 21, 2021, the EPA estimated the costs to implement the selected remedies for the Railroad Corridor will be approximately $4.1 million. In the ROD, the EPA also estimated the costs to implement the selected remedy (including institutional controls) for the four “areas of interest” in Operable Unit 1 would be approximately $1.8 million, with annual operating costs of ten thousand dollars. We are evaluating BNSF’s claim and are unable at this time to predict outcomes or estimate a range of reasonably possible losses. Yellowstone County Generating Station Air Permit On October 21, 2021, the Montana Environmental Information Center (MEIC) and the Sierra Club filed a lawsuit in Montana State Court, against the Montana Department of Environmental Quality (MDEQ) and NorthWestern, alleging that the environmental analysis conducted by MDEQ prior to issuance of the Yellowstone County Generating Station's air quality construction permit was inadequate. On April 4, 2023, the Montana District Court issued an order finding MDEQ's environmental analysis was deficient in not addressing exterior lighting and greenhouse gases and remanded it back to MDEQ to address the deficiencies and vacated the air quality permit pending that remand. As a result of the vacatur of the permit, we paused construction. On June 8, 2023, the Montana District Court granted our motion to stay the order vacating the air quality permit pending the outcome of our notice of appeal with the Montana Supreme Court. We recommenced construction in June 2023 and expect the plant to be operational by the end of the third quarter 2024. On May 10, 2023, Montana House Bill 971 was signed into law, preventing the MDEQ from considering climate impacts in its analysis of large projects such as coal mines and power plants, and on June 1, 2023, the MDEQ issued its supplemental air quality permit that contained the updated exterior lighting analysis, and the MDEQ indicated that no other analysis was necessary. The comment period concerning the MDEQ’s supplemental air quality permit ended on July 3, 2023. We expect to receive a final revised permit from the MDEQ during the third quarter of 2023. This current lawsuit, as well as additional potential legal challenges related to the Yellowstone County Generating Station, could delay the project timing and increase costs. Other Legal Proceedings We are also subject to various other legal proceedings, governmental audits and claims that arise in the ordinary course of business. In our opinion, the amount of ultimate liability with respect to these other actions will not materially affect our financial position, results of operations, or cash flows.

Organization, Consolidation and

Organization, Consolidation and Presentation of Financial Statements (Tables)	6 Months Ended
	Jun. 30, 2023
Cash, Cash Equivalents, and Restricted Cash [Abstract]
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the Condensed Consolidated Balance Sheets that sum to the total of the same such amounts shown in the Condensed Consolidated Statements of Cash Flows (in thousands): June 30, December 31, June 30, December 31, 2023 2022 2022 2021 Cash and cash equivalents $ 7,757 $ 8,489 $ 8,117 $ 2,820 Restricted cash 16,263 13,974 17,963 15,942 Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows $ 24,020 $ 22,463 $ 26,080 $ 18,762

Regulated Operations (Tables)

Regulated Operations (Tables)	6 Months Ended
Regulated Operations (Tables)	Jun. 30, 2023
Regulated Operations [Abstract]
Montana Rate Review	On August 8, 2022, we filed a Montana electric and natural gas rate review with the Montana Public Service Commission (MPSC) requesting an annual increase to electric and natural gas utility rates. On September 28, 2022, the MPSC approved interim rates effective October 1, 2022, subject to refund. Subsequently, we modified our request through rebuttal testimony. On April 3, 2023, we filed a settlement with certain parties, which is subject to approval by the MPSC. The details of our rebuttal request, interim rates granted, and settlement agreement are set forth below: Requested Revenue Increase Through Rebuttal Testimony (in millions) Electric Natural Gas Base Rates $90.6 $22.4 Power Cost & Credit Mechanism (PCCAM) (1) $69.7 n/a Property Tax (tracker base adjustment) (1) $14.5 $4.2 Total Revenue Increase Requested through Rebuttal Testimony $174.8 $26.6 Interim Revenue Increase Granted (in millions) Electric Natural Gas Base Rates $29.4 $1.7 PCCAM (1) $61.1 n/a Property Tax (tracker base adjustment) (1)(2) $10.8 $2.9 Total Interim Revenue Granted $101.3 $4.6 Requested Revenue Increase Through Settlement Agreement (in millions) Electric Natural Gas Base Rates $67.4 $14.1 PCCAM (1) $69.7 n/a Property Tax (tracker base adjustment) (1) $14.5 $4.2 Total Revenue Increase Requested Through Settlement Agreement $151.6 $18.3 (1) These items are flow-through costs. PCCAM reflects our fuel and purchased power costs. (2) Our requested interim property tax base increases went into effect on January 1, 2023, as part of our 2023 property tax tracker filing.

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
Income Taxes (Tables)	Jun. 30, 2023
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	The following table summarizes the differences between our effective tax rate and the federal statutory rate (in thousands): Three Months Ended June 30, 2023 2022 Income before income taxes $ 21,271 $ 31,221 Income tax calculated at federal statutory rate 4,467 21.0 % 6,554 21.0 % Permanent or flow-through adjustments: State income tax, net of federal provisions 273 1.3 431 1.4 Flow-through repairs deductions (1,708) (8.0) (3,313) (10.6) Production tax credits (1,147) (5.4) (2,558) (8.2) Amortization of excess deferred income tax (233) (1.1) (162) (0.5) Plant and depreciation flow-through items 201 0.9 398 1.3 Other, net 294 1.4 85 0.2 (2,320) (10.9) (5,119) (16.4) Income tax expense $ 2,147 10.1 % $ 1,435 4.6 % Six Months Ended June 30, 2023 2022 Income before income taxes $ 94,042 $ 91,442 Income tax calculated at federal statutory rate 19,749 21.0 % 19,200 21.0 % Permanent or flow through adjustments: State income, net of federal provisions 1,232 1.3 831 0.9 Flow-through repairs deductions (7,553) (8.0) (10,114) (11.1) Production tax credits (4,346) (4.6) (6,382) (7.0) Reduction to previously claimed alternative minimum tax credit 3,186 3.4 — — Amortization of excess deferred income tax (1,032) (1.1) (573) (0.6) Share-based compensation 388 0.4 (253) (0.3) Plant and depreciation flow through items 889 0.9 143 0.2 Other, net (125) (0.1) (306) (0.3) (7,361) (7.8) (16,654) (18.2) Income tax expense $ 12,388 13.2 % $ 2,546 2.8 %

Accumulated Other Comprehensive

Accumulated Other Comprehensive (Income) Loss (Tables)	6 Months Ended
Accumulated Other Comprehensive (Income) Loss (Tables)	Jun. 30, 2023
Statement of Financial Position [Abstract]
Comprehensive Income (Loss) [Table Text Block]	The following tables display the components of Other Comprehensive (Loss) Income, after-tax, and the related tax effects (in thousands): Three Months Ended June 30, 2023 June 30, 2022 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (1) $ — $ (1) $ 1 $ — $ 1 Reclassification of net income on derivative instruments 153 (40) 113 153 (40) 113 Postretirement medical liability adjustment (212) 45 (167) (212) 54 (158) Other comprehensive (loss) income $ (60) $ 5 $ (55) $ (58) $ 14 $ (44) Six Months Ended June 30, 2023 June 30, 2022 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (3) $ — $ (3) $ (1) $ — $ (1) Reclassification of net income on derivative instruments 306 (80) 226 306 (80) 226 Postretirement medical liability adjustment (424) 90 (334) (424) 108 (316) Other comprehensive (loss) income $ (121) $ 10 $ (111) $ (119) $ 28 $ (91)
Accumulated Other Comprehensive Income [Table Text Block]	Balances by classification included within accumulated other comprehensive loss (AOCL) on the Condensed Consolidated Balance Sheets are as follows, net of tax (in thousands): June 30, 2023 December 31, 2022 Foreign currency translation $ 1,432 $ 1,435 Derivative instruments designated as cash flow hedges (9,599) (9,825) Postretirement medical plans 208 542 Accumulated other comprehensive loss $ (7,959) $ (7,848)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following tables display the changes in AOCL by component, net of tax (in thousands): Three Months Ended June 30, 2023 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,712) $ 375 $ 1,433 $ (7,904) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (167) — (167) Net current-period other comprehensive income (loss) 113 (167) (1) (55) Ending balance $ (9,599) $ 208 $ 1,432 $ (7,959) Three Months Ended June 30, 2022 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (10,164) $ 1,366 $ 1,441 $ (7,357) Other comprehensive loss before reclassifications — — 1 1 Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (158) — (158) Net current-period other comprehensive income (loss) 113 (158) 1 (44) Ending balance $ (10,051) $ 1,208 $ 1,442 $ (7,401) Six Months Ended June 30, 2023 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Pension and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,825) $ 542 $ 1,435 $ (7,848) Other comprehensive loss before reclassifications — — (3) (3) Amounts reclassified from AOCL Interest Expense 226 — — 226 Amounts reclassified from AOCL — (334) — (334) Net current-period other comprehensive income (loss) 226 (334) (3) (111) Ending balance $ (9,599) $ 208 $ 1,432 $ (7,959) Six Months Ended June 30, 2022 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Pension and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (10,277) $ 1,524 $ 1,443 $ (7,310) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 226 — — 226 Amounts reclassified from AOCL — (316) — (316) Net current-period other comprehensive income (loss) 226 (316) (1) (91) Ending balance $ (10,051) $ 1,208 $ 1,442 $ (7,401)

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
Segment Information (Tables)	Jun. 30, 2023
Segment Reporting Information [Line Items]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	Financial data for the business segments are as follows (in thousands): Three Months Ended June 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 229,266 $ 61,236 $ — $ — $ 290,502 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 42,363 25,215 — — 67,578 Utility margin 186,903 36,021 — — 222,924 Operating and maintenance 41,368 13,472 — — 54,840 Administrative and general 21,635 8,321 (1) — 29,955 Property and other taxes 31,022 9,104 3 — 40,129 Depreciation and depletion 43,319 9,061 — — 52,380 Operating income (loss) 49,559 (3,937) (2) — 45,620 Interest expense, net (21,724) (4,490) (2,197) — (28,411) Other income (expense), net 2,954 1,144 (36) — 4,062 Income tax (expense) benefit (3,515) (373) 1,741 — (2,147) Net income (loss) $ 27,274 $ (7,656) $ (494) $ — $ 19,124 Total assets $ 5,878,433 $ 1,406,068 $ 9,741 $ — $ 7,294,242 Capital expenditures $ 94,690 $ 32,068 $ — $ — $ 126,758 Three Months Ended June 30, 2022 Electric Gas Other Eliminations Total Operating revenues $ 243,418 $ 79,586 $ — $ — $ 323,004 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 57,696 37,305 — — 95,001 Utility margin 185,722 42,281 — — 228,003 Operating and maintenance 40,822 12,515 — — 53,337 Administrative and general 20,115 7,171 (66) — 27,220 Property and other taxes 36,426 10,465 2 — 46,893 Depreciation and depletion 40,185 8,027 — — 48,212 Operating income 48,174 4,103 64 — 52,341 Interest expense, net (18,837) (3,323) (1,873) — (24,033) Other income, net 1,319 1,412 182 — 2,913 Income tax (expense) benefit (790) (1,000) 355 — (1,435) Net income (loss) $ 29,866 $ 1,192 $ (1,272) $ — $ 29,786 Total assets $ 5,593,989 $ 1,319,829 $ 6,479 $ — $ 6,920,297 Capital expenditures $ 91,673 $ 27,263 $ — $ — $ 118,936 Six Months Ended June 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 524,574 $ 220,470 $ — $ — $ 745,044 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 120,497 112,573 — — 233,070 Utility margin 404,077 107,897 — — 511,974 Operating and maintenance 83,781 26,920 — — 110,701 Administrative and general 46,603 18,087 13 — 64,703 Property and other taxes 69,273 20,002 5 — 89,280 Depreciation and depletion 87,217 18,411 — — 105,628 Operating income 117,203 24,477 (18) — 141,662 Interest expense, net (40,284) (7,741) (8,394) — (56,419) Other income (expense), net 6,320 2,559 (80) — 8,799 Income tax expense (10,143) (139) (2,106) — (12,388) Net income (loss) $ 73,096 $ 19,156 $ (10,598) $ — $ 81,654 Total assets $ 5,878,433 $ 1,406,068 $ 9,741 $ — $ 7,294,242 Capital expenditures $ 215,509 $ 47,853 $ — $ — $ 263,362 Six Months Ended June 30, 2022 Electric Gas Other Eliminations Total Operating revenues $ 515,145 $ 202,341 $ — $ — $ 717,486 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 135,319 94,755 — — 230,074 Utility margin 379,826 107,586 — — 487,412 Operating and maintenance 80,323 25,808 — — 106,131 Administrative and general 42,852 15,823 189 — 58,864 Property and other taxes 72,851 20,888 4 — 93,743 Depreciation and depletion 80,609 16,508 — — 97,117 Operating income (loss) 103,191 28,559 (193) — 131,557 Interest expense, net (37,806) (6,713) (3,230) — (47,749) Other income, net 4,301 2,942 391 — 7,634 Income tax (expense) benefit (1,784) (2,382) 1,620 — (2,546) Net income (loss) $ 67,902 $ 22,406 $ (1,412) $ — $ 88,896 Total assets $ 5,593,989 $ 1,319,829 $ 6,479 $ — $ 6,920,297 Capital expenditures $ 190,282 $ 44,156 $ — $ — $ 234,438

Revenue from Contracts with C_2

Revenue from Contracts with Customers Disaggregation of Revenue (Tables)	6 Months Ended
	Jun. 30, 2023
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	The following tables disaggregate our revenue by major source and customer class (in millions): Three Months Ended June 30, 2023 June 30, 2022 Electric Natural Gas Total Electric Natural Gas Total Montana $ 83.8 $ 17.6 $ 101.4 $ 70.7 $ 28.6 $ 99.3 South Dakota 15.7 8.4 24.1 15.6 9.4 25.0 Nebraska — 7.4 7.4 — 7.4 7.4 Residential 99.5 33.4 132.9 86.3 45.4 131.7 Montana 101.9 9.9 111.8 84.3 14.7 99.0 South Dakota 25.1 5.5 30.6 26.5 6.4 32.9 Nebraska — 4.7 4.7 — 4.5 4.5 Commercial 127.0 20.1 147.1 110.8 25.6 136.4 Industrial 10.8 0.2 11.0 9.0 0.2 9.2 Lighting, governmental, irrigation, and interdepartmental 8.7 0.3 9.0 8.3 0.5 8.8 Total Customer Revenues 246.0 54.0 300.0 214.4 71.7 286.1 Other tariff and contract based revenues 20.0 10.6 30.6 21.6 9.2 30.8 Total Revenue from Contracts with Customers 266.0 64.6 330.6 236.0 80.9 316.9 Regulatory amortization and other (36.7) (3.4) (40.1) 7.4 (1.3) 6.1 Total Revenues $ 229.3 $ 61.2 $ 290.5 $ 243.4 $ 79.6 $ 323.0 Six Months Ended June 30, 2023 June 30, 2022 Electric Natural Gas Total Electric Natural Gas Total Montana $ 209.3 $ 84.5 $ 293.8 $ 167.7 $ 80.9 $ 248.6 South Dakota 35.5 28.3 63.8 36.0 29.3 65.3 Nebraska — 28.0 28.0 — 22.8 22.8 Residential 244.8 140.8 385.6 203.7 133.0 336.7 Montana 214.5 46.3 260.8 170.8 41.8 212.6 South Dakota 50.3 19.8 70.1 54.1 20.9 75.0 Nebraska — 17.8 17.8 — 13.7 13.7 Commercial 264.8 83.9 348.7 224.9 76.4 301.3 Industrial 22.6 0.9 23.5 18.7 0.8 19.5 Lighting, governmental, irrigation, and interdepartmental 13.9 1.0 14.9 12.8 1.2 14.0 Total Customer Revenues 546.1 226.6 772.7 460.1 211.4 671.5 Other tariff and contract based revenues 41.3 22.9 64.2 41.7 19.2 60.9 Total Revenue from Contracts with Customers 587.4 249.5 836.9 501.8 230.6 732.4 Regulatory amortization and other (62.8) (29.1) (91.9) 13.3 (28.2) (14.9) Total Revenues $ 524.6 $ 220.4 $ 745.0 $ 515.1 $ 202.4 $ 717.5

Earnings Per Share (Tables)

Earnings Per Share (Tables)	6 Months Ended
Earnings Per Share (Tables)	Jun. 30, 2023
Earnings Per Share [Abstract]
Schedule of Weighted Average Number of Shares [Table Text Block]	Average shares used in computing the basic and diluted earnings per share are as follows: Three Months Ended June 30, 2023 June 30, 2022 Basic computation 59,804,283 54,271,862 Dilutive effect of: Performance share awards (1) 45,391 34,900 Forward equity sale (2) — 834,126 Diluted computation 59,849,674 55,140,888 Six Months Ended June 30, 2023 June 30, 2022 Basic computation 59,790,316 54,184,798 Dilutive effect of: Performance share awards (1) 29,200 23,072 Forward equity sale (2) — 772,755 Diluted computation 59,819,516 54,980,625 (1) Performance share awards are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the award. (2) Forward equity shares are included in diluted weighted average number of shares outstanding based upon what would be issued if the end of the most recent reporting period was the end of the term of the forward sale agreement.

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	6 Months Ended
Employee Benefit Plans (Tables)	Jun. 30, 2023
Retirement Benefits [Abstract]
Schedule of Defined Benefit Plans Disclosures [Table Text Block]	Net periodic benefit cost (credit) for our pension and other postretirement plans consists of the following (in thousands): Pension Benefits Other Postretirement Benefits Three Months Ended June 30, Three Months Ended June 30, 2023 2022 2023 2022 Components of Net Periodic Benefit Cost (Credit) Service cost $ 1,422 $ 2,228 $ 79 $ 84 Interest cost 6,482 4,725 161 88 Expected return on plan assets (6,671) (6,034) (273) (261) Amortization of prior service credit — — 29 (473) Recognized actuarial (gain) loss (3) 191 5 (10) Net periodic benefit cost (credit) $ 1,230 $ 1,110 $ 1 $ (572) Pension Benefits Other Postretirement Benefits Six Months Ended June 30, Six Months Ended June 30, 2023 2022 2023 2022 Components of Net Periodic Benefit Cost (Credit) Service cost $ 2,916 $ 5,112 $ 166 $ 175 Interest cost 13,047 9,393 337 179 Expected return on plan assets (13,357) (12,086) (548) (523) Amortization of prior service credit — — 58 (946) Recognized actuarial loss (gain) 137 191 36 (24) Net periodic benefit cost (credit) $ 2,743 $ 2,610 $ 49 $ (1,139)

Nature of Operations and Basi_2

Nature of Operations and Basis of Consolidation (Details)	Dec. 31, 2022 customers
Number of customers	764,200

Nature of Operations and Basi_3

Nature of Operations and Basis of Consolidation Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Cash, Cash Equivalents, and Restricted Cash [Abstract]
Cash and cash equivalents	$ 7,757	$ 8,489	$ 8,117	$ 2,820
Restricted cash	16,263	13,974	17,963	15,942
Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows	$ 24,020	$ 22,463	$ 26,080	$ 18,762

Regulatory Matters (Details)

Regulatory Matters (Details) $ in Millions	6 Months Ended
Regulatory Matters (Details) $ in Millions	Jun. 30, 2023 USD ($) Rate
Montana \| Electricity, US Regulated
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	$ 174.8
Public Utilities, Requested Return on Equity, Percentage	9.65%
Public Utilities, Requested Equity Capital Structure, Percentage \| Rate	48.02%
Interim rate increase, amount	$ 101.3
PCCAM base cost amount	138.7
Public Utilities, PCCAM base cost requested through settlement agreement	208.4
Montana \| Electricity, US Regulated \| Base electric rate
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	90.6
Montana \| Electricity, US Regulated \| PCCAM base amount
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	69.7
Montana \| Electricity, US Regulated \| Electric property tax tracker true-up
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	14.5
Montana \| Electricity, US Regulated \| Base electric interim rate
Public Utilities, General Disclosures [Line Items]
Interim rate increase, amount	29.4
Montana \| Electricity, US Regulated \| PCCAM base interim amount
Public Utilities, General Disclosures [Line Items]
Interim rate increase, amount	61.1
Montana \| Electricity, US Regulated \| Electric property tax tracker true-up interim amount
Public Utilities, General Disclosures [Line Items]
Interim rate increase, amount	10.8
Montana \| Electricity, US Regulated \| Base electric interim rate requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	67.4
Montana \| Electricity, US Regulated \| PCCAM base interim amount requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	69.7
Montana \| Electricity, US Regulated \| Electric property tax tracker true-up amount requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	14.5
Montana \| Electricity, US Regulated \| Total electric rates requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	151.6
Montana \| Natural Gas, US Regulated
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	$ 26.6
Public Utilities, Requested Return on Equity, Percentage	9.55%
Interim rate increase, amount	$ 4.6
Montana \| Natural Gas, US Regulated \| Base gas rate
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	22.4
Montana \| Natural Gas, US Regulated \| Gas property tax tracker true-up
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	4.2
Montana \| Natural Gas, US Regulated \| Base natural gas interim rates
Public Utilities, General Disclosures [Line Items]
Interim rate increase, amount	1.7
Montana \| Natural Gas, US Regulated \| Natural gas property tax tracker true-up interim amount
Public Utilities, General Disclosures [Line Items]
Interim rate increase, amount	2.9
Montana \| Natural Gas, US Regulated \| Base natural gas rate requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	14.1
Montana \| Natural Gas, US Regulated \| Natural gas property tax tracker true-up amount requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	4.2
Montana \| Natural Gas, US Regulated \| Total natural gas rates requested through settlement agreement
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	18.3
South Dakota
Public Utilities, General Disclosures [Line Items]
Rate base	$ 787.3
South Dakota \| Electricity, US Regulated
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.70%
Public Utilities, Requested Equity Capital Structure, Percentage \| Rate	50.50%
South Dakota \| Electricity, US Regulated \| Base electric rate
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	$ 30.9

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	6 Months Ended
Income Taxes (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Income Tax Contingency [Line Items]
Unrecognized tax benefit more likely than not percentage threshold	50%
Unrecognized tax benefits	$ 29.5
Unrecognized tax benefits that would impact effective tax rate	27.8
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 2.1	$ 1.4
Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Open Tax Year	2019

Income Taxes Effective Tax Rate

Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Effective tax rate reconciliation
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ 21,271	$ 31,221	$ 94,042	$ 91,442
Income tax calculated at federal statutory rate	4,467	6,554	19,749	19,200
State income tax, net of federal provisions	273	431	1,232	831
Flow-through repairs deductions	(1,708)	(3,313)	(7,553)	(10,114)
Production tax credits	(1,147)	(2,558)	(4,346)	(6,382)
Reduction to previously claimed alternative minimum tax credit			3,186	0
Amortization of excess deferred income tax	(233)	(162)	(1,032)	(573)
Share-based compensation			388	(253)
Plant and depreciation flow-through items	201	398	889	143
Other, net	294	85	(125)	(306)
Total Other Reconciling Items	(2,320)	(5,119)	(7,361)	(16,654)
Income tax expense	$ 2,147	$ 1,435	$ 12,388	$ 2,546
Income tax calculated at federal statutory rate	21%	21%	21%	21%
State income tax, net of federal provisions	1.30%	1.40%	1.30%	0.90%
Flow-through repairs deductions	(8.00%)	(10.60%)	(8.00%)	(11.10%)
Production tax credits	(5.40%)	(8.20%)	(4.60%)	(7.00%)
Reduction to previously claimed alternative minimum tax credit			3.40%	0%
Amortization of excess deferred income tax	(1.10%)	(0.50%)	(1.10%)	(0.60%)
Share-based compensation			0.40%	(0.30%)
Plant and depreciation flow-through items	0.90%	1.30%	0.90%	0.20%
Other, net	1.40%	0.20%	(0.10%)	(0.30%)
Total Other Reconciling Items	(10.90%)	(16.40%)	(7.80%)	(18.20%)
Income tax expense	10.10%	4.60%	13.20%	2.80%

Comprehensive Income (Loss) (De

Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Other Comprehensive Income (Loss), before Tax [Abstract]
Foreign currency translation adjustment	$ (1)	$ 1	$ (3)	$ (1)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(153)	(153)	(306)	(306)
Postretirement medical liability adjustment	(212)	(212)	(424)	(424)
Other Comprehensive Income (Loss), before Tax	(60)	(58)	(121)	(119)
Other Comprehensive Income (Loss), Tax [Abstract]
Foreign currency translation adjustment	0	0	0	0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	(40)	(40)	(80)	(80)
Postretirement medical liability adjustment	45	54	90	108
Other Comprehensive Income (Loss), Tax	5	14	10	28
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustment	(1)	1	(3)	(1)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(113)	(113)	(226)	(226)
Postretirement medical liability adjustment	(167)	(158)	(334)	(316)
Total Other Comprehensive Loss	$ (55)	$ (44)	$ (111)	$ (91)

Classification within AOCL on t

Classification within AOCL on the Condenced Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Foreign currency translation	$ 1,432		$ 1,435
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax	(9,599)		(9,825)
Postretirement medical plans	208		542
Accumulated other comprehensive loss	$ (7,959)	$ (7,904)	$ (7,848)	$ (7,401)	$ (7,357)	$ (7,310)

Changes in AOCI by Component (D

Changes in AOCI by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (7,904)	$ (7,357)	$ (7,848)	$ (7,310)
Other comprehensive loss before reclassifications	(1)	1	(3)	(1)
Amounts reclassified from AOCL	113	113	226	226
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(167)	(158)	(334)	(316)
Total Other Comprehensive Loss	(55)	(44)	(111)	(91)
Ending balance	(7,959)	(7,401)	(7,959)	(7,401)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(9,712)	(10,164)	(9,825)	(10,277)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from AOCL	113	113		226
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0			0
Total Other Comprehensive Loss	113	113	226	226
Ending balance	(9,599)	(10,051)	(9,599)	(10,051)
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	1,433	1,441	1,435	1,443
Other comprehensive loss before reclassifications	(1)	1	(3)	(1)
Amounts reclassified from AOCL	0	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	0	0
Total Other Comprehensive Loss	(1)	1	(3)	(1)
Ending balance	1,432	1,442	1,432	1,442
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	375	1,366	542	1,524
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from AOCL	0	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(167)	(158)	(334)	(316)
Total Other Comprehensive Loss	(167)	(158)	(334)	(316)
Ending balance	$ 208	$ 1,208	$ 208	$ 1,208

Financing Activities (Details)

Financing Activities (Details) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2023 USD ($) $ / shares shares
Equity Distribution Agreement
Debt Instrument [Line Items]
Common Stock Aggregate Gross Sales Price Maximum	$ 200
Issuance of shares \| shares	188,682
Common Stock Average Price \| $ / shares	$ 57.83
Stock Issued, Value, Net of Fees	$ 10.8
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	0.1
Montana First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 239
Debt Instrument, Interest Rate, Stated Percentage	5.57%
Long-term Debt, Maturity Date	Mar. 30, 2033
South Dakota First Mortgage Bonds Due 2033
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 31
Debt Instrument, Interest Rate, Stated Percentage	5.57%
Long-term Debt, Maturity Date	Mar. 30, 2033
South Dakota First Mortgage Bonds Due 2033, To Be Issued May 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 30
Montana First Mortgage Bonds Due May 1, 2033
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.42%
Long-term Debt, Maturity Date	May 01, 2033
Secured Debt Montana 2.00% Due 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 144.7
Debt Instrument, Interest Rate, Stated Percentage	3.88%
Long-term Debt, Maturity Date	Jul. 01, 2028
Secured Debt Montana 2.00% Due 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 144.7
Debt Instrument, Interest Rate, Stated Percentage	2%

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Segment Information (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Operating revenues	$ 290,502	$ 323,004	$ 745,044	$ 717,486
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	67,578	95,001	233,070	230,074
Utility margin	222,924	228,003	511,974	487,412
Operating and maintenance	54,840	53,337	110,701	106,131
Administrative and general	29,955	27,220	64,703	58,864
Property and other taxes	40,129	46,893	89,280	93,743
Depreciation and depletion	52,380	48,212	105,628	97,117
Operating income (loss)	45,620	52,341	141,662	131,557
Interest expense, net	(28,411)	(24,033)	(56,419)	(47,749)
Other income (expense), net	4,062	2,913	8,799	7,634
Income tax expense	(2,147)	(1,435)	(12,388)	(2,546)
Net Income	19,124	29,786	81,654	88,896
Total assets	7,294,242	6,920,297	7,294,242	6,920,297	$ 7,317,783
Capital expenditures	126,758	118,936	263,362	234,438
Operating Segments \| Electricity, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	229,266	243,418	524,574	515,145
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	42,363	57,696	120,497	135,319
Utility margin	186,903	185,722	404,077	379,826
Operating and maintenance	41,368	40,822	83,781	80,323
Administrative and general	21,635	20,115	46,603	42,852
Property and other taxes	31,022	36,426	69,273	72,851
Depreciation and depletion	43,319	40,185	87,217	80,609
Operating income (loss)	49,559	48,174	117,203	103,191
Interest expense, net	(21,724)	(18,837)	(40,284)	(37,806)
Other income (expense), net	2,954	1,319	6,320	4,301
Income tax expense	(3,515)	(790)	(10,143)	(1,784)
Net Income	27,274	29,866	73,096	67,902
Total assets	5,878,433	5,593,989	5,878,433	5,593,989
Capital expenditures	94,690	91,673	215,509	190,282
Operating Segments \| Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	61,236	79,586	220,470	202,341
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	25,215	37,305	112,573	94,755
Utility margin	36,021	42,281	107,897	107,586
Operating and maintenance	13,472	12,515	26,920	25,808
Administrative and general	8,321	7,171	18,087	15,823
Property and other taxes	9,104	10,465	20,002	20,888
Depreciation and depletion	9,061	8,027	18,411	16,508
Operating income (loss)	(3,937)	4,103	24,477	28,559
Interest expense, net	(4,490)	(3,323)	(7,741)	(6,713)
Other income (expense), net	1,144	1,412	2,559	2,942
Income tax expense	(373)	(1,000)	(139)	(2,382)
Net Income	(7,656)	1,192	19,156	22,406
Total assets	1,406,068	1,319,829	1,406,068	1,319,829
Capital expenditures	32,068	27,263	47,853	44,156
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	0	0	0	0
Utility margin	0	0	0	0
Operating and maintenance	0	0	0	0
Administrative and general	0	0	0	0
Property and other taxes	0	0	0	0
Depreciation and depletion	0	0	0	0
Operating income (loss)	0	0	0	0
Interest expense, net	0	0	0	0
Other income (expense), net	0	0	0	0
Income tax expense	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	0	0	0	0
All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	0	0	0	0
Utility margin	0	0	0	0
Operating and maintenance	0	0	0	0
Administrative and general	(1)	(66)	13	189
Property and other taxes	3	2	5	4
Depreciation and depletion	0	0	0	0
Operating income (loss)	(2)	64	(18)	(193)
Interest expense, net	(2,197)	(1,873)	(8,394)	(3,230)
Other income (expense), net	(36)	182	(80)	391
Income tax expense	1,741	355	(2,106)	1,620
Net Income	(494)	(1,272)	(10,598)	(1,412)
Total assets	9,741	6,479	9,741	6,479
Capital expenditures	$ 0	$ 0	$ 0	$ 0

Revenue from Contracts with C_3

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 330,600	$ 316,900	$ 836,900	$ 732,400
Total Revenues	290,502	323,004	745,044	717,486
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	132,900	131,700	385,600	336,700
Residential \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	101,400	99,300	293,800	248,600
Residential \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	24,100	25,000	63,800	65,300
Residential \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,400	7,400	28,000	22,800
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	147,100	136,400	348,700	301,300
Commercial \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	111,800	99,000	260,800	212,600
Commercial \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,600	32,900	70,100	75,000
Commercial \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,700	4,500	17,800	13,700
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,000	9,200	23,500	19,500
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,000	8,800	14,900	14,000
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300,000	286,100	772,700	671,500
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,600	30,800	64,200	60,900
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(40,100)	6,100	(91,900)	(14,900)
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	290,500	323,000	745,000	717,500
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	266,000	236,000	587,400	501,800
Electric Domestic Regulated [Member] \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	99,500	86,300	244,800	203,700
Electric Domestic Regulated [Member] \| Residential \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	83,800	70,700	209,300	167,700
Electric Domestic Regulated [Member] \| Residential \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	15,700	15,600	35,500	36,000
Electric Domestic Regulated [Member] \| Residential \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Electric Domestic Regulated [Member] \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	127,000	110,800	264,800	224,900
Electric Domestic Regulated [Member] \| Commercial \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	101,900	84,300	214,500	170,800
Electric Domestic Regulated [Member] \| Commercial \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	25,100	26,500	50,300	54,100
Electric Domestic Regulated [Member] \| Commercial \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Electric Domestic Regulated [Member] \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,800	9,000	22,600	18,700
Electric Domestic Regulated [Member] \| Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,700	8,300	13,900	12,800
Electric Domestic Regulated [Member] \| Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	246,000	214,400	546,100	460,100
Electric Domestic Regulated [Member] \| Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,000	21,600	41,300	41,700
Electric Domestic Regulated [Member] \| Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(36,700)	7,400	(62,800)	13,300
Electric Domestic Regulated [Member] \| Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	229,300	243,400	524,600	515,100
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	64,600	80,900	249,500	230,600
Gas Domestic Regulated [Member] \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	33,400	45,400	140,800	133,000
Gas Domestic Regulated [Member] \| Residential \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17,600	28,600	84,500	80,900
Gas Domestic Regulated [Member] \| Residential \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,400	9,400	28,300	29,300
Gas Domestic Regulated [Member] \| Residential \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,400	7,400	28,000	22,800
Gas Domestic Regulated [Member] \| Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,100	25,600	83,900	76,400
Gas Domestic Regulated [Member] \| Commercial \| Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,900	14,700	46,300	41,800
Gas Domestic Regulated [Member] \| Commercial \| South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,500	6,400	19,800	20,900
Gas Domestic Regulated [Member] \| Commercial \| Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,700	4,500	17,800	13,700
Gas Domestic Regulated [Member] \| Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	200	200	900	800
Gas Domestic Regulated [Member] \| Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300	500	1,000	1,200
Gas Domestic Regulated [Member] \| Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	54,000	71,700	226,600	211,400
Gas Domestic Regulated [Member] \| Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10,600	9,200	22,900	19,200
Gas Domestic Regulated [Member] \| Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(3,400)	(1,300)	(29,100)	(28,200)
Gas Domestic Regulated [Member] \| Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 61,200	$ 79,600	$ 220,400	$ 202,400

Earnings Per Share (Details)

Earnings Per Share (Details) - shares	3 Months Ended		6 Months Ended
Earnings Per Share (Details) - shares	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Basic computation	59,804,283	54,271,862	59,790,316	54,184,798
Performance share awards(1)	45,391	34,900	29,200	23,072
Incremental Common Shares Attributable to Dilutive Effect of Equity Forward Agreements	0	834,126	0	772,755
Diluted computation	59,849,674	55,140,888	59,819,516	54,980,625
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	21,890			36,296

Employee Benefit Plans Net Peri

Employee Benefit Plans Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Components of Net Periodic Benefit Cost (Income) [Abstract]
Minimum Expected Future Employer Pension Contributions, Remainder of Fiscal Year	$ 10,600		$ 10,600
Payment for pension benefits			600
Pension Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	1,422	$ 2,228	2,916	$ 5,112
Interest cost	6,482	4,725	13,047	9,393
Expected return on plan assets	(6,671)	(6,034)	(13,357)	(12,086)
Amortization of prior service credit	0	0	0	0
Recognized actuarial (gain) loss	(3)	191	137	191
Net periodic benefit cost (credit)	1,230	1,110	2,743	2,610
Other Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	79	84	166	175
Interest cost	161	88	337	179
Expected return on plan assets	(273)	(261)	(548)	(523)
Amortization of prior service credit	29	(473)	58	(946)
Recognized actuarial (gain) loss	5	(10)	36	(24)
Net periodic benefit cost (credit)	$ 1	$ (572)	$ 49	$ (1,139)

Commitments and Contingencies E

Commitments and Contingencies Environmental (Details) - Environmental remediation obligations [Member] $ in Millions	Jun. 30, 2023 USD ($)
Environmental remediation obligation, minimum	$ 20.9
Environmental remediation obligation, maximum	32
Accrual for Environmental Loss Contingencies, Gross	$ 25.7

Commitments and Contingencies L

Commitments and Contingencies Litigation (Details) $ in Millions	6 Months Ended
	Jun. 30, 2023 USD ($)
Riverbed Rents [Member]
Loss Contingencies [Line Items]
Annual riverbed rent exposure	$ 3.8
BNSF Railway Company
Loss Contingencies [Line Items]
Damages sought	4.4
EPA estimated costs to implement selected remedies
Loss Contingencies [Line Items]
Site Contingency, Loss Exposure Not Accrued, Best Estimate	4.1
EPA estimated costs to implement selected remedies for areas of interest in Operable Unit 1
Loss Contingencies [Line Items]
Site Contingency, Loss Exposure Not Accrued, Best Estimate	$ 1.8