NorthWestern Energy 10-Q 28 Oct 24 2024 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	9 Months Ended
	Sep. 30, 2024	Oct. 25, 2024
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2024
Entity Registrant Name	NORTHWESTERN ENERGY GROUP, INC.
Entity File Number	000-56598
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	93-2020320
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		61,314,217
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Central Index Key	0001993004

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues
Electric	$ 306,478	$ 280,030	$ 909,798	$ 804,604
Gas	38,683	41,060	230,634	261,530
Total Revenues	345,161	321,090	1,140,432	1,066,134
Operating expenses
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	87,888	88,943	339,089	322,013
Operating and maintenance	55,866	53,240	167,415	163,941
Administrative and general	34,924	29,355	106,650	94,058
Property and other taxes	41,596	41,763	125,023	131,043
Depreciation and depletion	56,954	52,159	170,630	157,787
Total Operating Expenses	277,228	265,460	908,807	868,842
Operating income	67,933	55,630	231,625	197,292
Interest expense, net	(33,397)	(28,725)	(96,251)	(85,144)
Other income, net	9,116	4,127	19,595	12,926
Income before income taxes	43,652	31,032	154,969	125,074
Income tax benefit (expense)	3,167	(1,697)	(11,410)	(14,085)
Net Income	$ 46,819	$ 29,335	$ 143,559	$ 110,989
Average Common Shares Outstanding	61,301,696	60,442,164	61,285,570	60,010,609
Basic Earnings per Average Common Share	$ 0.76	$ 0.48	$ 2.34	$ 1.85
Diluted Earnings per Average Common Share	0.76	0.48	2.34	1.85
Dividends Declared per Common Share	$ 0.650	$ 0.64	$ 1.95	$ 1.92

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Comprehensive Income (Loss)
Net Income	$ 46,819	$ 29,335	$ 143,559	$ 110,989
Other comprehensive income, net of tax:
Foreign currency translation adjustment	1	(7)	(1)	(10)
Defined benefit pension plan and postretirement medical liability adjustment	0	(168)	0	(502)
Reclassification of net losses on derivative instruments	113	113	339	339
Total Other Comprehensive Income (Loss)	114	(62)	338	(173)
Comprehensive Income	$ 46,933	$ 29,273	$ 143,897	$ 110,816

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEET - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Current Assets:
Cash and cash equivalents	$ 2,527	$ 9,164
Restricted cash	25,365	16,023
Accounts receivable, net	144,186	212,257
Inventories	121,568	114,539
Regulatory assets	34,334	29,626
Prepaid expenses and other	40,440	25,397
Total current assets	368,420	407,006
Property, plant, and equipment, net	6,304,721	6,039,801
Goodwill	357,586	357,586
Regulatory assets	766,229	743,945
Other noncurrent assets	57,118	52,314
Total Assets	7,854,074	7,600,652
Current Liabilities:
Current maturities of finance leases	3,529	3,338
Current portion of long-term debt	299,918	99,950
Short-Term Debt	100,000
Accounts payable	93,748	124,340
Accrued expenses and other	286,070	246,167
Regulatory liabilities	30,001	61,103
Total current liabilities	813,266	534,898
Long-term finance leases	2,798	5,461
Long-term debt	2,567,940	2,684,635
Deferred income taxes	640,082	600,520
Noncurrent regulatory liabilities	665,360	657,452
Other noncurrent liabilities	348,166	332,372
Total Liabilities	5,037,612	4,815,338
Commitments and Contingencies (Note 10)
Shareholders' Equity:
Common Stock, Value, Issued	648	648
Treasury stock at cost	(97,557)	(97,926)
Paid-in capital	2,084,560	2,078,753
Retained earnings	836,129	811,495
Accumulated other comprehensive loss	(7,318)	(7,656)
Total Shareholders' Equity	2,816,462	2,785,314
Total Liabilities and Shareholders' Equity	$ 7,854,074	$ 7,600,652
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	64,803,949
Common Stock, Shares, Outstanding	61,308,009
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	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 143,559	$ 110,989
Items not affecting cash:
Depreciation and depletion	170,630	157,787
Amortization of debt issuance costs, discount and deferred hedge gain	3,490	3,997
Stock-based compensation costs	5,291	5,119
Equity portion of allowance for funds used during construction	(15,371)	(12,530)
Gain on disposition of assets	(14)	(27)
Other Noncash Expense	4,159	0
Deferred income taxes	7,128	(13,281)
Changes in current assets and liabilities:
Accounts receivable	68,071	96,910
Inventories	(7,030)	(11,721)
Other current assets	(15,043)	389
Accounts payable	(14,235)	(60,815)
Accrued expenses and other	39,928	65,058
Regulatory assets	(4,708)	94,069
Regulatory liabilities	(31,102)	10,588
Other noncurrent assets and liabilities	(10,849)	(19,610)
Cash Provided by Operating Activities	343,904	426,922
Net Cash Provided by (Used in) Investing Activities [Abstract]
Property, plant, and equipment additions	(400,511)	(407,170)
Investment in equity securities	(4,599)	(3,804)
Cash Used in Investing Activities	(405,110)	(410,974)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from Issuance of Common Stock	0	73,613
Dividends on common stock	(118,925)	(115,048)
Issuance of long-term debt	215,000	300,000
Proceeds from (Repayments of) Short-Term Debt, Maturing in More than Three Months	100,000	0
Repayments on long-term debt	(100,000)	0
Line of credit repayments, net	(32,000)	(273,000)
Other financing activities, net	(164)	(2,336)
Cash Provided by (Used in) Financing Activities	63,911	(16,771)
Increase (Decrease) in Cash, Cash Equivalents, and Restricted Cash	2,705	(823)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance	25,187	22,463
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance	27,892	21,640
Cash Paid [Abstract]
Income Taxes Paid, Net	(4,469)	3,204
Interest Paid, Excluding Capitalized Interest, Operating Activities	92,562	64,533
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital expenditures included in accounts payable	25,966	43,389
Notes Issued	$ 0	$ 144,660

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, 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	110,989	0	0	0	110,989	0
Foreign currency translation adjustment	10	0	0	0	0	(10)
Reclassification of net losses on derivative instruments	339	0	0	0	0	339
Defined benefit pension plan and postretirement medical liability adjustment	(502)	$ 0	$ 0	0	0	(502)
Stock based compensation, shares		51	0
Stock based compensation, value	4,911	$ 0	$ 0	4,911	0	0
Issuance of shares		1,433
Issuance of shares, treasury stock			(14)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 270
Issuance of shares, value	74,552	$ 15		74,267	0	0
Dividends on common stock	$ (115,048)	$ 0	$ 0	0	(115,048)	0
Dividends per share	$ 1.92
Balance, shares at Sep. 30, 2023		64,762	3,520
Balance, end of period at Sep. 30, 2023	$ 2,740,414	$ 648	$ (98,122)	2,078,554	767,355	(8,021)
Balance, shares at Jun. 30, 2023		63,518	3,527
Balance, beginning of period at Jun. 30, 2023	2,686,724	$ 635	$ (98,302)	2,015,367	776,983	(7,959)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	29,335	0	0	0	29,335	0
Foreign currency translation adjustment	7	0	0	0	0	7
Reclassification of net losses on derivative instruments	113	0	0	0	0	113
Defined benefit pension plan and postretirement medical liability adjustment	(168)	$ 0	$ 0	0	0	(168)
Stock based compensation, shares		0	0
Stock based compensation, value	239	$ 0	$ 0	239	0	0
Issuance of shares		1,244
Issuance of shares, treasury stock			(7)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 180
Issuance of shares, value	63,141	$ 13		62,948	0	0
Dividends on common stock	$ (38,963)	$ 0	$ 0	0	(38,963)	0
Dividends per share	$ 0.64
Balance, shares at Sep. 30, 2023		64,762	3,520
Balance, end of period at Sep. 30, 2023	$ 2,740,414	$ 648	$ (98,122)	2,078,554	767,355	(8,021)
Balance, shares at Dec. 31, 2023		64,762	3,513
Balance, beginning of period at Dec. 31, 2023	2,785,314	$ 648	$ (97,926)	2,078,753	811,495	(7,656)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	143,559	0	0	0	143,559	0
Foreign currency translation adjustment	1	0	0	0	0	1
Reclassification of net losses on derivative instruments	339	0	0	0	0	339
Defined benefit pension plan and postretirement medical liability adjustment	0	$ 0	$ 0	0	0	0
Stock based compensation, shares		42	0
Stock based compensation, value	4,980	$ 0	$ (272)	5,252	0	0
Issuance of shares		0
Issuance of shares, treasury stock			(17)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 641
Issuance of shares, value	1,196	$ 0		555	0	0
Dividends on common stock	$ (118,925)	$ 0	$ 0	0	(118,925)	0
Dividends per share	$ 1.95
Balance, shares at Sep. 30, 2024		64,804	3,496
Balance, end of period at Sep. 30, 2024	$ 2,816,462	$ 648	$ (97,557)	2,084,560	836,129	(7,318)
Balance, shares at Jun. 30, 2024		64,803	3,504
Balance, beginning of period at Jun. 30, 2024	2,807,257	$ 648	$ (97,776)	2,082,857	828,960	(7,432)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	46,819	0	0	0	46,819	0
Foreign currency translation adjustment	(1)	0	0	0	0	(1)
Reclassification of net losses on derivative instruments	113	$ 0	$ 0	0	0	113
Defined benefit pension plan and postretirement medical liability adjustment	0
Stock based compensation, shares		1	0
Stock based compensation, value	1,481	$ 0	$ 0	1,481	0	0
Issuance of shares		0
Issuance of shares, treasury stock			(8)
Stock Issued During Period, Value, Treasury Stock Reissued			$ 219
Issuance of shares, value	441	$ 0		222	0	0
Dividends on common stock	$ (39,650)	$ 0	$ 0	0	(39,650)	0
Dividends per share	$ 0.650
Balance, shares at Sep. 30, 2024		64,804	3,496
Balance, end of period at Sep. 30, 2024	$ 2,816,462	$ 648	$ (97,557)	$ 2,084,560	$ 836,129	$ (7,318)

Nature of Operations and Basis

Nature of Operations and Basis of Consolidation	9 Months Ended
	Sep. 30, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure	Nature of Operations and Basis of Consolidation NorthWestern Energy Group, doing business as NorthWestern Energy, provides electricity and/or natural gas to approximately 775,300 customers in Montana, South Dakota, Nebraska and Yellowstone National Park, through its subsidiaries NorthWestern Corporation (NW Corp) and NorthWestern Energy Public Service Corporation (NWE Public Service). 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 September 30, 2024 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 the NorthWestern Energy Group Annual Report on Form 10-K for the year ended December 31, 2023 . Holding Company Reorganization On January 1, 2024, we completed the second and final phase of our holding company reorganization. NW Corp contributed the assets and liabilities of its South Dakota and Nebraska regulated utilities to NWE Public Service, and then distributed its equity interest in NWE Public Service and certain other subsidiaries to NorthWestern Energy Group, resulting in NW Corp owning and operating the Montana regulated utility and NWE Public Service owning and operating the Nebraska and South Dakota utilities, each as a direct subsidiary of NorthWestern Energy Group. 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): September 30, December 31, September 30, December 31, 2024 2023 2023 2022 Cash and cash equivalents $ 2,527 $ 9,164 $ 5,091 $ 8,489 Restricted cash 25,365 16,023 16,549 13,974 Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows $ 27,892 $ 25,187 $ 21,640 $ 22,463 Goodwill We completed our annual goodwill impairment test as of April 1, 2024, and no impairment was identified. We calculate the fair value of our reporting units by considering various factors, including valuation studies based primarily on a discounted cash flow analysis, with published industry valuations and market data as supporting information. Key assumptions in the determination of fair value include the use of an appropriate discount rate and estimated future cash flows. In estimating cash

Regulatory Matters

Regulatory Matters	9 Months Ended
	Sep. 30, 2024
Regulated Operations [Abstract]
Public Utilities Disclosure [Text Block]	Regulatory Matters Montana Rate Review In July 2024, we filed a Montana electric and natural gas rate review with the Montana Public Service Commission (MPSC). The filing requests a base rate annual revenue increase of $156.5 million ($69.4 million net with Property Tax and Power Cost and Credit Adjustment Mechanism (PCCAM) tracker adjustments) for electric and $28.6 million for natural gas. Our request is based on a return on equity of 10.80 percent with a capital structure including 46.81 percent equity, and forecasted 2024 electric and natural gas rate base of $3.45 billion and $731.9 million, respectively. The electric rate base investment includes the 175-megawatt natural gas-fired Yellowstone County Generating Station ("YCGS"), which was placed in service in October 2024. Our filing included a request for interim base rates to be effective October 1, 2024. Implementation of interim base rates, if any, has been delayed beyond our requested effective date as the MPSC has not yet made a decision on the interim rate request. The MPSC has developed its procedural schedule for our rate review request including a hearing scheduled to commence on April 22, 2025. If a final order is not received by May 23, 2025, which is 270 days from acceptance of our filing, we intend to implement, as permitted by the MPSC regulations, our requested rates, which will be subject to refund, until a final order is received. South Dakota Natural Gas Rate Review In June 2024, we filed a natural gas rate review (2023 test year) with the South Dakota Public Utilities Commission. The filing requests a base rate annual revenue increase of $6.0 million. Our request is based on a return on equity of 10.70 percent, a capital structure including 53.13 percent equity, and rate base of $95.6 million. If a final order is not received by December 21, 2024, interim base rates may go into effect. Nebraska Natural Gas Rate Review

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2024
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 September 30, 2024 2023 Income before income taxes $ 43,652 $ 31,032 Income tax calculated at federal statutory rate 9,167 21.0 % 6,516 21.0 % Permanent or flow-through adjustments: State income tax, net of federal provisions 61 0.1 121 0.4 Gas repairs safe harbor method change (6,994) (16.0) — — Flow-through repairs deductions (4,581) (10.5) (4,189) (13.5) Production tax credits (2,447) (5.6) (1,261) (4.1) Amortization of excess deferred income tax (219) (0.5) (323) (1.0) Income tax return to accrual adjustment — — 411 1.3 Plant and depreciation flow-through items 1,816 4.2 358 1.2 Other, net 30 0.0 64 0.2 (12,334) (28.3) (4,819) (15.5) Income tax (benefit) expense $ (3,167) (7.3) % $ 1,697 5.5 % Nine Months Ended September 30, 2024 2023 Income before income taxes $ 154,969 $ 125,074 Income tax calculated at federal statutory rate 32,544 21.0 % 26,265 21.0 % Permanent or flow through adjustments: State income, net of federal provisions 749 0.5 1,353 1.1 Flow-through repairs deductions (13,824) (8.9) (11,742) (9.4) Production tax credits (7,434) (4.8) (5,607) (4.5) Gas repairs safe harbor method change (6,994) (4.5) — — Amortization of excess deferred income tax (775) (0.5) (1,355) (1.1) Reduction to previously claimed alternative minimum tax credit — — 3,186 2.5 Income tax return to accrual adjustment — — 411 0.3 Plant and depreciation flow through items 5,955 3.8 1,247 1.0 Share-based compensation 298 0.2 388 0.3 Other, net 891 0.6 (61) 0.1 (21,134) (13.6) (12,180) (9.7) Income tax expense $ 11,410 7.4 % $ 14,085 11.3 % In 2023, the Internal Revenue Service (IRS) issued a safe harbor method of accounting for the repair and maintenance of natural gas transmission and distribution property. During the three months ended September 30, 2024, after completion of our impact analysis of the gas repairs safe harbor method change, we recorded an income tax benefit of approximately $7.0 million related to tax deductions for repair costs that were previously capitalized in the 2022 and prior tax years. 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 $26.9 million as of September 30, 2024, including approximately $24.3 million that, if recognized, would impact our effective tax rate. In the next twelve months we expect the statute of limitations to expire for certain uncertain tax benefits, which would result in a decrease to our total unrecognized tax benefits of approximately $16.9 million. Our policy is to recognize interest and penalties related to uncertain tax positions in income tax expense. As of September 30, 2024, we have accrued $6.8 million for the payment of interest and penalties on the Condensed Consolidated Balance Sheets. As of December 31, 2023, we had accrued $4.5 million for the payment of interest and penalties on the Condensed Consolidated Balance Sheets.

Comprehensive Income (Loss)

Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2024
Statement of Comprehensive Income [Abstract]
Comprehensive Income (Loss) Note [Text Block]	Comprehensive (Loss) Income The following tables display the components of Other Comprehensive Income (Loss), after-tax, and the related tax effects (in thousands): Three Months Ended September 30, 2024 September 30, 2023 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ 1 $ — $ 1 $ (7) $ — $ (7) Reclassification of net income on derivative instruments 153 (40) 113 153 (40) 113 Defined benefit pension plan and postretirement medical liability adjustment — — — (212) 44 (168) Other comprehensive income (loss) $ 154 $ (40) $ 114 $ (66) $ 4 $ (62) Nine Months Ended September 30, 2024 September 30, 2023 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (1) $ — $ (1) $ (10) $ — $ (10) Reclassification of net income on derivative instruments 459 (120) 339 459 (120) 339 Defined benefit pension plan and postretirement medical liability adjustment — — — (636) 134 (502) Other comprehensive income (loss) $ 458 $ (120) $ 338 $ (187) $ 14 $ (173) Balances by classification included within accumulated other comprehensive loss (AOCL) on the Condensed Consolidated Balance Sheets are as follows, net of tax (in thousands): September 30, 2024 December 31, 2023 Foreign currency translation $ 1,436 $ 1,437 Derivative instruments designated as cash flow hedges (9,034) (9,373) Defined benefit pension plan 280 280 Accumulated other comprehensive loss $ (7,318) $ (7,656) The following tables display the changes in AOCL by component, net of tax (in thousands): Three Months Ended September 30, 2024 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Defined Benefit Pension Plan Foreign Currency Translation Total Beginning balance $ (9,147) $ 280 $ 1,435 $ (7,432) Other comprehensive income before reclassifications — — 1 1 Amounts reclassified from AOCL Interest Expense 113 — — 113 Net current-period other comprehensive income 113 — 1 114 Ending balance $ (9,034) $ 280 $ 1,436 $ (7,318) Three Months Ended September 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 $ (9,599) $ 208 $ 1,432 $ (7,959) Other comprehensive loss before reclassifications — — (7) (7) Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (168) — (168) Net current-period other comprehensive income (loss) 113 (168) (7) (62) Ending balance $ (9,486) $ 40 $ 1,425 $ (8,021) Nine Months Ended September 30, 2024 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Defined Benefit Pension Plan and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,373) $ 280 $ 1,437 $ (7,656) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 339 — — 339 Net current-period other comprehensive income (loss) 339 — (1) 338 Ending balance $ (9,034) $ 280 $ 1,436 $ (7,318) Nine Months Ended September 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 — — (10) (10) Amounts reclassified from AOCL Interest Expense 339 — — 339 Amounts reclassified from AOCL — (502) — (502) Net current-period other comprehensive income (loss) 339 (502) (10) (173) Ending balance $ (9,486) $ 40 $ 1,425 $ (8,021)

Financing Activities

Financing Activities	9 Months Ended
	Sep. 30, 2024
Financing Activities [Abstract]
Financing Activities	(5) Financing Activities On March 28, 2024, NW Corp issued and sold $175.0 million aggregate principal amount of Montana First Mortgage Bonds at a fixed interest rate of 5.56 percent maturing on March 28, 2031. These bonds were issued in transactions exempt from the registration requirements of the Securities Act of 1933. Proceeds were used to redeem NW Corp's $100.0 million of Montana First Mortgage Bonds due this year and for other general utility purposes. The bonds are secured by NW Corp's electric and natural gas assets associated with its Montana utility operations. On March 28, 2024, NWE Public Service issued and sold $33.0 million aggregate principal amount of South Dakota First Mortgage Bonds at a fixed interest rate of 5.55 percent maturing on March 28, 2029, and $7.0 million aggregate principal amount of South Dakota First Mortgage Bonds at a fixed interest rate of 5.75 percent maturing on March 28, 2034. These bonds were issued in transactions exempt from the registration requirements of the Securities Act of 1933. Proceeds were used for general utility purposes. The bonds are secured by NWE Public Service's electric and natural gas assets associated with its South Dakota and Nebraska utility operations. On April 12, 2024, NorthWestern Energy Group entered into a $100.0 million Term Loan Credit Agreement (Term Loan) with a maturity date of April 11, 2025. Borrowings may be made at a variable interest rate equal to the Secured Overnight Financing Rate plus an applicable margin as provided in the Term Loan. These proceeds were used to repay a portion of our outstanding revolving credit facility borrowings and for general corporate purposes. The Term Loan provides for prepayment of the principal and interest; however, amounts prepaid may not be reborrowed. The Term Loan requires us to maintain a consolidated indebtedness to total capitalization ratio of 65 percent or less. It also contains covenants which, among other things, limit our ability to engage in any consolidation or merger or otherwise liquidate or dissolve, dispose of property, and restricts certain affiliate transactions. A default on the South Dakota or Montana First Mortgage Bonds would trigger a cross default on the Term Loan; however a default on the Term Loan would not trigger a default on the South Dakota or Montana First Mortgage Bonds.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
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 September 30, 2024 Electric Gas Other Eliminations Total Operating revenues $ 306,478 $ 38,683 $ — $ — $ 345,161 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 80,761 7,127 — — 87,888 Utility margin 225,717 31,556 — — 257,273 Operating and maintenance 42,491 13,375 — — 55,866 Administrative and general 24,892 9,887 145 — 34,924 Property and other taxes 32,251 9,345 — — 41,596 Depreciation and depletion 47,540 9,414 — — 56,954 Operating income (loss) 78,543 (10,465) (145) — 67,933 Interest expense, net (24,188) (7,537) (1,672) — (33,397) Other income, net 6,057 3,017 42 — 9,116 Income tax (expense) benefit (7,635) 9,734 1,068 — 3,167 Net income (loss) $ 52,777 $ (5,251) $ (707) $ — $ 46,819 Total assets $ 6,256,750 $ 1,578,075 $ 19,249 $ — $ 7,854,074 Capital expenditures $ 109,925 $ 43,225 $ — $ — $ 153,150 Three Months Ended September 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 280,030 $ 41,060 $ — $ — $ 321,090 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 77,995 10,948 — — 88,943 Utility margin 202,035 30,112 — — 232,147 Operating and maintenance 39,990 13,250 — — 53,240 Administrative and general 20,682 8,249 424 — 29,355 Property and other taxes 33,740 9,574 (1,551) — 41,763 Depreciation and depletion 43,230 8,929 — — 52,159 Operating income (loss) 64,393 (9,890) 1,127 — 55,630 Interest expense, net (21,300) (4,426) (2,999) — (28,725) Other income (expense), net 3,380 1,328 (581) — 4,127 Income tax (expense) benefit (3,223) (41) 1,567 — (1,697) Net income (loss) $ 43,250 $ (13,029) $ (886) $ — $ 29,335 Total assets $ 5,963,950 $ 1,454,445 $ 11,104 $ — $ 7,429,499 Capital expenditures $ 110,804 $ 46,359 $ — $ — $ 157,163 Nine Months Ended September 30, 2024 Electric Gas Other Eliminations Total Operating revenues $ 909,798 $ 230,634 $ — $ — $ 1,140,432 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 256,989 82,100 — — 339,089 Utility margin 652,809 148,534 — — 801,343 Operating and maintenance 126,257 41,158 — — 167,415 Administrative and general 76,105 27,754 2,791 — 106,650 Property and other taxes 96,557 28,465 1 — 125,023 Depreciation and depletion 142,390 28,240 — — 170,630 Operating income (loss) 211,500 22,917 (2,792) — 231,625 Interest expense, net (72,143) (20,933) (3,175) — (96,251) Other income (expense), net 15,549 4,998 (952) — 19,595 Income tax (expense) benefit (18,809) 6,865 534 — (11,410) Net income (loss) $ 136,097 $ 13,847 $ (6,385) $ — $ 143,559 Total assets $ 6,256,750 $ 1,578,075 $ 19,249 $ — $ 7,854,074 Capital expenditures $ 312,773 $ 87,738 $ — $ — $ 400,511 Nine Months Ended September 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 804,604 $ 261,530 $ — $ — $ 1,066,134 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 198,492 123,521 — — 322,013 Utility margin 606,112 138,009 — — 744,121 Operating and maintenance 123,771 40,170 — — 163,941 Administrative and general 67,285 26,336 437 — 94,058 Property and other taxes 103,013 29,576 (1,546) — 131,043 Depreciation and depletion 130,447 27,340 — — 157,787 Operating income 181,596 14,587 1,109 — 197,292 Interest expense, net (61,584) (12,167) (11,393) — (85,144) Other income (expense), net 9,700 3,887 (661) — 12,926 Income tax expense (13,366) (180) (539) — (14,085) Net income (loss) $ 116,346 $ 6,127 $ (11,484) $ — $ 110,989 Total assets $ 5,963,950 $ 1,454,445 $ 11,104 $ — $ 7,429,499 Capital expenditures $ 326,313 $ 94,212 $ — $ — $ 420,525

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2024
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 0-30 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 0-30 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 September 30, 2024 September 30, 2023 Electric Natural Gas Total Electric Natural Gas Total Montana $ 100.7 $ 8.4 $ 109.1 $ 96.8 $ 9.6 $ 106.4 South Dakota 19.1 1.7 20.8 18.0 2.0 20.0 Nebraska — 1.8 1.8 — 2.2 2.2 Residential 119.8 11.9 131.7 114.8 13.8 128.6 Montana 109.6 6.2 115.8 110.1 6.1 116.2 South Dakota 30.1 1.3 31.4 27.5 1.5 29.0 Nebraska — 0.8 0.8 — 1.3 1.3 Commercial 139.7 8.3 148.0 137.6 8.9 146.5 Industrial 11.8 0.1 11.9 11.4 0.1 11.5 Lighting, governmental, irrigation, and interdepartmental 14.1 0.2 14.3 13.2 0.2 13.4 Total Customer Revenues 285.4 20.5 305.9 277.0 23.0 300.0 Other tariff and contract based revenues 28.0 9.3 37.3 22.1 10.2 32.3 Total Revenue from Contracts with Customers 313.4 29.8 343.2 299.1 33.2 332.3 Regulatory amortization and other (6.9) 8.9 2.0 (19.1) 7.9 (11.2) Total Revenues $ 306.5 $ 38.7 $ 345.2 $ 280.0 $ 41.1 $ 321.1 Nine Months Ended September 30, 2024 September 30, 2023 Electric Natural Gas Total Electric Natural Gas Total Montana $ 304.1 $ 75.9 $ 380.0 $ 306.1 $ 94.1 $ 400.2 South Dakota 53.8 21.2 75.0 53.4 30.3 83.7 Nebraska — 16.1 16.1 — 30.2 30.2 Residential 357.9 113.2 471.1 359.5 154.6 514.1 Montana 310.8 42.0 352.8 324.6 52.4 377.0 South Dakota 84.2 14.3 98.5 77.8 21.3 99.1 Nebraska — 9.0 9.0 — 19.1 19.1 Commercial 395.0 65.3 460.3 402.4 92.8 495.2 Industrial 34.8 0.7 35.5 34.0 1.0 35.0 Lighting, governmental, irrigation, and interdepartmental 27.4 1.1 28.5 27.2 1.3 28.5 Total Customer Revenues 815.1 180.3 995.4 823.1 249.7 1,072.8 Other tariff and contract based revenues 77.4 30.9 108.3 63.5 33.1 96.6 Total Revenue from Contracts with Customers 892.5 211.2 1,103.7 886.6 282.8 1,169.4 Regulatory amortization and other 17.3 19.4 36.7 (82.0) (21.3) (103.3) Total Revenues $ 909.8 $ 230.6 $ 1,140.4 $ 804.6 $ 261.5 $ 1,066.1

Earnings Per Share

Earnings Per Share	3 Months Ended
	Sep. 30, 2024
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. Average shares used in computing the basic and diluted earnings per share are as follows: Three Months Ended September 30, 2024 September 30, 2023 Basic computation 61,301,696 60,442,164 Dilutive effect of: Performance share awards (1) 95,279 35,533 Diluted computation 61,396,975 60,477,697 Nine Months Ended September 30, 2024 September 30, 2023 Basic computation 61,285,570 60,010,609 Dilutive effect of: Performance share awards (1) 69,136 31,311 Diluted computation 61,354,706 60,041,920 (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. As of September 30, 2024, there were 16,015 shares from performance and restricted share awards which were antidilutive and excluded from the earnings per share calculations, compared to 32,649 shares as of September 30, 2023.

Employee Benefit Plans

Employee Benefit Plans	9 Months Ended
	Sep. 30, 2024
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 September 30, Three Months Ended September 30, 2024 2023 2024 2023 Components of Net Periodic Benefit Cost (Credit) Service cost $ 1,398 $ 1,459 $ 77 $ 84 Interest cost 5,736 6,524 139 168 Expected return on plan assets (6,331) (6,679) (320) (274) Amortization of prior service credit — — — 29 Recognized actuarial loss (gain) 8 68 (18) 18 Net periodic benefit cost (credit) $ 811 $ 1,372 $ (122) $ 25 Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Components of Net Periodic Benefit Cost (Credit) Service cost $ 4,194 $ 4,375 $ 231 $ 250 Interest cost 17,208 19,571 418 505 Expected return on plan assets (18,994) (20,036) (960) (822) Amortization of prior service credit — — — 87 Recognized actuarial loss (gain) 25 205 (55) 54 Net periodic benefit cost (credit) $ 2,433 $ 4,115 $ (366) $ 74

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and Contingencies ENVIRONMENTAL LIABILITIES AND REGULATION Except as set forth below, the circumstances set forth in Note 18 - Commitments and Contingencies to the financial statements included in the NorthWestern Energy Group Annual Report on Form 10-K for the year ended December 31, 2023 appropriately represent, in all material respects, the current status of our environmental liabilities and regulation. Environmental Protection Agency (EPA) Rules On April 25, 2024, the EPA released final rules related to greenhouse gas (GHG) emission standards (GHG Rules) for existing coal-fired facilities and new coal and natural gas-fired facilities as well as final rules strengthening the MATS requirements (MATS Rules). Compliance with the rules will require expensive upgrades at Colstrip Units 3 and 4 with proposed compliance dates that may not be achievable and / or require technology that is unproven, resulting in significant impacts to costs of the facilities. The final MATS and GHG Rules require compliance as early as 2027 and 2032, respectively. Previous efforts by the EPA were met with extensive litigation, and this time is no different. We, along with many other utilities, electric cooperatives, organizations, and states, have petitioned for judicial review of the GHG and MATS Rules with the U.S. Court of Appeals for the D.C. Circuit. The United States Supreme Court denied the multiple stay requests related to the MATS Rule and the GHG Rule. The litigation on the merits continues for both the MATS and GHG rules in the D.C. Circuit Court of Appeals, and decisions are expected in 2025. If the MATS Rules and GHG Rules are implemented, it would result in additional material compliance costs. We will continue working with federal and state regulatory authorities, other utilities, and stakeholders to seek relief from the MATS and GHG regulations that, in our view, disproportionately impact customers in our region. These GHG Rules and MATS Rules as well as 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. 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 the NorthWestern Energy Group Annual Report on Form 10-K for the year ended December 31, 2023 . 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. On August 25, 2023, the Federal District Court issued its Findings of Fact, Conclusions of Law, and Order (the "Order"), which found all but one of the segments of the riverbeds in dispute not navigable, and thus not owned by the State of Montana. The one segment found navigable, and thus owned by the State, was the segment on which the Black Eagle development was located. The State filed a motion to pursue an interlocutory appeal of the Order, and on January 2, 2024, the Federal District Court certified the Order for appeal to the 9th Circuit Court of Appeals. Briefing in the appeal is underway. Damages were bifurcated by agreement and will be tried separately for the Black Eagle segment, and any other segments found navigable, should the State prevail on appeal. We dispute the State’s claims and intend to continue to vigorously defend the lawsuit. If the Federal District Court calculates damages as the State District Court did in 2008, we do not anticipate the resulting annual rent for the Black Eagle segment would have a material impact to our financial position or results of operations. 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. On September 17, 2024, the owners agreed to stay the Arbitration for 120 days. 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; though the number of plaintiffs has since decreased to 57. 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. We do not anticipate that the amount of ultimate liability, if any, will have a material effect on our financial position, results of operations, or cash flows. Yellowstone County Generating Station Air Permit On October 21, 2021, the Montana Environmental Information Center and the Sierra Club filed a lawsuit in Montana State District 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 appeal to the Montana Supreme Court. Oral argument was held May 15, 2024. We recommenced construction in June 2023 and placed the plant in service in October 2024. The ultimate resolution of the lawsuit challenging the Yellowstone County Generating Station air quality permit could impact our ability to operate the facility. During the litigation of the air permit, Montana House Bill 971 was signed into law, preventing the MDEQ from, except under certain exceptions, evaluating greenhouse gas emissions and corresponding impacts to the climate in environmental reviews of large projects such as coal mines and power plants. On August 4, 2023, the Montana First Judicial District Court in Held v. State of Montana, a separate case by Montana youths alleging climate damages, issued its order finding House Bill 971 unconstitutional delaying the issuance of the revised Yellowstone County Generating Station's air permit. The Montana Supreme Court granted NorthWestern permission to participate as amicus in the Held appeal. The Montana Supreme Court heard oral argument on the Held appeal on July 10, 2024. 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.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pay vs Performance Disclosure
Net Income	$ 46,819	$ 29,335	$ 143,559	$ 110,989

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Sep. 30, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Organization, Consolidation and

Organization, Consolidation and Presentation of Financial Statements (Tables)	9 Months Ended
	Sep. 30, 2024
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): September 30, December 31, September 30, December 31, 2024 2023 2023 2022 Cash and cash equivalents $ 2,527 $ 9,164 $ 5,091 $ 8,489 Restricted cash 25,365 16,023 16,549 13,974 Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows $ 27,892 $ 25,187 $ 21,640 $ 22,463 Goodwill We completed our annual goodwill impairment test as of April 1, 2024, and no impairment was identified. We calculate the fair value of our reporting units by considering various factors, including valuation studies based primarily on a discounted cash flow analysis, with published industry valuations and market data as supporting information. Key assumptions in the determination of fair value include the use of an appropriate discount rate and estimated future cash flows. In estimating cash

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2024
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 September 30, 2024 2023 Income before income taxes $ 43,652 $ 31,032 Income tax calculated at federal statutory rate 9,167 21.0 % 6,516 21.0 % Permanent or flow-through adjustments: State income tax, net of federal provisions 61 0.1 121 0.4 Gas repairs safe harbor method change (6,994) (16.0) — — Flow-through repairs deductions (4,581) (10.5) (4,189) (13.5) Production tax credits (2,447) (5.6) (1,261) (4.1) Amortization of excess deferred income tax (219) (0.5) (323) (1.0) Income tax return to accrual adjustment — — 411 1.3 Plant and depreciation flow-through items 1,816 4.2 358 1.2 Other, net 30 0.0 64 0.2 (12,334) (28.3) (4,819) (15.5) Income tax (benefit) expense $ (3,167) (7.3) % $ 1,697 5.5 % Nine Months Ended September 30, 2024 2023 Income before income taxes $ 154,969 $ 125,074 Income tax calculated at federal statutory rate 32,544 21.0 % 26,265 21.0 % Permanent or flow through adjustments: State income, net of federal provisions 749 0.5 1,353 1.1 Flow-through repairs deductions (13,824) (8.9) (11,742) (9.4) Production tax credits (7,434) (4.8) (5,607) (4.5) Gas repairs safe harbor method change (6,994) (4.5) — — Amortization of excess deferred income tax (775) (0.5) (1,355) (1.1) Reduction to previously claimed alternative minimum tax credit — — 3,186 2.5 Income tax return to accrual adjustment — — 411 0.3 Plant and depreciation flow through items 5,955 3.8 1,247 1.0 Share-based compensation 298 0.2 388 0.3 Other, net 891 0.6 (61) 0.1 (21,134) (13.6) (12,180) (9.7) Income tax expense $ 11,410 7.4 % $ 14,085 11.3 % In 2023, the Internal Revenue Service (IRS) issued a safe harbor method of accounting for the repair and maintenance of natural gas transmission and distribution property. During the three months ended September 30, 2024, after completion of our impact analysis of the gas repairs safe harbor method change, we recorded an income tax benefit of approximately $7.0 million related to tax deductions for repair costs that were previously capitalized in the 2022 and prior tax years.

Accumulated Other Comprehensive

Accumulated Other Comprehensive (Income) Loss (Tables)	9 Months Ended
	Sep. 30, 2024
Statement of Financial Position [Abstract]
Comprehensive Income (Loss) [Table Text Block]	The following tables display the components of Other Comprehensive Income (Loss), after-tax, and the related tax effects (in thousands): Three Months Ended September 30, 2024 September 30, 2023 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ 1 $ — $ 1 $ (7) $ — $ (7) Reclassification of net income on derivative instruments 153 (40) 113 153 (40) 113 Defined benefit pension plan and postretirement medical liability adjustment — — — (212) 44 (168) Other comprehensive income (loss) $ 154 $ (40) $ 114 $ (66) $ 4 $ (62) Nine Months Ended September 30, 2024 September 30, 2023 Before-Tax Amount Tax Expense Net-of-Tax Amount Before-Tax Amount Tax Expense Net-of-Tax Amount Foreign currency translation adjustment $ (1) $ — $ (1) $ (10) $ — $ (10) Reclassification of net income on derivative instruments 459 (120) 339 459 (120) 339 Defined benefit pension plan and postretirement medical liability adjustment — — — (636) 134 (502) Other comprehensive income (loss) $ 458 $ (120) $ 338 $ (187) $ 14 $ (173)
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): September 30, 2024 December 31, 2023 Foreign currency translation $ 1,436 $ 1,437 Derivative instruments designated as cash flow hedges (9,034) (9,373) Defined benefit pension plan 280 280 Accumulated other comprehensive loss $ (7,318) $ (7,656)
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 September 30, 2024 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Defined Benefit Pension Plan Foreign Currency Translation Total Beginning balance $ (9,147) $ 280 $ 1,435 $ (7,432) Other comprehensive income before reclassifications — — 1 1 Amounts reclassified from AOCL Interest Expense 113 — — 113 Net current-period other comprehensive income 113 — 1 114 Ending balance $ (9,034) $ 280 $ 1,436 $ (7,318) Three Months Ended September 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 $ (9,599) $ 208 $ 1,432 $ (7,959) Other comprehensive loss before reclassifications — — (7) (7) Amounts reclassified from AOCL Interest Expense 113 — — 113 Amounts reclassified from AOCL — (168) — (168) Net current-period other comprehensive income (loss) 113 (168) (7) (62) Ending balance $ (9,486) $ 40 $ 1,425 $ (8,021) Nine Months Ended September 30, 2024 Affected Line Item in the Condensed Consolidated Statements of Income Interest Rate Derivative Instruments Designated as Cash Flow Hedges Defined Benefit Pension Plan and Postretirement Medical Plans Foreign Currency Translation Total Beginning balance $ (9,373) $ 280 $ 1,437 $ (7,656) Other comprehensive loss before reclassifications — — (1) (1) Amounts reclassified from AOCL Interest Expense 339 — — 339 Net current-period other comprehensive income (loss) 339 — (1) 338 Ending balance $ (9,034) $ 280 $ 1,436 $ (7,318) Nine Months Ended September 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 — — (10) (10) Amounts reclassified from AOCL Interest Expense 339 — — 339 Amounts reclassified from AOCL — (502) — (502) Net current-period other comprehensive income (loss) 339 (502) (10) (173) Ending balance $ (9,486) $ 40 $ 1,425 $ (8,021)

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	Financial data for the business segments are as follows (in thousands): Three Months Ended September 30, 2024 Electric Gas Other Eliminations Total Operating revenues $ 306,478 $ 38,683 $ — $ — $ 345,161 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 80,761 7,127 — — 87,888 Utility margin 225,717 31,556 — — 257,273 Operating and maintenance 42,491 13,375 — — 55,866 Administrative and general 24,892 9,887 145 — 34,924 Property and other taxes 32,251 9,345 — — 41,596 Depreciation and depletion 47,540 9,414 — — 56,954 Operating income (loss) 78,543 (10,465) (145) — 67,933 Interest expense, net (24,188) (7,537) (1,672) — (33,397) Other income, net 6,057 3,017 42 — 9,116 Income tax (expense) benefit (7,635) 9,734 1,068 — 3,167 Net income (loss) $ 52,777 $ (5,251) $ (707) $ — $ 46,819 Total assets $ 6,256,750 $ 1,578,075 $ 19,249 $ — $ 7,854,074 Capital expenditures $ 109,925 $ 43,225 $ — $ — $ 153,150 Three Months Ended September 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 280,030 $ 41,060 $ — $ — $ 321,090 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 77,995 10,948 — — 88,943 Utility margin 202,035 30,112 — — 232,147 Operating and maintenance 39,990 13,250 — — 53,240 Administrative and general 20,682 8,249 424 — 29,355 Property and other taxes 33,740 9,574 (1,551) — 41,763 Depreciation and depletion 43,230 8,929 — — 52,159 Operating income (loss) 64,393 (9,890) 1,127 — 55,630 Interest expense, net (21,300) (4,426) (2,999) — (28,725) Other income (expense), net 3,380 1,328 (581) — 4,127 Income tax (expense) benefit (3,223) (41) 1,567 — (1,697) Net income (loss) $ 43,250 $ (13,029) $ (886) $ — $ 29,335 Total assets $ 5,963,950 $ 1,454,445 $ 11,104 $ — $ 7,429,499 Capital expenditures $ 110,804 $ 46,359 $ — $ — $ 157,163 Nine Months Ended September 30, 2024 Electric Gas Other Eliminations Total Operating revenues $ 909,798 $ 230,634 $ — $ — $ 1,140,432 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 256,989 82,100 — — 339,089 Utility margin 652,809 148,534 — — 801,343 Operating and maintenance 126,257 41,158 — — 167,415 Administrative and general 76,105 27,754 2,791 — 106,650 Property and other taxes 96,557 28,465 1 — 125,023 Depreciation and depletion 142,390 28,240 — — 170,630 Operating income (loss) 211,500 22,917 (2,792) — 231,625 Interest expense, net (72,143) (20,933) (3,175) — (96,251) Other income (expense), net 15,549 4,998 (952) — 19,595 Income tax (expense) benefit (18,809) 6,865 534 — (11,410) Net income (loss) $ 136,097 $ 13,847 $ (6,385) $ — $ 143,559 Total assets $ 6,256,750 $ 1,578,075 $ 19,249 $ — $ 7,854,074 Capital expenditures $ 312,773 $ 87,738 $ — $ — $ 400,511 Nine Months Ended September 30, 2023 Electric Gas Other Eliminations Total Operating revenues $ 804,604 $ 261,530 $ — $ — $ 1,066,134 Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below) 198,492 123,521 — — 322,013 Utility margin 606,112 138,009 — — 744,121 Operating and maintenance 123,771 40,170 — — 163,941 Administrative and general 67,285 26,336 437 — 94,058 Property and other taxes 103,013 29,576 (1,546) — 131,043 Depreciation and depletion 130,447 27,340 — — 157,787 Operating income 181,596 14,587 1,109 — 197,292 Interest expense, net (61,584) (12,167) (11,393) — (85,144) Other income (expense), net 9,700 3,887 (661) — 12,926 Income tax expense (13,366) (180) (539) — (14,085) Net income (loss) $ 116,346 $ 6,127 $ (11,484) $ — $ 110,989 Total assets $ 5,963,950 $ 1,454,445 $ 11,104 $ — $ 7,429,499 Capital expenditures $ 326,313 $ 94,212 $ — $ — $ 420,525

Revenue from Contracts with C_2

Revenue from Contracts with Customers Disaggregation of Revenue (Tables)	9 Months Ended
	Sep. 30, 2024
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 September 30, 2024 September 30, 2023 Electric Natural Gas Total Electric Natural Gas Total Montana $ 100.7 $ 8.4 $ 109.1 $ 96.8 $ 9.6 $ 106.4 South Dakota 19.1 1.7 20.8 18.0 2.0 20.0 Nebraska — 1.8 1.8 — 2.2 2.2 Residential 119.8 11.9 131.7 114.8 13.8 128.6 Montana 109.6 6.2 115.8 110.1 6.1 116.2 South Dakota 30.1 1.3 31.4 27.5 1.5 29.0 Nebraska — 0.8 0.8 — 1.3 1.3 Commercial 139.7 8.3 148.0 137.6 8.9 146.5 Industrial 11.8 0.1 11.9 11.4 0.1 11.5 Lighting, governmental, irrigation, and interdepartmental 14.1 0.2 14.3 13.2 0.2 13.4 Total Customer Revenues 285.4 20.5 305.9 277.0 23.0 300.0 Other tariff and contract based revenues 28.0 9.3 37.3 22.1 10.2 32.3 Total Revenue from Contracts with Customers 313.4 29.8 343.2 299.1 33.2 332.3 Regulatory amortization and other (6.9) 8.9 2.0 (19.1) 7.9 (11.2) Total Revenues $ 306.5 $ 38.7 $ 345.2 $ 280.0 $ 41.1 $ 321.1 Nine Months Ended September 30, 2024 September 30, 2023 Electric Natural Gas Total Electric Natural Gas Total Montana $ 304.1 $ 75.9 $ 380.0 $ 306.1 $ 94.1 $ 400.2 South Dakota 53.8 21.2 75.0 53.4 30.3 83.7 Nebraska — 16.1 16.1 — 30.2 30.2 Residential 357.9 113.2 471.1 359.5 154.6 514.1 Montana 310.8 42.0 352.8 324.6 52.4 377.0 South Dakota 84.2 14.3 98.5 77.8 21.3 99.1 Nebraska — 9.0 9.0 — 19.1 19.1 Commercial 395.0 65.3 460.3 402.4 92.8 495.2 Industrial 34.8 0.7 35.5 34.0 1.0 35.0 Lighting, governmental, irrigation, and interdepartmental 27.4 1.1 28.5 27.2 1.3 28.5 Total Customer Revenues 815.1 180.3 995.4 823.1 249.7 1,072.8 Other tariff and contract based revenues 77.4 30.9 108.3 63.5 33.1 96.6 Total Revenue from Contracts with Customers 892.5 211.2 1,103.7 886.6 282.8 1,169.4 Regulatory amortization and other 17.3 19.4 36.7 (82.0) (21.3) (103.3) Total Revenues $ 909.8 $ 230.6 $ 1,140.4 $ 804.6 $ 261.5 $ 1,066.1

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2024
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 September 30, 2024 September 30, 2023 Basic computation 61,301,696 60,442,164 Dilutive effect of: Performance share awards (1) 95,279 35,533 Diluted computation 61,396,975 60,477,697 Nine Months Ended September 30, 2024 September 30, 2023 Basic computation 61,285,570 60,010,609 Dilutive effect of: Performance share awards (1) 69,136 31,311 Diluted computation 61,354,706 60,041,920 (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.

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2024
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 September 30, Three Months Ended September 30, 2024 2023 2024 2023 Components of Net Periodic Benefit Cost (Credit) Service cost $ 1,398 $ 1,459 $ 77 $ 84 Interest cost 5,736 6,524 139 168 Expected return on plan assets (6,331) (6,679) (320) (274) Amortization of prior service credit — — — 29 Recognized actuarial loss (gain) 8 68 (18) 18 Net periodic benefit cost (credit) $ 811 $ 1,372 $ (122) $ 25 Pension Benefits Other Postretirement Benefits Nine Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 Components of Net Periodic Benefit Cost (Credit) Service cost $ 4,194 $ 4,375 $ 231 $ 250 Interest cost 17,208 19,571 418 505 Expected return on plan assets (18,994) (20,036) (960) (822) Amortization of prior service credit — — — 87 Recognized actuarial loss (gain) 25 205 (55) 54 Net periodic benefit cost (credit) $ 2,433 $ 4,115 $ (366) $ 74

Nature of Operations and Basi_2

Nature of Operations and Basis of Consolidation (Details)	Dec. 31, 2023 customers
Number of customers	775,300

Nature of Operations and Basi_3

Nature of Operations and Basis of Consolidation Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Cash, Cash Equivalents, and Restricted Cash [Abstract]
Cash and cash equivalents	$ 2,527	$ 9,164	$ 5,091	$ 8,489
Restricted cash	25,365	16,023	16,549	13,974
Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statements of Cash Flows	$ 27,892	$ 25,187	$ 21,640	$ 22,463

Regulatory Matters (Details)

Regulatory Matters (Details) $ in Millions	9 Months Ended
	Sep. 30, 2024 USD ($) Rate
Electricity, US Regulated | Montana
Schedule of Regulated Operations [Line Items]
Rate base	$ 3,450
Electricity, US Regulated | Base electric rate | Montana
Schedule of Regulated Operations [Line Items]
Requested rate increase (decrease), amount	156.5
Electricity, US Regulated | Base electric rate - net of PCCAM and Property Tax | Montana
Schedule of Regulated Operations [Line Items]
Requested rate increase (decrease), amount	$ 69.4
Natural Gas, US Regulated | Montana
Schedule of Regulated Operations [Line Items]
Public Utilities, Requested Return on Equity, Percentage | Rate	10.80%
Public Utilities, Requested Equity Capital Structure, Percentage | Rate	46.81%
Rate base	$ 731.9
Natural Gas, US Regulated | South Dakota
Schedule of Regulated Operations [Line Items]
Public Utilities, Requested Return on Equity, Percentage | Rate	10.70%
Public Utilities, Requested Equity Capital Structure, Percentage | Rate	53.13%
Rate base	$ 95.6
Natural Gas, US Regulated | Nebraska
Schedule of Regulated Operations [Line Items]
Public Utilities, Requested Return on Equity, Percentage | Rate	10.70%
Public Utilities, Requested Equity Capital Structure, Percentage | Rate	53.13%
Rate base	$ 47.4
Natural Gas, US Regulated | Base gas rate | Montana
Schedule of Regulated Operations [Line Items]
Requested rate increase (decrease), amount	28.6
Natural Gas, US Regulated | Base gas rate | South Dakota
Schedule of Regulated Operations [Line Items]
Requested rate increase (decrease), amount	6
Natural Gas, US Regulated | Base gas rate | Nebraska
Schedule of Regulated Operations [Line Items]
Requested rate increase (decrease), amount	3.6
Public Utilities, Interim Rate Increase (Decrease), Amount	$ 2.3

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Dec. 31, 2023
Income Tax Contingency [Line Items]
Unrecognized tax benefit more likely than not percentage threshold		50%
Unrecognized tax benefits	$ 26.9	$ 26.9
Unrecognized tax benefits that would impact effective tax rate	24.3	24.3
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	6.8	6.8	$ 4.5
Decrease in Unrecognized Tax Benefits is Reasonably Possible	16.9	$ 16.9
Other Tax Expense (Benefit)	7
Other Tax Expense (Benefit)	$ 7
Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Open Tax Year		2020

Income Taxes Effective Tax Rate

Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Effective tax rate reconciliation
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ 43,652	$ 31,032	$ 154,969	$ 125,074
Income tax calculated at federal statutory rate	9,167	6,516	32,544	26,265
State income tax, net of federal provisions	61	121	749	1,353
Effective Income Tax Rate Reconciliation, Gas Repairs Safe Harbor Method Change, Amount	(6,994)	0	(6,994)	0
Flow-through repairs deductions	(4,581)	(4,189)	(13,824)	(11,742)
Production tax credits	(2,447)	(1,261)	(7,434)	(5,607)
Amortization of excess deferred income tax	(219)	(323)	(775)	(1,355)
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount	0	411	0	411
Reduction to previously claimed alternative minimum tax credit			0	3,186
Plant and depreciation flow-through items	1,816	358	5,955	1,247
Share-based compensation			298	388
Other, net	30	64	891	(61)
Effective Income Tax Rate Reconciliation, Total Reconciling Items, Amount	(12,334)	(4,819)	(21,134)	(12,180)
Income tax expense	$ (3,167)	$ 1,697	$ 11,410	$ 14,085
Income tax calculated at federal statutory rate	21%	21%	21%	21%
State income tax, net of federal provisions	0.10%	0.40%	0.50%	1.10%
Effective Income Tax Rate Reconciliation, Gas Repairs Safe Harbor Method Change, Percent	(16.00%)	0%	(4.50%)	0%
Flow-through repairs deductions	(10.50%)	(13.50%)	(8.90%)	(9.40%)
Production tax credits	(5.60%)	(4.10%)	(4.80%)	(4.50%)
Amortization of excess deferred income tax	(0.50%)	(1.00%)	(0.50%)	(1.10%)
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent	0%	1.30%	0%	0.30%
Reduction to previously claimed alternative minimum tax credit			0%	2.50%
Plant and depreciation flow-through items	4.20%	1.20%	3.80%	1%
Share-based compensation			0.20%	0.30%
Other, net	0%	0.20%	0.60%	0.10%
Effective Income Tax Rate Reconciliation, Total Reconciling Items, Percent	(28.30%)	(15.50%)	(13.60%)	(9.70%)
Income tax expense	(7.30%)	5.50%	7.40%	11.30%

Comprehensive Income (Loss) (De

Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Other Comprehensive Income (Loss), before Tax [Abstract]
Foreign currency translation adjustment	$ 1	$ (7)	$ (1)	$ (10)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(153)	(153)	(459)	(459)
Defined benefit pension plan and postretirement medical liability adjustment	0	(212)	0	(636)
Other Comprehensive Income (Loss), before Tax	154	(66)	458	(187)
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)	(120)	(120)
Defined benefit pension plan and postretirement medical liability adjustment	0	44	0	134
Other Comprehensive Income (Loss), Tax	(40)	4	(120)	14
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustment	1	(7)	(1)	(10)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(113)	(113)	(339)	(339)
Defined benefit pension plan and postretirement medical liability adjustment	0	(168)	0	(502)
Total Other Comprehensive Income (Loss)	$ 114	$ (62)	$ 338	$ (173)

Classification within AOCL on t

Classification within AOCL on the Condenced Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Foreign currency translation	$ 1,436		$ 1,437
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax	(9,034)		(9,373)
Defined benefit pension plan	280		280
Accumulated other comprehensive loss	$ (7,318)	$ (7,432)	$ (7,656)	$ (8,021)	$ (7,959)	$ (7,848)

Changes in AOCI by Component (D

Changes in AOCI by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ (7,432)	$ (7,959)	$ (7,656)	$ (7,848)
Other comprehensive income before reclassifications	1	(7)	(1)	(10)
Amounts reclassified from AOCL	113	113	339	339
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(168)		(502)
Total Other Comprehensive Income (Loss)	114	(62)	338	(173)
Ending balance	(7,318)	(8,021)	(7,318)	(8,021)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(9,147)	(9,599)	(9,373)	(9,825)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from AOCL	113	113	339	339
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0		0
Total Other Comprehensive Income (Loss)	113	113	339	339
Ending balance	(9,034)	(9,486)	(9,034)	(9,486)
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	1,435	1,432	1,437	1,435
Other comprehensive income before reclassifications	1	(7)	(1)	(10)
Amounts reclassified from AOCL	0	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0		0
Total Other Comprehensive Income (Loss)	1	(7)	(1)	(10)
Ending balance	1,436	1,425	1,436	1,425
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	280	208	280	542
Other comprehensive income 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		(168)		(502)
Total Other Comprehensive Income (Loss)	0	(168)	0	(502)
Ending balance	$ 280	$ 40	$ 280	$ 40

Financing Activities (Details)

Financing Activities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Short-term Debt [Line Items]
Short-Term Debt	$ 100,000	$ 0
Term Loan Due 2025
Short-term Debt [Line Items]
Short-Term Debt	$ 100,000
Debt Instrument, Maturity Date	Apr. 11, 2025
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage	65%
Montana First Mortgage Bonds Due March 28, 2031
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 175,000
Debt Instrument, Interest Rate, Stated Percentage	5.56%
Long-term Debt, Maturity Date	Mar. 28, 2031
South Dakota First Mortgage Bonds Due 2031
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 33,000
Debt Instrument, Interest Rate, Stated Percentage	5.55%
Long-term Debt, Maturity Date	Mar. 28, 2029
South Dakota First Mortgage Bonds Due 2034
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 7,000
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Long-term Debt, Maturity Date	Mar. 28, 2034
Montana First Mortgage Bonds Due 2024
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	$ 100,000

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Segment Reporting Information [Line Items]
Operating revenues	$ 345,161	$ 321,090	$ 1,140,432	$ 1,066,134
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	87,888	88,943	339,089	322,013
Utility margin	257,273	232,147	801,343	744,121
Operating and maintenance	55,866	53,240	167,415	163,941
Administrative and general	34,924	29,355	106,650	94,058
Property and other taxes	41,596	41,763	125,023	131,043
Depreciation and depletion	56,954	52,159	170,630	157,787
Operating income (loss)	67,933	55,630	231,625	197,292
Interest expense, net	(33,397)	(28,725)	(96,251)	(85,144)
Other income, net	9,116	4,127	19,595	12,926
Income tax benefit (expense)	3,167	(1,697)	(11,410)	(14,085)
Net Income	46,819	29,335	143,559	110,989
Total assets	7,854,074	7,429,499	7,854,074	7,429,499	$ 7,600,652
Capital expenditures	153,150	157,163	400,511	420,525
Operating Segments | Electricity, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	306,478	280,030	909,798	804,604
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	80,761	77,995	256,989	198,492
Utility margin	225,717	202,035	652,809	606,112
Operating and maintenance	42,491	39,990	126,257	123,771
Administrative and general	24,892	20,682	76,105	67,285
Property and other taxes	32,251	33,740	96,557	103,013
Depreciation and depletion	47,540	43,230	142,390	130,447
Operating income (loss)	78,543	64,393	211,500	181,596
Interest expense, net	(24,188)	(21,300)	(72,143)	(61,584)
Other income, net	6,057	3,380	15,549	9,700
Income tax benefit (expense)	(7,635)	(3,223)	(18,809)	(13,366)
Net Income	52,777	43,250	136,097	116,346
Total assets	6,256,750	5,963,950	6,256,750	5,963,950
Capital expenditures	109,925	110,804	312,773	326,313
Operating Segments | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Operating revenues	38,683	41,060	230,634	261,530
Fuel, purchased supply and direct transmission expense (exclusive of depreciation and depletion shown separately below)	7,127	10,948	82,100	123,521
Utility margin	31,556	30,112	148,534	138,009
Operating and maintenance	13,375	13,250	41,158	40,170
Administrative and general	9,887	8,249	27,754	26,336
Property and other taxes	9,345	9,574	28,465	29,576
Depreciation and depletion	9,414	8,929	28,240	27,340
Operating income (loss)	(10,465)	(9,890)	22,917	14,587
Interest expense, net	(7,537)	(4,426)	(20,933)	(12,167)
Other income, net	3,017	1,328	4,998	3,887
Income tax benefit (expense)	9,734	(41)	6,865	(180)
Net Income	(5,251)	(13,029)	13,847	6,127
Total assets	1,578,075	1,454,445	1,578,075	1,454,445
Capital expenditures	43,225	46,359	87,738	94,212
Operating Segments | Product and Service, Other
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	145	424	2,791	437
Property and other taxes	0	(1,551)	1	(1,546)
Depreciation and depletion	0	0	0	0
Operating income (loss)	(145)	1,127	(2,792)	1,109
Interest expense, net	(1,672)	(2,999)	(3,175)	(11,393)
Other income, net	42	(581)	(952)	(661)
Income tax benefit (expense)	1,068	1,567	534	(539)
Net Income	(707)	(886)	(6,385)	(11,484)
Total assets	19,249	11,104	19,249	11,104
Capital expenditures	0	0	0	0
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, net	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

Revenue from Contracts with C_3

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 343,200	$ 332,300	$ 1,103,700	$ 1,169,400
Total Revenues	345,161	321,090	1,140,432	1,066,134
Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	131,700	128,600	471,100	514,100
Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109,100	106,400	380,000	400,200
Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,800	20,000	75,000	83,700
Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,800	2,200	16,100	30,200
Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148,000	146,500	460,300	495,200
Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	115,800	116,200	352,800	377,000
Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31,400	29,000	98,500	99,100
Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	800	1,300	9,000	19,100
Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,900	11,500	35,500	35,000
Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,300	13,400	28,500	28,500
Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	305,900	300,000	995,400	1,072,800
Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37,300	32,300	108,300	96,600
Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	2,000	(11,200)	36,700	(103,300)
Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	345,200	321,100	1,140,400	1,066,100
Electric Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	313,400	299,100	892,500	886,600
Electric Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	119,800	114,800	357,900	359,500
Electric Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100,700	96,800	304,100	306,100
Electric Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	19,100	18,000	53,800	53,400
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	139,700	137,600	395,000	402,400
Electric Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109,600	110,100	310,800	324,600
Electric Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,100	27,500	84,200	77,800
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	11,800	11,400	34,800	34,000
Electric Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14,100	13,200	27,400	27,200
Electric Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	285,400	277,000	815,100	823,100
Electric Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	28,000	22,100	77,400	63,500
Electric Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	(6,900)	(19,100)	17,300	(82,000)
Electric Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	306,500	280,000	909,800	804,600
Gas Domestic Regulated [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,800	33,200	211,200	282,800
Gas Domestic Regulated [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,900	13,800	113,200	154,600
Gas Domestic Regulated [Member] | Residential | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,400	9,600	75,900	94,100
Gas Domestic Regulated [Member] | Residential | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,700	2,000	21,200	30,300
Gas Domestic Regulated [Member] | Residential | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,800	2,200	16,100	30,200
Gas Domestic Regulated [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8,300	8,900	65,300	92,800
Gas Domestic Regulated [Member] | Commercial | Montana
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	6,200	6,100	42,000	52,400
Gas Domestic Regulated [Member] | Commercial | South Dakota
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,300	1,500	14,300	21,300
Gas Domestic Regulated [Member] | Commercial | Nebraska
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	800	1,300	9,000	19,100
Gas Domestic Regulated [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100	100	700	1,000
Gas Domestic Regulated [Member] | Other, Lighting, Governmental, Irrigation, and Interdepartmental
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	200	200	1,100	1,300
Gas Domestic Regulated [Member] | Total Customer Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,500	23,000	180,300	249,700
Gas Domestic Regulated [Member] | Other Tariff and Contract Based Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9,300	10,200	30,900	33,100
Gas Domestic Regulated [Member] | Regulatory amortization
Disaggregation of Revenue [Line Items]
Regulatory amortization	8,900	7,900	19,400	(21,300)
Gas Domestic Regulated [Member] | Total Revenues
Disaggregation of Revenue [Line Items]
Total Revenues	$ 38,700	$ 41,100	$ 230,600	$ 261,500

Earnings Per Share (Details)

Earnings Per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Basic computation	61,301,696	60,442,164	61,285,570	60,010,609
Performance share awards(1)	95,279	35,533	69,136	31,311
Diluted computation	61,396,975	60,477,697	61,354,706	60,041,920
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	16,015			32,649

Employee Benefit Plans Net Peri

Employee Benefit Plans Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Components of Net Periodic Benefit Cost (Income) [Abstract]
Minimum Expected Future Employer Pension Contributions, Remainder of Fiscal Year	$ 2,600		$ 2,600
Payment for Pension Benefits			8,600
Pension Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	1,398	$ 1,459	4,194	$ 4,375
Interest cost	5,736	6,524	17,208	19,571
Expected return on plan assets	(6,331)	(6,679)	(18,994)	(20,036)
Amortization of prior service credit	0	0	0	0
Recognized actuarial loss (gain)	8	68	25	205
Net periodic benefit cost (credit)	811	1,372	2,433	4,115
Other Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	77	84	231	250
Interest cost	139	168	418	505
Expected return on plan assets	(320)	(274)	(960)	(822)
Amortization of prior service credit	0	29	0	87
Recognized actuarial loss (gain)	(18)	18	(55)	54
Net periodic benefit cost (credit)	$ (122)	$ 25	$ (366)	$ 74