CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss) Attributable to Parent	$ 260,849	$ 182,717	$ 210,708
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	704,783	647,755	656,323
Conservation amortization	103,147	99,585	96,571
Deferred Income Taxes and Tax Credits	(1,228)	(6,287)	7,475
Unrealized Gain (Loss) on Derivatives	(13,785)	26,807	3,574
Gain (Loss) on Extinguishment of Debt	0	13,546	0
Afudc Equity	(27,806)	(23,223)	(15,802)
Production tax credits	(45,562)	(39,761)	(68,622)
Other non-cash	(9,284)		(4,639)
Other non-cash		9,069
Payment for Pension Benefits	18,000	18,000	18,000
Increase (Decrease) in Other Regulatory Assets	126,625	152,417	79,233
increase (decrease) in purchased gas	(29,720)	(45,111)	132,766
Increase (Decrease) in Other Operating Assets	24,761	3,171	16,098
Change in certain current assets and liabilities:
Increase (Decrease) in Accounts and Other Receivables	96,498	32,994	(3,058)
Increase (Decrease) in Materials and Supplies	(5,046)	2,649	6,018
Increase Decrease In Fuel And Gas Inventories	10,598	(3,287)	(1,268)
increase (decrease) in purchased gas adjustment, short-term	0	0	(9,921)
Increase (Decrease) in Prepaid Expense and Other Assets	997	18,242	1,103
Increase (Decrease) in Accounts Payable	84,775	16,516	(116,311)
Increase (Decrease) in Income Taxes Payable	16,646	10,773	(18,133)
Increase (Decrease) in Other Accounts Payable and Accrued Liabilities	(3,224)	(30,854)	15,163
Net Cash Provided by (Used in) Operating Activities, Total	826,598	727,568	527,336
Net Cash Provided by (Used in) Investing Activities
Construction expenditures excluding equity allowance for funds used during construction	922,144	908,136	959,387
Payments for (Proceeds from) Other Investing Activities	(1,367)	(5,340)	(6,908)
Net Cash Provided by (Used in) Investing Activities, Total	(920,777)	(902,796)	(952,479)
Net Cash Provided by (Used in) Financing Activities
Proceeds from (Repayments of) Short-term Debt	(233,800)	197,800	(203,297)
Payments of Dividends	106,420	45,421	64,220
Proceeds from Contributions from Parent	210,000	4,575	0
Proceeds from Issuance of Long-term Debt	961,538	644,690	689,351
Repayments of Long-term Debt	502,410	450,000	0
Repayments of Debt	234,000	159,400	0
Proceeds from (Payments for) Other Financing Activities	20,570	(1,311)	13,893
Net Cash Provided by (Used in) Financing Activities, Total	115,478	190,933	435,727
Net increase (decrease) in cash, cash equivalents, and restricted cash	21,299	15,705	10,584
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	81,851	66,146	55,562
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	103,150	81,851	66,146
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	329,894	336,441	328,703
Income Taxes Paid, Net	22,647	4,974	10,616
Non-cash financing and investing activities:
Capital Expenditures Incurred but Not yet Paid	89,958	58,304	58,329
Recognition of new right-of-use assets, Finance Lease	105,176	0	0
Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss) Attributable to Parent	336,063	274,280	292,924
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	704,372	647,546	656,220
Conservation amortization	103,147	99,585	96,571
Deferred Income Taxes and Tax Credits	(8,652)	15,271	20,474
Unrealized Gain (Loss) on Derivatives	(13,785)	26,807	3,574
Afudc Equity	(27,806)	(23,223)	(15,802)
Production tax credits	(45,562)	(39,761)	(68,622)
Other non-cash	(19,761)	(1,575)	(15,154)
Payment for Pension Benefits	18,000	18,000	18,000
Increase (Decrease) in Other Regulatory Assets	126,625	152,417	79,173
increase (decrease) in purchased gas	(29,720)	(45,111)	132,766
Increase (Decrease) in Other Operating Assets	14,097	(8,764)	8,967
Change in certain current assets and liabilities:
Increase (Decrease) in Accounts and Other Receivables	96,487	33,835	(7,650)
Increase (Decrease) in Materials and Supplies	(5,046)	2,649	6,018
Increase Decrease In Fuel And Gas Inventories	10,598	(3,287)	(1,210)
increase (decrease) in purchased gas adjustment, short-term	0	0	(9,921)
Increase (Decrease) in Prepaid Expense and Other Assets	997	18,242	1,103
Increase (Decrease) in Accounts Payable	92,007	16,549	(116,370)
Increase (Decrease) in Income Taxes Payable	26,152	7,277	(18,016)
Increase (Decrease) in Other Accounts Payable and Accrued Liabilities	6,256	(29,965)	15,371
Net Cash Provided by (Used in) Operating Activities, Total	920,393	824,810	623,924
Net Cash Provided by (Used in) Investing Activities
Construction expenditures excluding equity allowance for funds used during construction	908,273	876,437	919,271
Payments for (Proceeds from) Other Investing Activities	(1,367)	(5,340)	(6,908)
Net Cash Provided by (Used in) Investing Activities, Total	(906,906)	(871,097)	(912,363)
Net Cash Provided by (Used in) Financing Activities
Proceeds from (Repayments of) Short-term Debt	(233,800)	197,800	(203,297)
Payments of Dividends	229,857	149,072	164,575
Proceeds from Contributions from Parent	0	0	210,000
Proceeds from Issuance of Long-term Debt	446,063	0	443,151
Repayments of Long-term Debt	2,410	0	0
Proceeds from (Payments for) Other Financing Activities	22,043	13,389	14,558
Net Cash Provided by (Used in) Financing Activities, Total	2,039	62,117	299,837
Net increase (decrease) in cash, cash equivalents, and restricted cash	15,526	15,830	11,398
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	80,721	64,891	53,493
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	96,247	80,721	64,891
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	223,484	228,420	219,665
Income Taxes Paid, Net	38,442	11,521	19,269
Non-cash financing and investing activities:
Capital Expenditures Incurred but Not yet Paid	89,958	58,304	58,329
Recognition of new right-of-use assets, Finance Lease	105,176	0	0
Colstrip Units 1 – 4 Common Facilities
Non-cash financing and investing activities:
Increase (Decrease) in Regulatory Assets and Liabilities	$ 0	$ 0	$ 4,163