CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss) Attributable to Parent	$ 210,708,000	$ 235,622,000	$ 175,194,000
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	656,323,000	666,432,000	481,969,000
Conservation amortization	96,571,000	111,714,000	121,216,000
Deferred Income Taxes and Tax Credits	7,475,000	19,457,000	254,524,000
Unrealized Gain (Loss) on Derivatives	(3,574,000)	41,662,000	(30,650,000)
Afudc Equity	15,802,000	17,191,000	15,027,000
Production tax credits	(68,622,000)	(83,976,000)	(53,331,000)
Other non-cash	(4,639,000)	15,339,000	17,568,000
Payment for Pension Benefits	18,000,000	18,000,000	18,000,000
Increase (Decrease) in Other Regulatory Assets	79,233,000	71,348,000	88,875,000
increase (decrease) in purchased gas	132,766,000	0	0
Increase (Decrease) in Other Operating Assets	16,098,000	(2,695,000)	27,411,000
Change in certain current assets and liabilities:
Increase (Decrease) in Accounts and Other Receivables	(3,058,000)	(17,659,000)	(132,000)
Increase (Decrease) in Materials and Supplies	6,018,000	9,177,000	625,000
Increase Decrease In Fuel And Gas Inventories	(1,268,000)	3,443,000	(8,266,000)
increase (decrease) in purchased gas adjustment, short-term	(9,921,000)	25,972,000	(18,836,000)
Increase (Decrease) in Prepaid Expense and Other Assets	1,103,000	3,679,000	(21,050,000)
Increase (Decrease) in Accounts Payable	(116,311,000)	117,270,000	26,396,000
Increase (Decrease) in Income Taxes Payable	(18,133,000)	164,000	6,520,000
Increase (Decrease) in Other Accounts Payable and Accrued Liabilities	15,163,000	(7,723,000)	13,079,000
Net Cash Provided by (Used in) Operating Activities, Total	527,336,000	904,181,000	972,131,000
Net Cash Provided by (Used in) Investing Activities
Construction expenditures excluding equity allowance for funds used during construction	959,387,000	1,072,670,000	1,040,135,000
Payments for (Proceeds from) Other Investing Activities	(6,908,000)	(2,097,000)	195,000
Net Cash Provided by (Used in) Investing Activities, Total	(952,479,000)	(1,070,573,000)	(1,040,330,000)
Net Cash Provided by (Used in) Financing Activities
Proceeds from (Repayments of) Short-term Debt	(203,297,000)	49,834,000	83,700,000
Payments of Dividends	64,220,000	77,204,000	123,307,000
Proceeds from Issuance of Long-term Debt	689,351,000	804,050,000	90,120,000
Repayments of Long-term Debt	0	600,000,000	0
Proceeds from (Payments for) Other Financing Activities	13,893,000	8,513,000	13,151,000
Net Cash Provided by (Used in) Financing Activities, Total	435,727,000	185,193,000	63,664,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	10,584,000	18,801,000	(4,535,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	55,562,000	36,761,000	41,296,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	66,146,000	55,562,000	36,761,000
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	328,703,000	322,476,000	326,798,000
Income Taxes Paid, Net	10,616,000	8,303,000	1,649,000
Non-cash financing and investing activities:
Capital Expenditures Incurred but Not yet Paid	58,329,000	97,673,000	92,959,000
Increase (Decrease) in Regulatory Liabilities	0	0
Hydro Treasury Grants
Non-cash financing and investing activities:
Increase (Decrease) in Regulatory Liabilities			95,935,000
Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income (Loss) Attributable to Parent	292,924,000	317,162,000	320,054,000
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	656,220,000	666,324,000	481,955,000
Conservation amortization	96,571,000	111,714,000	121,216,000
Deferred Income Taxes and Tax Credits	20,474,000	30,995,000	210,842,000
Unrealized Gain (Loss) on Derivatives	(3,574,000)	41,662,000	(30,790,000)
Afudc Equity	15,802,000	17,191,000	15,027,000
Production tax credits	(68,622,000)	(83,976,000)	(53,331,000)
Other non-cash	(15,154,000)	4,428,000	6,445,000
Payment for Pension Benefits	18,000,000	18,000,000	18,000,000
Increase (Decrease) in Other Regulatory Assets	79,173,000	71,348,000	88,875,000
increase (decrease) in purchased gas	132,766,000	0	0
Increase (Decrease) in Other Operating Assets	8,967,000	(16,917,000)	14,547,000
Change in certain current assets and liabilities:
Increase (Decrease) in Accounts and Other Receivables	(7,650,000)	(12,626,000)	(13,285,000)
Increase (Decrease) in Materials and Supplies	6,018,000	9,177,000	625,000
Increase Decrease In Fuel And Gas Inventories	(1,210,000)	3,443,000	(8,266,000)
increase (decrease) in purchased gas adjustment, short-term	(9,921,000)	25,972,000	(18,836,000)
Increase (Decrease) in Prepaid Expense and Other Assets	1,103,000	3,679,000	(21,050,000)
Increase (Decrease) in Accounts Payable	(116,370,000)	117,397,000	26,396,000
Increase (Decrease) in Income Taxes Payable	(18,016,000)	930,000	5,635,000
Increase (Decrease) in Other Accounts Payable and Accrued Liabilities	15,371,000	(8,141,000)	12,438,000
Net Cash Provided by (Used in) Operating Activities, Total	623,924,000	995,904,000	1,086,803,000
Net Cash Provided by (Used in) Investing Activities
Construction expenditures excluding equity allowance for funds used during construction	919,271,000	1,010,506,000	963,652,000
Payments for (Proceeds from) Other Investing Activities	(6,908,000)	(2,097,000)	(241,000)
Net Cash Provided by (Used in) Investing Activities, Total	(912,363,000)	(1,008,409,000)	(963,411,000)
Net Cash Provided by (Used in) Financing Activities
Proceeds from (Repayments of) Short-term Debt	(203,297,000)	49,834,000	83,700,000
Payments of Dividends	164,575,000	173,716,000	227,783,000
Proceeds from Contributions from Parent	210,000,000	0	0
Proceeds from Issuance of Long-term Debt	443,151,000	594,750,000	0
Repayments of Long-term Debt	0	450,000,000	0
Proceeds from (Payments for) Other Financing Activities	14,558,000	9,121,000	15,801,000
Net Cash Provided by (Used in) Financing Activities, Total	299,837,000	29,989,000	(128,282,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	11,398,000	17,484,000	(4,890,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	53,493,000	36,009,000	40,899,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	64,891,000	53,493,000	36,009,000
Supplemental cash flow information:
Cash payments for interest (net of capitalized interest)	219,665,000	221,155,000	224,423,000
Income Taxes Paid, Net	19,269,000	18,124,000	3,058,000
Non-cash financing and investing activities:
Capital Expenditures Incurred but Not yet Paid	58,329,000	97,673,000	92,959,000
Subsidiaries [Member] | Hydro Treasury Grants
Non-cash financing and investing activities:
Increase (Decrease) in Regulatory Liabilities	0	0	95,935,000
Colstrip Units 1 – 4 Common Facilities
Non-cash financing and investing activities:
Increase (Decrease) in Regulatory Assets and Liabilities	$ 4,163,000	$ (3,086,000)	$ (49,177,000)