Regulation and Rates Net regulatory assets and liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory Assets	$ 861,678,000	$ 804,901,000
Regulatory Liabilities	1,833,655,000	1,885,888,000
Deferred income tax charge
Regulatory Assets [Line Items]
Regulatory Liabilities	$ 757,000	608,000
Regulatory liabilities related to power contracts
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period, Min	6 years
Net Regulatory Assets, Remaining Amortization Period, Max	33 years
Regulatory Liabilities	$ 156,597,000	162,711,000
Various other regulatory liabilities
Regulatory Assets [Line Items]
Regulatory Liabilities	1,265,000	1,323,000
Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets	(971,977,000)	(1,080,987,000)
Requlatory Assets Related to Power Contracts
Regulatory Assets [Line Items]
Regulatory Assets	$ 14,146,000	16,693,000
Net Regulatory Assets, Remaining Amortization Period, Min	6 years
Net Regulatory Assets, Remaining Amortization Period, Max	33 years
Subsidiaries [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities Reclassified from Accumulated Depreciation	$ 469,900,000	424,700,000
Subsidiaries [Member] | Deferred income tax charge
Regulatory Assets [Line Items]
Regulatory Liabilities	946,936,000	976,582,000
Subsidiaries [Member] | Cost of removal
Regulatory Assets [Line Items]
Regulatory Liabilities	$ 469,922,000	424,727,000
Subsidiaries [Member] | Treasury grants
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period	18 years
Regulatory Liabilities	$ 101,981,000	168,884,000
Subsidiaries [Member] | Production tax credits [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	85,323,000	93,616,000
Subsidiaries [Member] | Gain on Sale Shuffleton [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	12,483,000	0
Subsidiaries [Member] | Microsoft special contract [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	12,661,000	0
Subsidiaries [Member] | Repurposed production tax credits [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	23,171,000	0
Subsidiaries [Member] | Accumulated Provision for Rate Refund [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	0	34,579,000
Subsidiaries [Member] | Deferred decoupling revenue, net [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	8,500,000	13,758,000
Subsidiaries [Member] | Various other regulatory liabilities
Regulatory Assets [Line Items]
Regulatory Liabilities	15,573,000	10,316,000
Subsidiaries [Member] | Liabilities, Total
Regulatory Assets [Line Items]
Regulatory Liabilities	(1,676,550,000)	1,722,462,000
Subsidiaries [Member] | Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets	(829,018,000)	(934,254,000)
Subsidiaries [Member] | Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 121,894,000	118,331,000
Net Regulatory Assets, Remaining Amortization Period, Min	1 year
Net Regulatory Assets, Remaining Amortization Period, Max	4 years
Subsidiaries [Member] | Chelan PUD contract initiation
Regulatory Assets [Line Items]
Regulatory Assets	$ 83,875,000	90,964,000
Net Regulatory Assets, Remaining Amortization Period	11 years 9 months 18 days
Subsidiaries [Member] | Environmental remediation
Regulatory Assets [Line Items]
Regulatory Assets	$ 68,486,000	76,345,000
Subsidiaries [Member] | Lower Snake River
Regulatory Assets [Line Items]
Regulatory Assets	$ 62,899,000	67,021,000
Net Regulatory Assets, Remaining Amortization Period	17 years 4 months 24 days
Subsidiaries [Member] | Deferred decoupling revenue, net [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 43,509,000	65,779,000
Net Regulatory Assets, Remaining Amortization Period	2 years
Subsidiaries [Member] | Baker Dam Licensing Operating Maintenance Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 56,427,000	55,607,000
Subsidiaries [Member] | Deferred Washington Commission AFUDC
Regulatory Assets [Line Items]
Regulatory Assets	$ 57,553,000	52,029,000
Net Regulatory Assets, Remaining Amortization Period	30 years
Subsidiaries [Member] | Property tax tracker
Regulatory Assets [Line Items]
Regulatory Assets	$ 22,442,000	45,621,000
Net Regulatory Assets, Remaining Amortization Period	2 years
Subsidiaries [Member] | Unamortized loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory Assets	$ 40,177,000	42,378,000
Net Regulatory Assets, Remaining Amortization Period, Min	2 years
Net Regulatory Assets, Remaining Amortization Period, Max	48 years
Subsidiaries [Member] | Colstrip Regulatory Asset
Regulatory Assets [Line Items]
Regulatory Assets	$ 0	37,674,000
Subsidiaries [Member] | Energy Conservation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	25,272,000	30,701,000
Subsidiaries [Member] | GTZ depreciation expense deferral [Member]
Regulatory Assets [Line Items]
Regulatory Assets	22,148,000	0
Subsidiaries [Member] | Advanced metering infrastructure [Member]
Regulatory Assets [Line Items]
Regulatory Assets	14,845,000	0
Subsidiaries [Member] | Generation plant major maintenance [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 12,744,000	15,027,000
Net Regulatory Assets, Remaining Amortization Period, Min	3 years
Net Regulatory Assets, Remaining Amortization Period, Max	10 years
Subsidiaries [Member] | PGA deferral of unrealized losses on derivative instruments
Regulatory Assets [Line Items]
Regulatory Assets	$ 0	14,739,000
Subsidiaries [Member] | White River relicensing and other costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 6,399,000	12,966,000
Net Regulatory Assets, Remaining Amortization Period	1 year
Subsidiaries [Member] | Mint Farm ownership and operating costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 10,318,000	12,319,000
Net Regulatory Assets, Remaining Amortization Period	5 years 3 months 18 days
Subsidiaries [Member] | PGA receivable [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 132,766,000	9,922,000
Net Regulatory Assets, Remaining Amortization Period	2 years
Subsidiaries [Member] | Snoqualmie Licensing Operating Maintenance Costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 7,442,000	7,407,000
Subsidiaries [Member] | Colstrip major maintenance [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 2,929,000	6,841,000
Net Regulatory Assets, Remaining Amortization Period	0 years
Subsidiaries [Member] | PCA Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 41,745,000	4,735,000
Subsidiaries [Member] | Colstrip common property
Regulatory Assets [Line Items]
Regulatory Assets	$ 3,188,000	3,903,000
Net Regulatory Assets, Remaining Amortization Period	4 years 4 months 24 days
Subsidiaries [Member] | Ferndale
Regulatory Assets [Line Items]
Regulatory Assets	$ 0	3,316,000
Net Regulatory Assets, Remaining Amortization Period	0 years
Subsidiaries [Member] | Various other regulatory assets
Regulatory Assets [Line Items]
Regulatory Assets	$ 10,474,000	14,583,000
Subsidiaries [Member] | Assets, Total
Regulatory Assets [Line Items]
Regulatory Assets	$ 847,532,000	$ 788,208,000