Regulation and Rates Net regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax charge
Regulatory Assets [Line Items]
Regulatory Liabilities	$ 608	$ 634
Regulatory liabilities related to power contracts
Regulatory Assets [Line Items]
Regulatory Liabilities	162,711	174,918
Various other regulatory liabilities
Regulatory Assets [Line Items]
Regulatory Liabilities	1,323	1,314
Liabilities, Total
Regulatory Assets [Line Items]
Regulatory Liabilities	1,885,888	1,934,182
Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets	(1,080,987)
Various other regulatory assets
Regulatory Assets [Line Items]
Regulatory Assets	0	(8)
Requlatory Assets Related to Power Contracts
Regulatory Assets [Line Items]
Regulatory Assets	16,693	19,454
Assets, Total
Regulatory Assets [Line Items]
Regulatory Assets	804,901	972,562
Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets		(961,620)
Subsidiaries [Member] | Deferred income tax charge
Regulatory Assets [Line Items]
Regulatory Liabilities	976,582	1,012,260
Subsidiaries [Member] | Cost of removal
Regulatory Assets [Line Items]
Regulatory Liabilities	$ 424,727	389,579
Subsidiaries [Member] | Treasury grants
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period	20 years
Regulatory Liabilities	$ 168,884	205,775
Subsidiaries [Member] | Production tax credits [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	93,616	93,616
Subsidiaries [Member] | Accumulated Provision for Rate Refund [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	34,579	0
Subsidiaries [Member] | Decoupling rate of return sharing [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	0	18,400
Subsidiaries [Member] | Deferral and interest decoupling revenue [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	$ 13,758	7,896
Subsidiaries [Member] | Deferred decoupling revenue, net [Member]
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period	2 years
Regulatory Liabilities	$ 13,758	26,296
Subsidiaries [Member] | PGA payable
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period	1 year
Regulatory Liabilities	$ 0	16,051
Subsidiaries [Member] | Summit purchase option buy-out
Regulatory Assets [Line Items]
Net Regulatory Assets, Remaining Amortization Period	1 year 10 months
Regulatory Liabilities	$ 2,888	4,463
Subsidiaries [Member] | Various other regulatory liabilities
Regulatory Assets [Line Items]
Regulatory Liabilities	7,428	10,544
Subsidiaries [Member] | Liabilities, Total
Regulatory Assets [Line Items]
Regulatory Liabilities	1,722,462	1,758,584
Subsidiaries [Member] | Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets	(934,254)
Subsidiaries [Member] | Storm Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	118,331	128,508
Subsidiaries [Member] | Colstrip Regulatory Asset [Domain]
Regulatory Assets [Line Items]
Regulatory Assets	37,674	127,627
Subsidiaries [Member] | Deferred decoupling revenue, net [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 65,779	98,769
Net Regulatory Assets, Remaining Amortization Period	2 years
Subsidiaries [Member] | Chelan PUD contract initiation
Regulatory Assets [Line Items]
Regulatory Assets	$ 90,964	98,052
Net Regulatory Assets, Remaining Amortization Period	12 years 10 months
Subsidiaries [Member] | Environmental remediation
Regulatory Assets [Line Items]
Regulatory Assets	$ 76,345	81,550
Subsidiaries [Member] | Lower Snake River
Regulatory Assets [Line Items]
Regulatory Assets	$ 67,021	70,975
Net Regulatory Assets, Remaining Amortization Period	18 years 5 months
Subsidiaries [Member] | Baker Dam Licensing Operating Maintenance Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 55,607	54,817
Subsidiaries [Member] | Deferred Washington Commission AFUDC
Regulatory Assets [Line Items]
Regulatory Assets	$ 52,029	50,301
Net Regulatory Assets, Remaining Amortization Period	10 years
Subsidiaries [Member] | Unamortized loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory Assets	$ 42,378	39,674
Subsidiaries [Member] | Property tax tracker
Regulatory Assets [Line Items]
Regulatory Assets	$ 45,621	36,517
Net Regulatory Assets, Remaining Amortization Period	2 years
Subsidiaries [Member] | Energy Conservation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 30,701	35,538
Subsidiaries [Member] | PGA deferral of unrealized losses on derivative instruments
Regulatory Assets [Line Items]
Regulatory Assets	14,739	26,030
Subsidiaries [Member] | White River relicensing and other costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 12,966	19,502
Net Regulatory Assets, Remaining Amortization Period	3 years
Subsidiaries [Member] | Generation plant major maintenance [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 15,027	17,216
Subsidiaries [Member] | Mint Farm ownership and operating costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 12,319	14,319
Net Regulatory Assets, Remaining Amortization Period	6 years 4 months
Subsidiaries [Member] | Colstrip major maintenance [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 6,841	8,723
Net Regulatory Assets, Remaining Amortization Period	6 months
Subsidiaries [Member] | Snoqualmie Licensing Operating Maintenance Costs
Regulatory Assets [Line Items]
Regulatory Assets	$ 7,407	7,341
Subsidiaries [Member] | Ferndale
Regulatory Assets [Line Items]
Regulatory Assets	$ 3,316	7,295
Net Regulatory Assets, Remaining Amortization Period	10 months
Subsidiaries [Member] | Colstrip common property
Regulatory Assets [Line Items]
Regulatory Assets	$ 3,903	4,618
Net Regulatory Assets, Remaining Amortization Period	6 years 5 months
Subsidiaries [Member] | PCA Mechanism [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 4,735	4,576
Subsidiaries [Member] | Electron Unrecovered Loss
Regulatory Assets [Line Items]
Regulatory Assets	$ 0	3,786
Net Regulatory Assets, Remaining Amortization Period	1 year
Subsidiaries [Member] | PGA receivable [Domain]
Regulatory Assets [Line Items]
Regulatory Assets	$ 9,922	0
Net Regulatory Assets, Remaining Amortization Period	1 year
Subsidiaries [Member] | Various other regulatory assets
Regulatory Assets [Line Items]
Regulatory Assets	$ 14,583	17,382
Subsidiaries [Member] | Assets, Total
Regulatory Assets [Line Items]
Regulatory Assets	788,208	953,116
Subsidiaries [Member] | Net Regulatory Assets
Regulatory Assets [Line Items]
Net Regulatory Assets		(805,468)
Colstrip Regulatory Asset [Domain]
Regulatory Assets [Line Items]
Regulatory Assets	$ 130,700	$ 127,600