Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Regulated Asset Liability [Line Items]
Regulatory assets	$ 27,404	$ 44,750
Regulatory Assets, Noncurrent	581,495	619,399
Regulatory liabilities	88,500	48,264
Non-current regulatory liabilities	799,661	800,089
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	26,512	0
Non-current regulatory liabilities	428,825	460,542
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	31,515	37,474
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	9,160	5,816
Non-current regulatory liabilities	34,212	24,057
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	290,568	285,786
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	30,994	18,638
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	21,313	4,974
Non-current regulatory liabilities	15,062	11,066
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	21,750	24,991
Regulatory Assets, Noncurrent	11,277	18,967
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	5,571	19,759
Regulatory Assets, Noncurrent	13,308	2,600
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	204,129	209,115
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	134,078	169,704
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	91,925	90,315
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	43,299	43,954
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	21,932	24,620
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	23,773	24,330
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	83	0
Regulatory Assets, Noncurrent	$ 37,774	$ 35,794