Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Regulated Asset Liability [Line Items]
Regulatory assets	$ 21,525	$ 44,750
Regulatory Assets, Noncurrent	576,863	619,399
Regulatory liabilities	94,978	48,264
Non-current regulatory liabilities	798,440	800,089
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	36,850	0
Non-current regulatory liabilities	433,714	460,542
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	35,442	37,474
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	9,792	5,816
Non-current regulatory liabilities	30,219	24,057
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	8,853	0
Non-current regulatory liabilities	426	5,816
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	293,965	285,786
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	36,345	18,638
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	4,041	4,974
Non-current regulatory liabilities	3,771	5,250
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	19,238	24,991
Regulatory Assets, Noncurrent	9,555	18,967
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	2,287	19,759
Regulatory Assets, Noncurrent	18,099	2,600
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	201,620	209,115
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	131,972	169,704
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	90,285	90,315
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	42,971	43,954
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	19,774	24,620
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	24,158	24,330
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	$ 38,429	$ 35,794