Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Regulated Asset Liability [Line Items]
Regulatory assets	$ 38,785	$ 48,552
Regulatory Assets, Noncurrent	630,451	614,354
Regulatory liabilities	37,954	113,209
Non-current regulatory liabilities	789,655	780,701
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	23,079	27,997
Non-current regulatory liabilities	418,039	425,613
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	1,852	40,713
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	6,612	25,072
Non-current regulatory liabilities	32,617	16,933
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	437	6,782
Non-current regulatory liabilities	2,861	204
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	302,734	297,379
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	17,659	28,078
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	5,974	12,645
Non-current regulatory liabilities	15,745	12,494
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	25,125	41,428
Regulatory Assets, Noncurrent	10,764	16,866
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	6,498	3,408
Regulatory Assets, Noncurrent	16,167	17,501
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	221,454	228,062
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	155,787	133,854
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	95,217	91,188
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	41,988	42,643
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	13,705	19,674
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	25,273	24,334
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	7,162	3,716
Regulatory Assets, Noncurrent	$ 50,096	$ 40,232