Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Regulated Asset Liability [Line Items]
Regulatory assets	$ 13,057	$ 21,851
Regulatory Assets, Noncurrent	750,866	670,802
Regulatory liabilities	66,336	51,715
Non-current regulatory liabilities	783,376	775,436
Provision for Rate Refunds [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	8,490	3,565
Non-current regulatory liabilities	0	0
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	23,975	23,803
Non-current regulatory liabilities	404,046	407,549
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	5,449	3,189
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	23,595	14,155
Non-current regulatory liabilities	20,716	23,544
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	363	255
Non-current regulatory liabilities	5,972	2,398
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	317,203	312,403
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	16,136	17,088
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	4,464	6,748
Non-current regulatory liabilities	19,303	12,454
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	78	6,310
Regulatory Assets, Noncurrent	0	264
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	10,927	12,098
Regulatory Assets, Noncurrent	15,504	14,806
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	206,112	208,754
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	246,566	168,594
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	96,525	95,752
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	41,005	41,332
AFUDC above FERC allowed rate
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	41,333	40,749
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	8,670	12,170
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	31,678	31,291
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	2,052	3,443
Regulatory Assets, Noncurrent	$ 63,473	$ 57,090