Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020
Regulated Asset Liability [Line Items]
Regulatory assets		$ 27,174	$ 13,673
Non-current regulatory assets		870,095	750,443
Regulatory liabilities		70,176	46,435
Non-current regulatory liabilities		882,443	784,820
Income Tax Related [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	[1]	45,431	14,952
Non-current regulatory liabilities	[1]	485,165	399,677
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		14,996	20,299
Non-current regulatory liabilities		13,118	17,570
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		849	1,447
Non-current regulatory liabilities		3,754	1,519
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		336,940	325,832
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		15,637	15,046
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		12,680	10,949
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		8,900	9,737
Non-current regulatory liabilities		15,149	14,227
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		5,652	2,073
Non-current regulatory assets		1,410	5,722
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		9,768	7,123
Non-current regulatory assets		13,911	17,123
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		194,005	198,746
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		197,302	214,851
Income Tax Related [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	[1]	0	0
Non-current regulatory assets	[1]	238,234	108,517
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		39,484	40,043
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		48,961	47,393
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		1,449	3,814
Utility Plant to be Abandoned [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		27,599	28,916
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		14,292	8,166
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		11,754	4,477
Non-current regulatory assets		$ 93,448	$ 77,152

[1] In 2021, the Company received regulatory approval in all jurisdictions to change to flow-through tax treatment of certain basis adjustments, which was $127.5 million.