Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Regulated Asset Liability [Line Items]
Regulatory assets		$ 37,768	$ 13,673
Non-current regulatory assets		875,952	750,443
Regulatory liabilities		75,559	46,435
Non-current regulatory liabilities		886,351	784,820
Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	[1]	52,843	14,952
Non-current regulatory liabilities	[1]	475,607	399,677
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		10,512	20,299
Non-current regulatory liabilities		9,136	17,570
Decoupling Rebate [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		2,717	1,447
Non-current regulatory liabilities		10,283	1,519
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		344,061	325,832
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		16,757	15,046
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		0	0
Non-current regulatory liabilities		12,673	10,949
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities		9,487	9,737
Non-current regulatory liabilities		17,834	14,227
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		11,159	2,073
Non-current regulatory assets		0	5,722
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		9,036	7,123
Non-current regulatory assets		12,641	17,123
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		9,967	2,308
Non-current regulatory assets		4,758	0
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		7,606	1,775
Non-current regulatory assets		4,043	1,562
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		192,220	198,746
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		195,909	214,851
Deferred Income Taxes [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	[1]	0	0
Non-current regulatory assets	[1]	242,782	108,517
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		39,205	40,043
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		45,093	47,393
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		3,124	3,814
Utility Plant to be Abandoned [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		29,684	28,916
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	0
Non-current regulatory assets		15,648	8,166
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets		0	394
Non-current regulatory assets		$ 90,845	$ 75,590

[1] In 2021, the Company received regulatory approval in all jurisdictions to change to flow-through tax treatment of certain basis adjustments, which was $128.8 million as of September 30, 2021.