Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Regulated Asset Liability [Line Items]
Regulatory assets	$ 34,733	$ 43,783
Non-current regulatory assets	838,623	860,626
Regulatory liabilities	109,278	77,149
Non-current regulatory liabilities	850,702	861,515
Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	66,234	56,331
Non-current regulatory liabilities	434,525	458,789
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	23,477	0
Non-current regulatory liabilities	1,096	0
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	1,540	6,457
Non-current regulatory liabilities	5,485	5,434
Decoupling Rebate [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	3,245	3,049
Non-current regulatory liabilities	14,164	6,259
Utility Plant Retirement Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	350,592	350,190
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	17,348	15,062
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	12,191	12,500
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	14,782	11,312
Non-current regulatory liabilities	15,301	13,281
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	12,447
Non-current regulatory assets	0	2,938
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	9,798	9,907
Non-current regulatory assets	6,106	14,625
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	18,353	14,095
Non-current regulatory assets	8,401	6,932
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	5,978	7,334
Non-current regulatory assets	2,089	3,501
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	164,207	165,696
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	191,087	199,754
Deferred Income Taxes [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	240,502	244,154
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	38,647	38,926
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	49,522	48,455
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	3,003	3,974
Utility Plant to be Abandoned [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	26,238	26,771
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	15,117	13,591
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	6,582	6,768
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Non-current regulatory assets	35,101	36,008
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	604	0
Non-current regulatory assets	$ 52,021	$ 48,533