Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	$ 130,389	$ 146,327
Regulatory Assets, Non-Current	844,767	894,168
Regulatory Liabilities, Current	94,385	76,007
Regulatory Liabilities, Non-Current	829,061	856,666
Other Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	14,753	25,129
Regulatory Liabilities, Non-Current	54,559	56,582
Excess Deferred Income Taxes [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	14,230	14,510
Regulatory Liabilities, Non-Current	286,185	293,029
Deferred Climate Commitment Act Revenues [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	34,670	0
Regulatory Liabilities, Non-Current	9,435	37,231
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	250	0
Regulatory Liabilities, Non-Current	2,463	4,000
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	10,469	9,296
Regulatory Liabilities, Non-Current	0	0
Decoupling Rebate [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	9,648	18,680
Regulatory Liabilities, Non-Current	2,869	6,344
Utility Plant Retirement Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	0	0
Regulatory Liabilities, Non-Current	432,227	417,027
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	0	0
Regulatory Liabilities, Non-Current	24,405	23,752
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	0	0
Regulatory Liabilities, Non-Current	10,787	10,172
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities, Current	10,365	8,392
Regulatory Liabilities, Non-Current	6,131	8,529
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	41,242	51,419
Regulatory Assets, Non-Current	10,362	17,720
Deferred Climate Commitment Act Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	39,558	0
Regulatory Assets, Non-Current	20,530	46,022
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	17,859	29,190
Regulatory Assets, Non-Current	12,895	20,654
Wildfire Resiliency [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	10,487	0
Regulatory Assets, Non-Current	12,490	23,737
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	8,133	4,638
Regulatory Assets, Non-Current	8,218	5,469
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	9,452	60,667
Regulatory Assets, Non-Current	0	0
Deferred Income Taxes [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	247,393	244,303
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	112,730	117,658
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	175,820	179,489
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	48,727	49,985
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	36,134	36,692
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	27,827	29,345
Utility Plant Abandoned [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	36,878	38,274
Colstrip Excess Depreciation [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	22,347	19,429
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	11,945	12,142
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Non-Current	16,068	10,033
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets, Current	3,658	413
Regulatory Assets, Non-Current	$ 44,403	$ 43,216