Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Regulated Asset Liability [Line Items]
Earning a return, asset	$ 543,185
Not earning a return, asset	480,348
Pending regulatory treatment, asset	16,962
Regulatory assets	146,327	$ 193,787
Non-current regulatory assets	894,168	833,328
Earning a return, liability	816,631
Not earning a return, liability	94,342
Pending Regulatory Treatment Liability	21,700
Regulatory liabilities	76,007	95,665
Non-current regulatory liabilities	856,666	840,837
Deferred natural gas costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	9,296
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	9,296	0
Non-current regulatory liabilities	0	0
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	4,000
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	4,000	0
Utility Plant Retirement Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	417,027
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	417,027	376,817
Excess Deferred Income Taxes [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	307,539
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	14,510	15,310
Non-current regulatory liabilities	293,029	314,096
Other Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	81,711
Pending Regulatory Treatment Liability	0
Regulatory liabilities	25,129	57,957
Non-current regulatory liabilities	56,582	76,638
Climate Commitment Act [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	37,231
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	37,231	0
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	12,216
Not earning a return, liability	0
Pending Regulatory Treatment Liability	11,536
Regulatory liabilities	0	0
Non-current regulatory liabilities	$ 23,752	24,204
Decoupling Rebates [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory liability	2 years
Earning a return, liability	$ 25,024
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	18,680	9,469
Non-current regulatory liabilities	6,344	20,476
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	8
Pending Regulatory Treatment Liability	10,164
Regulatory liabilities	0	0
Non-current regulatory liabilities	10,172	11,874
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	4,298
Not earning a return, liability	12,623
Pending Regulatory Treatment Liability	0
Regulatory liabilities	8,392	12,929
Non-current regulatory liabilities	8,529	16,732
Deferred Income Tax [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	244,303
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	244,303	$ 240,325
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets		12 years
Earning a return, asset	0
Not earning a return, asset	117,658
Pending regulatory treatment, asset	0
Regulatory assets	0	$ 0
Non-current regulatory assets	117,658	135,337
Climate Commitment Act [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	46,022
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	46,022	0
Energy Commodity Derivatives [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	69,139
Pending regulatory treatment, asset	0
Regulatory assets	51,419	112,090
Non-current regulatory assets	17,720	18,185
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	5,701
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 5,701	6,177
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	36 years
Earning a return, asset	$ 36,692
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	36,692	37,809
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	10,033
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 10,033	3,683
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	2 years
Earning a return, asset	$ 10,107
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	4,638	6,250
Non-current regulatory assets	5,469	5,449
Utility Plant Abandoned [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	34,852
Not earning a return, asset	3,422
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	38,274	24,389
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	178,898
Not earning a return, asset	0
Pending regulatory treatment, asset	591
Regulatory assets	0	0
Non-current regulatory assets	179,489	185,919
Deferred power costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	49,844
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	29,190	23,356
Non-current regulatory assets	20,654	24,043
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	60,667
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	60,667	52,091
Non-current regulatory assets	0	0
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	49,985
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	49,985	51,649
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	0
Pending regulatory treatment, asset	12,142
Regulatory assets	0	0
Non-current regulatory assets	12,142	9,793
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	29,345
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	29,345	32,381
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	41,072
Not earning a return, asset	35,793
Pending regulatory treatment, asset	4,229
Regulatory assets	413	0
Non-current regulatory assets	$ 80,681	$ 58,189