Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Regulated Asset Liability [Line Items]
Earning a return, asset	$ 712,914
Not earning a return, asset	285,816
Pending regulatory treatment, asset	28,385
Regulatory assets	193,787	$ 43,783
Non-current regulatory assets	833,328	860,626
Earning a return, liability	853,865
Not earning a return, liability	51,628
Pending Regulatory Treatment Liability	31,009
Regulatory liabilities	95,665	77,149
Non-current regulatory liabilities	840,837	861,515
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	6,457
Non-current regulatory liabilities	0	5,434
Utility Plant Retirement Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	376,817
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	376,817	350,190
Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	427,365
Not earning a return, liability	27,458
Pending Regulatory Treatment Liability	9,178
Regulatory liabilities	73,267	56,331
Non-current regulatory liabilities	390,734	458,789
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	13,020
Not earning a return, liability	0
Pending Regulatory Treatment Liability	11,184
Regulatory liabilities	0	0
Non-current regulatory liabilities	$ 24,204	15,062
Decoupling Rebates [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory liability	3 years
Earning a return, liability	$ 29,945
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	9,469	3,049
Non-current regulatory liabilities	20,476	6,259
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	1,227
Pending Regulatory Treatment Liability	10,647
Regulatory liabilities	0	0
Non-current regulatory liabilities	11,874	12,500
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	6,718
Not earning a return, liability	22,943
Pending Regulatory Treatment Liability	0
Regulatory liabilities	12,929	11,312
Non-current regulatory liabilities	16,732	13,281
Deferred Income Tax [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	240,325
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	240,325	244,154
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	135,337
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	135,337	165,696
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	130,275
Pending regulatory treatment, asset	0
Regulatory assets	112,090	12,447
Non-current regulatory assets	18,185	2,938
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	6,177
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 6,177	6,768
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	37 years
Earning a return, asset	$ 37,809
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	37,809	38,926
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	3,683
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 3,683	3,974
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	3 years
Earning a return, asset	$ 11,699
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	6,250	9,907
Non-current regulatory assets	5,449	14,625
Utility Plant Abandoned [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	24,389
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	24,389	26,771
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	168,832
Not earning a return, asset	0
Pending regulatory treatment, asset	17,087
Regulatory assets	0	0
Non-current regulatory assets	185,919	199,754
Deferred power costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	47,399
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	23,356	7,334
Non-current regulatory assets	24,043	3,501
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	52,091
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	52,091	14,095
Non-current regulatory assets	0	6,932
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	51,649
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	51,649	48,455
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	1,650
Pending regulatory treatment, asset	8,143
Regulatory assets	0	0
Non-current regulatory assets	9,793	13,591
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	32,381
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	32,381	36,008
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	40,163
Not earning a return, asset	14,871
Pending regulatory treatment, asset	3,155
Regulatory assets	0	0
Non-current regulatory assets	$ 58,189	$ 48,533

Pending regulatory treatment, liability

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Regulated Asset Liability [Line Items]

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Earning a return includes either interest on the regulatory asset, or a return on the investment as a component of rate base or the weighted cost of capital.

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Earning a return includes either interest on the regulatory liability, or a return on the investment as a component of rate base or the weighted cost of capital.

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Remaining amounts of regulatory liabilities for which no interest during recovery period is provided.

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Amount of regulatory costs not yet approved by the regulatory authority.

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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Amortization period for the recovery of regulatory liability, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

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