Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Regulated Asset Liability [Line Items]
Earning a return, asset	$ 652,603
Not earning a return, asset	193,618
Pending regulatory treatment, asset	58,188
Regulatory assets	43,783	$ 13,673
Non-current regulatory assets	860,626	750,443
Earning a return, liability	738,730
Not earning a return, liability	42,099
Pending Regulatory Treatment Liability	157,835
Regulatory liabilities	77,149	46,435
Non-current regulatory liabilities	861,515	784,820
Natural Gas Deferrals [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	874
Non-current regulatory liabilities	0	0
Deferred Power Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	11,891
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	6,457	20,299
Non-current regulatory liabilities	5,434	17,570
Utility Plant Retirement Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	350,190
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	350,190	325,832
Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	346,847
Not earning a return, liability	24,411
Pending Regulatory Treatment Liability	143,862
Regulatory liabilities	56,331	14,952
Non-current regulatory liabilities	458,789	399,677
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	13,589
Not earning a return, liability	0
Pending Regulatory Treatment Liability	1,473
Regulatory liabilities	0	0
Non-current regulatory liabilities	$ 15,062	15,046
Decoupling Rebates [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory liability	1 year
Earning a return, liability	$ 9,308
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	3,049	1,447
Non-current regulatory liabilities	6,259	1,519
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	0
Pending Regulatory Treatment Liability	12,500
Regulatory liabilities	0	0
Non-current regulatory liabilities	12,500	10,949
Other Regulatory Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	6,905
Not earning a return, liability	17,688
Pending Regulatory Treatment Liability	0
Regulatory liabilities	11,312	8,863
Non-current regulatory liabilities	13,281	14,227
Deferred Income Tax [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	244,154
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	244,154	108,517
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	165,696
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	165,696	198,746
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	15,385
Pending regulatory treatment, asset	0
Regulatory assets	12,447	2,073
Non-current regulatory assets	2,938	5,722
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	6,768
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 6,768	7,512
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	38 years
Earning a return, asset	$ 38,926
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	38,926	40,043
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	3,974
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 3,974	3,814
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	2 years
Earning a return, asset	$ 24,532
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	9,907	7,123
Non-current regulatory assets	14,625	17,123
Utility Plant to be Abandoned [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	26,771
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	26,771	28,916
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	158,082
Not earning a return, asset	0
Pending regulatory treatment, asset	41,672
Regulatory assets	0	0
Non-current regulatory assets	199,754	214,851
Deferred power costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	10,835
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	7,334	1,775
Non-current regulatory assets	3,501	1,562
Deferred Natural Gas Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	21,027
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	14,095	2,308
Non-current regulatory assets	6,932	0
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	48,455
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	48,455	47,393
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	0
Pending regulatory treatment, asset	13,591
Regulatory assets	0	0
Non-current regulatory assets	13,591	8,166
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	36,008
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	36,008	26,379
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	37,045
Not earning a return, asset	8,563
Pending regulatory treatment, asset	2,925
Regulatory assets	0	394
Non-current regulatory assets	$ 48,533	$ 41,699

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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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