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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

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 + Details
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