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Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Regulatory Assets [Line Items]    
Regulatory assets $ 2,054 $ 13,565
Regulatory assets, noncurrent 35,796 15,951
Total regulatory assets 37,850 29,516
Regulatory Liabilities [Line Items]    
Regulatory liabilities 21,374 145,243
Regulatory liabilities 252,272 271,706
Total regulatory liabilities 273,646 416,949
Customer tax refund [Domain]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 0 125,506
Fuel recovery    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 1,637 0
Postretirement benefits    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 42,415 42,207
Deferred federal taxes-liability    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 18,643 18,643
Regulatory liabilities 169,240 187,883
Deferred state taxes - liability    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 1,094 1,094
Regulatory liabilities 9,932 11,026
Asset retirement obligations, net [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 30,685 30,590
Fuel recovery    
Regulatory Assets [Line Items]    
Regulatory assets 19 11,871
Levelized depreciation    
Regulatory Assets [Line Items]    
Regulatory assets 2,035 1,694
Regulatory assets, noncurrent 9,519 11,601
Grossed-up deferred taxes on equity funds used during construction    
Regulatory Assets [Line Items]    
Regulatory assets, noncurrent 3,969 4,350
Washington State Carbon and Greenhouse Tax [Member]    
Regulatory Assets [Line Items]    
Regulatory assets $ 22,308 $ 0