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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 22,517 $ 21,900
Regulatory assets, long-term 158,515 168,395
Regulatory liabilities, current 25,323 16,663
Regulatory liabilities, long -term 230,733 233,973
Deferred Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 130,555 134,719
Plant Removal Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 5,893 0
Regulatory liabilities, long -term 96,734 98,707
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 5,692 10,947
Regulatory liabilities, long -term $ 0 0
Regulatory Liabilities - Remaining Recovery/Refund Period 1 year  
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current $ 5,265 3,581
Regulatory liabilities, long -term 2,507 470
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 1,959 1,959
Regulatory liabilities, long -term $ 0 0
Regulatory Liabilities - Remaining Recovery/Refund Period 1 year  
Derivative Instruments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current $ 6,136 0
Regulatory liabilities, long -term 787 0
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 378 176
Regulatory liabilities, long -term 150 77
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 11,037 11,037
Regulatory assets, long-term 139,467 146,071
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 7,229 8,871
Regulatory assets, long-term $ 8,225 9,373
Regulatory Assets - Remaining Recovery/Refund Period 3 years  
Asset Retirement Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 0 0
Regulatory assets, long-term 7,065 8,462
ISO Cost Recovery Trackers    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 270 1,079
Regulatory assets, long-term $ 1,027 867
Regulatory Assets - Remaining Recovery/Refund Period 2 years  
Unrecovered Project Costs    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 3,227 361
Regulatory assets, long-term $ 1,456 2,989
Regulatory Assets - Remaining Recovery/Refund Period 3 years  
Deferred Rate Case Expenses    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 637 360
Regulatory assets, long-term 936 230
Debt Reacquisition Premiums    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 117 192
Regulatory assets, long-term $ 262 341
Regulatory Assets - Remaining Recovery/Refund Period 12 years  
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 0 0
Regulatory assets, long-term $ 77 $ 62