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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 21,419 $ 21,900
Regulatory assets, long-term 167,556 168,395
Regulatory liabilities, current 12,517 16,663
Regulatory liabilities, long -term 234,986 233,973
Deferred Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 133,306 134,719
Plant Removal Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 100,616 98,707
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 7,072 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 $ 3,306 3,581
Regulatory liabilities, long -term 837 470
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  
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current $ 180 176
Regulatory liabilities, long -term 227 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 143,996 146,071
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 6,891 8,871
Regulatory assets, long-term $ 10,418 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 8,539 8,462
ISO Cost Recovery Trackers    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 809 1,079
Regulatory assets, long-term $ 703 867
Regulatory Assets - Remaining Recovery/Refund Period 2 years  
Unrecovered Project Costs    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 1,556 361
Regulatory assets, long-term $ 2,937 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 $ 493 360
Regulatory assets, long-term 165 230
Debt Reacquisition Premiums    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 180 192
Regulatory assets, long-term $ 302 341
Regulatory Assets - Remaining Recovery/Refund Period 12 years  
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 453 0
Regulatory assets, long-term $ 496 $ 62