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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Feb. 01, 2022
Dec. 03, 2020
Dec. 31, 2022
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     $ 24,999 $ 24,999 $ 27,342  
Regulatory assets, long-term     94,655 94,655 125,508  
Regulatory liabilities, current     17,300 17,300 24,844  
Regulatory liabilities, long -term     244,497 244,497 234,430  
Reduction of regulatory assets       (2,464) (4,803) $ (4,070)
Minnesota Rate Case | MPUC            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Revenue, approved rate increase, amount   $ 6,900 15,300 4,100    
Revenue, approved rate increase, percent   3.20%        
Revenue requirement amount $ 209,000          
Approved return on base rate, percent 7.18%          
Approved return on equity, percent 9.48%          
Approved equity ratio, percent 52.50%          
Recovery period 5 years          
Approved cap of annual base revenues, percent 4.00%          
Excess capacity auction revenue       5,300    
Reduction of regulatory assets       2,600    
Refund liability       1,800    
Deferred Income Taxes            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     0 0 0  
Regulatory liabilities, long -term     131,480 131,480 129,437  
Plant Removal Obligations            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     8,509 8,509 8,306  
Regulatory liabilities, long -term     105,733 105,733 101,595  
Fuel Clause Adjustments            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     365 365 1,554  
Regulatory liabilities, long -term     0 $ 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     2,504 $ 2,504 5,772  
Regulatory liabilities, long -term     7,136 7,136 3,336  
Pension and Other Postretirement Benefit Plans            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     5,589 5,589 2,603  
Regulatory liabilities, long -term     0 $ 0 0  
Regulatory liabilities - remaining recovery/refund period       1 year    
Derivative Instruments            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     0 $ 0 6,214  
Regulatory liabilities, long -term     0 $ 0 0  
Regulatory liabilities - remaining recovery/refund period       1 year    
Other            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory liabilities, current     333 $ 333 395  
Regulatory liabilities, long -term     148 148 62  
Pension and Other Postretirement Benefit Plans            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     0 0 7,791  
Regulatory assets, long-term     88,354 88,354 114,961  
Alternative Revenue Program Riders            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     5,679 5,679 11,889  
Regulatory assets, long-term     2,508 $ 2,508 5,564  
Regulatory assets - Remaining recovery/refund period       2 years    
Asset Retirement Obligations            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     0 $ 0 0  
Regulatory assets, long-term     1,467 1,467 742  
ISO Cost Recovery Trackers            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     575 575 0  
Regulatory assets, long-term     314 $ 314 1,342  
Regulatory assets - Remaining recovery/refund period       2 years    
Unrecovered Project Costs            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     320 $ 320 2,136  
Regulatory assets, long-term     990 $ 990 1,455  
Regulatory assets - Remaining recovery/refund period       5 years    
Deferred Rate Case Expenses            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     377 $ 377 607  
Regulatory assets, long-term     754 $ 754 1,131  
Regulatory assets - Remaining recovery/refund period       2 years    
Debt Reacquisition Premiums            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     25 $ 25 100  
Regulatory assets, long-term     216 $ 216 240  
Regulatory assets - Remaining recovery/refund period       10 years    
Fuel Clause Adjustments            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     10,893 $ 10,893 4,819  
Regulatory assets, long-term     0 $ 0 0  
Regulatory assets - Remaining recovery/refund period       1 year    
Derivative Instruments            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     7,130 $ 7,130 0  
Regulatory assets, long-term     0 $ 0 0  
Regulatory assets - Remaining recovery/refund period       1 year    
Other            
Schedule of Regulatory Assets and Liabilities [Line Items]            
Regulatory assets - current     0 $ 0 0  
Regulatory assets, long-term     $ 52 $ 52 $ 73