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Commitment, Contingencies And Guarantees: Power Purchase and Transmission Services Agreements (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
USD ($)
MW
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Platte River Power Authority - Unit Contingent Energy | Colorado Electric | Wind      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 60    
Cost of Purchased Power | $ $ 4,246 $ 2,791 $ 0
Platte River Power Authority - Unit Contingent Energy | Colorado Electric | Coal      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 25    
Cost of Purchased Power | $ $ 4,447 4,524 1,802
Platte River Power Authority - Unit Contingent Energy | South Dakota Electric | Wind      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 12    
Cost of Purchased Power | $ $ 596 715 688
TC Colorado Solar, LLC | Colorado Electric | Solar      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 200    
Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility 15 years    
PacifiCorp Purchase Power Agreement | South Dakota Electric | Coal      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 50    
Cost of Purchased Power | $ $ 8,923 5,897 7,477
PacifiCorp Transmission | South Dakota Electric      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 50    
Cost of Purchased Power | $ $ 1,783 1,776 1,741
Fall River Solar | South Dakota Electric | Solar      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 80    
Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility 20 years    
Happy Jack Wind Purchase Power Agreement | Wyoming Electric | Wind      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 30    
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased 50.00%    
Cost of Purchased Power | $ $ 3,544 4,531 3,936
Silver Sage Wind Power Purchase Agreement | Wyoming Electric | Wind      
Long-term Purchase Commitment [Line Items]      
Number of Megawatts Capacity Purchased 30    
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased 67.00%    
Cost of Purchased Power | $ $ 4,717 $ 6,203 $ 5,366