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Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Derivative [Line Items]          
Regulatory assets $ 504,859   $ 504,859   $ 652,981
Regulatory liabilities 562,957   562,957   518,573
Gas price derivatives          
Derivative [Line Items]          
Regulatory assets 3,849   3,849   $ 8,793
Not Designated as Hedging Instrument          
Derivative [Line Items]          
Derivative, Gain (Loss) on Derivative, Net $ 255 $ 1,617 $ (2,445) $ 2,779  
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold  
Not Designated as Hedging Instrument | Natural Gas          
Derivative [Line Items]          
Derivative, Loss on Derivative     $ (2,445)    
Derivative, Loss, Statement of Income or Comprehensive Income [Extensible Enumeration] Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold Fuel, Purchased Power and Cost of Gas Sold  
Derivative, Gain on Derivative $ 255 $ 1,617   $ 2,779