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Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($)
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Derivative [Line Items]      
Regulatory assets $ 431,100,000   $ 480,100,000
Regulatory liabilities 553,400,000   566,600,000
Gas price derivatives      
Derivative [Line Items]      
Regulatory assets 0   $ 5,100,000
Not Designated as Hedging Instrument      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net $ 600 $ (3,100)  
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  
Not Designated as Hedging Instrument | Natural Gas      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net $ 600 $ (3,100)  
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