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Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Dec. 31, 2021
Derivative [Line Items]      
Regulatory assets $ 723,344   $ 796,599
Gas price derivatives      
Derivative [Line Items]      
Regulatory assets 220   $ 2,584
Not Designated as Hedging Instrument      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net 3,494 $ (1,158)  
Not Designated as Hedging Instrument | Fuel, purchased power and cost of natural gas sold | Electricity      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net 0 (1,524)  
Not Designated as Hedging Instrument | Fuel, purchased power and cost of natural gas sold | Natural Gas      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net $ 3,494 $ 366