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DERIVATIVE FINANCIAL INSTRUMENTS: Derivative Instruments, Gain (Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Financial Swaps | Purchased power      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] $ 8,542 $ 2,373 $ 1,959
Financial Swaps | Fuel Expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] 16,209 68,489 12,180
Financial Swaps | Operating revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] 4,216 6,249 1,046
Forward contracts | Purchased power      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] 4,035 2,994 1,099
Forward contracts | Fuel Expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] 866 136 194
Forward contracts | Operating revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] $ 2,280 $ 1,090 $ 1,966
[1] Excludes unrealized gains or losses on derivatives, which are recorded on the balance sheet as regulatory assets or regulatory liabilities.