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DERIVATIVE FINANCIAL INSTRUMENTS: Derivative Instruments, Gain (Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Financial Swaps | Operating revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] $ 559 $ 5,189 $ 4,216
Financial Swaps | Purchased power      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] (3,967) (7,101) (8,542)
Financial Swaps | Fuel Expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] (37,659) (63,380) (16,209)
Forward contracts | Operating revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] 917 1,885 2,280
Forward contracts | Purchased power      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] (1,017) (3,742) (4,035)
Forward contracts | Fuel Expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, Gain (Loss) on Derivative, Net [1] $ (1,457) $ (2,510) $ (866)
[1] Excludes unrealized gains or losses on derivatives, which are recorded on the balance sheet as regulatory assets or regulatory liabilities.