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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Condensed Statement of Income Captions [Line Items]    
Interest Expense $ 37,825 $ 35,781
Fuel, purchased power and cost of natural gas sold 293,147 187,879
Operations and maintenance 129,679 125,466
Income before Income Taxes 100,981 113,226
Income tax benefit (expense) (494) (16,002)
Net income (loss) 100,487 97,224
Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Net income (loss) (911) (1,393)
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Income before Income Taxes (744) (1,199)
Income tax benefit (expense) 198 285
Net income (loss) (546) (914)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Operations and maintenance 25 30
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Operations and maintenance (598) (597)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Income before Income Taxes (573) (567)
Income tax benefit (expense) 208 88
Net income (loss) (365) (479)
Interest rate swaps | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Interest Expense (713) (713)
Commodity Contract | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income    
Condensed Statement of Income Captions [Line Items]    
Fuel, purchased power and cost of natural gas sold $ (31) $ (486)