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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Condensed Statement of Income Captions [Line Items]                
Interest Expense $ 38,604     $ 36,521     $ 115,098 $ 108,067
Fuel, purchased power and cost of natural gas sold 94,057     71,686     495,678 331,194
Operations and maintenance 122,277     122,759     375,201 365,533
Income tax benefit (expense) (5,253)     (4,651)     (6,333) (25,484)
Net income (loss) 48,162 $ 28,287 $ 100,487 40,349 $ 24,694 $ 97,224 176,936 162,267
Reclassification out of Accumulated Other Comprehensive Income                
Condensed Statement of Income Captions [Line Items]                
Net income (loss) (847)     (1,111)     (2,701) (3,497)
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedging                
Condensed Statement of Income Captions [Line Items]                
Income before Income Taxes (381)     (890)     (1,782) (2,872)
Income tax benefit (expense) (26)     209     308 680
Net income (loss) (407)     (681)     (1,474) (2,192)
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedging | Interest rate swaps                
Condensed Statement of Income Captions [Line Items]                
Interest Expense (712)     (712)     (2,138) (2,138)
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedging | Commodity Contract                
Condensed Statement of Income Captions [Line Items]                
Fuel, purchased power and cost of natural gas sold 331     (178)     356 (734)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost                
Condensed Statement of Income Captions [Line Items]                
Operations and maintenance 25     24     74 79
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss)                
Condensed Statement of Income Captions [Line Items]                
Operations and maintenance (598)     (597)     (1,793) (1,791)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment                
Condensed Statement of Income Captions [Line Items]                
Income before Income Taxes (573)     (573)     (1,719) (1,712)
Income tax benefit (expense) 133     143     492 407
Net income (loss) $ (440)     $ (430)     $ (1,227) $ (1,305)