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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, 2023
Jun. 30, 2023
Mar. 31, 2023
Sep. 30, 2022
Jun. 30, 2022
Mar. 31, 2022
Sep. 30, 2023
Sep. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Interest expense $ 44,477     $ 40,580     $ 131,809 $ 118,454
Fuel, purchased power and cost of natural gas sold 102,241     168,535     749,753 793,632
Operations and maintenance 125,767     134,449     412,522 403,549
Income Tax Expense (Benefit) (7,366)     (2,090)     (15,950) (15,920)
Net income 48,760 $ 26,544 $ 117,380 37,834 $ 35,846 $ 121,024 192,684 194,704
Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Operations and maintenance 0     24     0 70
Net income (600)     (651)     (3,597) 814
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Income before income taxes (746)     (669)     (4,611) 1,482
Income Tax Expense (Benefit) 170     124     1,081 (332)
Net income (576)     (545)     (3,530) 1,150
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Swap                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Interest expense (712)     (712)     (2,138) (2,138)
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Fuel, purchased power and cost of natural gas sold (34)     43     (2,473) 3,620
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Operations and maintenance (43)     (188)     (129) (563)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Income before income taxes (43)     (164)     (129) (493)
Income Tax Expense (Benefit) 19     58     62 157
Net income $ (24)     $ (106)     $ (67) $ (336)