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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Interest expense $ 162,584 $ 154,112 $ 144,931
Fuel, purchased power and cost of natural gas sold 1,230,550 741,934 492,404
Operations and maintenance 548,430 501,690 495,404
Income Tax Expense (Benefit) (25,205) (7,169) (32,918)
Net income 270,758 251,260 $ 242,763
Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Net income (544) (2,280)  
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 (142) (800)  
Income Tax Expense (Benefit) 58 175  
Net income (84) (625)  
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 (2,850) (2,851)  
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 2,708 2,051  
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost | Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Operations and maintenance 93 98  
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 (751) (2,391)  
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 (658) (2,293)  
Income Tax Expense (Benefit) 198 638  
Net income $ (460) $ (1,655)