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Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Interest expense $ 154,112 $ 144,931 $ 139,291
Fuel, purchased power and cost of natural gas sold 741,934 492,404 570,829
Operations and maintenance 501,690 495,404 495,994
Income Tax Expense (Benefit) (7,169) (32,918) (29,580)
Net income 251,260 242,763 $ 213,322
Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Net income (2,280) (4,418)  
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 (800) (3,452)  
Income Tax Expense (Benefit) 175 383  
Net income (625) (3,069)  
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 (2,851) (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 2,051 (601)  
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 98 103  
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 (2,391) (2,387)  
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 (2,293) (2,284)  
Income Tax Expense (Benefit) 638 935  
Net income $ (1,655) $ (1,349)