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Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Interest expense $ 144,931 $ 139,291 $ 141,616
Fuel, purchased power and cost of natural gas sold 492,404 570,829 625,610
Operations and maintenance 495,404 495,994 481,706
Income before income taxes 275,681 242,902 255,882
Income Tax Expense (Benefit) (32,918) (29,580) 23,667
Net income 242,763 213,322 $ 272,662
Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Net income (4,418) (2,944)  
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 (3,452) (2,434)  
Income Tax Expense (Benefit) 383 611  
Net income (3,069) (1,823)  
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 (601) 417  
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 103 77  
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 (2,387) (745)  
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 (2,284) (668)  
Income Tax Expense (Benefit) 935 (453)  
Net income $ (1,349) $ (1,121)