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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Interest expense $ 195.2 $ 180.0 $ 162.6
Fuel, purchased power and cost of natural gas sold 730.3 982.9 1,230.6
Operations and maintenance 557.0 552.0 548.4
Income Tax Expense (Benefit) (36.3) (25.6) (25.2)
Net income 283.7 276.0 $ 270.8
Reclassification out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Operations and maintenance 0.0 0.0  
Net income (5.1) (4.7)  
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 (6.4) (5.9)  
Income Tax Expense (Benefit) 1.4 1.4  
Net income (5.0) (4.5)  
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 (2.9) (2.9)  
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 (3.5) (3.0)  
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 (0.2) (0.2)  
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 (0.2) (0.2)  
Income Tax Expense (Benefit) 0.1 0.0  
Net income $ (0.1) $ (0.2)