v3.25.4
Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]      
Interest expense $ 206.9 $ 195.2 $ 180.0
Fuel, purchased power and cost of natural gas sold 831.5 730.3 982.9
Operations and maintenance 589.8 557.0 552.0
Income tax benefit (expense) (43.7) (36.3) (25.6)
Net income 299.8 283.7 $ 276.0
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 (2.4) (5.1)  
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 (3.9) (6.4)  
Income tax benefit (expense) 0.9 1.4  
Net income (3.0) (5.0)  
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 (1.0) (3.5)  
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 0.8 (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 0.8 (0.2)  
Income tax benefit (expense) (0.2) 0.1  
Net income $ 0.6 $ (0.1)