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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Sep. 30, 2024
Jun. 30, 2024
Mar. 31, 2024
Sep. 30, 2025
Sep. 30, 2024
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Interest expense $ 50.6     $ 50.6     $ 152.2 $ 144.8
Fuel, purchased power and cost of natural gas sold 112.0     94.5     595.7 518.2
Income tax (expense) benefit (4.0)     (2.9)     (26.5) (23.6)
Operations and maintenance 150.8     145.6     452.2 420.8
Net income 25.9 $ 28.8 $ 136.4 26.4 $ 24.7 $ 131.6 191.1 182.6
Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Net income (0.6)     (0.6)     (2.1) (4.3)
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 (0.8)     (0.8)     (2.6) (5.4)
Income tax (expense) benefit 0.2     0.2     0.6 1.2
Net income (0.6)     (0.6)     (2.0) (4.2)
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 (0.7)     (0.8)     (2.1) (2.2)
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 (0.1)     (0.0)     (0.5) (3.2)
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 0.0     0.0     (0.1) (0.1)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income                
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                
Net income $ 0.0     $ 0.0     $ (0.1) $ (0.1)