v3.26.1
Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Jun. 30, 2026
Jun. 30, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]            
Interest expense $ 52.7   $ 49.8   $ 105.9 $ 101.6
Fuel, purchased power and cost of natural gas sold 113.5   124.0   451.3 483.8
Income tax (expense) benefit (5.2)   (4.4)   (22.8) (22.5)
Operations and maintenance 150.5   147.6   298.6 301.3
Net income 40.4 $ 133.1 28.8 $ 136.4 173.5 165.2
Reclassification out of Accumulated Other Comprehensive Income            
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]            
Net income (1.1)   (0.7)   (3.0) (1.6)
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 (1.4)   (0.9)   (3.7) (2.0)
Income tax (expense) benefit 0.3   0.2   0.8 0.5
Net income (1.1)   (0.7)   (2.9) (1.5)
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 (0.8)   (0.8)   (1.4) (1.4)
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 (0.6)   (0.1)   (2.3) (0.6)
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)