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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]    
Interest expense $ 46.0 $ 44.1
Fuel, purchased power and cost of natural gas sold 316.6 526.3
Income tax expense (16.9) (14.7)
Net income 131.6 117.4
Reclassification out of Accumulated Other Comprehensive Income    
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]    
Net income (2.6) (2.1)
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 (3.3) (2.7)
Income tax expense 0.7 0.6
Net income (2.6) (2.1)
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) (0.7)
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 $ (2.6) $ (2.0)