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Other Comprehensive Income: Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2024
Mar. 31, 2024
Jun. 30, 2023
Mar. 31, 2023
Jun. 30, 2024
Jun. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]            
Interest expense $ 48.2   $ 43.3   $ 94.2 $ 87.3
Fuel, purchased power and cost of natural gas sold 107.1   121.3   423.7 647.5
Operations and maintenance 141.7   145.8   275.2 286.7
Income tax expense (3.7)   6.1   (20.6) (8.6)
Net income 24.7 $ 131.6 26.6 $ 117.4 156.2 143.9
Reclassification out of Accumulated Other Comprehensive Income            
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]            
Net income (1.0)   (0.9)   (3.7) (3.0)
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 (1.3)   (1.2)   (4.6) (3.9)
Income tax expense 0.3   0.3   1.0 0.9
Net income (1.0)   (0.9)   (3.6) (3.0)
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 (0.7)   (0.7)   (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)   (0.5)   (3.2) (2.5)
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 0.0   0.0   (0.1) 0.0
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 0.0   0.0   (0.1) 0.0
Net income $ 0.0   $ 0.0   $ (0.1) $ 0.0