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Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax $ 1.2 $ 87.1 $ 28.4  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 2.3 2.6 1.5  
Net current-period other comprehensive income (loss) 3.5 89.7 29.9  
Accumulated other comprehensive loss (33.6) (37.1) (126.8) $ (156.7)
Gains and Losses on Securities        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 3.1 (13.7) (3.5)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.8 0.4 (0.4)  
Net current-period other comprehensive income (loss) 3.9 (13.3) (3.9)  
Accumulated other comprehensive loss (7.3) (11.2) 2.1 6.0
Gains and Losses on Cash Flow Hedges        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (0.5) 109.7 25.3  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.3 0.2 0.1  
Net current-period other comprehensive income (loss)   109.9 25.4  
Accumulated other comprehensive loss (12.8) (12.6) (122.5) (147.9)
Pension and OPEB Items        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (1.4) (8.9) 6.6  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 1.2 2.0 1.8  
Net current-period other comprehensive income (loss) (0.2) (6.9) 8.4  
Accumulated other comprehensive loss $ (13.5) $ (13.3) $ (6.4) $ (14.8)