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Summary of Significant Accounting Policies - Summary of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Balance $ 4,640.9 $ 4,511.6
Balance 4,797.1 4,617.1
Accumulated Other Comprehensive Income (Loss) [Member]    
Balance (2.7) (7.2)
Amounts reclassified from accumulated other comprehensive income (loss) 0.3 0.2
Regulatory classification of certain pension costs   3.9
Balance (2.4) (3.1)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]    
Balance (8.8) (12.9)
Amounts reclassified from accumulated other comprehensive income (loss) 0.4 0.3
Regulatory classification of certain pension costs   3.9
Balance (8.4) (8.7)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]    
Balance 6.1 5.7
Amounts reclassified from accumulated other comprehensive income (loss) (0.1) (0.1)
Regulatory classification of certain pension costs   0.0
Balance $ 6.0 $ 5.6