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Summary of Significant Accounting Policies - Summary of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Accumulated other comprehensive income (loss) $ (7.2) $ (11.9)
Amounts reclassified from accumulated other comprehensive income (loss) 0.0 0.2
Settlement cost 0.0 0.9
Regulatory classification of certain pension costs 3.9  
Accumulated other comprehensive income (loss) (3.3) (10.8)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]    
Accumulated other comprehensive income (loss) (12.9) (18.5)
Amounts reclassified from accumulated other comprehensive income (loss) 0.1 0.3
Settlement cost   0.9
Regulatory classification of certain pension costs 3.9  
Accumulated other comprehensive income (loss) (8.9) (17.3)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]    
Accumulated other comprehensive income (loss) 5.7 6.6
Amounts reclassified from accumulated other comprehensive income (loss) (0.1) (0.1)
Settlement cost   0.0
Regulatory classification of certain pension costs 0.0  
Accumulated other comprehensive income (loss) $ 5.6 $ 6.5