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Accumulated Other Comprehensive Loss (AOCL) - Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect $ (100.2) $ (96.3)
Income tax effect 2.3 2.3
Accumulated other comprehensive loss (97.9) (94.0)
Defined benefit plan and other postretirement liabilities    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect (101.8) (98.0)
Income tax effect 2.6 2.6
Unrealized gain (loss) on financial instruments    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect 1.6 1.7
Income tax effect $ (0.3) $ (0.3)