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Accumulated Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) Included in Computation of Net Periodic Pension and Postretirement Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
Dec. 31, 2023
Dec. 31, 2022
Sep. 30, 2021
Prior Service Costs        
Reclassification out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax $ 0.2 $ 0.5 $ 0.8 $ 0.9
Settlement        
Reclassification out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax     33.6  
Actuarial (Gains) Losses        
Reclassification out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax 0.3 (2.6) 1.3 5.2
Employee Pension and Postretirement Benefits, Net of Tax        
Reclassification out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax 0.5 (2.1) 35.7 6.1
Tax benefit (0.1) 0.5 (8.3) (1.5)
Net of tax $ 0.4 $ (1.6) $ 27.4 $ 4.6