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Savings, Pension and Other Postretirement Employee Benefit Plans - Pre-tax Components of Net Periodic Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 3.5 $ 2.2 $ 1.8
Interest cost 12.5 8.9 8.4
Expected return on plan assets (15.2) (11.3) (10.6)
Amortization of actuarial loss 0.1 6.2 10.2
Net periodic cost 0.8 5.9 9.9
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.2 0.3 0.4
Interest cost 2.8 2.1 2.0
Expected return on plan assets 0.0 0.0 0.0
Amortization of actuarial loss (0.4) 0.0 0.3
Net periodic cost $ 2.6 $ 2.4 $ 2.7