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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 2.2 $ 2.4 $ 1.8
Interest cost 10.4 12.4 12.0
Expected return on plan assets (15.0) (16.5) (17.0)
Amortization of prior service cost (credit) 0.0 0.0 0.0
Amortization of actuarial loss 9.8 7.3 10.1
Net periodic cost 7.5 5.6 6.9
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.1 0.1 0.1
Interest cost 2.3 2.8 2.4
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service cost (credit) 0.0 0.0 (1.7)
Amortization of actuarial loss 0.0 (0.3) (0.9)
Net periodic cost $ 2.4 $ 2.6 $ (0.1)