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Savings, Pension and Other Postretirement Employee Benefit Plans - Changes in Benefit Obligation, Plan Assets and Funded Status for Company-Sponsored Benefit Plans (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Noncurrent assets $ 1.0    
Current liability for pension and other postretirement employee benefits (6.2) $ (7.4)  
Liability for pension and other postretirement employee benefits (80.5) (76.6)  
Liability, Defined Benefit Pension Plan   (9.9)  
Liability, Other Postretirement Defined Benefit Plan (74.2) (66.4)  
Pension Benefit Plans      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 316.5 294.2  
Service cost 2.2 2.4 $ 1.8
Interest cost 10.4 12.4 12.0
Actuarial losses (gains) 26.9 29.0  
Defined Benefit Plan, Benefit Obligation, Benefits Paid (21.1) (21.5)  
Benefit obligation at end of year 334.9 316.5 294.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 306.6 268.8  
Actual return on plan assets 50.0 58.8  
Employer contribution 0.5 0.5  
Defined Benefit Plan, Plan Assets, Benefits Paid 21.1 21.5  
Fair value of plan assets at end of year 335.9 306.6 268.8
Funded status at end of year 1.0 (9.9)  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Noncurrent assets 13.5 7.7  
Current liability for pension and other postretirement employee benefits (0.4) (0.4)  
Liability for pension and other postretirement employee benefits (12.0) (17.2)  
Net actuarial loss (gain) 73.3 91.4  
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 66.4 60.3  
Service cost 0.1 0.1 0.1
Interest cost 2.3 2.8 2.4
Actuarial losses (gains) 11.0 9.6  
Defined Benefit Plan, Benefit Obligation, Benefits Paid (5.6) (6.4)  
Benefit obligation at end of year 74.2 66.4 60.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 0.0 0.0  
Actual return on plan assets 0.0 0.0  
Employer contribution 5.6 7.0  
Defined Benefit Plan, Plan Assets, Benefits Paid 5.6 7.0  
Fair value of plan assets at end of year 0.0 0.0 $ 0.0
Funded status at end of year (74.2) (66.4)  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Noncurrent assets 0.0 0.0  
Current liability for pension and other postretirement employee benefits (5.8) (7.0)  
Liability for pension and other postretirement employee benefits (68.4) (59.4)  
Net actuarial loss (gain) $ 5.9 $ (5.1)