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Pension and Postretirement Benefit Costs (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Components of net periodic benefit cost    
Net periodic pension (benefit) cost $ 2.8 $ 4.9
Defined benefit pension plans    
Components of net periodic benefit cost    
Service cost for benefits earned 0.0 0.5
Interest cost on projected benefit obligation 7.0 8.3
Expected return on plan assets (7.4) (7.8)
Net periodic pension (benefit) cost $ (0.4) 1.0
Other Defined Benefit Plan Information [Abstract]    
Funding threshold 80.00%  
Postretirement benefit plans    
Components of net periodic postretirement benefits cost    
Service cost for benefits earned $ 1.1 1.2
Interest cost on accumulated postretirement benefit obligation 5.5 6.3
Expected return on plan assets (0.4) (0.1)
Amortization of prior service credit (2.2) (2.2)
Net periodic postretirement benefit cost $ 4.0 $ 5.2