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Pension and Postretirement Benefit Costs (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Components of net periodic benefit cost        
Net periodic pension (benefit) cost $ 2.7 $ 4.8 $ 5.5 $ 9.7
Defined benefit pension plans        
Components of net periodic benefit cost        
Service cost for benefits earned 0.1 0.5 0.1 1.0
Interest cost on projected benefit obligation 7.0 8.4 14.0 16.7
Expected return on plan assets (7.4) (7.8) (14.8) (15.6)
Net periodic pension (benefit) cost $ (0.3) 1.1 $ (0.7) 2.1
Other Defined Benefit Plan Information [Abstract]        
Funding threshold 80.00%   80.00%  
Postretirement benefit plans        
Components of net periodic postretirement benefits cost        
Service cost for benefits earned $ 1.1 1.2 $ 2.2 2.4
Interest cost on accumulated postretirement benefit obligation 5.4 6.3 10.9 12.6
Expected return on plan assets (0.4) (0.1) (0.8) (0.2)
Amortization of prior service credit (2.2) (2.2) (4.4) (4.4)
Net periodic postretirement benefit cost $ 3.9 $ 5.2 $ 7.9 $ 10.4