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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 358.4 $ 314.4  
Service cost 4.9 4.0 $ 4.3
Interest cost 11.4 13.3 12.4
Actuarial (gain) loss 31.1 46.5  
Medicare Part D 0.0 0.0  
Benefits paid (20.0) (19.8)  
Benefit obligation at end of year 385.8 358.4 314.4
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 107.7 108.7  
Service cost 0.2 0.2 0.2
Interest cost 3.1 4.5 4.2
Actuarial (gain) loss 2.7 0.1  
Medicare Part D 0.2 0.3  
Benefits paid (6.0) (6.1)  
Benefit obligation at end of year $ 107.9 $ 107.7 $ 108.7