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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 314.4 $ 344.1  
Service cost 4.0 4.3 $ 4.4
Interest cost 13.3 12.4 13.3
Plan amendments 0.0 0.5  
Actuarial (gain) loss 46.5 (28.0)  
Medicare Part D 0.0 0.0  
Benefits paid (19.8) (18.9)  
Benefit obligation at end of year 358.4 314.4 344.1
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 108.7 120.8  
Service cost 0.2 0.2 0.8
Interest cost 4.5 4.2 5.3
Plan amendments 0.0 0.1  
Actuarial (gain) loss 0.1 (10.5)  
Medicare Part D 0.3 0.3  
Benefits paid (6.1) (6.4)  
Benefit obligation at end of year $ 107.7 $ 108.7 $ 120.8