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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Change in benefit obligation [Roll Forward]      
Plan amendments   $ (18.8)  
Defined Benefit Pension Plans [Member]      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 344.1 328.0  
Service cost 4.3 4.4 $ 4.6
Interest cost 12.4 13.3 13.9
Plan amendments 0.5 0.0  
Actuarial (gain) loss (28.0) 18.5  
Medicare Part D 0.0 0.0  
Benefits paid (18.9) (20.1)  
Benefit obligation at end of year 314.4 344.1 328.0
Other Postretirement Benefit Plans [Member]      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 120.8 133.9  
Service cost 0.2 0.8 1.0
Interest cost 4.2 5.3 5.6
Plan amendments 0.1 (27.4)  
Actuarial (gain) loss (10.5) 14.7  
Medicare Part D 0.3 0.4  
Benefits paid (6.4) (6.9)  
Benefit obligation at end of year $ 108.7 $ 120.8 $ 133.9