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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 385.8 $ 358.4  
Service cost 4.7 4.9 $ 4.0
Interest cost 9.6 11.4 13.3
Actuarial (gain) loss (18.8) 31.1  
Medicare Part D 0.0 0.0  
Benefits paid (21.2) (20.0)  
Benefit obligation at end of year 360.1 385.8 358.4
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 107.9 107.7  
Service cost 0.2 0.2 0.2
Interest cost 2.4 3.1 4.5
Actuarial (gain) loss (5.0) 2.7  
Medicare Part D 0.2 0.2  
Benefits paid (6.1) (6.0)  
Benefit obligation at end of year $ 99.6 $ 107.9 $ 107.7