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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 360.1 $ 385.8  
Service cost 4.3 4.7 $ 4.9
Interest cost 10.3 9.6 11.4
Actuarial (gain) loss (87.7) (18.8)  
Medicare Part D 0.0 0.0  
Benefits paid (24.0) (21.2)  
Curtailment 0.0 0.0  
Benefit obligation at end of year 263.0 360.1 385.8
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 99.6 107.9  
Service cost 0.2 0.2 0.2
Interest cost 2.9 2.4 3.1
Actuarial (gain) loss (23.2) (5.0)  
Medicare Part D 0.2 0.2  
Benefits paid (6.1) (6.1)  
Curtailment 0.1 0.0  
Benefit obligation at end of year $ 73.7 $ 99.6 $ 107.9