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Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 263.0 $ 360.1  
Service cost 2.4 4.3 $ 4.7
Interest cost 14.0 10.3 9.6
Plan amendments 1.1 0.0  
Actuarial (gain) loss 10.1 (87.7)  
Medicare Part D 0.0 0.0  
Benefits paid (18.8) (24.0)  
Curtailment 0.0 0.0  
Benefit obligation at end of year 271.8 263.0 360.1
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 73.7 99.6  
Service cost 0.1 0.2 0.2
Interest cost 3.8 2.9 2.4
Plan amendments 0.0 0.0  
Actuarial (gain) loss 4.5 (23.2)  
Medicare Part D 0.3 0.2  
Benefits paid (6.7) (6.1)  
Curtailment 0.0 0.1  
Benefit obligation at end of year $ 75.7 $ 73.7 $ 99.6