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Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 216.6 $ 329.7
Actual return on plan assets 20.7 (93.3)
Employer contributions 6.3 4.2
Medicare Part D subsidy received 0.0 0.0
Benefits paid (18.8) (24.0)
Fair value of assets at end of year 224.8 216.6
OPEB    
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 0.0 0.0
Actual return on plan assets 0.0 0.0
Employer contributions 6.4 5.9
Medicare Part D subsidy received 0.3 0.2
Benefits paid (6.7) (6.1)
Fair value of assets at end of year $ 0.0 $ 0.0