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Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Pension    
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 318.8 $ 295.8
Actual return on plan assets 30.3 41.1
Employer contributions 1.8 1.9
Medicare Part D subsidy received 0.0 0.0
Benefits paid (21.2) (20.0)
Fair value of assets at end of year 329.7 318.8
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 5.9 5.8
Medicare Part D subsidy received 0.2 0.2
Benefits paid (6.1) (6.0)
Fair value of assets at end of year $ 0.0 $ 0.0