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Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 295.9  
Employer contributions 1.9 $ 1.8
Fair value of assets at end of year 318.8 295.9
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets 318.8 295.9
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 295.8 261.7
Actual return on plan assets 41.1 52.2
Employer contributions 1.9 1.8
Medicare Part D subsidy received 0.0 0.0
Benefits paid (20.0) (19.8)
Fair value of assets at end of year 318.8 295.8
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 5.8 5.8
Medicare Part D subsidy received 0.2 0.3
Benefits paid (6.0) (6.1)
Fair value of assets at end of year $ 0.0 $ 0.0