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Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 261.7  
Employer contributions 1.8 $ 1.8
Fair value of assets at end of year 295.9 261.7
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets 295.9 261.7
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 261.7 303.4
Actual return on plan assets 52.2 (24.6)
Employer contributions 1.8 1.8
Medicare Part D subsidy received 0.0 0.0
Benefits paid (19.8) (18.9)
Fair value of assets at end of year 295.9 261.7
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 6.1
Medicare Part D subsidy received 0.3 0.3
Benefits paid (6.1) (6.4)
Fair value of assets at end of year $ 0.0 $ 0.0