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Postretirement Benefits Other Than Pensions - Components Of Comprehensive OPEB (Income) Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Unrecognized Postretirement Benefit Costs Arising During the Year $ 8.0 $ (3.9) $ (14.2)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)   (1.3)  
Amortization of Accumulated Unrecognized Pension Loss (1.1) 0.6 (5.3)
Other Postretirement Benefit Plan, Defined Benefit [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost Earned During the Year 0.2 0.1 0.1
Interest Cost on Accumulated Postretirement Benefit Obligation 0.4 0.4 0.8
Amortization of Actuarial Loss (1.8) (1.8) (1.4)
Pension Expense (Income) Recognized in Consolidated Statements of Income (2.5) (1.3) (0.5)
Unrecognized Postretirement Benefit Costs Arising During the Year (3.0) (0.5) (1.3)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (1.3)    
Plan Amendments     (11.0)
Prior Service Credit Arising During the Year 1.3 0.0 0.0
Amortization of Accumulated Unrecognized Pension Loss 1.8 1.8 1.4
Comprehensive OPEB Expense $ (2.4) $ (1.3) $ (11.4)