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Benefit Plans - Postretirement cost sensitivities (Details) - Postemployment Retirement Benefits
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Sensitivity to Trend Assumptions  
One-percentage point increase, effect on service cost and interest cost components $ 1.0
One-percentage point increase, effect on year-end benefit obligations 5.0
One-percentage point decrease, effect on service cost and interest cost components (1.0)
One-percentage point decrease, effect on year-end benefit obligations $ (7.0)