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Other Postretirement Employee Benefit Plans (Effect Of One-Percentage-Point Change In Assumed Health Care Cost Trend Rates) (Details) - Other postretirement benefits
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on the total of service and interest cost components, 1% increase $ 0.3
Effect on the total of service and interest cost components, 1% decrease (0.3)
Effect on postretirement medical benefit obligation, 1% increase 4.9
Effect on postretirement medical benefit obligation, 1% decrease $ (4.3)