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Benefit Obligations (Schedule of Impact of One-Percentage-Point Change in Assumed Health Care Cost Trend) (Details) - Other Postretirement Benefits Plan [Member] - Domestic Plan [Member]
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Postretirement obligations, impact of 1% decrease $ 2
Postretirement obligations, impact of 1% increase 2
Service and interest cost, impact of 1% decrease 0
Service and interest cost, impact of 1% increase $ 0