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Pension and Other Postretirement Benefit Plans - One Percentage Point Change in Health Care Cost Trend Rate (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Retirement Benefits [Abstract]  
Effect on total service and interest cost components, Increase $ 13
Effect on benefit obligation, Increase 125
Effect on total service and interest cost components, Decrease (11)
Effect on benefit obligation, Decrease $ (104)