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Pension and Other Benefit Plans - Effect of One Percentage Point Change in Assumed Health Care Cost Trend Rates (Detail) - Postretirement medical plan
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on total service and interest cost components, One Percentage Point Increase $ 3
Effect on accumulated postretirement benefit obligation, One Percentage Point Increase 34
Effect on total service and interest cost components, One Percentage Point Decrease (3)
Effect on accumulated postretirement benefit obligation, One Percentage Point Decrease $ (30)