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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, 2015
USD ($)
Pension Plans Postretirement And Other Employee Benefits [Line Items]  
Effect on total service and interest cost components, One percentage point increase $ 7
Effect on accumulated postretirement benefit obligation, One percentage point increase 37
Effect on total service and interest cost components, One percentage point decrease (6)
Effect on accumulated postretirement benefit obligation, One percentage point decrease $ (33)