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One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on postretirement benefit obligation, Increase $ 49
Effect on annual service and interest cost, Increase 3
Effect on postretirement benefit obligation, Decrease (40)
Effect on annual service and interest cost, Decrease $ (2)