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Effect of One-Percentage-Point Change in Assumed Health Care Trend Rate (Detail) - Other Postretirement Benefits
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on total service and interest cost, 1% Increase $ 11
Effect on post retirement benefit obligation, 1% Increase 170
Effect on total service and interest cost,1% Decrease (8)
Effect on post retirement benefit obligation, 1% Decrease $ (149)