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One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]  
Effect on postretirement benefit obligation, Increase $ 90us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation
Effect on annual service and interest cost, Increase 6us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents
Effect on postretirement benefit obligation, Decrease (71)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1
Effect on annual service and interest cost, Decrease $ (5)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1