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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2 $ 3
Interest cost 4 5
Net loss from experience differences 3 2
Prior service credit [1] (10) (2)
Net periodic benefit cost $ (1) $ 8
[1] For the three months ended March 31, 2017, amortization of prior service credit includes an $8 million gain related to a change in the eligibility requirement and a change in benefits to Medicare-eligible participants.