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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2 $ 3 $ 4 $ 6
Interest cost 3 5 7 10
Net loss from experience differences 4 1 7 3
Prior service credit [1] (10) (1) (20) (3)
Net periodic benefit cost $ (1) $ 8 $ (2) $ 16
[1] For the three and six months ended June 30, 2017, amortization of prior service credit includes an $8 million and $16 million gain respectively, related to a change in the eligibility requirement and a change in benefits to Medicare-eligible participants.