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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1 $ 3 $ 5 $ 9
Interest cost 4 6 11 16
Net loss from experience differences 4 2 11 5
Prior service credit [1] (10) $ (11) (30) (14)
Net periodic benefit cost $ (1)   $ (3) $ 16
[1] For the three and nine months ended September 30, 2017, amortization of prior service credit includes an $8 million and $24 million gain respectively, related to a change in the eligibility requirement and a change in benefits to Medicare-eligible participants.