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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Service cost   $ 7 $ 12 $ 15
Interest cost   15 20 22
Net loss from experience differences $ (10) 14 10 13
Prior service credit [1]   (40) (20) (7)
Net periodic benefit cost   $ (4) $ 22 $ 43
[1] In the fourth quarter of 2016, the prior service credit included a one-time $9 million curtailment gain related to a change in the eligibility requirement resulting in ongoing amortization of $10 million. In 2017, we continue to amortize prior service credit and recorded $40 million on a full year basis.