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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 3 $ 4
Interest cost 5 6
Net loss from experience differences 2 3
Prior service credit (2) (2)
Net periodic benefit cost $ 8 $ 11