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Components of Net Periodic Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 15 $ 13 $ 15
Interest cost 22 22 20
Net loss from experience differences 13 5 12
Prior service credit (7) (10) (12)
Net periodic benefit cost $ 43 $ 30 $ 35