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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, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 3 $ 3 $ 6 $ 7
Interest cost 5 6 10 12
Net loss from experience differences 1 4 3 7
Prior service credit (1) (2) (3) (4)
Net periodic benefit cost $ 8 $ 11 $ 16 $ 22