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Components of Net Postretirement Health Care Costs (Detail) - Postretirement Benefit Plans - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 3 $ 3 $ 7 $ 6
Interest cost 6 6 12 11
Net loss from experience differences 4 1 7 3
Prior service credit (2) (2) (4) (5)
Net periodic benefit cost $ 11 $ 8 $ 22 $ 15