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Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 27 $ 31 $ 55 $ 62
Interest cost 50 50 101 100
Expected return on plan assets (54) (49) (108) (98)
Amortization of unrecognized (gain) loss and prior service cost (credit) 20 22 38 44
Settlement loss 1   2 1
Total 44 54 88 109
Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 6 5 10 10
Interest cost 22 21 44 41
Expected return on plan assets (1) (1) (1) (1)
Amortization of unrecognized (gain) loss and prior service cost (credit) (14) (14) (27) (27)
Total $ 13 $ 11 $ 26 $ 23