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Pension and Other Postretirement Benefits - 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 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 41 $ 44 $ 81 $ 88
Interest cost 122 145 244 289
Expected return on plan assets (186) (189) (371) (377)
Recognized net actuarial loss 102 117 204 235
Amortization of prior service benefit 4 4 8 8
Settlements 2   2 1
Special termination benefits   10 1 12
Net periodic benefit cost 85 131 169 256
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4 4 7 7
Interest cost 19 23 37 46
Recognized net actuarial loss 5 5 11 9
Amortization of prior service benefit (7) (10) (13) (19)
Special termination benefits   1   1
Net periodic benefit cost $ 21 22 $ 42 43
Curtailments   $ (1)   $ (1)