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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 44 $ 42 $ 132 $ 125
Interest cost 144 157 433 476
Expected return on plan assets (189) (198) (566) (586)
Recognized net actuarial loss 117 97 352 292
Amortization of prior service cost (benefit) 4 5 12 14
Settlements 13 18 14 18
Curtailments 9   9  
Special termination benefits     12  
Net periodic benefit cost 142 121 398 339
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4 4 11 12
Interest cost 23 28 69 86
Recognized net actuarial loss 4 4 13 10
Amortization of prior service cost (benefit) (9) (6) (28) (15)
Curtailments (5)   (6)  
Special termination benefits     1  
Net periodic benefit cost $ 17 $ 30 $ 60 $ 93