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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, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 44 $ 42 $ 88 $ 83
Interest cost 145 159 289 319
Expected return on plan assets (189) (195) (377) (388)
Recognized net actuarial loss 117 98 235 195
Amortization of prior service cost (benefit) 4 5 8 9
Settlement     1  
Special termination benefits 10   12  
Net periodic benefit cost 131 109 256 218
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4 4 7 8
Interest cost 23 29 46 58
Recognized net actuarial loss 5 3 9 6
Amortization of prior service cost (benefit) (10) (4) (19) (9)
Curtailment (1)   (1)  
Special termination benefits 1   1  
Net periodic benefit cost $ 22 $ 32 $ 43 $ 63