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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Pension Benefits [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 49 $ 46 $ 148 $ 139
Interest cost 150 160 453 480
Expected return on plan assets (196) (202) (592) (605)
Recognized net actuarial loss 122 97 369 288
Amortization of prior service cost (benefit) 4 5 14 14
Settlement     2  
Curtailment 6   6  
Special termination benefits 72   72  
Net periodic benefit cost 207 106 472 316
Other Postretirement Benefits [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4 3 13 10
Interest cost 28 33 85 99
Recognized net actuarial loss 9 7 26 19
Amortization of prior service cost (benefit) (4) (4) (13) (12)
Net periodic benefit cost $ 37 $ 39 $ 111 $ 116