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Pension and Other Postretirement Benefit - Schedule of Components of Net Periodic Benefit Cost (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Pension Benefits [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 46 $ 41 $ 139 $ 123
Interest cost 160 169 480 509
Expected return on plan assets (202) (202) (605) (605)
Amortization of prior service benefit 5 5 14 14
Recognized net actuarial loss 97 63 288 187
Net periodic benefit cost 106 76 316 228
Other Postretirement Benefits [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 3 5 10 13
Interest cost 33 40 99 119
Expected return on plan assets   (1)   (2)
Amortization of prior service benefit (4) (5) (12) (13)
Recognized net actuarial loss 7 7 19 20
Net periodic benefit cost $ 39 $ 46 $ 116 $ 137