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Components of Net Periodic Benefit Cost (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Pension Benefits
   
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 33 $ 25
Interest cost 47 46
Expected return on plan assets (40) (35)
Amortization of unrecognized (gain) loss and prior service cost 18 5
Net periodic benefit costs 58 41
Other Postretirement Benefits
   
Defined Benefit Plan Disclosure [Line Items]    
Service cost 14 13
Interest cost 28 31
Expected return on plan assets (1) (1)
Amortization of unrecognized (gain) loss and prior service cost 3 (1)
Net periodic benefit costs $ 44 $ 42