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Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 112 $ 124 $ 98
Interest cost 200 200 201
Expected return on plan assets (216) (194) (180)
Amortization of unrecognized actuarial (gain) loss 76 85 12
Other 5 4 1
Net periodic benefit cost 177 219 132
Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 19 21 19
Interest cost 86 82 88
Expected return on plan assets (2) (2) (2)
Curtailment gain (107)    
Amortization of unrecognized actuarial (gain) loss (19) (22) (47)
Amortization of prior service credits (31) (32) (31)
Net periodic benefit cost $ (54) $ 47 $ 27