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Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 195 $ 112 $ 124
Interest cost 220 200 200
Expected return on plan assets (243) (216) (194)
Amortization of unrecognized actuarial (gain) loss 128 76 85
Other 5 5 4
Net periodic benefit cost (credit) 305 177 219
Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 13 19 21
Interest cost 66 86 82
Expected return on plan assets (2) (2) (2)
Curtailment gain   (107)  
Amortization of unrecognized actuarial (gain) loss (33) (19) (22)
Amortization of prior service credits (37) (31) (32)
Net periodic benefit cost (credit) $ 7 $ (54) $ 47