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Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Pension Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 52 $ 46 $ 104 $ 93
Interest cost 56 55 112 110
Expected return on plan assets (94) (71) (189) (142)
Prior service cost (credit) amortization       (1)
Actuarial loss (gain) amortization 36 32 73 64
Net periodic benefit cost 50 62 100 124
Postretirement Welfare Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost   1 1 2
Expected return on plan assets (1)   (2) (1)
Prior service cost (credit) amortization   (1) (1) (2)
Actuarial loss (gain) amortization (1) (2) (3) (3)
Net periodic benefit cost $ (2) $ (2) $ (5) $ (4)