Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 47	$ 45	$ 140	$ 133
Interest cost	55	53	165	158
Expected return on plan assets	(71)	(66)	(213)	(198)
Prior service cost (credit) amortization		(2)	(1)	(4)
Actuarial loss (gain) amortization	31	44	95	131
Net periodic benefit cost	62	74	186	220
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			2	2
Expected return on plan assets	(1)	(1)	(2)	(1)
Prior service cost (credit) amortization	(1)		(3)	(2)
Actuarial loss (gain) amortization	(1)	(1)	(4)	(3)
Net periodic benefit cost	$ (3)	$ (2)	$ (7)	$ (4)