Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 46	$ 44	$ 93	$ 88
Interest cost	55	53	110	105
Expected return on plan assets	(71)	(66)	(142)	(132)
Prior service cost (credit) amortization		(1)	(1)	(2)
Actuarial loss (gain) amortization	32	43	64	87
Net periodic benefit cost	62	73	124	146
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1	2	2
Expected return on plan assets			(1)
Prior service cost (credit) amortization	(1)	(1)	(2)	(2)
Actuarial loss (gain) amortization	(2)	(1)	(3)	(2)
Net periodic benefit cost	$ (2)	$ (1)	$ (4)	$ (2)