Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 52	$ 47
Interest cost	56	55
Expected return on plan assets	(95)	(71)
Prior service cost (credit) amortization		(1)
Actuarial loss (gain) amortization	37	32
Net periodic benefit cost	50	62
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1
Expected return on plan assets	(1)	(1)
Prior service cost (credit) amortization	(1)	(1)
Actuarial loss (gain) amortization	(2)	(1)
Net periodic benefit cost	$ (3)	$ (2)