Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 48	$ 52
Interest cost	62	56
Expected return on plan assets	(95)	(95)
Actuarial loss (gain) amortization	24	37
Net periodic benefit cost	39	50
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	(1)	(2)
Net periodic benefit cost	$ (2)	$ (3)