Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 59	$ 48
Interest cost	58	62
Expected return on plan assets	(101)	(95)
Actuarial loss (gain) amortization	34	24
Net periodic benefit cost	50	39
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)