Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 66	$ 59
Interest cost	55	58
Expected return on plan assets	(112)	(101)
Actuarial loss (gain) amortization	42	34
Net periodic benefit cost	51	50
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1
Expected return on plan assets		(1)
Prior service cost (credit) amortization	(1)	(1)
Actuarial loss (gain) amortization	(2)	(2)
Net periodic benefit cost	$ (3)	$ (3)