Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 66	$ 59	$ 132	$ 118
Interest cost		55	58	110	117
Expected return on plan assets		(113)	(101)	(225)	(201)
Prior service cost (credit) amortization		(1)		(1)
Actuarial loss (gain) amortization		43	34	85	67
Net periodic benefit cost	[1]	50	50	101	101
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1		1
Expected return on plan assets			(1)		(2)
Prior service cost (credit) amortization		(1)	(1)	(2)	(2)
Actuarial loss (gain) amortization		(1)	(2)	(3)	(3)
Net periodic benefit cost	[1]	$ (2)	$ (3)	$ (5)	$ (6)

[1] (a) service cost is included in employee benefits expense on the consolidated statements of income .All other components are included in other noninterest expenses in other expenses expenses on the consolidated statement of income.