Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 66	$ 58	$ 198	$ 176
Interest cost		54	59	164	176
Expected return on plan assets		(112)	(101)	(337)	(302)
Prior service cost (credit) amortization				(1)
Actuarial loss (gain) amortization		42	34	127	101
Net periodic benefit cost	[1]	50	50	151	151
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1		1	1
Expected return on plan assets					(2)
Prior service cost (credit) amortization		(1)		(3)	(2)
Actuarial loss (gain) amortization		(2)	(2)	(5)	(5)
Net periodic benefit cost	[1]	$ (2)	$ (2)	$ (7)	$ (8)

[1] Service cost is included in employee benefits expense on the Consolidated Statement of Income. All other components are included in other noninterest expense on the Consolidated Statement of Income.