Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 56	$ 69	$ 168	$ 206
Interest cost		93	62	278	185
Expected return on plan assets		(137)	(119)	(410)	(358)
Prior service cost (credit) amortization			(1)	(1)	(2)
Actuarial loss (gain) amortization		1	35	3	105
Net periodic benefit cost	[1]	13	46	38	136
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1		2
Expected return on plan assets		(1)		(2)
Prior service cost (credit) amortization		(1)		(2)	(2)
Actuarial loss (gain) amortization		(2)	(2)	(6)	(5)
Net periodic benefit cost	[1]	$ (3)	$ (2)	$ (8)	$ (7)

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