Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in plan assets:
Fair value of plan assets at beginning of year	$ 2,938
Fair value of plan assets at end of year	3,141	$ 2,938
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,379	2,420
Service cost	11	18	$ 20
Interest cost	$ 115	$ 82	$ 62
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense	Other Noninterest Expense
Actuarial (gain) loss	$ 13	$ (636)
Business combinations		633
Benefits paid	(149)	(138)
Benefit obligation at end of year	2,369	2,379	$ 2,420
Change in plan assets:
Fair value of plan assets at beginning of year	2,942	2,596
Actual return on plan assets	334	(386)
Employer contributions	18	14
Business combinations		856
Benefits paid	(149)	(138)
Fair value of plan assets at end of year	3,145	2,942	2,596
Funded status	776	563
Prepaid asset recognized in the Consolidated Balance Sheet	922	715
Accrued liability recognized in the Consolidated Balance Sheet	(146)	(152)
Net accrued asset (liability) recognized in the Consolidated Balance Sheet	776	563
Net loss (gain)	191	309
Pre-tax adjustment to accumulated other comprehensive income	191	309
Taxes	(49)	(80)
Net adjustment to accumulated other comprehensive income	142	229
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	60	52
Service cost	2	3	1
Interest cost	$ 3	$ 2	$ 1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense	Other Noninterest Expense
Plan participants’ contributions	$ 1	$ 2
Actuarial (gain) loss	(5)	(22)
Plan amendment		13
Business combinations		15
Medicare Part D reimbursement		1
Benefits paid	(4)	(6)
Benefit obligation at end of year	57	60	$ 52
Change in plan assets:
Employer contributions	3	3
Plan participants’ contributions	1	2
Medicare Part D reimbursement		1
Benefits paid	(4)	(6)
Funded status	(57)	(60)
Accrued liability recognized in the Consolidated Balance Sheet	(57)	(60)
Net accrued asset (liability) recognized in the Consolidated Balance Sheet	(57)	(60)
Net loss (gain)	(37)	(35)
Net prior service cost (credit)	1	(1)
Pre-tax adjustment to accumulated other comprehensive income	(36)	(36)
Taxes	9	9
Net adjustment to accumulated other comprehensive income	$ (27)	$ (27)