Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2025		Sep. 30, 2024	Sep. 30, 2025		Sep. 30, 2024
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	$ 614		$ 583	$ 537	[1]	$ 591
Provision charged (credited) to income	(54)		(30)	87		12
Charge-offs	(41)		(8)	(112)		(64)
Recoveries	6		2	13		8
Net (charge-offs) recoveries	(35)		(6)	(99)		(56)
Allowance for credit losses - end of period	525	[1]	547	525	[1]	547
Commercial loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(106)		(57)
Commercial loans | Real estate, including construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	180		145	144		144
Provision charged (credited) to income	(5)		17	58		18
Charge-offs	(2)		0	(29)		0
Recoveries	1		0	1		0
Net (charge-offs) recoveries	(1)		0	(28)		0
Allowance for credit losses - end of period	174		162	174		162
Commercial loans | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	430		421	381		429
Provision charged (credited) to income	(51)		(48)	37		(5)
Charge-offs	(37)		(5)	(77)		(57)
Recoveries	3		0	4		1
Net (charge-offs) recoveries	(34)		(5)	(73)		(56)
Allowance for credit losses - end of period	345		368	345		368
Commercial loans | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	1		1	1		1
Provision charged (credited) to income	(1)		0	(1)		0
Charge-offs	0		0	0		0
Recoveries	0		0	0		0
Net (charge-offs) recoveries	0		0	0		0
Allowance for credit losses - end of period	0		1	0		1
Consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(6)		(7)
Consumer loans | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	(17)		(7)	(12)		(9)
Provision charged (credited) to income	3		(2)	(5)		(2)
Charge-offs	(1)		0	(1)		0
Recoveries	1		1	4		3
Net (charge-offs) recoveries	0		1	3		3
Allowance for credit losses - end of period	(14)		(8)	(14)		(8)
Consumer loans | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	5		7	6		7
Provision charged (credited) to income	0		1	(2)		0
Charge-offs	0		0	0		0
Recoveries	0		0	1		1
Net (charge-offs) recoveries	0		0	1		1
Allowance for credit losses - end of period	5		8	5		8
Consumer loans | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	12		13	13		16
Provision charged (credited) to income	0		1	1		0
Charge-offs	(1)		(2)	(5)		(6)
Recoveries	1		1	3		3
Net (charge-offs) recoveries	0		(1)	(2)		(3)
Allowance for credit losses - end of period	12		13	12		13
Consumer loans | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	3		3	4		3
Provision charged (credited) to income	0		1	(1)		1
Charge-offs	0		(1)	0		(1)
Recoveries	0		0	0		0
Net (charge-offs) recoveries	0		(1)	0		(1)
Allowance for credit losses - end of period	$ 3		$ 3	$ 3		$ 3

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at September 30, 2025 and December 31, 2024. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 15, "Variable Interest Entities," for additional information. September 30, 2025December 31, 2024(in millions)AssetsOther assets$7 $11 Total assets$7 $11 LiabilitiesInterest, taxes and other liabilities$14 $4 Total liabilities$14 $4