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, 2024		Sep. 30, 2023	Sep. 30, 2024		Sep. 30, 2023
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	$ 583		$ 605	$ 591	[1]	$ 584
Provision charged (credited) to income	(30)		2	12		32
Charge-offs	(8)		(36)	(64)		(52)
Recoveries	2		5	8		12
Net (charge-offs) recoveries	(6)		(31)	(56)		(40)
Allowance for credit losses - end of period	547	[1]	576	547	[1]	576
Commercial loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(57)		(40)
Commercial loans | Real estate, including construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	145		146	144		200
Provision charged (credited) to income	17		(3)	18		(57)
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	162		143	162		143
Commercial loans | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	300		295	302		230
Provision charged (credited) to income	(33)		24	13		93
Charge-offs	(5)		(34)	(54)		(40)
Recoveries	0		3	1		5
Net (charge-offs) recoveries	(5)		(31)	(53)		(35)
Allowance for credit losses - end of period	262		288	262		288
Commercial loans | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	121		151	127		120
Provision charged (credited) to income	(15)		(21)	(18)		10
Charge-offs	0		0	(3)		0
Recoveries	0		0	0		0
Net (charge-offs) recoveries	0		0	(3)		0
Allowance for credit losses - end of period	106		130	106		130
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	0		0	0		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	1		1	1		1
Consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(7)		(12)
Consumer loans | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	(7)		(13)	(9)		11
Provision charged (credited) to income	(2)		2	(2)		(20)
Charge-offs	0		0	0		(4)
Recoveries	1		0	3		2
Net (charge-offs) recoveries	1		0	3		(2)
Allowance for credit losses - end of period	(8)		(11)	(8)		(11)
Consumer loans | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	7		7	7		2
Provision charged (credited) to income	1		(1)	0		4
Charge-offs	0		0	0		(1)
Recoveries	0		1	1		2
Net (charge-offs) recoveries	0		1	1		1
Allowance for credit losses - end of period	8		7	8		7
Consumer loans | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	13		15	16		16
Provision charged (credited) to income	1		2	0		3
Charge-offs	(2)		(2)	(6)		(6)
Recoveries	1		1	3		3
Net (charge-offs) recoveries	(1)		(1)	(3)		(3)
Allowance for credit losses - end of period	13		16	13		16
Consumer loans | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	3		3	3		4
Provision charged (credited) to income	1		(1)	1		(1)
Charge-offs	(1)		0	(1)		(1)
Recoveries	0		0	0		0
Net (charge-offs) recoveries	(1)		0	(1)		(1)
Allowance for credit losses - end of period	$ 3		$ 2	$ 3		$ 2

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at September 30, 2024 and December 31, 2023. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 15, "Variable Interest Entities," for additional information. September 30, 2024December 31, 2023(in millions)AssetsOther assets$13 $23 Total assets$13 $23 LiabilitiesInterest, taxes and other liabilities$21 $17 Total liabilities$21 $17