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
	Mar. 31, 2024		Mar. 31, 2023
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	$ 591	[1]	$ 584
Provision charged (credited) to income	(10)		12
Charge-offs	(3)		(11)
Recoveries	3		3
Net (charge-offs) recoveries	0		(8)
Allowance for credit losses - end of period	581	[1]	588
Commercial loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(1)		(5)
Commercial loans | Real Estate, including Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	144		200
Provision charged (credited) to income	(10)		(28)
Charge-offs	0		0
Recoveries	0		0
Net (charge-offs) recoveries	0		0
Allowance for credit losses - end of period	134		172
Commercial loans | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	302		230
Provision charged (credited) to income	7		38
Charge-offs	(1)		(5)
Recoveries	1		1
Net (charge-offs) recoveries	0		(4)
Allowance for credit losses - end of period	309		264
Commercial loans | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	127		120
Provision charged (credited) to income	(8)		12
Charge-offs	0		0
Recoveries	0		0
Net (charge-offs) recoveries	0		0
Allowance for credit losses - end of period	119		132
Commercial loans | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	1		1
Provision charged (credited) to income	0		0
Charge-offs	0		0
Recoveries	0		0
Net (charge-offs) recoveries	0		0
Allowance for credit losses - end of period	1		1
Consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(2)		(6)
Consumer loans | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	(9)		11
Provision charged (credited) to income	1		(10)
Charge-offs	0		(3)
Recoveries	1		0
Net (charge-offs) recoveries	1		(3)
Allowance for credit losses - end of period	(7)		(2)
Consumer loans | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	7		2
Provision charged (credited) to income	1		1
Charge-offs	0		(1)
Recoveries	0		1
Net (charge-offs) recoveries	0		0
Allowance for credit losses - end of period	8		3
Consumer loans | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	16		16
Provision charged (credited) to income	0		0
Charge-offs	(2)		(2)
Recoveries	1		1
Net (charge-offs) recoveries	(1)		(1)
Allowance for credit losses - end of period	15		15
Consumer loans | Consumer Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	3		4
Provision charged (credited) to income	(1)		(1)
Charge-offs	0		0
Recoveries	0		0
Net (charge-offs) recoveries	0		0
Allowance for credit losses - end of period	$ 2		$ 3

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at March 31, 2024 and December 31, 2023. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 15, "Variable Interest Entities," for additional information. March 31, 2024December 31, 2023(in millions)AssetsOther assets$21 $23 Total assets$21 $23 LiabilitiesInterest, taxes and other liabilities$28 $17 Total liabilities$28 $17