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			12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				$ 584	[1]	$ 447		$ 1,015
Provision charged (credited) to income				51		143		(437)
Charge-offs				(62)		(26)		(165)
Recoveries				18		20		34
Net (charge-offs) recoveries				(44)		(6)		(131)
Allowance for credit losses - end of period	$ 447			591	[1]	584	[1]	447
Allowance for credit loss increase (decrease)	14
Exit of Mass Market Retail Banking Business
Allowance for Loan and Lease Losses [Roll Forward]
Reduction to the provision for credit losses			$ 101
Commercial loans | Exit of Mass Market Retail Banking Business
Allowance for Loan and Lease Losses [Roll Forward]
Transfer to held-for-sale, allowance for credit loss			0
Consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(4)
Reduction to the provision for credit losses	22
Consumer loans | Exit of Mass Market Retail Banking Business
Allowance for Loan and Lease Losses [Roll Forward]
Reduction to the provision for credit losses			100
Commercial loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(48)
Commercial loans | Real Estate, including Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				200		73		145
Provision charged (credited) to income				(56)		129		(72)
Charge-offs				0		(3)		0
Recoveries				0		1		0
Net (charge-offs) recoveries				0		(2)		0
Allowance for credit losses - end of period	73			144		200		73
Reduction to the provision for credit losses		$ 24
Commercial loans | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				230		243		375
Provision charged (credited) to income				111		(8)		(107)
Charge-offs				(48)		(9)		(29)
Recoveries				9		4		4
Net (charge-offs) recoveries				(39)		(5)		(25)
Allowance for credit losses - end of period	243			302		230		243
Commercial loans | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				120		100		287
Provision charged (credited) to income				7		29		(174)
Charge-offs				0		(9)		(13)
Recoveries				0		0		0
Net (charge-offs) recoveries				0		(9)		(13)
Allowance for credit losses - end of period	100			127		120		100
Commercial loans | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				1		4		7
Provision charged (credited) to income				0		(3)		(3)
Charge-offs				0		0		0
Recoveries				0		0		0
Net (charge-offs) recoveries				0		0		0
Allowance for credit losses - end of period	4			1		1		4
Consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(14)
Consumer loans | Exit of Mass Market Retail Banking Business
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs			(56)
Reduction to the provision for credit losses			$ 100
Consumer loans | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				11		8		(9)
Provision charged (credited) to income				(19)		0		21
Charge-offs				(4)		(2)		(19)
Recoveries				3		5		15
Net (charge-offs) recoveries				(1)		3		(4)
Allowance for credit losses - end of period	8			(9)		11		8
Consumer loans | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				2		5		22
Provision charged (credited) to income				4		(6)		(18)
Charge-offs				(1)		(1)		(5)
Recoveries				2		4		6
Net (charge-offs) recoveries				1		3		1
Allowance for credit losses - end of period	5			7		2		5
Consumer loans | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				16		14		161
Provision charged (credited) to income				4		(2)		(66)
Charge-offs				(8)		(1)		(89)
Recoveries				4		5		8
Net (charge-offs) recoveries				(4)		4		(81)
Allowance for credit losses - end of period	14			16		16		14
Consumer loans | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period				4		0		27
Provision charged (credited) to income				0		4		(18)
Charge-offs				(1)		(1)		(10)
Recoveries				0		1		1
Net (charge-offs) recoveries				(1)		0		(9)
Allowance for credit losses - end of period	$ 0			$ 3		$ 4		$ 0

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at December 31, 2023 and 2022. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 27, "Variable Interest Entities," for additional information. At December 31,20232022(in millions)AssetsLoans, net$— $162 Other assets23 44 Total assets$23 $206 LiabilitiesInterest, taxes and other liabilities$17 $22 Total liabilities$17 $22