Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) - USD ($)	3 Months Ended				6 Months Ended		12 Months Ended
	Dec. 31, 2021		Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2021		Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							$ 1,015,000,000	[1]	$ 637,000,000		$ 541,000,000
Provision charged (credited) to income							(437,000,000)		736,000,000		195,000,000
Charge offs							(165,000,000)		(228,000,000)		(125,000,000)
Recoveries							34,000,000		40,000,000		26,000,000
Net (charge offs) recoveries							(131,000,000)		(188,000,000)		(99,000,000)
Allowance for credit losses - end of period	$ 447,000,000	[1]			$ 447,000,000	[1]	447,000,000	[1]	1,015,000,000	[1]	637,000,000
Reduction to the provision for credit losses	22,000,000
Increase in allowance for credit losses	14,000,000
Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Reduction to the provision for credit losses				$ 101,000,000
Lower of amortized cost or fair value adjustment				0
Commercial loans: | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Reduction to the provision for credit losses				0
Lower of amortized cost or fair value adjustment				0
Commercial loans: | Real Estate, including Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							145,000,000		153,000,000		116,000,000
Provision charged (credited) to income							(72,000,000)		116,000,000		37,000,000
Charge offs							0		(12,000,000)		0
Recoveries							0		0		0
Net (charge offs) recoveries							0		(12,000,000)		0
Allowance for credit losses - end of period	73,000,000				73,000,000		73,000,000		145,000,000		153,000,000
Reduction to the provision for credit losses			$ 24,000,000
Commercial loans: | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							375,000,000		239,000,000		219,000,000
Provision charged (credited) to income							(107,000,000)		274,000,000		62,000,000
Charge offs							(29,000,000)		(90,000,000)		(45,000,000)
Recoveries							4,000,000		12,000,000		3,000,000
Net (charge offs) recoveries							(25,000,000)		(78,000,000)		(42,000,000)
Allowance for credit losses - end of period	243,000,000				243,000,000		243,000,000		375,000,000		239,000,000
Commercial loans: | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							287,000,000		106,000,000		108,000,000
Provision charged (credited) to income							(174,000,000)		149,000,000		1,000,000
Charge offs							(13,000,000)		(20,000,000)		(3,000,000)
Recoveries							0		1,000,000		0
Net (charge offs) recoveries							(13,000,000)		(19,000,000)		(3,000,000)
Allowance for credit losses - end of period	100,000,000				100,000,000		100,000,000		287,000,000		106,000,000
Commercial loans: | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							7,000,000		9,000,000		15,000,000
Provision charged (credited) to income							(3,000,000)		2,000,000		(6,000,000)
Charge offs							0		0		0
Recoveries							0		1,000,000		0
Net (charge offs) recoveries							0		1,000,000		0
Allowance for credit losses - end of period	4,000,000				4,000,000		4,000,000		7,000,000		9,000,000
Consumer loans:
Allowance for Loan and Lease Losses [Roll Forward]
Charge offs	(4,000,000)
Reduction to the provision for credit losses	22,000,000
Consumer loans: | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio
Allowance for Loan and Lease Losses [Roll Forward]
Charge offs				(56,000,000)
Reduction to the provision for credit losses				100,000,000
Lower of amortized cost or fair value adjustment				$ 0	7,000,000
Consumer loans: | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							(9,000,000)		12,000,000		13,000,000
Provision charged (credited) to income							21,000,000		55,000,000		(3,000,000)
Charge offs							(19,000,000)		(2,000,000)		(8,000,000)
Recoveries							15,000,000		12,000,000		10,000,000
Net (charge offs) recoveries							(4,000,000)		10,000,000		2,000,000
Allowance for credit losses - end of period	8,000,000				8,000,000		8,000,000		(9,000,000)		12,000,000
Consumer loans: | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							22,000,000		6,000,000		7,000,000
Provision charged (credited) to income							(18,000,000)		6,000,000		(2,000,000)
Charge offs							(5,000,000)		(3,000,000)		(4,000,000)
Recoveries							6,000,000		6,000,000		5,000,000
Net (charge offs) recoveries							1,000,000		3,000,000		1,000,000
Allowance for credit losses - end of period	5,000,000				5,000,000		5,000,000		22,000,000		6,000,000
Consumer loans: | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							161,000,000		105,000,000		58,000,000
Provision charged (credited) to income							(66,000,000)		107,000,000		101,000,000
Charge offs							(89,000,000)		(90,000,000)		(60,000,000)
Recoveries							8,000,000		7,000,000		6,000,000
Net (charge offs) recoveries							(81,000,000)		(83,000,000)		(54,000,000)
Allowance for credit losses - end of period	14,000,000				14,000,000		14,000,000		161,000,000		105,000,000
Consumer loans: | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period							27,000,000		7,000,000		5,000,000
Provision charged (credited) to income							(18,000,000)		27,000,000		5,000,000
Charge offs							(10,000,000)		(11,000,000)		(5,000,000)
Recoveries							1,000,000		1,000,000		2,000,000
Net (charge offs) recoveries							(9,000,000)		(10,000,000)		(3,000,000)
Allowance for credit losses - end of period	$ 0				$ 0		$ 0		27,000,000		7,000,000
Cumulative Effect, Period of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(170,000,000)
Allowance for credit losses - end of period											(170,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Commercial loans: | Real Estate, including Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(112,000,000)
Allowance for credit losses - end of period											(112,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Commercial loans: | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(60,000,000)
Allowance for credit losses - end of period											(60,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Commercial loans: | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									51,000,000
Allowance for credit losses - end of period											51,000,000
Cumulative Effect, Period of Adoption, Adjustment | Commercial loans: | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(5,000,000)
Allowance for credit losses - end of period											(5,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Consumer loans: | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(86,000,000)
Allowance for credit losses - end of period											(86,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Consumer loans: | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									7,000,000
Allowance for credit losses - end of period											7,000,000
Cumulative Effect, Period of Adoption, Adjustment | Consumer loans: | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									32,000,000
Allowance for credit losses - end of period											32,000,000
Cumulative Effect, Period of Adoption, Adjustment | Consumer loans: | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									3,000,000
Allowance for credit losses - end of period											3,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									467,000,000
Allowance for credit losses - end of period											467,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial loans: | Real Estate, including Construction
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									41,000,000
Allowance for credit losses - end of period											41,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial loans: | Business and corporate banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									179,000,000
Allowance for credit losses - end of period											179,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial loans: | Global banking
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									157,000,000
Allowance for credit losses - end of period											157,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial loans: | Other commercial
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									4,000,000
Allowance for credit losses - end of period											4,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer loans: | Residential Mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									(74,000,000)
Allowance for credit losses - end of period											(74,000,000)
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer loans: | Home equity mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									13,000,000
Allowance for credit losses - end of period											13,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer loans: | Credit cards
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									137,000,000
Allowance for credit losses - end of period											137,000,000
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer loans: | Other consumer
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period									$ 10,000,000
Allowance for credit losses - end of period											$ 10,000,000

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at December 31, 2021 and 2020. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 27, "Variable Interest Entities," for additional information. At December 31,20212020(in millions)AssetsLoans$46 $10 Other assets55 79 Total assets$101 $89 LiabilitiesInterest, taxes and other liabilities$9 $10 Total liabilities$9 $10