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, 2021		Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		$ 547			$ 1,192	$ 1,015
Provision charged (credited) to income		(64)	[1]		(75)	(410)	[1]	$ 774
Charge offs		(16)			(44)	(155)	[1]	(184)
Recoveries		8			11	25		27
Net (charge offs) recoveries		(8)			(33)	(130)		(157)
Allowance for credit losses - end of period		475		$ 547	1,084	475		1,084
Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Reduction to the provision for credit losses				101
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		117			120	145
Provision charged (credited) to income		(2)	[1]		43	(30)	[1]	122
Charge offs		0			(12)	0	[1]	(12)
Recoveries		0			0	0		0
Net (charge offs) recoveries		0			(12)	0		(12)
Allowance for credit losses - end of period		115		117	151	115		151
Reduction to the provision for credit losses		24
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		284			434	375
Provision charged (credited) to income		(38)	[1]		(36)	(115)	[1]	283
Charge offs		(13)			(2)	(29)	[1]	(69)
Recoveries		1			5	3		8
Net (charge offs) recoveries		(12)			3	(26)		(61)
Allowance for credit losses - end of period		234		284	401	234		401
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		131			424	287
Provision charged (credited) to income		(9)	[1]		(103)	(153)	[1]	183
Charge offs		0			(1)	(12)	[1]	(20)
Recoveries		0			0	0		0
Net (charge offs) recoveries		0			(1)	(12)		(20)
Allowance for credit losses - end of period		122		131	320	122		320
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		6			5	7
Provision charged (credited) to income		(2)	[1]		3	(3)	[1]	3
Charge offs		0			0	0	[1]	0
Recoveries		0			0	0		1
Net (charge offs) recoveries		0			0	0		1
Allowance for credit losses - end of period		4		6	8	4		8
Consumer Loans [Member] | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Charge offs						56
Reduction to the provision for credit losses				100		100
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		(6)			(19)	(9)
Provision charged (credited) to income		(9)	[1]		12	(3)	[1]	62
Charge offs		0			(1)	(10)	[1]	(1)
Recoveries		4			3	11		8
Net (charge offs) recoveries		4			2	1		7
Allowance for credit losses - end of period		(11)		(6)	(5)	(11)		(5)
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		15			16	22
Provision charged (credited) to income		(2)	[1]		(1)	(10)	[1]	1
Charge offs		(3)			(1)	(5)	[1]	(3)
Recoveries		1			1	4		4
Net (charge offs) recoveries		(2)			0	(1)		1
Allowance for credit losses - end of period		11		15	15	11		15
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		0			190	161
Provision charged (credited) to income		(1)	[1]		(2)	(78)	[1]	96
Charge offs		0			(23)	(88)	[1]	(71)
Recoveries		1			2	5		5
Net (charge offs) recoveries		1			(21)	(83)		(66)
Allowance for credit losses - end of period		0		0	167	0		167
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period		0			22	27
Provision charged (credited) to income		(1)	[1]		9	(18)	[1]	24
Charge offs		0			(4)	(11)	[1]	(8)
Recoveries		1			0	2		1
Net (charge offs) recoveries		1			(4)	(9)		(7)
Allowance for credit losses - end of period		$ 0		$ 0	$ 27	$ 0		27
As Previously Reported [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								637
As Previously Reported [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								153
As Previously Reported [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								239
As Previously Reported [Member] | Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								106
As Previously Reported [Member] | Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								9
As Previously Reported [Member] | Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								12
As Previously Reported [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								6
As Previously Reported [Member] | Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								105
As Previously Reported [Member] | Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								7
Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							(170)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							(112)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							(60)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							51
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							(5)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							(86)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							7
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							32
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period	[2]							3
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								467
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								41
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								179
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								157
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								4
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								(74)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								13
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								137
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses - beginning of period								$ 10

[1] During the second quarter of 2021, we made the decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale. As a result of transferring these loans to held for sale, during the nine months ended September 30, 2021, we recognized $56 million of the existing allowance for credit losses on consumer loans as charge-offs, primarily related to non-performing credit cards, and released $100 million of the existing allowance for credit losses on consumer loans as reductions to the provision for credit losses, primarily related to credit cards. The existing commercial allowance for credit losses on the retail business banking loan portfolio transferred to held for sale was not material. See Note 3, "Branch Assets and Liabilities Held for Sale."

[2] (2)See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," in our 2020 Form 10-K for additional discussion.