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, 2021	Mar. 31, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		$ 1,015
Provision charged (credited) to income		(145)	$ 512
Charge offs		(25)	(56)
Recoveries		8	8
Net (charge offs) recoveries		(17)	(48)
Allowance for credit losses – end of period		853	931
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		145
Provision charged (credited) to income		(25)	51
Charge offs		0	0
Recoveries		0	0
Net (charge offs) recoveries		0	0
Allowance for credit losses – end of period		120	92
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		375
Provision charged (credited) to income		(53)	213
Charge offs		(2)	(12)
Recoveries		1	2
Net (charge offs) recoveries		(1)	(10)
Allowance for credit losses – end of period		321	382
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		287
Provision charged (credited) to income		(65)	110
Charge offs		0	(19)
Recoveries		0	0
Net (charge offs) recoveries		0	(19)
Allowance for credit losses – end of period		222	248
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		7
Provision charged (credited) to income		0	1
Charge offs		0	0
Recoveries		0	0
Net (charge offs) recoveries		0	0
Allowance for credit losses – end of period		7	5
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		(9)
Provision charged (credited) to income		(2)	30
Charge offs		(1)	1
Recoveries		3	2
Net (charge offs) recoveries		2	3
Allowance for credit losses – end of period		(9)	(41)
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		22
Provision charged (credited) to income		(3)	3
Charge offs		0	(2)
Recoveries		2	2
Net (charge offs) recoveries		2	0
Allowance for credit losses – end of period		21	16
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		161
Provision charged (credited) to income		(1)	95
Charge offs		(18)	(22)
Recoveries		2	2
Net (charge offs) recoveries		(16)	(20)
Allowance for credit losses – end of period		144	212
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period		27
Provision charged (credited) to income		4	9
Charge offs		(4)	(2)
Recoveries		0	0
Net (charge offs) recoveries		(4)	(2)
Allowance for credit losses – end of period		$ 27	17
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	[1]		(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	[1]		(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	[1]		(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	[1]		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	[1]		(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	[1]		(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	[1]		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	[1]		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	[1]		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] See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," in our 2020 Form 10-K for additional discussion.