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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				$ 541	$ 637	$ 541	$ 681
Provision charged (credited) to income	$ (3)	$ 94	$ 46	58	736	195	(73)
Charge offs					(228)	(125)	(142)
Recoveries					40	26	75
Net (charge offs) recoveries					(188)	(99)	(67)
Allowance for credit losses – end of period	637				1,015	637	541
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				116	153	116	82
Provision charged (credited) to income					116	37	34
Charge offs					(12)	0	0
Recoveries					0	0	0
Net (charge offs) recoveries					(12)	0	0
Allowance for credit losses – end of period	153				145	153	116
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				219	239	219	244
Provision charged (credited) to income					274	62	(31)
Charge offs					(90)	(45)	(41)
Recoveries					12	3	47
Net (charge offs) recoveries					(78)	(42)	6
Allowance for credit losses – end of period	239				375	239	219
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				108	106	108	264
Provision charged (credited) to income					149	1	(109)
Charge offs					(20)	(3)	(48)
Recoveries					1	0	1
Net (charge offs) recoveries					(19)	(3)	(47)
Allowance for credit losses – end of period	106				287	106	108
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				15	9	15	18
Provision charged (credited) to income					2	(6)	(3)
Charge offs					0	0	0
Recoveries					1	0	0
Net (charge offs) recoveries					1	0	0
Allowance for credit losses – end of period	9				7	9	15
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				13	12	13	25
Provision charged (credited) to income					55	(3)	(18)
Charge offs					(2)	(8)	(7)
Recoveries					12	10	13
Net (charge offs) recoveries					10	2	6
Allowance for credit losses – end of period	12				(9)	12	13
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				7	6	7	11
Provision charged (credited) to income					6	(2)	(3)
Charge offs					(3)	(4)	(7)
Recoveries					6	5	6
Net (charge offs) recoveries					3	1	(1)
Allowance for credit losses – end of period	6				22	6	7
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				58	105	58	32
Provision charged (credited) to income					107	101	55
Charge offs					(90)	(60)	(35)
Recoveries					7	6	6
Net (charge offs) recoveries					(83)	(54)	(29)
Allowance for credit losses – end of period	105				161	105	58
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period				$ 5	7	5	5
Provision charged (credited) to income					27	5	2
Charge offs					(11)	(5)	(4)
Recoveries					1	2	2
Net (charge offs) recoveries					(10)	(3)	(2)
Allowance for credit losses – end of period	7				27	7	$ 5
As Previously Reported [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period					637
Allowance for credit losses – end of period	637					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
Allowance for credit losses – end of period	153					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
Allowance for credit losses – end of period	239					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
Allowance for credit losses – end of period	106					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
Allowance for credit losses – end of period	9					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
Allowance for credit losses – end of period	12					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
Allowance for credit losses – end of period	6					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
Allowance for credit losses – end of period	105					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
Allowance for credit losses – end of period	7					7
Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period					(170)
Allowance for credit losses – end of period	(170)					(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					(112)
Allowance for credit losses – end of period	(112)					(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					(60)
Allowance for credit losses – end of period	(60)					(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					51
Allowance for credit losses – end of period	51					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					(5)
Allowance for credit losses – end of period	(5)					(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					(86)
Allowance for credit losses – end of period	(86)					(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					7
Allowance for credit losses – end of period	7					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					32
Allowance for credit losses – end of period	32					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					3
Allowance for credit losses – end of period	3					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
Allowance for credit losses – end of period	467					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
Allowance for credit losses – end of period	41					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
Allowance for credit losses – end of period	179					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
Allowance for credit losses – end of period	157					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
Allowance for credit losses – end of period	4					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)
Allowance for credit losses – end of period	(74)					(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
Allowance for credit losses – end of period	13					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
Allowance for credit losses – end of period	137					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
Allowance for credit losses – end of period	$ 10					$ 10