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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	$ 1,192	$ 613	$ 637	$ 541
Provision charged (credited) to income	(75)	94	774	198
Charge offs	(44)	(37)	(184)	(81)
Recoveries	11	6	27	18
Net (charge offs) recoveries	(33)	(31)	(157)	(63)
Allowance for credit losses – end of period	1,084	676	1,084	676
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	120	149		116
Provision charged (credited) to income	43	27	122	60
Charge offs	(12)	0	(12)	0
Recoveries	0	0	0	0
Net (charge offs) recoveries	(12)	0	(12)	0
Allowance for credit losses – end of period	151	176	151	176
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	434	251		219
Provision charged (credited) to income	(36)	32	283	67
Charge offs	(2)	(18)	(69)	(22)
Recoveries	5	2	8	3
Net (charge offs) recoveries	3	(16)	(61)	(19)
Allowance for credit losses – end of period	401	267	401	267
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	424	110		108
Provision charged (credited) to income	(103)	1	183	6
Charge offs	(1)	0	(20)	(3)
Recoveries	0	0	0	0
Net (charge offs) recoveries	(1)	0	(20)	(3)
Allowance for credit losses – end of period	320	111	320	111
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	5	9		15
Provision charged (credited) to income	3	0	3	(6)
Charge offs	0	0	0	0
Recoveries	0	0	1	0
Net (charge offs) recoveries	0	0	1	0
Allowance for credit losses – end of period	8	9	8	9
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	(19)	11		13
Provision charged (credited) to income	12	(1)	62	(1)
Charge offs	(1)	(1)	(1)	(8)
Recoveries	3	2	8	7
Net (charge offs) recoveries	2	1	7	(1)
Allowance for credit losses – end of period	(5)	11	(5)	11
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	16	7		7
Provision charged (credited) to income	(1)	(1)	1	(1)
Charge offs	(1)	(1)	(3)	(3)
Recoveries	1	1	4	3
Net (charge offs) recoveries	0	0	1	0
Allowance for credit losses – end of period	15	6	15	6
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	190	71		58
Provision charged (credited) to income	(2)	34	96	69
Charge offs	(23)	(17)	(71)	(42)
Recoveries	2	1	5	4
Net (charge offs) recoveries	(21)	(16)	(66)	(38)
Allowance for credit losses – end of period	167	89	167	89
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	22	5		5
Provision charged (credited) to income	9	2	24	4
Charge offs	(4)	0	(8)	(3)
Recoveries	0	0	1	1
Net (charge offs) recoveries	(4)	0	(7)	(2)
Allowance for credit losses – end of period	$ 27	$ 7	27	$ 7
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			(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)
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)
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
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)
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)
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
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
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
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