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, 2020	Mar. 31, 2019	Dec. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 637	$ 541
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			$ (170)
Provision charged (credited) to income	512	58
Charge offs	(56)	(21)
Recoveries	8	5
Net (charge offs) recoveries	(48)	(16)
Balance at end of period	931	583
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		116
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			(112)
Provision charged (credited) to income	51	3
Charge offs	0	0
Recoveries	0	0
Net (charge offs) recoveries	0	0
Balance at end of period	92	119
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		219
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			(60)
Provision charged (credited) to income	213	22
Charge offs	(12)	(2)
Recoveries	2	0
Net (charge offs) recoveries	(10)	(2)
Balance at end of period	382	239
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		108
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			51
Provision charged (credited) to income	110	14
Charge offs	(19)	0
Recoveries	0	0
Net (charge offs) recoveries	(19)	0
Balance at end of period	248	122
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		15
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			(5)
Provision charged (credited) to income	1	0
Charge offs	0	0
Recoveries	0	0
Net (charge offs) recoveries	0	0
Balance at end of period	5	15
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		13
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			(86)
Provision charged (credited) to income	30	(1)
Charge offs	1	(4)
Recoveries	2	2
Net (charge offs) recoveries	3	(2)
Balance at end of period	(41)	10
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		7
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			7
Provision charged (credited) to income	3	1
Charge offs	(2)	(1)
Recoveries	2	1
Net (charge offs) recoveries	0	0
Balance at end of period	16	8
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		58
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			32
Provision charged (credited) to income	95	16
Charge offs	(22)	(13)
Recoveries	2	2
Net (charge offs) recoveries	(20)	(11)
Balance at end of period	212	63
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		5
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			$ 3
Provision charged (credited) to income	9	3
Charge offs	(2)	(1)
Recoveries	0	0
Net (charge offs) recoveries	(2)	(1)
Balance at end of period	17	$ 7
As Previously Reported [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	637
As Previously Reported [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	153
As Previously Reported [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	239
As Previously Reported [Member] | Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	106
As Previously Reported [Member] | Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	9
As Previously Reported [Member] | Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	12
As Previously Reported [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	6
As Previously Reported [Member] | Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	105
As Previously Reported [Member] | Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	7
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at 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]
Balance at beginning of period	$ 10