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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	$ 931	$ 583	$ 637	$ 541
Provision charged (credited) to income	337	46	849	104
Charge offs	(84)	(23)	(140)	(44)
Recoveries	8	7	16	12
Net (charge offs) recoveries	(76)	(16)	(124)	(32)
Allowance for credit losses – end of period	1,192	613	1,192	613
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	92	119		116
Provision charged (credited) to income	28	30	79	33
Charge offs	0	0	0	0
Recoveries	0	0	0	0
Net (charge offs) recoveries	0	0	0	0
Allowance for credit losses – end of period	120	149	120	149
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	382	239		219
Provision charged (credited) to income	106	13	319	35
Charge offs	(55)	(2)	(67)	(4)
Recoveries	1	1	3	1
Net (charge offs) recoveries	(54)	(1)	(64)	(3)
Allowance for credit losses – end of period	434	251	434	251
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	248	122		108
Provision charged (credited) to income	176	(9)	286	5
Charge offs	0	(3)	(19)	(3)
Recoveries	0	0	0	0
Net (charge offs) recoveries	0	(3)	(19)	(3)
Allowance for credit losses – end of period	424	110	424	110
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	5	15		15
Provision charged (credited) to income	(1)	(6)	0	(6)
Charge offs	0	0	0	0
Recoveries	1	0	1	0
Net (charge offs) recoveries	1	0	1	0
Allowance for credit losses – end of period	5	9	5	9
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	(41)	10		13
Provision charged (credited) to income	20	1	50	0
Charge offs	(1)	(3)	0	(7)
Recoveries	3	3	5	5
Net (charge offs) recoveries	2	0	5	(2)
Allowance for credit losses – end of period	(19)	11	(19)	11
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	16	8		7
Provision charged (credited) to income	(1)	(1)	2	0
Charge offs	0	(1)	(2)	(2)
Recoveries	1	1	3	2
Net (charge offs) recoveries	1	0	1	0
Allowance for credit losses – end of period	16	7	16	7
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	212	63		58
Provision charged (credited) to income	3	19	98	35
Charge offs	(26)	(12)	(48)	(25)
Recoveries	1	1	3	3
Net (charge offs) recoveries	(25)	(11)	(45)	(22)
Allowance for credit losses – end of period	190	71	190	71
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses – beginning of period	17	7		5
Provision charged (credited) to income	6	(1)	15	2
Charge offs	(2)	(2)	(4)	(3)
Recoveries	1	1	1	1
Net (charge offs) recoveries	(1)	(1)	(3)	(2)
Allowance for credit losses – end of period	$ 22	$ 5	22	$ 5
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