Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 992	$ 831
Current	[1]	65,417	67,722
Total loans		66,409	68,553
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		195	125
Current	[1]	45,588	48,086
Total loans		45,783	48,211
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		47	8
Current	[1]	11,236	11,493
Total loans		11,283	11,501
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		71	95
Current	[1]	15,093	13,384
Total loans	[2]	15,164	13,479
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		7	0
Current	[1]	15,498	17,915
Total loans	[3]	15,505	17,915
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		70	22
Current	[1]	3,761	5,294
Total loans		3,831	5,316
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		797	706
Current	[1]	19,829	19,636
Total loans		20,626	20,342
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		713	614
Current	[1]	17,726	17,187
Total loans		18,439	17,801
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		38	34
Current	[1]	727	819
Total loans		765	853
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		35	48
Current	[1]	1,072	1,357
Total loans		1,107	1,405
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		11	10
Current	[1]	304	273
Total loans	[4]	315	283
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		534	470
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		138	89
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		47	7
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		21	60
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0	0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		70	22
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		396	381
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		364	342
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		10	10
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		16	24
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		6	5
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		458	361
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		57	36
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0	1
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		50	35
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		7	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		401	325
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		349	272
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		28	24
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		19	24
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 5	$ 5

[1] Loans less than 30 days past due are presented as current.

[2] Includes loans funded under the Paycheck Protection Program ("PPP") which totaled $1,186 million at September 30, 2020. PPP loans are fully guaranteed by the Small Business Administration, if certain conditions are met.

[3] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.

[4] Includes certain student loans that we have elected to designate under the fair value option and are therefore carried at fair value, which totaled $34 million at September 30, 2020. See Note 10, "Fair Value Option," for further details.