Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 878	$ 831
Current	[1]	71,590	67,722
Total loans		72,468	68,553
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		80	125
Current	[1]	51,833	48,086
Total loans		51,913	48,211
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		11	8
Current	[1]	11,409	11,493
Total loans		11,420	11,501
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		37	95
Current	[1]	17,308	13,384
Total loans	[2]	17,345	13,479
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		0	0
Current	[1]	18,668	17,915
Total loans	[3]	18,668	17,915
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		32	22
Current	[1]	4,448	5,294
Total loans		4,480	5,316
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		798	706
Current	[1]	19,757	19,636
Total loans		20,555	20,342
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		702	614
Current	[1]	17,579	17,187
Total loans		18,281	17,801
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		37	34
Current	[1]	755	819
Total loans		792	853
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		47	48
Current	[1]	1,128	1,357
Total loans		1,175	1,405
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		12	10
Current	[1]	295	273
Total loans	[4]	307	283
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		489	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		63	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		11	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		20	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		32	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		426	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		387	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		13	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		20	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		389	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		17	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		17	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		0	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		372	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		315	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		24	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		27	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		$ 6	$ 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 $1,197 million at June 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 $35 million at June 30, 2020. See Note 10, "Fair Value Option," for further details.