Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		$ 873	$ 831
Current	[1]	80,855	67,722
Total loans		81,728	68,553
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		150	125
Current	[1]	61,096	48,086
Total loans		61,246	48,211
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		6	8
Current	[1]	11,363	11,493
Total loans		11,369	11,501
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		112	95
Current	[1]	19,508	13,384
Total loans		19,620	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]	24,141	17,915
Total loans	[2]	24,141	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]	6,084	5,294
Total loans		6,116	5,316
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		723	706
Current	[1]	19,759	19,636
Total loans		20,482	20,342
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		625	614
Current	[1]	17,409	17,187
Total loans		18,034	17,801
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		33	34
Current	[1]	781	819
Total loans		814	853
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		53	48
Current	[1]	1,247	1,357
Total loans		1,300	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]	322	273
Total loans	[3]	334	283
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Days Past Due, 30 Days or More		548	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		140	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		4	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		104	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		408	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		366	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		25	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		7	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		325	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		10	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		2	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		8	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		315	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		259	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		23	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		28	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] 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.

[3] Includes certain student loans that we have elected to designate under the fair value option and are therefore carried at fair value, which totaled $37 million at March 31, 2020. See Note 10, "Fair Value Option," for further details.