Loans - Summary of Past Due Status of Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2021		Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans		$ 55,526			$ 62,088
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		38,914			41,599
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total loans		8,250			10,464
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[1]	12,788			13,479
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[2]	11,440			13,519
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total loans		6,436			4,137
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[3]	16,612	[4]		20,489
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total loans		16,093			18,377
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total loans		369			727
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total loans		0			1,066
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[5]	150			319
Financial Asset, Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total loans		722			1,141
Financial Asset, Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		225			307
Financial Asset, Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total loans		61			78
Financial Asset, Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans		125			145
Financial Asset, Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans		23			60
Financial Asset, Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total loans		16			24
Financial Asset, Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		497	[4]		834
Financial Asset, Past Due [Member] | Consumer Loans [Member] | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member] | Loans Held for Sale [Member]
Financing Receivable, Past Due [Line Items]
Total loans				$ 115
Financial Asset, Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total loans		467			746
Financial Asset, Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total loans		24			33
Financial Asset, Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total loans		0			42
Financial Asset, Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total loans		6			13
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total loans		407			704
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		196			228
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total loans		56			78
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans		124			126
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans		0			0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total loans		16			24
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		211	[4]		476
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total loans		203			435
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total loans		5			11
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total loans		0			23
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total loans		3			7
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total loans		315			437
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		29			79
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 loans		5			0
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 loans		1			19
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 loans		23			60
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 loans		0			0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans		286	[4]		358
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 loans		264			311
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 loans		19			22
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 loans		0			19
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 loans		3			6
Financial Asset, Not Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	54,804			60,947
Financial Asset, Not Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	38,689			41,292
Financial Asset, Not Past Due [Member] | Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	8,189			10,386
Financial Asset, Not Past Due [Member] | Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	12,663			13,334
Financial Asset, Not Past Due [Member] | Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	11,417			13,459
Financial Asset, Not Past Due [Member] | Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	6,420			4,113
Financial Asset, Not Past Due [Member] | Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	16,115	[4]		19,655
Financial Asset, Not Past Due [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	15,626			17,631
Financial Asset, Not Past Due [Member] | Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	345			694
Financial Asset, Not Past Due [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	0			1,024
Financial Asset, Not Past Due [Member] | Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Total loans	[6]	$ 144			$ 306

[1] Includes loans funded under the Paycheck Protection Program ("PPP") which totaled $437 million and $1,043 million at September 30, 2021 and December 31, 2020, respectively. PPP loans are fully guaranteed by the Small Business Administration, if certain conditions are met.

[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] The decrease in loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021. See Note 8, "Loans Held for Sale," for additional information.

[4] The decrease in past due loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021, including $115 million of consumer loans which were past due 30 days or more.

[5] Includes certain student loans that we have elected to designate under the fair value option and are therefore carried at fair value, which totaled $27 million and $32 million at September 30, 2021 and December 31, 2020, respectively. See Note 11, "Fair Value Option," for further details.

[6] Loans less than 30 days past due are presented as current.(2)The decrease in past due loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021, including $115 million of consumer loans which were past due 30 days or more.