Loans - Components of Loans (Details) - USD ($) $ in Millions		Sep. 30, 2021		Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 55,526		$ 62,088
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		38,914		41,599
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		8,250		10,464
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[1]	12,788		13,479
Loans funded under the Payroll Protection Program		437		1,043
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[2]	11,440		13,519
Commercial Loans [Member] | Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		6,436		4,137
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[3]	16,612	[4]	20,489
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		16,093		18,377
Consumer Loans [Member] | Home Equity Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		369		727
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		0		1,066
Consumer Loans [Member] | Other consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[5]	150		319
Consumer Loans [Member] | Student Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, designated under fair value option		27		32
Affiliates [Member] | Commercial Loans [Member] | Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	[6]	2,857		1,100
Other [Member] | Commercial Loans [Member] | Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 3,579		$ 3,037

[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] See Note 15, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.