Related Party Transactions - Summary of Related Party Transactions (Details) - USD ($) $ in Millions		Sep. 30, 2021		Dec. 31, 2020		Sep. 30, 2020
Assets:
Cash and due from banks		$ 1,018		$ 1,302
Interest bearing deposits with banks		62,140		14,353
Securities purchased under agreements to resell		7,480		35,746
Trading assets		21,985		27,284
Other		5,409		6,084
Total assets		201,150	[1]	196,434	[1]	$ 200,946
Liabilities:
Deposits		152,368		145,150		$ 150,328
Trading liabilities		2,670		5,397
Short-term borrowings		6,377		4,952
Long-term debt		18,105		19,979
Other		3,233		2,665
Total liabilities	[1]	182,913		178,143
Affiliated Entity [Member]
Assets:
Cash and due from banks		313		516
Interest bearing deposits with banks		2		187
Securities purchased under agreements to resell	[2]	873		3,941
Trading assets		190		314
Loans		2,857		1,100
Other	[3]	289		319
Total assets		4,524		6,377
Liabilities:
Deposits		15,624		15,163
Trading liabilities	[4]	223		2,375
Short-term borrowings		390		270
Long-term debt		5,528		2,878
Other	[3]	203		268
Total liabilities		$ 21,968		$ 20,954

[1] Assets

[2] Reflects purchases of securities under which other HSBC affiliates have agreed to repurchase.

[3] Other assets and other liabilities primarily consist of derivative balances associated with hedging activities and other miscellaneous account receivables and payables.

[4] The decrease in trading liabilities at September 30, 2021 primarily reflects a decrease in borrowing of gold inventory from HSBC Bank plc to support client activity levels.