Related Party Transactions - Summary of Related Party Transactions (Details) - USD ($) $ in Millions		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Assets:
Cash and due from banks		$ 1,302		$ 1,744
Interest bearing deposits with banks		14,353		2,038
Securities purchased under agreements to resell		35,746		17,838
Trading assets		27,284		28,452
Loans		62,088		68,553
Other		6,084		6,723
Total assets		196,434	[1]	175,375	[1]	$ 172,448
Liabilities:
Deposits		145,150		119,693		110,955
Trading liabilities		5,397		3,235
Short-term borrowings		4,952		3,659		$ 4,180
Long-term debt		19,979		26,697
Other		2,665		3,835
Total liabilities	[1]	178,143		157,119
Affiliated Entity [Member]
Assets:
Cash and due from banks		516		850
Interest bearing deposits with banks		187		40
Securities purchased under agreements to resell	[2]	3,941		4,600
Trading assets		314		79
Loans		1,100		2,343
Other	[3]	319		456
Total assets		6,377		8,368
Liabilities:
Deposits		15,163		9,000
Trading liabilities	[4]	2,375		293
Short-term borrowings		270		1,166
Long-term debt		2,878		7,848
Other	[3]	268		931
Total liabilities		$ 20,954		$ 19,238

[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 increase in trading liabilities at December 31, 2020 primarily reflects an increase in borrowing of gold inventory from HSBC Bank plc to support client activity levels