Related Party Transactions - Summary of Related Party Transactions (Details) - USD ($) $ in Millions		Mar. 31, 2021		Dec. 31, 2020		Mar. 31, 2020
Assets:
Cash and due from banks		$ 1,447		$ 1,302
Interest bearing deposits with banks		53,607		14,353
Securities purchased under agreements to resell		3,760		35,746
Trading assets		25,822		27,284
Loans		61,526		62,088
Other		6,109		6,084
Total assets		194,856	[1]	196,434	[1]	$ 205,077
Liabilities:
Deposits		148,582		145,150		$ 136,723
Trading liabilities		2,319		5,397
Short-term borrowings		5,243		4,952
Long-term debt		18,179		19,979
Other		2,559		2,665
Total liabilities	[1]	176,882		178,143
Affiliated Entity [Member]
Assets:
Cash and due from banks		607		516
Interest bearing deposits with banks		104		187
Securities purchased under agreements to resell	[2]	724		3,941
Trading assets		98		314
Loans		1,652		1,100
Other	[3]	402		319
Total assets		3,587		6,377
Liabilities:
Deposits		15,943		15,163
Trading liabilities	[4]	44		2,375
Short-term borrowings		660		270
Long-term debt		3,528		2,878
Other	[3]	269		268
Total liabilities		$ 20,444		$ 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 March 31, 2021 primarily reflects a decrease in borrowing of gold inventory from HSBC Bank plc to support client activity levels.