Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Interest income	$ 2,675	$ 3,572	$ 4,990
Interest expense	(592)	(1,398)	(2,878)
Net interest income	2,083	2,174	2,112
Trading revenue (expense)	24	273	591
Servicing and other fees from HSBC affiliates: [Abstract]
Gain on instruments designated at fair value and related derivatives	35	34	(13)
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(1,620)	(1,551)	(1,638)
Affiliated Entity
Related Party Transaction [Line Items]
Interest income	26	55	168
Interest expense	(237)	(354)	(538)
Net interest income	(211)	(299)	(370)
Trading revenue (expense)	(3,182)	(93)	(2,669)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain on instruments designated at fair value and related derivatives	1,128	699	1,986
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(1,620)	(1,551)	(1,638)
Rental income from HSBC affiliates, net	42	44	55
Stock based compensation expense	(15)	(22)	(25)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(118)	(89)	(100)
Other HSBC affiliates
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(441)	(396)	(385)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(1,061)	(1,066)	(1,153)
Servicing and other fees from HSBC affiliates
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	321	347	354
Servicing and other fees from HSBC affiliates | Affiliated Entity
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	321	347	354
Servicing and other fees from HSBC affiliates | HSBC Bank plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	176	186	171
Servicing and other fees from HSBC affiliates | HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	91	107	111
Servicing and other fees from HSBC affiliates | Other HSBC affiliates
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	$ 54	$ 54	$ 72