Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Income (Expense):
Interest income	$ 1,696	$ 1,880
Net interest income	552	526
Trading revenue (expense)	197	193
Servicing and other fees from HSBC affiliates:
Gain (loss) on instruments designated at fair value and related derivatives	(5)	(3)
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(460)	(404)
Related Party
Income (Expense):
Interest income	80	71
Interest expense	(181)	(162)
Net interest income	(101)	(91)
Trading revenue (expense)	1,328	(484)
Servicing and other fees from HSBC affiliates:
Gain (loss) on instruments designated at fair value and related derivatives	(204)	(187)
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(460)	(404)
Rental income from HSBC affiliates, net	13	12
HSBC Technology & Services (USA) Inc. ("HTSU")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(253)	(226)
HSBC Markets (USA) Inc. ("HMUS")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(54)	(55)
Other HSBC affiliates
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(153)	(123)
Servicing and other fees from HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	102	102
Servicing and other fees from HSBC affiliates | Related Party
Servicing and other fees from HSBC affiliates:
Other revenues	102	102
Servicing and other fees from HSBC affiliates | HSBC Bank plc
Servicing and other fees from HSBC affiliates:
Other revenues	58	67
Servicing and other fees from HSBC affiliates | HSBC Markets (USA) Inc. ("HMUS")
Servicing and other fees from HSBC affiliates:
Other revenues	22	18
Servicing and other fees from HSBC affiliates | Other HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	$ 22	$ 17