Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Income (Expense):
Interest income	$ 1,946	$ 1,792	$ 5,756	$ 5,119
Net interest income	439	372	1,277	1,289
Trading revenue (expense)	208	277	717	730
Servicing and other fees from HSBC affiliates:
Gain (loss) on instruments designated at fair value and related derivatives	(3)	(9)	(33)	(29)
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(428)	(426)	(1,246)	(1,235)
Related Party
Income (Expense):
Interest income	66	56	181	170
Interest expense	(140)	(132)	(423)	(375)
Net interest income	(74)	(76)	(242)	(205)
Trading revenue (expense)	(2,194)	1,043	(5,275)	(372)
Servicing and other fees from HSBC affiliates:
Gain (loss) on instruments designated at fair value and related derivatives	218	(233)	316	467
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(428)	(426)	(1,246)	(1,235)
Rental income from HSBC affiliates, net	11	8	39	24
Stock based compensation expense	(7)	(4)	(22)	(14)
HSBC Markets (USA) Inc. ("HMUS")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(66)	(59)	(183)	(167)
Other HSBC affiliates
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(124)	(126)	(356)	(362)
HSBC Technology & Services (USA) Inc. ("HTSU")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(238)	(241)	(707)	(706)
Servicing and other fees from HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	99	87	280	238
Servicing and other fees from HSBC affiliates | Related Party
Servicing and other fees from HSBC affiliates:
Other revenues	99	87	280	238
Servicing and other fees from HSBC affiliates | HSBC Bank plc
Servicing and other fees from HSBC affiliates:
Other revenues	50	47	147	125
Servicing and other fees from HSBC affiliates | HSBC Markets (USA) Inc. ("HMUS")
Servicing and other fees from HSBC affiliates:
Other revenues	24	21	74	61
Servicing and other fees from HSBC affiliates | Other HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	$ 25	$ 19	$ 59	$ 52