Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Income (Expense):
Interest income	$ 1,904	$ 1,946	$ 5,692	$ 5,756
Net interest income	525	439	1,574	1,277
Trading revenue (expense)	186	208	570	717
Servicing and other fees from HSBC affiliates:
Gain on instruments designated at fair value and related derivatives	(3)	(3)	(7)	(33)
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(432)	(428)	(1,292)	(1,246)
Related Party
Income (Expense):
Interest income	76	66	223	181
Interest expense	(201)	(140)	(557)	(423)
Net interest income	(125)	(74)	(334)	(242)
Trading revenue (expense)	274	(2,194)	(2,259)	(5,275)
Servicing and other fees from HSBC affiliates:
Gain on instruments designated at fair value and related derivatives	381	218	645	316
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(432)	(428)	(1,292)	(1,246)
Rental income from HSBC affiliates, net	12	11	35	39
Stock based compensation expense	(7)	(7)	(23)	(22)
HSBC Technology & Services (USA) Inc. ("HTSU")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(249)	(238)	(725)	(707)
HSBC Markets (USA) Inc. ("HMUS")
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(43)	(66)	(162)	(183)
Other HSBC affiliates
Support services from HSBC affiliates:
Total support services from HSBC affiliates	(140)	(124)	(405)	(356)
Servicing and other fees from HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	99	99	299	280
Servicing and other fees from HSBC affiliates | Related Party
Servicing and other fees from HSBC affiliates:
Other revenues	99	99	299	280
Servicing and other fees from HSBC affiliates | HSBC Bank plc
Servicing and other fees from HSBC affiliates:
Other revenues	57	50	181	147
Servicing and other fees from HSBC affiliates | HSBC Markets (USA) Inc. ("HMUS")
Servicing and other fees from HSBC affiliates:
Other revenues	23	24	65	74
Servicing and other fees from HSBC affiliates | Other HSBC affiliates
Servicing and other fees from HSBC affiliates:
Other revenues	$ 19	$ 25	$ 53	$ 59