Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Interest income		$ 1,273	$ 1,188	$ 3,851	$ 3,409
Interest expense		(770)	(629)	(2,246)	(1,743)
Net interest income		503	559	1,605	1,666
Trading revenue (expense)		178	154	434	509
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		298	320	873	954
Gain (loss) on instruments designated at fair value and related derivatives		(19)	29	(45)	63
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(394)	(399)	(1,185)	(1,215)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		44	26	134	72
Interest expense		(151)	(96)	(427)	(264)
Net interest income		(107)	(70)	(293)	(192)
Trading revenue (expense)		551	(504)	(972)	27
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives		98	241	1,518	(62)
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(394)	(399)	(1,185)	(1,215)
Rental income from HSBC affiliates, net	[1]	16	10	40	36
Stock based compensation expense with HSBC	[2]	(6)	(6)	(20)	(21)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(24)	(22)	(74)	(78)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(96)	(76)	(262)	(228)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(274)	(301)	(849)	(909)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		88	103	257	289
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		88	103	257	289
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank Plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		40	48	121	132
Servicing and other fees from HSBC affiliates [Member] | HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		27	31	84	92
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		$ 21	$ 24	$ 52	$ 65

[1] We receive rental income from our affiliates, and in some cases pay rental expense to our affiliates, for rent on certain office space. Net rental income from our affiliates is recorded as a component of occupancy expense, net in our consolidated statement of income (loss).

[2] Employees may participate in one or more stock compensation plans sponsored by HSBC. These expenses are included in salaries and employee benefits in our consolidated statement of income (loss). Certain employees are also eligible to participate in a defined benefit pension plan and other postretirement plans sponsored by HSBC North America which are discussed in Note 12, "Pension and Other Postretirement Benefits."