Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Interest income		$ 804	$ 1,273	$ 2,789	$ 3,851
Interest expense		(256)	(770)	(1,199)	(2,246)
Net interest income		548	503	1,590	1,605
Trading revenue (expense)		49	178	255	434
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		269	298	814	873
Gain (loss) on instruments designated at fair value and related derivatives		15	(19)	15	(45)
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(391)	(394)	(1,162)	(1,185)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		9	44	47	134
Interest expense		(77)	(151)	(276)	(427)
Net interest income		(68)	(107)	(229)	(293)
Trading revenue (expense)		(1,755)	551	(2,372)	(972)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives		307	98	(203)	1,518
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(391)	(394)	(1,162)	(1,185)
Rental income from HSBC affiliates, net	[1]	13	16	31	40
Stock based compensation expense	[2]	(3)	(6)	(16)	(20)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(17)	(24)	(63)	(74)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(89)	(96)	(279)	(262)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(285)	(274)	(820)	(849)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		79	88	255	257
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		79	88	255	257
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		43	40	131	121
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	27	84	84
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		$ 9	$ 21	$ 40	$ 52

[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."