Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Interest income		$ 1,301	$ 1,147	$ 2,578	$ 2,221
Interest expense		(761)	(592)	(1,476)	(1,114)
Net interest income		540	555	1,102	1,107
Trading revenue (expense)		123	192	256	355
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		303	323	575	634
Gain (loss) on instruments designated at fair value and related derivatives		(17)	4	(26)	34
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(411)	(405)	(791)	(816)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		47	24	90	46
Interest expense		(152)	(90)	(276)	(168)
Net interest income		(105)	(66)	(186)	(122)
Trading revenue (expense)		(9)	(627)	(1,523)	531
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives		412	135	1,420	(303)
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(411)	(405)	(791)	(816)
Rental income from HSBC affiliates, net	[1]	12	14	24	26
Stock based compensation expense with HSBC	[2]	(8)	(10)	(14)	(15)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(23)	(25)	(50)	(56)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(90)	(70)	(166)	(152)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(298)	(310)	(575)	(608)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		88	87	169	186
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		88	87	169	186
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank Plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		46	43	81	84
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	30	57	61
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		$ 15	$ 14	$ 31	$ 41

[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.

[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. 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."