Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Related Party Transaction [Line Items]
Interest income										$ 4,655		$ 4,123		$ 3,910
Interest expense										(2,403)		(1,850)		(1,426)
Net interest income		$ 586	$ 559	$ 555	$ 552	$ 534	$ 560	$ 582	$ 597	2,252		2,273		2,484
Trading revenue (expense)										625		354		227
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										1,236		1,233		1,235
Gain (loss) on instruments designated at fair value and related derivatives										19		43		(71)
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,603)		(1,549)		(1,495)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income										107		65		122
Interest expense										(367)		(267)		(166)
Net interest income										(260)		(202)		(44)
Trading revenue (expense)										1,370		(615)		(1,297)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives										(1,052)		1,108		467
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,603)		(1,549)		(1,495)
Rental income from HSBC affiliates, net										48	[1]	54	[1]	60
Stock based compensation expense with HSBC	[2]									(25)		(31)		(39)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(105)		(121)		(214)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(300)		(263)		(254)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,198)		(1,165)		(1,027)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										356		348		289
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										356		348		289
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank Plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										176		154		156
Servicing and other fees from HSBC affiliates [Member] | HSBC Finance Corporation (HSBC Finance) [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										3		44		40
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										112		71		34
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										65		$ 79		$ 59
Servicing and other fees from HSBC affiliates [Member] | Out of Period Adjustments [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										$ 10

[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 20, "Pension and Other Postretirement Benefits."