Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Related Party Transaction [Line Items]
Interest income										$ 4,990		$ 4,655		$ 4,123
Interest expense										(2,878)		(2,403)		(1,850)
Net interest income		$ 507	$ 503	$ 540	$ 562	$ 586	$ 559	$ 555	$ 552	2,112		2,252		2,273
Trading revenue (expense)										591		625		354
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										1,184		1,236		1,233
Gain (loss) on instruments designated at fair value and related derivatives										(13)		19		43
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,638)		(1,603)		(1,549)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income										168		107		65
Interest expense										(538)		(367)		(267)
Net interest income										(370)		(260)		(202)
Trading revenue (expense)										(2,669)		1,370		(615)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives										1,986		(1,052)		1,108
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,638)		(1,603)		(1,549)
Rental income from HSBC affiliates, net										55	[1]	48	[1]	54
Stock based compensation expense with HSBC	[2]									(25)		(25)		(31)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(100)		(105)		(121)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(385)		(300)		(263)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,153)		(1,198)		(1,165)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										354		356		348
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										354		356		348
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										171		176		154
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										111		112		71
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues										$ 72		$ 68		$ 123

[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 21, "Pension and Other Postretirement Benefits."