Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Related Party Transaction [Line Items]
Interest income									$ 3,572		$ 4,990		$ 4,655
Interest expense									(1,398)		(2,878)		(2,403)
Net interest income	$ 584	$ 548	$ 538	$ 504	$ 507	$ 503	$ 540	$ 562	2,174		2,112		2,252
Trading revenue (expense)									273		591		625
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues									1,091		1,184		1,236
Gain (loss) on instruments designated at fair value and related derivatives									34		(13)		19
Support services from HSBC affiliates:
Total support services from HSBC affiliates									(1,551)		(1,638)		(1,603)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income									55		168		107
Interest expense									(354)		(538)		(367)
Net interest income									(299)		(370)		(260)
Trading revenue (expense)									(93)		(2,669)		1,370
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives									699		1,986		(1,052)
Support services from HSBC affiliates:
Total support services from HSBC affiliates									(1,551)		(1,638)		(1,603)
Rental income from HSBC affiliates, net									44	[1]	55	[1]	48
Stock based compensation expense									(22)	[2]	(25)	[2]	(25)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates									(89)		(100)		(105)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates									(396)		(385)		(300)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates									(1,066)		(1,153)		(1,198)
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues									347		354		356
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues									347		354		356
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues									186		171		176
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									107		111		112
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues									$ 54		$ 72		$ 68

[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."