Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Interest income		$ 1,047	$ 966	$ 3,104	$ 2,926
Interest expense		(487)	(362)	(1,365)	(1,030)
Net interest income	[1]	560	604	1,739	1,896
Trading revenue (expense)		81	120	222	185
Servicing and other fees with HSBC affiliates:
Total affiliate income		52	50	186	155
Gain on instruments designated at fair value and related derivatives		7	(88)	40	90
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(340)	(354)	(1,034)	(1,029)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		16	30	51	90
Interest expense		(72)	(38)	(208)	(102)
Net interest income		(56)	(8)	(157)	(12)
Trading revenue (expense)		0	(661)	(806)	138
Servicing and other fees with HSBC affiliates:
Total affiliate income		52	50	186	155
Gain on instruments designated at fair value and related derivatives		347	193	871	228
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(340)	(354)	(1,034)	(1,029)
Stock based compensation expense with HSBC	[2]	(8)	(7)	(26)	(24)
HSBC Bank Plc [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		22	22	67	70
HSBC Finance Corporation [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		3	8	40	29
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		6	5	16	15
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(14)	(42)	(38)	(150)
Other HSBC affiliates [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		21	15	63	41
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(58)	(60)	(150)	(145)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		$ (268)	$ (252)	$ (846)	$ (734)

[1] Net interest income of each segment represents the difference between actual interest earned on assets and interest paid on liabilities of the segment adjusted for a funding charge or credit. Segments are charged a cost to fund assets (e.g. customer loans) and receive a funding credit for funds provided (e.g. customer deposits) based on equivalent market rates. The objective of these charges/credits is to transfer interest rate risk from the segments to one centralized unit in Balance Sheet Management and more appropriately reflect the profitability of the segments.

[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.