Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Interest income		$ 1,074	$ 1,015
Interest expense		(522)	(418)
Net interest income	[1]	552	597
Trading revenue		163	70
Servicing and other fees with HSBC affiliates:
Total affiliate income		99	114
Gain (loss) on instruments designated at fair value and related derivatives		30	34
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(411)	(384)
Occupancy expense, net		43	41
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		22	24
Interest expense		(78)	(66)
Net interest income		(56)	(42)
Trading revenue		1,158	85
Servicing and other fees with HSBC affiliates:
Total affiliate income		99	114
Gain (loss) on instruments designated at fair value and related derivatives		(438)	360
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(411)	(384)
Occupancy expense, net		12	13
Stock based compensation expense with HSBC	[2]	(5)	(7)
HSBC Bank Plc [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		41	41
HSBC Finance Corporation [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		1	34
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		31	17
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(31)	(29)
Other HSBC affiliates [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		26	22
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(82)	(62)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(298)	(293)
Reclassification/Out of Period Adjustments [Member] | Affiliated Entity [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income		$ 10	32
Support services from HSBC affiliates:
Total support services from HSBC affiliates			45
Occupancy expense, net			$ (13)

[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] During the fourth quarter of 2017, we changed our presentation for certain cost reimbursements that were previously netted as an offset to affiliate expense and began presenting these reimbursements gross in affiliate income for consistency in presentation across similar transactions. Separately, we also concluded that rental revenue we receive from our affiliates for rent on certain office space would be better presented as a reduction to occupancy expense as opposed to a reduction to affiliates expense. As a result, we have reclassified prior period amounts in order to conform to the current year presentation, which increased servicing and other fees from HSBC affiliates $32 million, increased support services from HSBC affiliates $45 million and decreased occupancy expense, net $13 million during the three months ended March 31, 2017. Reported net income (loss) for the three months ended March 31, 2017 remained unaffected.