Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Related Party Transaction [Line Items]
Interest income		$ 1,147	$ 1,042	$ 2,221		$ 2,057
Interest expense		(592)	(460)	(1,114)		(878)
Net interest income	[1]	555	582	1,107		1,179
Trading revenue (expense)		192	71	355		141
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		323	300	634		629
Gain (loss) on instruments designated at fair value and related derivatives		4	(1)	34		33
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(405)	(399)	(816)		(783)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		24	11	46		35
Interest expense		(90)	(70)	(168)		(136)
Net interest income		(66)	(59)	(122)		(101)
Trading revenue (expense)		(627)	(891)	531		(806)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives		135	164	(303)		524
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(405)	(399)	(816)		(783)
Rental income from HSBC affiliates, net		14	15	26		28
Stock based compensation expense with HSBC		(10)	(11)	(15)	[2]	(18)	[2]
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(25)	(31)	(56)		(60)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(70)	(66)	(152)		(128)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(310)	(302)	(608)		(595)
Reclassification/Out of Period Adjustments [Member] | Affiliated Entity [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates			44			89
Rental income from HSBC affiliates, net			15			28
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		87	81	186		195
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		87	81	186		195
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank Plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		43	34	84		75
Servicing and other fees from HSBC affiliates [Member] | HSBC Finance Corporation (HSBC Finance) [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		1	3	2		37
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		30	18	61		35
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		$ 13	26	39		48
Servicing and other fees from HSBC affiliates [Member] | Reclassification/Out of Period Adjustments [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues			$ 29	$ 10		$ 61

[1] Net interest income of each segment represents the difference between actual interest earned on assets and interest incurred 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 affiliate 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 by $29 million and $61 million, respectively, increased support services from HSBC affiliates by $44 million and $89 million, respectively, and increased rental income from HSBC affiliates, net by $15 million and $28 million, respectively, during the three and six months ended June 30, 2017. Reported net income for the three and six months ended June 30, 2017 remained unaffected.