Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Interest income		$ 1,188	$ 1,047	$ 3,409	$ 3,104
Interest expense		(629)	(487)	(1,743)	(1,365)
Net interest income	[1]	559	560	1,666	1,739
Trading revenue (expense)		154	81	509	222
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		320	299	954	928
Gain (loss) on instruments designated at fair value and related derivatives		29	7	63	40
Support services from HSBC affiliates:
Total support services from HSBC affiliates		(399)	(378)	(1,215)	(1,161)
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income		26	16	72	51
Interest expense		(96)	(72)	(264)	(208)
Net interest income		(70)	(56)	(192)	(157)
Trading revenue (expense)		(504)	0	27	(806)
Servicing and other fees from HSBC affiliates: [Abstract]
Gain (loss) on instruments designated at fair value and related derivatives	[2]	241	347	(62)	871
Support services from HSBC affiliates:
Total support services from HSBC affiliates	[2]	(399)	(378)	(1,215)	(1,161)
Rental income from HSBC affiliates, net	[2],[3]	10	13	36	41
Stock based compensation expense with HSBC	[4]	(6)	(8)	(21)	(26)
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates	[2]	(22)	(30)	(78)	(90)
Other HSBC affiliates [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates	[2]	(76)	(71)	(228)	(199)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates	[2]	(301)	(277)	(909)	(872)
Reclassification/Out of Period Adjustments [Member] | Affiliated Entity [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates			38		127
Rental income from HSBC affiliates, net			13		41
Servicing and other fees from HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues		103	77	289	272
Servicing and other fees from HSBC affiliates [Member] | Affiliated Entity [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	[2]	103	77	289	272
Servicing and other fees from HSBC affiliates [Member] | HSBC Bank Plc [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	[2]	48	35	132	110
Servicing and other fees from HSBC affiliates [Member] | HSBC Finance Corporation (HSBC Finance) [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	[2]	2	3	4	40
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	[2]	31	18	92	53
Servicing and other fees from HSBC affiliates [Member] | Other HSBC affiliates [Member]
Servicing and other fees from HSBC affiliates: [Abstract]
Other revenues	[2]	$ 22	21	61	69
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			$ 25	$ 10	$ 86

[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 $25 million and $86 million, respectively, increased support services from HSBC affiliates by $38 million and $127 million, respectively, and increased rental income from HSBC affiliates, net by $13 million and $41 million, respectively, during the three and nine months ended September 30, 2017. Reported net income for the three and nine months ended September 30, 2017 remained unaffected.

[3] We receive rental revenue 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.

[4] 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."