Related Party Transactions - Summary of Related Party Transactions (Income - Expense) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Related Party Transaction [Line Items]
Interest income										$ 4,123		$ 3,910		$ 3,501
Interest expense										(1,850)		(1,426)		(1,031)
Net interest income		$ 534	$ 560	$ 582	$ 597	$ 588	$ 604	$ 638	$ 654	2,273	[1]	2,484	[1]	2,470	[1]
Trading revenue (expense)										354		227		74
Servicing and other fees with HSBC affiliates:
Total affiliate income										348		289		276
Total other revenues		458	393	558	593	282	338	307	477	2,002	[2]	1,404	[2]	1,735	[2]
Gain on instruments designated at fair value and related derivatives										43		(71)		264
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,549)		(1,495)		(1,556)
Occupancy expense, net										(202)		(171)		(172)
Total operating expenses		$ (903)	$ (820)	$ (849)	$ (819)	$ (899)	$ (821)	$ (837)	$ (741)	(3,391)	[2]	(3,298)	[2],[3]	(3,284)	[2],[3]
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Interest income										65		122		117
Interest expense										(267)		(166)		(63)
Net interest income										(202)		(44)		54
Trading revenue (expense)										(615)		(1,297)		402
Servicing and other fees with HSBC affiliates:
Total affiliate income										348		289		276
Gain on instruments designated at fair value and related derivatives										1,108		467		(116)
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(1,549)		(1,495)		(1,556)
Occupancy expense, net										54		60		58
Stock based compensation expense with HSBC	[4]									(31)		(39)		(49)
HSBC Bank Plc [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income										154		156		145
HSBC Finance Corporation [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income										44		40		53
HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income										71		34		25
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(121)		(214)		(283)
Other HSBC affiliates [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income										79		59		53
Support services from HSBC affiliates:
Total support services from HSBC affiliates										(263)		(254)		(243)
HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates										$ (1,165)		(1,027)		(1,030)
As Previously Reported [Member]
Servicing and other fees with HSBC affiliates:
Total other revenues												1,332		1,672
Support services from HSBC affiliates:
Occupancy expense, net												(231)		(230)
Total operating expenses												(3,226)		(3,221)
As Previously Reported [Member] | Affiliated Entity [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income												217		213
Support services from HSBC affiliates:
Total support services from HSBC affiliates												(1,363)		(1,435)
As Previously Reported [Member] | HSBC Bank Plc [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income												95		82
As Previously Reported [Member] | HSBC Finance Corporation [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income												40		53
As Previously Reported [Member] | HSBC Markets (USA) Inc. (HMUS) and subsidiaries [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income												23		25
Support services from HSBC affiliates:
Total support services from HSBC affiliates												(176)		(256)
As Previously Reported [Member] | Other HSBC affiliates [Member]
Servicing and other fees with HSBC affiliates:
Total affiliate income												59		53
Support services from HSBC affiliates:
Total support services from HSBC affiliates												(193)		(180)
As Previously Reported [Member] | HSBC Technology and Services (USA) (HTSU) [Member]
Support services from HSBC affiliates:
Total support services from HSBC affiliates												$ (994)		$ (999)

[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] Expenses for the segments include fully apportioned corporate overhead expenses.

[3] In 2017, we changed our presentation for certain cost reimbursements that were previously netted as an offset to affiliate expense. We now present these reimbursements gross in affiliate income. As a result, we have reclassified prior year amounts in order to conform to the current year presentation, which increased both RBWM other operating income and RBWM operating expenses $11 million during the year ended December 31, 2016 and also increased both GB&M other operating income and GB&M operating expenses $61 million and $63 million during the years ended December 31, 2016 and 2015, respectively. See Note 21, "Related Party Transactions," in the accompanying consolidated financial statements for additional information.

[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 (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 20, "Pension and Other Postretirement Benefits.