Related Party Transactions - Additional Information (Detail)			1 Months Ended	3 Months Ended			12 Months Ended
		Oct. 17, 2013 USD ($)	Nov. 30, 2013 USD ($)	Mar. 31, 2016 USD ($) borrowing	Mar. 31, 2015 USD ($)	Dec. 31, 2010 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Oct. 31, 2015 USD ($)
Related Party Transaction [Line Items]
General term of securities purchased under an agreement to resell				120 days
Due to affiliates				$ 5,774,000,000			$ 5,925,000,000
Long-term debt				7,502,000,000			9,510,000,000
Litigation judgment claims		$ 2,500,000,000
Notional values of derivative contracts				7,074,000,000			8,933,000,000
Fair value of our agreements with affiliate counterparties required the affiliate to provide collateral				537,000,000			491,000,000
HSBC USA, Inc. [Member]
Related Party Transaction [Line Items]
Due to affiliates				$ 3,012,000,000			3,012,000,000
HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Debt instrument, term				364 days
Maximum maturity period for borrowings				5 years
HSBC Holdings plc [Member]
Related Party Transaction [Line Items]
Due to affiliates				$ 812,000,000			813,000,000
HSBC North America [Member]
Related Party Transaction [Line Items]
Due to affiliates				1,600,000,000			1,600,000,000
Annual fee to guarantor					$ 2,000,000		0
HSBC North America [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Borrowing capacity of credit facility				$ 1,000,000,000			1,000,000,000			$ 1,000,000,000
Debt instrument, term				364 days
HSBC affiliates [Member]
Related Party Transaction [Line Items]
Due to affiliates	[1]			$ 5,774,000,000			5,925,000,000
Interest bearing deposits with banks					2,001,000,000
Interest income on deposits				0
HSBC Bank USA [Member]
Related Party Transaction [Line Items]
Notional values of derivative contracts				7,100,000,000			8,900,000,000
Receivables services to affiliates				666,000,000			696,000,000
Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Number of shares of stock issued (shares) | shares						1,000
Value of issued stock						$ 1,000,000,000
Dividend paid on preferred stock				22,000,000	$ 22,000,000		86,000,000	$ 86,000,000	$ 86,000,000
Amended Credit Agreement [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Borrowing capacity of credit facility				5,000,000,000
Amended Credit Agreement Maturing September 2017 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Long-term debt due to affiliate				512,000,000
Amended Credit Agreement Maturing January 2018 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Due to affiliates				1,500,000,000
Amended Credit Agreement Maturing September 2018 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Due to affiliates				1,000,000,000
Maturing May 2017 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Borrowing capacity of credit facility				$ 1,000,000,000			1,000,000,000
Debt instrument, term				364 days
Amount of credit facility outstanding				$ 0			$ 0
Subordinated Debt [Member] | Public Subordinated Debt Issue Maturing 2021 [Member]
Related Party Transaction [Line Items]
Long-term debt				3,000,000,000
Delayed Draw Term Loan [Member] | HSBC North America [Member]
Related Party Transaction [Line Items]
Loan agreements with affiliates				$ 600,000,000
Number of draws allowed | borrowing				3
Amount per draw				$ 200,000,000
Delayed Draw Term Loan [Member] | Minimum [Member] | HSBC North America [Member]
Related Party Transaction [Line Items]
Maturity year of long term debt				2034
Delayed Draw Term Loan [Member] | Maximum [Member] | HSBC North America [Member]
Related Party Transaction [Line Items]
Maturity year of long term debt				2035
Securities Litigation [Member]
Related Party Transaction [Line Items]
Litigation judgment claims			$ 2,500,000,000

[1] Due to affiliates includes amounts owed to HSBC and its subsidiaries as a result of direct debt issuances and excludes preferred stock.