Related Party Transactions - Additional Information (Detail)			1 Months Ended		3 Months Ended			7 Months Ended	9 Months Ended		12 Months Ended
		Oct. 17, 2013 USD ($)	Oct. 31, 2016 USD ($)	Nov. 30, 2013 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2010 shares	Jul. 31, 2016 USD ($) borrowing	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 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					$ 4,824,000,000				$ 4,824,000,000		$ 5,925,000,000
Long-term debt					4,494,000,000				4,494,000,000		9,510,000,000
Litigation judgment claims		$ 2,500,000,000
Notional values of derivative contracts					1,765,000,000				1,765,000,000		8,933,000,000
Fair value of our agreements with affiliate counterparties required the affiliate to provide collateral					189,000,000				189,000,000		491,000,000
HSBC USA, Inc. [Member]
Related Party Transaction [Line Items]
Due to affiliates					3,012,000,000				3,012,000,000		3,012,000,000
HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Borrowing capacity of credit facility					5,000,000,000				$ 5,000,000,000
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				$ 812,000,000		813,000,000
HSBC North America [Member]
Related Party Transaction [Line Items]
Due to affiliates					1,000,000,000				1,000,000,000		1,600,000,000
Annual fee to guarantor										$ 4,000,000
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	$ 1,000,000,000
Debt instrument, term									364 days
Amount of credit facility outstanding					0				$ 0		0
HSBC affiliates [Member]
Related Party Transaction [Line Items]
Due to affiliates	[1]				4,824,000,000				4,824,000,000		5,925,000,000
Interest bearing deposits with banks						$ 1,503,000,000				1,503,000,000
Interest income on deposits					0				0
HSBC Securities (USA) Inc. [Member]
Related Party Transaction [Line Items]
Receivable transaction assistance fees (less than $1 million for the 3 and 9 months ended September 30, 2015)					1,000,000	1,000,000			5,000,000	1,000,000
HSBC Bank USA [Member]
Related Party Transaction [Line Items]
Notional values of derivative contracts					1,800,000,000				1,800,000,000		8,900,000,000
Receivables services to affiliates					586,000,000				586,000,000		696,000,000
Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Number of shares of stock issued (shares) | shares							1,000
Dividend paid on preferred stock					21,000,000	$ 21,000,000			64,000,000	$ 64,000,000
Credit Agreement Maturing September 2017 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Due to affiliates					512,000,000				512,000,000
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				1,500,000,000
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				1,000,000,000
Terminated September 2016 [Member] | HSBC USA, Inc. [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Borrowing capacity of credit facility											$ 1,000,000,000
Debt instrument, term											364 days
Amount of credit facility outstanding					0				0		$ 0
Subordinated Debt [Member] | Public Subordinated Debt Issue Maturing 2021 [Member]
Related Party Transaction [Line Items]
Long-term debt					$ 3,000,000,000				$ 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
Subsequent Event [Member] | HSBC North America [Member] | Revolving Credit Facility [Member]
Related Party Transaction [Line Items]
Loan agreement repaid with proceeds from receivable sales			$ 1,000,000,000

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