Intangible Assets - Residential Mortgage Loans Serviced for Others (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Goodwill And Intangible Assets Disclosure [Line Items]
Outstanding principal balances at period end	$ 33,584	$ 37,839
Custodial balances maintained and included in noninterest bearing deposits at period end	$ 838	$ 838

Custodial balances maintained and included in noninterest bearing deposits at period end.

No definition available.

Name:	hsbcusa_CustodialBalancesMaintainedAndIncludedInNoninterestBearingDepositsAtPeriodEnd
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant