Guarantee Arrangements and Pledged Assets - Outstanding Repurchase Demands Received From GSEs and Other Third Parties (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Commitments And Guarantee Obligations [Line Items]
Outstanding repurchase demands on loans sourced from our legacy broker channel	$ 143	$ 87

Outstanding repurchase demands on loans sourced from our legacy broker channel.

No definition available.

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