Loans - Delinquency Ratio for Consumer Loan (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Residential mortgage loan delinquency carried at lower of amortized cost or fair value less cost to sell	$ 970	$ 803

Residential mortgage loan delinquency carried at lower of amortized cost or fair value less cost to sell.

No definition available.

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