Regulatory Capital - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Parenthetical) (Detail) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Consolidated assets under tier one common capital	$ 50

Well Capitalized Tier One Common Ratio For Entities With At Least Fifty Billion Assets

No definition available.

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