CONSOLIDATED STATEMENT OF CASH FLOWS (UNAUDITED) (Parenthetical) $ in Millions	Sep. 30, 2021 USD ($)
Retail Branches Sold [Member]
Cash and other assets	$ 55

Disposal Group, Including Discontinued Operation, Cash and Other Assets

No definition available.

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