Loans - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Receivables [Abstract]
Net deferred origination costs (fees)	$ 46	$ 79
Net unamortized premium (discount)	$ 9	$ 3

Loans and Leases Receivable, Unamortized Premium (Discount), Net

No definition available.

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