Allowance for Credit Losses - ECL Estimates (Details) - scenario	Sep. 30, 2021	Jun. 30, 2021
Credit Loss [Abstract]
Number of forward-looking economic scenarios	3
Number of updated forward-looking economic scenarios	4	4

Number of Updated Forward-looking Economic Scenarios

No definition available.

Name:	hsbcusa_NumberOfUpdatedForwardLookingEconomicScenarios
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant