Loans - Contractual Maturity of Loans Outstanding by Repricing Characteristic (Details) $ in Millions	Dec. 31, 2020 USD ($)
Receivables [Abstract]
Receivables at predetermined interest rates due after one year but within five years	$ 1,802
Receivables at floating or adjustable rates after one but within five years	19,348
Total after one but within five years	21,150
Receivables at predetermined interest rates after five years	5,233
Receivables at floating or adjustable rates after five years	11,545
Total after five years	$ 16,778

Financing Receivable, before Allowance for Credit Loss, Due after One but within Five Years

No definition available.

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

Financing Receivable, before Allowance for Credit Loss, Due after Year Five

No definition available.

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

Financing Receivable with Fixed Rates of Interest, due after Five Years

No definition available.

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

Financing Receivable with Fixed Rates of Interest, due after One but within Five Years

No definition available.

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

Financing Receivable with Variable Rates of Interest, due after Five Years

No definition available.

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

Financing Receivable with Variable Rates of Interest, due after One but within Five Years

No definition available.

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

No definition available.

Name:	us-gaap_ReceivablesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration