Loans - Delinquency Ratio for Consumer Loan - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Residential Mortgage Loan Delinquency Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell	$ 240	$ 254
Residential Mortgage Loan Delinquency Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell, Held-for-Sale	1	1
Consumer Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Mortgage Loans in Process of Foreclosure, Amount	$ 140	$ 125

Residential mortgage loan delinquency carried at lower of amortized cost or fair value less cost to sell.

No definition available.

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

- Definition

Residential Mortgage Loan Delinquency Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell, Held-for-Sale

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL49126937-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
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Balance Type:
Period Type: