Equity Subordination in Borrower's Capital Structure, Percent

No definition available.

Name:	hsbcusa_EquitySubordinationinBorrowersCapitalStructurePercent
Namespace Prefix:	hsbcusa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Financial Guarantee of Outstanding Debt by Borrowers' Parent, Percent

+ References

No definition available.

+ Details

Name:	hsbcusa_FinancialGuaranteeofOutstandingDebtbyBorrowersParentPercent
Namespace Prefix:	hsbcusa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Gain from Make-whole Payment from Investment Guarantor

+ References

No definition available.

+ Details

Name:	hsbcusa_GainfromMakewholePaymentfromInvestmentGuarantor
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Investment in Structured Note Vehicle

+ References

No definition available.

+ Details

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

- Definition

Number of Structured Note Vehicles Unwound

+ References

No definition available.

+ Details

Name:	hsbcusa_NumberofStructuredNoteVehiclesUnwound
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5728-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5747-111685

+ Details

Name:	us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=hsbcusa_StructuredNoteVehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=hsbcusa_TradingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=hsbcusa_LoanstoSpecialPurposeEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Variable Interest Entities - Additional Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) structured_note_vehicle	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Maximum exposure to loss		$ 3,541	$ 6,676
Number of structured note vehicles unwound \| structured_note_vehicle	1
Gain from make-whole payment from third-party investment guarantor	$ 11
Structured note vehicles [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Maximum exposure to loss		$ 3,013	$ 5,896
Trading Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investment in structured note vehicle	$ 1,081
Loans to Refinancing Special Purpose Entity [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity subordination in borrower's capital structure (percent)		15.00%
Financial guarantee of outstanding debt by borrowers' parent (percent)		25.00%