Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Effect of basis adjustments for active fair value hedges on the carrying value of debt	$ 132	$ 104
Amortization of basis adjustments related to terminated and/or re-designated fair value hedge relationships of our debt	5	6	$ 6
Accumulated unamortized basis adjustment for active fair value hedges which increased (decreased) the carrying amount of our available for sale securities	279	258
Amortization of basis adjustments related to terminated and/or re-designated cash flow hedge relationships	17	17	$ 11
Next twelve months, reclassification of remaining losses to earnings resulting from cash flow hedges	19
Aggregate fair value of all derivative instruments with credit-risk related contingent features	1,063	586
Collateral already posted aggregate fair value	758	581
Long-term Debt [Member]
Derivative [Line Items]
Accumulated unamortized basis adjustment for fair value hedges which increased (decreased) the carrying amount of our debt	$ 7	$ 12

Amortization of Basis Adjustments Related To Terminated and or Redesignated Cash Flow Hedge Relationships

No definition available.

Name:	hsbcusa_AmortizationofBasisAdjustmentsRelatedToTerminatedAndOrRedesignatedCashFlowRelationships
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amortization of basis adjustments related to terminated and/or redesignated fair value hedge relationships of Company debt.

+ References

No definition available.

+ Details

Name:	hsbcusa_AmortizationofBasisAdjustmentsRelatedToTerminatedandorRedesignatedFairValueHedgeRelationshipsofCompanyDebt
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Basis Adjustments for Active Fair Value Hedges on Debt

+ References

No definition available.

+ Details

Name:	hsbcusa_BasisAdjustmentsforActiveFairValueHedgesonDebt
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Increase (Decrease) In Accumulated Unamortized Adjustments For Fair Value Hedges As Available For Sale Securities

+ References

No definition available.

+ Details

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

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

+ Details

Name:	us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

+ Details

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

- Definition

Amount as of the balance sheet date of the unamortized adjustment to the carrying value of an interest-bearing hedged item made under an effective fair value hedge that is amortized upon discontinuation of the fair value hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 35
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82917030&loc=d3e75592-113984

+ Details

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

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: