Fair Value Option - Summary of Fair Value and Unpaid Principal Balance (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Loans Receivable [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Commercial loans held for sale, Fair Value	$ 649	$ 151
Commercial loans held for sale, Unpaid Principal Balance	666	159
Fair value over (under) unpaid principal balance	(17)	(8)
Securities purchased under resale agreements [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	2
Securities purchased under resale agreements, Fair Value	769	0
Securities purchased under resale agreements Unpaid Principal Balance	767
Securities sold under repurchase agreements [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	5	6
Securities sold under repurchase agreements, Fair Value	2,675	1,976
Securities sold under repurchase agreements, Unpaid Principal Balance	2,670	1,970
Long-term debt [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	347	257
Fixed rate long-term debt, Fair Value	2,097	2,007
Fixed rate long-term debt, Unpaid Principal Balance	1,750	1,750
Deposits [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	111	(97)
Hybrid Instruments, Fair Value	7,472	6,919
Hybrid Instruments, Unpaid Principal Balance	7,361	7,016
Structured Notes [Member] \| Long-term debt [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	106	(159)
Long-term debt, Fair value	7,902	7,164
Hybrid Instruments, Unpaid Principal Balance	$ 7,796	$ 7,323

Fair Value Option, Fair Value Carrying Amount, Liabilities, Hybrid Instruments

No definition available.

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Namespace Prefix:	hsbcusa_
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Balance Type:	credit
Period Type:	instant

- Definition

Fair Value Over (Under) Unpaid Principal Balance

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No definition available.

+ Details

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

- Definition

Fixed-rate debt accounted for under fair value option.

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No definition available.

+ Details

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

- Definition

Securities Purchased under Agreements to Resell, Fair Value Option

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No definition available.

+ Details

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

- Definition

Unpaid Principal Balance Commercial Syndicated Loans Held For Sale Carried at Fair Value

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No definition available.

+ Details

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

- Definition

Unpaid principal balance on fixed-rate debt accounted for under FVO.

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No definition available.

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

- Definition

Unpaid Principal Balance On Hybrid Instruments at Fair Value Option

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No definition available.

+ Details

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

- Definition

Unpaid Principal Balance, Repurchase Agreements

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No definition available.

+ Details

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

- Definition

Unpaid Principal Balance, Resale Agreements

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No definition available.

+ Details

Name:	hsbcusa_UnpaidPrincipalBalanceResaleAgreements
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.

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No definition available.

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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No definition available.

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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No definition available.

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

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

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No definition available.

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

- Details

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- Details

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- Details

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- Details

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