Fair Value Option - Summary of Fair Value and Unpaid Principal Balance (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Loans Receivable [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Commercial loans held for sale, Fair Value	$ 175	$ 384
Commercial loans held for sale, Unpaid Principal Balance	181	390
Short-term Borrowings [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Repurchase agreements, Fair Value	2,064	0
Repurchase agreements, Unpaid Principal Balance	2,060
Long-term debt [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fixed rate long-term debt, Fair Value	2,014	2,179
Fixed rate long-term debt, Unpaid Principal Balance	1,750	1,750
Hybrid Instruments, Fair Value	6,620	6,612
Hybrid Instruments, Unpaid Principal Balance	7,046	6,275
Deposits [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Hybrid Instruments, Fair Value	6,485	7,346
Hybrid Instruments, Unpaid Principal Balance	$ 6,486	$ 7,176

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

No definition available.

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

- Definition

Fixed-rate debt accounted for under fair value option.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

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.

+ References

No definition available.

+ Details

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

+ References

No definition available.

+ Details

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

- Definition

Unpaid Principle Balance, Repurchase Agreements

+ References

No definition available.

+ Details

Name:	hsbcusa_UnpaidPrincipleBalanceRepurchaseAgreements
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_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.

+ References

No definition available.

+ Details

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

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_EligibleItemOrGroupForFairValueOptionAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EligibleItemOrGroupForFairValueOptionAxis=us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EligibleItemOrGroupForFairValueOptionAxis=us-gaap_BorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EligibleItemOrGroupForFairValueOptionAxis=us-gaap_DepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: