Fair Value Option - Summary of Fair Value and Unpaid Principal Balance (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Student Loans Held for Investment [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, designated under fair value option	$ 32
Loans held for investment, Unpaid Principal Balance	34
Fair value over (under) unpaid principal balance	(2)
Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Commercial loans held for sale, Fair Value	36	$ 178
Commercial loans held for sale, Unpaid Principal Balance	36	189
Fair value over (under) unpaid principal balance	0	(11)
Securities sold under repurchase agreements [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance		0
Securities sold under repurchase agreements, Fair Value	0	373
Securities sold under repurchase agreements, Unpaid Principal Balance		373
Long-term debt [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	289	218
Fixed rate long-term debt, Fair Value	1,030	959
Fixed rate long-term debt, Unpaid Principal Balance	741	741
Long-term debt [Member] \| Structured Notes [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	1,363	1,201
Hybrid instruments, Unpaid Principal Balance	8,332	8,187
Hybrid instruments, structured notes, Fair Value	9,695	9,388
Structured Deposits [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	311	(282)
Hybrid instruments, structured deposits, Fair Value	4,155	7,209
Hybrid instruments, Unpaid Principal Balance	$ 3,844	$ 7,491

Fair Value, Option, Loans Held as Assets

No definition available.

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

- Definition

Fair Value Over (Under) Unpaid Principal Balance

+ References

No definition available.

+ Details

Name:	hsbcusa_FairValueOverUnderUnpaidPrincipalBalance
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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, Loans Receivable

+ References

No definition available.

+ Details

Name:	hsbcusa_UnpaidPrincipalBalanceLoansReceivable
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 Principal Balance, Repurchase Agreements

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

Name:	us-gaap_DepositsFairValueDisclosure
Namespace Prefix:	us-gaap_
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

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.

+ References

No definition available.

+ Details

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.

+ 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_FinancialInstrumentAxis=us-gaap_StudentLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=hsbcusa_StructuredNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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