Fair Value Option - Summary of Fair Value and Unpaid Principal Balance (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Student Loans Held for Investment [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, designated under fair value option	$ 30	$ 32
Loans held for investment, Unpaid Principal Balance	32	34
Fair value over (under) unpaid principal balance	(2)	(2)
Loans Held For Sale [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Commercial loans held for sale, Fair Value	83	36
Commercial loans held for sale, Unpaid Principal Balance	83	36
Fair value over (under) unpaid principal balance	0	0
Long-term debt [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	192	289
Fixed rate long-term debt, Fair Value	933	1,030
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	1,524	1,363
Hybrid instruments, Unpaid Principal Balance	7,413	8,332
Hybrid instruments, structured notes, Fair Value	8,937	9,695
Structured Deposits [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value over (under) unpaid principal balance	317	311
Hybrid instruments, structured deposits, Fair Value	3,765	4,155
Hybrid instruments, Unpaid Principal Balance	$ 3,448	$ 3,844

Fair Value, Option, Loans Held as Assets

No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Fair Value Over (Under) Unpaid Principal Balance

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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.

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No definition available.

+ Details

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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

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No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Unpaid Principal Balance, Loans Receivable

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No definition available.

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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.

+ Details

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- Definition

Unpaid Principal Balance On Hybrid Instruments at Fair Value Option

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No definition available.

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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.

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No definition available.

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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.

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No definition available.

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- 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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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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Balance Type:	credit
Period Type:	instant

- Details

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- Details

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