Fair Value Measurements - Carrying Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Federal funds sold and securities purchased under agreements to resell	$ 35,746	$ 17,838
Securities held-to-maturity	8,981	13,293
Loans, net of allowance for credit losses	61,073	67,916	$ 68,437
Loans held for sale	337	289
Deposits	145,150	119,693	$ 110,955
Long-term debt	19,979	26,697
Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	129	212
Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	208	77
Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	208	77
Level 1 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	1,302
Short-term financial assets		1,744
Federal funds sold and securities purchased under agreements to resell	0	0
Securities held-to-maturity, net of allowance for credit losses	0
Securities held-to-maturity		0
Short-term financial liabilities	0	0
Deposits	0	0
Long-term debt	0	0
Level 1 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	0	0
Level 1 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Level 1 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	0	0
Level 2 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	14,353
Short-term financial assets		2,038
Federal funds sold and securities purchased under agreements to resell	35,746	17,838
Securities held-to-maturity, net of allowance for credit losses	9,369
Securities held-to-maturity		13,431
Short-term financial liabilities	4,952	3,286
Deposits	141,001	112,425
Long-term debt	9,720	16,696
Level 2 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	93	34
Level 2 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Level 2 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	217	77
Level 3 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	12
Short-term financial assets		18
Federal funds sold and securities purchased under agreements to resell	0	0
Securities held-to-maturity, net of allowance for credit losses	0
Securities held-to-maturity		0
Short-term financial liabilities	13	18
Deposits	0	0
Long-term debt	0	0
Level 3 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	41,417	49,252
Loans held for sale	0	0
Level 3 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	19,865	19,889
Level 3 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	0	1
Carrying Value [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	15,667
Short-term financial assets		3,800
Federal funds sold and securities purchased under agreements to resell	35,746	17,838
Securities held-to-maturity, net of allowance for credit losses	8,981
Securities held-to-maturity		13,293
Short-term financial liabilities	4,965	3,304
Deposits	140,995	112,484
Long-term debt	9,254	16,350
Carrying Value [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	40,785	47,704
Loans held for sale	93	34
Carrying Value [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	20,256	20,212
Carrying Value [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	208	77
Estimate of fair value [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	15,667
Short-term financial assets		3,800
Federal funds sold and securities purchased under agreements to resell	35,746	17,838
Securities held-to-maturity, net of allowance for credit losses	9,369
Securities held-to-maturity		13,431
Short-term financial liabilities	4,965	3,304
Deposits	141,001	112,425
Long-term debt	9,720	16,696
Estimate of fair value [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	41,417	49,252
Loans held for sale	93	34
Estimate of fair value [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	19,865	19,889
Estimate of fair value [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	$ 217	$ 78

Debt Securities, Held-to-maturity, Net of Allowance for Credit Losses

No definition available.

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

- Definition

Short Term Financial Assets

+ References

No definition available.

+ Details

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

- Definition

Short Term Financial Assets, Net of Allowance for Credit Losses

+ References

No definition available.

+ Details

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

- Definition

Short-term Financial Liabilities

+ References

No definition available.

+ Details

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

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Includes: (1) the amount outstanding of funds lent to other depository institutions, securities brokers, or securities dealers in the form of Federal Funds sold; for example, immediately available funds lent under agreements or contracts that mature in one business day or roll over under a continuing contract, regardless of the nature of the transaction or the collateral involved, excluding overnight lending for commercial and industrial purposes. Also include Federal Funds sold under agreements to resell on a gross basis, excluding (1) sales of term Federal Funds, (2) due bills representing purchases of securities or other assets by the reporting bank that have not yet been delivered and similar instruments, (3) resale agreements that mature in more than one business day involving assets other than securities, and (4) yield maintenance dollar repurchase agreements (Federal Funds Sold) and (2) the dollar amount outstanding of funds lent in the form of security resale agreements regardless of maturity, if the agreement requires the bank to resell the identical security purchased or a security that meets the definition of substantially the same in the case of a dollar roll. Also include purchases of participations in pools of securities, regardless of maturity (Securities Purchased Under Agreements to Re-sell).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(2)(i)(A))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(3))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Amount, before valuation allowance, of financing receivable classified as held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=120238483&loc=d3e48678-111004

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: