Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans	$ 73,875	$ 68,978	$ 72,563
TDR loans carried at lower of amortized cost or fair value less cost to sell	672	615	655
Affiliated Entity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans	3,274	2,274	6,750
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans		19,636	19,528
Consumer Loans [Member] \| Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans	17,181	$ 17,383	$ 17,273
Provision for credit losses and charge-offs related to lower of amortized cost or fair value adjustment attributable to credit facotrs for loans transferred to held for sale	11
Additional charge-off for the existing allowance for credit losses on collateral dependent loans transferred to held for sale	$ 22

Allowance for Loan and Lease Losses, Write-offs, Collateral Dependent Loans Transferred to Held for Sale

No definition available.

Name:	hsbcusa_AllowanceforLoanandLeaseLossesWriteoffsCollateralDependentLoansTransferredtoHeldforSale
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Provision for Loan and Lease Losses, Recorded Lower of Amortized Cost or Fair Value Adjustment Attributable to Credit Factors for Loans Transferred to Held for Sale

+ References

No definition available.

+ Details

Name:	hsbcusa_ProvisionforLoanandLeaseLossesRecordedLowerofAmortizedCostorFairValueAdjustmentAttributabletoCreditFactorsforLoansTransferredtoHeldforSale
Namespace Prefix:	hsbcusa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Residential Mortgage Loan TDR Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell

+ References

No definition available.

+ Details

Name:	hsbcusa_ResidentialMortgageLoanTDRCarriedAtLowerOfAmortizedCostOrFairValueLessCostToSell
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.

+ References

No definition available.

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
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: