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Fair Value Measurements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair value of assets $ 3,498us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,690us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,504us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,150us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Loss severity, trust preferred securities 1.8hsbcusa_DebtObligationCollateralizedLossSeverityRate      
Fair Value Liability, Transfers Out of Level 3 121us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 211us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3    
Fair Value Liability, Transfers Into Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 (68)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3    
Average deviation of implied volatility, foreign currency pair 7.00%hsbcusa_DerivativeInstrumentForeignCurrencyTradeMarketImpliedVolatilityDeviation      
Average deviation of implied volatility, at-the-money equity option 6.00%hsbcusa_DerivativeInstrumentEquityTradeMarketImpliedVolatility      
Revaluation period 45 days      
Weighted Average [Member] | Residential Mortgage [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair Value Inputs, Loss Severity 68.00%us-gaap_FairValueInputsLossSeverity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember		      
Weighted Average [Member] | Impaired Loans [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair Value Inputs, Loss Severity 32.00%us-gaap_FairValueInputsLossSeverity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_ImpairedLoansMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember		      
Derivatives [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair value of assets 463us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeAssetsMember		 694us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeAssetsMember		    
Derivative Liabilities [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair value of assets 208us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeLiabilitiesMember		 392us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeLiabilitiesMember		    
Credit contracts [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair value of assets 192us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] 181us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] 210us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] 191us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1]
Government debt securities issued by foreign entities [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair value of assets   117us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   121us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Deposits [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Fair Value Liability, Transfers Out of Level 3 101us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 140us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 [2]    
Fair Value Liability, Transfers Into Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 (68)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 [2]    
Other liabilities [Member] | Standby Letters of Credit [Member]        
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]        
Deferred fees on standby letters of credit $ 52us-gaap_DeferredRevenueAndCredits
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_StandbyLettersOfCreditMember		   $ 50us-gaap_DeferredRevenueAndCredits
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_StandbyLettersOfCreditMember		  
[1] Level 3 net derivatives included derivative assets of $463 million and derivative liabilities of $208 million as of March 31, 2015 and derivative assets of $694 million and derivative liabilities of $392 million as of March 31, 2014.
[2] See Note 10, "Fair Value Option," for additional information.