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Fair Value Measurements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jun. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]          
Fair value of assets $ 3,504us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,504us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,150us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 3,690us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue  
Loss severity, trust preferred securities   1.8hsbcusa_DebtObligationCollateralizedLossSeverityRate      
Fair Value Assets, Transfers Out of Level 3   99us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 76us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3    
Fair Value Liability, Transfers Out of Level 3   880us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 742us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3    
Fair Value Liability, Transfers Into Level 3   (208)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 (343)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3    
Transfers Into Level 3   12us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 (45)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3    
Average deviation of implied volatility, foreign currency pair   3.00%hsbcusa_DerivativeInstrumentForeignCurrencyTradeMarketImpliedVolatilityDeviation      
Average deviation of implied volatility, at-the-money equity option   6.00%hsbcusa_DerivativeInstrumentEquityTradeMarketImpliedVolatility      
Reduction in trading revenue due to change in derivative valuation adjustment probability of default calculations       47hsbcusa_IncreaseDecreaseinTradingRevenueDuetoChangeinDerivativeValuationAdjustmentProbabilityofDefaultCalculations  
Reduction in trading revenue due to funding fair value adjustment applied to uncollateralized derivatives 9hsbcusa_IncreaseDecreaseinTradingRevenueDuetoFundingFairValueAdjustmentAppliedtoUncollateralizedDerivatives        
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 500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeAssetsMember		 500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeAssetsMember		 805us-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 248us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeLiabilitiesMember		 248us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= hsbcusa_DerivativeLiabilitiesMember		 517us-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 210us-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] 571us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1]  
Fair Value Assets, Transfers Out of Level 3   (114)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1]    
Transfers Into Level 3   0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] (45)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
/ 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 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 121us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 311us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		  
Fair Value Assets, Transfers Out of Level 3   119us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   119us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Transfers Into Level 3   0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
/ 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   678us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 516us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 [2]    
Fair Value Liability, Transfers Into Level 3   (201)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3
/ us-gaap_EligibleItemOrGroupForFairValueOptionAxis
= us-gaap_DepositsMember		 (305)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 $ 50us-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		 $ 46us-gaap_DeferredRevenueAndCredits
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_StandbyLettersOfCreditMember		    
[1] Level 3 net derivatives included derivative assets of $500 million and derivative liabilities of $248 million as of December 31, 2014 and derivative assets of $805 million and derivative liabilities of $517 million as of December 31, 2013.
[2] See Note 16, "Fair Value Option," for additional information.