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Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities $ 32,663us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 29,654us-gaap_AvailableForSaleSecuritiesAmortizedCost
Unrealized Gains, Available-for-sale securities 571us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 621us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Unrealized Losses, Available-for-sale securities (138)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (135)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair Value, Available-for-sale securities 33,096us-gaap_AvailableForSaleSecurities 30,140us-gaap_AvailableForSaleSecurities
Amortized Cost, Held-to-Maturity Securities 14,015us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment 13,469us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Unrealized Gains, Held-to-maturity securities 338us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 229us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Unrealized Losses, Held-to-maturity securities (3)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss (3)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Held-to-maturity Securities, Fair Value 14,350us-gaap_HeldToMaturitySecuritiesFairValue 13,695us-gaap_HeldToMaturitySecuritiesFairValue
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 10,720us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 11,793us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Unrealized Gains, Available-for-sale securities 235us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 276us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Unrealized Losses, Available-for-sale securities (56)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 (58)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Fair Value, Available-for-sale securities 10,899us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 12,011us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 670us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1] 520us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]
Unrealized Gains, Available-for-sale securities 8us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1] 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]
Unrealized Losses, Available-for-sale securities (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1] (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]
Fair Value, Available-for-sale securities 677us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1] 524us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]
Amortized Cost, Held-to-Maturity Securities 5,280us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2] 4,868us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2]
Unrealized Gains, Held-to-maturity securities 177us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2] 120us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2]
Unrealized Losses, Held-to-maturity securities (1)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2] (1)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2]
Held-to-maturity Securities, Fair Value 5,456us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2] 4,987us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [2]
Collateralized mortgage obligations [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 35us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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= hsbcusa_CollateralizedMortgageObligationsIssuedbyU.S.GovernmentSponsoredEnterprisesMember		 [1]
Unrealized Gains, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_CollateralizedMortgageObligationsIssuedbyU.S.GovernmentSponsoredEnterprisesMember		 [1] 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Unrealized Losses, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fair Value, Available-for-sale securities 35us-gaap_AvailableForSaleSecurities
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= hsbcusa_CollateralizedMortgageObligationsIssuedbyU.S.GovernmentSponsoredEnterprisesMember		 [1] 35us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_CollateralizedMortgageObligationsIssuedbyU.S.GovernmentSponsoredEnterprisesMember		 [1]
Direct agency obligations [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 4,143us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1] 3,995us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1]
Unrealized Gains, Available-for-sale securities 206us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1] 217us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1]
Unrealized Losses, Available-for-sale securities (6)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1]
Fair Value, Available-for-sale securities 4,343us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1] 4,206us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_DirectAgencyObligationsUSGovernmentSponsoredEnterprisesMember		 [1]
Mortgage-backed securities, issued or guaranteed by US Government agency [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 11,457us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 7,985us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Unrealized Gains, Available-for-sale securities 93us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 101us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Unrealized Losses, Available-for-sale securities (26)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 (27)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Fair Value, Available-for-sale securities 11,524us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 8,059us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Amortized Cost, Held-to-Maturity Securities 3,482us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 3,700us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Unrealized Gains, Held-to-maturity securities 59us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 53us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
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Unrealized Losses, Held-to-maturity securities 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
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= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Held-to-maturity Securities, Fair Value 3,541us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember		 3,752us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_MortgagebackedSecuritiesIssuedorGuaranteedbyUSGovernmentAgencyMember
Collateralized mortgage obligations, issued or guaranteed by US Government agency [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 378us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Unrealized Gains, Available-for-sale securities 7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Unrealized Losses, Available-for-sale securities (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fair Value, Available-for-sale securities 384us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Amortized Cost, Held-to-Maturity Securities 5,222us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
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Unrealized Gains, Held-to-maturity securities 100us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Unrealized Losses, Held-to-maturity securities (2)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
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Held-to-maturity Securities, Fair Value 5,320us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_CollateralizedMortgageObligationsIssuedorGuaranteedbyUSGovernmentAgencyMember		 4,920us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_CollateralizedMortgageObligationsIssuedorGuaranteedbyUSGovernmentAgencyMember
Obligations of U.S. states and political subdivision [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 742us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 661us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Unrealized Gains, Available-for-sale securities 10us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 10us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Unrealized Losses, Available-for-sale securities (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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= us-gaap_USStatesAndPoliticalSubdivisionsMember		 (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fair Value, Available-for-sale securities 748us-gaap_AvailableForSaleSecurities
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Amortized Cost, Held-to-Maturity Securities 21us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
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= us-gaap_USStatesAndPoliticalSubdivisionsMember		 23us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
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= us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Gains, Held-to-maturity securities 1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Unrealized Losses, Held-to-maturity securities 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Held-to-maturity Securities, Fair Value 22us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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= us-gaap_USStatesAndPoliticalSubdivisionsMember
Residential Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Held-to-Maturity Securities 10us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
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Unrealized Gains, Held-to-maturity securities 1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
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Unrealized Losses, Held-to-maturity securities 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
Held-to-maturity Securities, Fair Value 11us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 29us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 43us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Unrealized Gains, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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= us-gaap_CommercialMortgageBackedSecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Unrealized Losses, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fair Value, Available-for-sale securities 29us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Home Equity Backed Securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 94us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember
Unrealized Gains, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember
Unrealized Losses, Available-for-sale securities (9)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember		 (8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember
Fair Value, Available-for-sale securities 85us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember		 89us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_HomeEquityBackedSecuritiesMember
Other securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 112us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 110us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Unrealized Gains, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Fair Value, Available-for-sale securities 94us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Foreign debt securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 4,121us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 [3]
Unrealized Gains, Available-for-sale securities 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 [3]
Unrealized Losses, Available-for-sale securities (17)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 [3]
Fair Value, Available-for-sale securities 4,113us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 [3] 3,915us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 [3]
Foreign debt securities [Member] | Foreign Government [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 1,053us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_GuaranteeObligationsByNatureAxis
= hsbcusa_ForeignGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember		 689us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_GuaranteeObligationsByNatureAxis
= hsbcusa_ForeignGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= hsbcusa_ForeignDebtSecuritiesMember
Equity securities [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 162us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Unrealized Losses, Available-for-sale securities 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fair Value, Available-for-sale securities 165us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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US Government sponsored enterprises [Member] | Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 672us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_GuaranteeObligationsByNatureAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 521us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Amortized Cost, Held-to-Maturity Securities 3,539us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_GuaranteeObligationsByNatureAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
US Government sponsored enterprises [Member] | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]    
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 33us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_GuaranteeObligationsByNatureAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Amortized Cost, Held-to-Maturity Securities $ 1,741us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 $ 1,683us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
[1] Includes securities at amortized cost of $672 million and $521 million issued or guaranteed by FNMA at March 31, 2015 and December 31, 2014, respectively, and $33 million and $34 million issued or guaranteed by the Federal Home Loan Mortgage Corporation ("FHLMC") at March 31, 2015 and December 31, 2014, respectively.
[2] Includes securities at amortized cost of $3,539 million and $3,185 million issued or guaranteed by FNMA at March 31, 2015 and December 31, 2014, respectively, and $1,741 million and $1,683 million issued and guaranteed by FHLMC at March 31, 2015 and December 31, 2014, respectively.
[3] At March 31, 2015 and December 31, 2014, foreign debt securities consisted of $1,053 million and $689 million, respectively, of securities fully backed by foreign governments. The remainder of foreign debt securities represents public sector entity, bank or corporate debt.