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Loans - Trouble Debt Restructuring Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans $ 1,225hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment $ 1,275hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
Interest income recognized on TDR loans 11hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized 15hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
Commercial Loans [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 223hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember		 326hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
Interest income recognized on TDR loans 2hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember		 6hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
Commercial Loans [Member] | Construction and Other Real Estate [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 182hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 264hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Interest income recognized on TDR loans 1hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 6hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Commercial Loans [Member] | Business and Corporate Banking [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 34hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_BusinessandCorporateBankingMember		 19hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_BusinessandCorporateBankingMember
Interest income recognized on TDR loans 1hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_BusinessandCorporateBankingMember		 0hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_BusinessandCorporateBankingMember
Commercial Loans [Member] | Global Banking [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 7hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_GlobalBankingMember		 25hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_GlobalBankingMember
Commercial Loans [Member] | Other Commercial [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 0hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_OtherCommercialMember		 18hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= hsbcusa_OtherCommercialMember
Consumer Loans [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 1,002hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember		 949hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
Interest income recognized on TDR loans 9hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember		 9hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
Consumer Loans [Member] | Residential Mortgage [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 976hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 922hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Interest income recognized on TDR loans 9hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 9hsbcusa_TroubleDebtRestructuringInterestIncomeRecognized
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
Consumer Loans [Member] | Home Equity Line of Credit [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans 20hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember		 19hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
Consumer Loans [Member] | Credit Cards [Member]    
Financing Receivable, Modifications [Line Items]    
Average balance of TDR loans $ 6hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember		 $ 8hsbcusa_TroubleDebtRestructuringAverageRecordedInvestment
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember