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Loans and Leases - Narrative (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
loan
Mar. 31, 2014
Dec. 31, 2014
loan
Financing Receivable, Recorded Investment [Line Items]      
Aggregate short-duration advances $ 4,080,000,000us-gaap_BankOverdrafts   $ 3,540,000,000us-gaap_BankOverdrafts
Loans and leases, allowance for losses 41,000,000us-gaap_LoansAndLeasesReceivableAllowance   38,000,000us-gaap_LoansAndLeasesReceivableAllowance
Loans modified in troubled debt restructurings 0us-gaap_FinancingReceivableModificationsNumberOfContracts2   0us-gaap_FinancingReceivableModificationsNumberOfContracts2
Provision for loan losses 4,000,000us-gaap_ProvisionForLoanAndLeaseLosses 2,000,000us-gaap_ProvisionForLoanAndLeaseLosses  
Institutional      
Financing Receivable, Recorded Investment [Line Items]      
Loans more than 90 days past due 0stt_FinancingReceivableRecordedInvestmentEqualToGreaterThan90Day
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_InstitutionalMember
  0stt_FinancingReceivableRecordedInvestmentEqualToGreaterThan90Day
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_InstitutionalMember
Institutional | Senior Secured Bank Loans      
Financing Receivable, Recorded Investment [Line Items]      
Commercial and financial 2,530,000,000us-gaap_LoansReceivableCommercialFinancialInstitutions
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= stt_SeniorSecuredBankLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_InstitutionalMember
  2,070,000,000us-gaap_LoansReceivableCommercialFinancialInstitutions
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= stt_SeniorSecuredBankLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_InstitutionalMember
Commercial Real Estate      
Financing Receivable, Recorded Investment [Line Items]      
Loans more than 90 days past due 0stt_FinancingReceivableRecordedInvestmentEqualToGreaterThan90Day
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
  0stt_FinancingReceivableRecordedInvestmentEqualToGreaterThan90Day
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
Commercial and Financial      
Financing Receivable, Recorded Investment [Line Items]      
Loans and leases, allowance for losses 11,000,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_CommercialAndFinancialMember
   
Commercial and Financial | Senior Secured Bank Loans      
Financing Receivable, Recorded Investment [Line Items]      
Loans and leases, allowance for losses $ 30,000,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= stt_SeniorSecuredBankLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_CommercialAndFinancialMember
  $ 26,000,000us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= stt_SeniorSecuredBankLoansMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= stt_CommercialAndFinancialMember