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Commitments and Guarantees - Narrative (Details) (USD $)
In Billions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Loss Contingencies [Line Items]    
Unfunded commitments to extend credit $ 23.88stt_UnfundedCommitmentsToExtendCredit $ 24.43stt_UnfundedCommitmentsToExtendCredit
Unfunded commitments to extend credit, short term 78.00%stt_UnfundedCommitmentsToExtendCreditShortTerm  
Unfunded Commitment to Extend Credit, Term 1 year  
Cash collateral provided for securities lending 19.85stt_CashCollateralProvidedForSecuritiesLending 15.94stt_CashCollateralProvidedForSecuritiesLending
Accrued expenses and other liabilities    
Loss Contingencies [Line Items]    
Cash collateral received in connection to securities finance activities $ 6.96stt_SecuritiesFinanceCollateralObligationtoReturnCash
/ us-gaap_BalanceSheetLocationAxis
= stt_AccruedExpensesAndOtherLiabilitiesMember		 $ 6.48stt_SecuritiesFinanceCollateralObligationtoReturnCash
/ us-gaap_BalanceSheetLocationAxis
= stt_AccruedExpensesAndOtherLiabilitiesMember