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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Obligations to Purchase Financial Instruments at a Future Date (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Sep. 30, 2014
Financial Instruments Sold, Not yet Purchased, at Fair Value $ 613.3us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue $ 264.0us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue
Financial instrument sold, not yet purchased [Member]    
Derivative, Fair Value, Net $ 82.0us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_BalanceSheetLocationAxis
= intl_FinancialInstrumentSoldNotYetPurchasedMember		 $ 84.4us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_BalanceSheetLocationAxis
= intl_FinancialInstrumentSoldNotYetPurchasedMember