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Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
US Government Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Debt Securities, Amortized Cost Basis $ 967.8us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember		 $ 666.8us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Available-for-sale Securities, Gross Unrealized Gains 1.2us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Available-for-sale Securities, Gross Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember		 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Available-for-sale Securities, Debt Securities 969.0us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember		 666.8us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Deposits and Receivables from Exchange Clearing Organizations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain (Loss)   $ 0.1us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1
/ intl_DepositsAndReceivablesFromExchangeClearingOrganizationsAxis
= intl_DepositsAndReceivablesFromExchangeClearingOrganizationsMember