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Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Sep. 30, 2014
Business Acquisition, Contingent Consideration [Line Items]          
Business Combination, Contingent Consideration, Liability $ 3.1us-gaap_BusinessCombinationContingentConsiderationLiability   $ 3.1us-gaap_BusinessCombinationContingentConsiderationLiability   $ 5.5us-gaap_BusinessCombinationContingentConsiderationLiability
Change in fair value of contingent consideration for acquisitions 0.1us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1 0.1us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1 1.7us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1 0.3us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1  
Forward Insight Commodities Acquisition [Member]          
Business Acquisition, Contingent Consideration [Line Items]          
Present Value of Estimated Total Purchase Price 0.4intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_ForwardInsightCommoditiesAcquisitionMember
  0.4intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_ForwardInsightCommoditiesAcquisitionMember
   
Tradewire Acquisition [Member] [Member]          
Business Acquisition, Contingent Consideration [Line Items]          
Business Combination, Contingent Consideration, Liability 1.6us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_TradewireAcquisitionMemberMember
  1.6us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_TradewireAcquisitionMemberMember
   
Present Value of Estimated Total Purchase Price 3.6intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_TradewireAcquisitionMemberMember
  3.6intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_TradewireAcquisitionMemberMember
   
Forward Insight Commodities [Member]          
Business Acquisition, Contingent Consideration [Line Items]          
Business Combination, Contingent Consideration, Liability 0.1us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_ForwardInsightCommoditiesMember
  0.1us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_ForwardInsightCommoditiesMember
   
G.X. Clarke (INTL FCStone Partners) [Member]          
Business Acquisition, Contingent Consideration [Line Items]          
Business Combination, Contingent Consideration, Liability 1.5us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
  1.5us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
   
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High 1.5us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
  1.5us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
   
Present Value of Estimated Total Purchase Price 28.9intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
  28.9intl_PresentValueofEstimatedTotalPurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= intl_G.X.ClarkeINTLFCStonePartnersMember
   
Contingent Consideration [Member]          
Business Acquisition, Contingent Consideration [Line Items]          
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) 0.1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember
0.1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember
0.2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember
0.3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember
 
Change in fair value of contingent consideration for acquisitions     $ 0.2us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember
$ 0.3us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember