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Assets and Liabilities, at Fair Value - Contingent Consideration (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Payments to Acquire Businesses, Gross $ (3.4)us-gaap_PaymentsToAcquireBusinessesGross  
Change in fair value of contingent consideration for acquisitions 0.1us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1 0.2us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
Contingent Consideration [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in fair value of contingent consideration for acquisitions $ 0.1us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember		 $ 0.2us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
/ us-gaap_FairValueByLiabilityClassAxis
= intl_ContingentConsiderationMember