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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 | Mar. 31, 2017 | Sep. 30, 2016
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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] |  |
Financial Instruments Sold, Not yet Purchased, at Fair Value | $ 1,081.2 | $ 839.4

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- Definition Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] + References No definition available. + Details
Name: | intl_FinancialInstrumentsWithOffBalanceSheetRiskAndConcentrationsOfCreditRiskAbstract
Namespace Prefix: | intl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value of obligations incurred from short sales. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -Section 35 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35711043&loc=d3e42429-110968 + Details
Name: | us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant