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Commitments and Contingencies - Contingencies and Litigation (Details) $ in Millions | 3 Months Ended
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Dec. 31, 2017 USD ($)
Commitments and Contingencies [Abstract] |
Excess segregated funds returned | $ 15.5

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- Definition Commitments and Contingencies [Abstract] + References No definition available. + Details
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Namespace Prefix: | intl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Excess segregated funds returned + References No definition available. + Details
Name: | intl_ExcessSegregatedFundsReturned
Namespace Prefix: | intl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration