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Intangible Assets Intangible Assets Acquired (Details) $ in Millions | 9 Months Ended
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Jun. 30, 2019 USD ($)
Intangible Assets Acquired [Abstract] |
Indefinite-lived Intangible Assets Acquired | $ 2.7

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- Definition Intangible Assets Acquired [Abstract] + References No definition available. + Details
Name: | intl_IntangibleAssetsAcquiredAbstract
Namespace Prefix: | intl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of increase in assets, excluding financial assets and goodwill, lacking physical substance with an indefinite life, from an acquisition. + References No definition available. + Details
Name: | us-gaap_IndefinitelivedIntangibleAssetsAcquired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration