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Acquisitions ICAP (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2018 USD ($)
Business Acquisition [Line Items] |
Finite-lived Intangible Assets Acquired | $ 1.4
ICAP [Member] [Member] |
Business Acquisition [Line Items] |
Finite-lived Intangible Assets Acquired | $ 6.0

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition. + References No definition available. + Details
Name: | us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=intl_ICAPMemberMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |