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Basis of Presentation and Consolidation and Recently Issued Accounting Standards (Details) $ in Millions | 3 Months Ended
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Dec. 31, 2018 USD ($)
Prior Period Reclassification Adjustment | $ 7.5
Number of different types of foreign currencies | 140
Number of accounts for customers company-wide | 20,000

X
- Definition Number of different types of foreign currencies the company trades + References No definition available. + Details
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Namespace Prefix: | intl_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of accounts for the customers the company provides services. + References No definition available. + Details
Name: | intl_Numberofaccountsforcustomerscompanywide
Namespace Prefix: | intl_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of a reclassification adjustment made to prior period financial statement amounts. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration