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Description of Business and Significant Accounting Policies - Accounts, Customers and Other Facts (Details) | 12 Months Ended
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Sep. 30, 2018
Accounting Policies [Abstract] |
Number of different types of foreign currencies | 140
Number of accounts for customers company-wide | 20,000

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- 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

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- 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

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration