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Receivables from customers, net and notes receivable, net - Allowance for Customer Receivables (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Sep. 30, 2017
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Receivables from customers and notes receivable, net [Abstract] |  |
Allowance For Doubtful Accounts - Deposits with and Receivables from Broker-Dealers, Clearings Organizations, and Counterparties | $ 48.0 | $ 47.0
Allowance for Doubtful Accounts Receivable | $ 10.2 | $ 7.6

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- Definition Allowance For Doubtful Accounts - Deposits with and Receivables from Broker-Dealers, Clearings Organizations, and Counterparties + References No definition available. + Details
Name: | intl_AllowanceForDoubtfulAccountsDepositswithandReceivablesfromBrokerDealersClearingsOrganizationsandCounterparties
Namespace Prefix: | intl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Receivables from customers and notes receivable, net [Abstract] + References No definition available. + Details
Name: | intl_ReceivablesFromCustomersAndNotesReceivableNetAbstract
Namespace Prefix: | intl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AllowanceForDoubtfulAccountsReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant