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Retirement Plans Retirement Plans - Defined Contribution Plans (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount | $ 7.5 | $ 6.8 | $ 6.1
UK Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage | 100.00% |  |
UK Plan [Member] | Maximum [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 10.00% |  |
401k Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 62.50% |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage | 8.00% |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, Two to Three Years | 33.00% |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, Three to Four Years | 66.00% |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, All Years | 100.00% |  |
401k Plan [Member] | Maximum [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 80.00% |  |
401k Plan [Member] | Minimum [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 1.00% |  |

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- Definition Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, All Years + References No definition available. + Details
Name: | intl_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentageAllYears
Namespace Prefix: | intl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, Three to Four Years + References No definition available. + Details
Name: | intl_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentageThreeToFourYears
Namespace Prefix: | intl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage, Two to Three Years + References No definition available. + Details
Name: | intl_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentageTwoToThreeYears
Namespace Prefix: | intl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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_DefinedContributionPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PlanNameAxis=intl_UkPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PlanNameAxis=intl_A401KPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |