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v3.21.2

Retirement Plans - Defined Contribution Plans (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, employer discretionary contribution amount | $ 15.2 | $ 10.1 | $ 7.5
401k Plan |  |  |
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 | Minimum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, maximum annual contributions per employee, percent | 1.00% |  |
401k Plan | Maximum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, maximum annual contributions per employee, percent | 80.00% |  |
UK Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, employers matching contribution, annual vesting percentage | 100.00% |  |
UK Plan | Maximum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 10.00% |  |

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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: | dtr-types: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: | dtr-types: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
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Data Type: | dtr-types: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
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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
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types: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
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
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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
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
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- Details
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- Details
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Period Type: |

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- Details
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |