Allowance for Doubtful Accounts - Allowance for Bad Debts (Details) $ in Millions	6 Months Ended
Mar. 31, 2024 USD ($)
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance as of September 30, 2023	$ 59.9
Recovery of bad debts	(2.1)
Allowance charge-offs	(1.2)
Other	0.4
Balance as of March 31, 2024	57.0
Bad debt expense	$ 1.4

SEC Schedule, 12-09, Valuation Allowances and Reserves, Credit Loss Expense (Reversal) Not Included in Allowance

No definition available.

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Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of valuation and qualifying accounts and reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09(Column B))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09(Column E))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-4

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in valuation and qualifying accounts and reserves from charge to accounts other than cost and expense.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of decrease in valuation and qualifying accounts and reserves.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase in valuation and qualifying accounts and reserves from recovery of amount previously written off or reestablishment of reserve previously utilized.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09(Column C)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-4

+ Details

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Balance Type:	credit
Period Type:	duration