Revenue from Contracts with Clients - Narrative (Details)	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	5.50%	5.00%	0.90%
Term of risk management consulting contracts	6 months

Revenues From Contracts With Clients % Of Total Revenues

No definition available.

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Namespace Prefix:	intl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration