Revenue from Contracts with Clients - Narrative (Details)	3 Months Ended	9 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	4.50%	6.00%	5.50%	4.70%

Revenues From Contracts With Clients % Of Total Revenues

No definition available.

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