Revenue from Contracts with Clients - Narrative (Details)	3 Months Ended	6 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	5.90%	5.00%	6.20%	4.00%

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