Revenue from Contracts with Clients - Narrative (Details)	3 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	6.40%	2.80%

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