Revenue from Contracts with Clients - Narrative (Details)	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	5.00%	0.90%	1.40%
Term of risk management consulting contracts	6 months

Revenues from contracts with clients as a % of total revenues

No definition available.

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