Revenue from Contracts with Clients - Narrative (Details)	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Total revenues (percentage)	0.90%	1.40%	1.70%
Operating revenues (percentage)	37.20%	40.90%	47.40%
Term of risk management consulting contracts	6 months

Revenues from contracts with clients as a % of operating revenues

No definition available.

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Namespace Prefix:	intl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration