Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Statement of Cash Flows [Abstract]
Non cash equivalent segregated assets	$ 471.1	$ 51.8	$ 5.8
Segregated cash and other non cash equivalent assets included with in deposits and receivables from broker dealers clearing organizations and counter parties	2,629.5	2,662.0	2,218.3
Non segregated cash and other non cash equivalent assets included with in deposits and receivables from broker dealers clearing organizations and counter parties	$ 5,146.7	$ 2,007.0	$ 2,712.6

Non Segregated Cash And Other Non Cash Equivalent Assets Included With In Deposits And Receivables From Broker Dealers Clearing Organizations And Counter Parties

No definition available.

Name:	intl_NonSegregatedCashAndOtherNonCashEquivalentAssetsIncludedWithInDepositsAndReceivablesFromBrokerDealersClearingOrganizationsAndCounterParties
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant