Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2025	Jun. 30, 2024
Statement of Cash Flows [Abstract]
Non-cash equivalent segregated assets	$ 13.4	$ 48.4
Non-segregated cash and other non-cash equivalent assets included within deposits and receivables from broker-dealers, clearing organizations, and counterparties	$ 4,631.5	$ 5,114.7

Non-segregated Cash And Other Non-cash Equivalent Assets Included Within Deposits And Receivables From Broker-dealers, Clearing Organizations, And Counterparties

No definition available.

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