Commitments and Contingencies - Contingencies and Litigation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2014
Commitments and Contingencies [Abstract]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 0.5
Excess segregated funds returned	15.5intl_ExcessSegregatedFundsReturned
Excess segregated funds invested	21.9intl_ExcessSegregatedFundsInvested
Pre and Post Transfers of Funds related to Sentinel	$ 15.5intl_PreandPostTransfersofFundsSentinel

Pre and Post Transfers of Funds - Sentinel represents transfers or withdrawals of funds received by the Company and other FCMs within 90 days prior to the filing of the Sentinel bankruptcy petition, as well as avoidance of post-petition distributions and disallowance of the proof of claim filed by FCStone, LLC.

No definition available.

Name:	intl_PreandPostTransfersofFundsSentinel
Namespace Prefix:	intl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration