Commitments and Contingencies - Contingencies and Litigation (Details) $ in Millions	12 Months Ended
Sep. 30, 2016 USD ($)
Commitments and Contingencies [Abstract]
Excess segregated funds returned	$ 15.5
Excess segregated funds invested	21.9
Pre and Post Transfers of Funds - Sentinel	15.5
Post Transfers of Funds - Sentinel	$ 14.5

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