Commitments and Contingencies Commitments and Contingencies - Contingencies and Litigation (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Mar. 31, 2018
Excess segregated funds invested		$ 15.5
Pre and Post Transfers of Funds - Sentinel		$ 21.9
Litigation Settlement, Amount Awarded from Other Party	$ 1.9

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