Physical Commodities Inventory - Inventory Serving as Collateral (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Sep. 30, 2012
Physical Commodities Inventory [Abstract]
Inventory serving as collateral	$ 20.3	$ 129.1

Physical commodities inventory serving as collateral under one of the company's credit facilities.

No definition available.

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