Physical Commodities Inventory Level 4 - Inventory Serving as Collateral (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011
Physical Commodities Inventory [Abstract]
Inventory serving as collateral	$ 99.7	$ 159.4

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