Acquisitions Acquisitions (Details) (Tradewire Acquisition [Member] [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Tradewire Acquisition [Member] [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Amortizable Intangibles	$ 3.1

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Amortizable Intangibles

No definition available.

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