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Goodwill - Goodwill by Segment (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Sep. 30, 2014
Goodwill [Line Items]    
Goodwill $ 47.5us-gaap_Goodwill $ 47.5us-gaap_Goodwill
Commercial Hedging [Member]    
Goodwill [Line Items]    
Goodwill 30.7us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_CommodityAndRiskManagementMember		 30.7us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_CommodityAndRiskManagementMember
Global Payments [Member]    
Goodwill [Line Items]    
Goodwill 6.3us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_ForeignExchangeSegmentMember		 6.3us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_ForeignExchangeSegmentMember
Physical Commodities [Member]    
Goodwill [Line Items]    
Goodwill 2.4us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_OtherSegmentMember		 2.4us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_OtherSegmentMember
Securities [Member]    
Goodwill [Line Items]    
Goodwill $ 8.1us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_SecuritiesSegmentMember		 $ 8.1us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= intl_SecuritiesSegmentMember