Segment Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 13,627.7us-gaap_Revenues	$ 7,908.7us-gaap_Revenues
Operating revenues	137.5us-gaap_GrossProfit	112.9us-gaap_GrossProfit
Net operating revenues	93.2us-gaap_RevenuesNetOfInterestExpense	73.4us-gaap_RevenuesNetOfInterestExpense
Net Segment Contribution	69.9intl_NetSegmentContribution	57.2intl_NetSegmentContribution
Segment Income	40.9intl_SegmentIncome	29.4intl_SegmentIncome
Costs not allocated to operating segments	27.3	25.5
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	13.6	3.9
Assets	3,127.2us-gaap_Assets		3,039.7us-gaap_Assets
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	68.4	45.6
Operating revenues	68.4	45.6
Net operating revenues	57.0	36.2
Net Segment Contribution	40.6	26.8
Segment Income	24.9	12.1
Assets	1,398.7		1,400.9
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	15.2	13.6
Operating revenues	15.2	13.6
Net operating revenues	13.5	11.8
Net Segment Contribution	10.8	9.2
Segment Income	8.2	7.0
Assets	56.7		51.9
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	17.2	21.4
Operating revenues	17.2	21.4
Net operating revenues	10.0	15.6
Net Segment Contribution	6.6	11.9
Segment Income	1.7	7.2
Assets	220.3		235.5
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	13,496.7	7,801.0
Operating revenues	6.5	5.2
Net operating revenues	5.6	4.5
Net Segment Contribution	4.4	3.6
Segment Income	2.5	1.8
Assets	136.4		116.8
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	31.2	27.5
Operating revenues	31.2	27.5
Net operating revenues	9.5	7.1
Net Segment Contribution	7.5	5.7
Segment Income	3.6	1.3
Assets	1,192.7		1,136.2
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(1.0)	(0.4)
Operating revenues	(1.0)	(0.4)
Net operating revenues	(2.4)	(1.8)
Assets	$ 122.4		$ 98.4

Costs not allocated to operating segments

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Net segment contribution equals revenues less cost of sales, transaction-based clearing expenses, variable bonus compensation, introducing broker commissions and interest expense.

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Segment Income equals net contribution less non-variable direct segment costs.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-02.18)

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-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h)(1)(i))

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income after deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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