Segment Analysis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 14,442.0us-gaap_Revenues	$ 8,452.9us-gaap_Revenues	$ 28,069.7us-gaap_Revenues	$ 16,361.6us-gaap_Revenues
Operating revenues	156.5us-gaap_GrossProfit	129.2us-gaap_GrossProfit	294.0us-gaap_GrossProfit	242.1us-gaap_GrossProfit
Net operating revenues	107.9us-gaap_RevenuesNetOfInterestExpense	85.9us-gaap_RevenuesNetOfInterestExpense	201.1us-gaap_RevenuesNetOfInterestExpense	159.3us-gaap_RevenuesNetOfInterestExpense
Net Segment Contribution	83.7intl_NetSegmentContribution	65.9intl_NetSegmentContribution	153.6intl_NetSegmentContribution	123.1intl_NetSegmentContribution
Segment Income	49.1intl_SegmentIncome	36.5intl_SegmentIncome	90.0intl_SegmentIncome	65.9intl_SegmentIncome
Costs not allocated to operating segments	31.0	26.2	58.3	51.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	18.1	10.3	31.7	14.2
Assets	4,462.6us-gaap_Assets		4,462.6us-gaap_Assets		3,039.7us-gaap_Assets
Commercial Hedging [Member]
Segment Reporting Information [Line Items]
Revenues	64.7	63.8	133.1	109.4
Operating revenues	64.7	63.8	133.1	109.4
Net operating revenues	53.7	52.6	110.7	88.8
Net Segment Contribution	39.3	38.3	79.9	65.1
Segment Income	21.7	22.6	46.6	34.7
Assets	1,328.4		1,328.4		1,400.9
Global Payments [Member]
Segment Reporting Information [Line Items]
Revenues	18.4	12.8	33.6	26.4
Operating revenues	18.4	12.8	33.6	26.4
Net operating revenues	16.1	11.0	29.6	22.8
Net Segment Contribution	12.8	8.6	23.6	17.8
Segment Income	9.9	6.3	18.1	13.3
Assets	66.7		66.7		51.9
Securities Segment [Member]
Segment Reporting Information [Line Items]
Revenues	36.1	17.2	53.3	38.6
Operating revenues	36.1	17.2	53.3	38.6
Net operating revenues	25.5	11.8	35.5	27.4
Net Segment Contribution	19.6	8.9	26.2	20.8
Segment Income	12.2	3.8	13.9	11.0
Assets	1,655.1		1,655.1		235.5
Physical Commodities [Member]
Segment Reporting Information [Line Items]
Revenues	14,291.9	8,328.8	27,788.6	16,129.8
Operating revenues	6.4	5.1	12.9	10.3
Net operating revenues	5.9	4.2	11.5	8.7
Net Segment Contribution	4.5	3.6	8.9	7.2
Segment Income	2.4	1.7	4.9	3.5
Assets	165.8		165.8		116.8
Clearing and Execution Services Segment [Member]
Segment Reporting Information [Line Items]
Revenues	31.5	30.9	62.7	58.4
Operating revenues	31.5	30.9	62.7	58.4
Net operating revenues	9.2	8.4	18.7	15.5
Net Segment Contribution	7.5	6.5	15.0	12.2
Segment Income	2.9	2.1	6.5	3.4
Assets	1,149.8		1,149.8		1,136.2
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	(0.6)	(0.6)	(1.6)	(1.0)
Operating revenues	(0.6)	(0.6)	(1.6)	(1.0)
Net operating revenues	(2.5)	(2.1)	(4.9)	(3.9)
Assets	$ 96.8		$ 96.8		$ 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.

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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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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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