Acquisitions Acquisitions (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Deferred consideration payable related to acquisitions			$ 5.0	$ 0
After-tax net income over year one			5.0intl_Aftertaxnetincomeoveryearone
Gross Profit	156.5us-gaap_GrossProfit	129.2us-gaap_GrossProfit	294.0us-gaap_GrossProfit	242.1us-gaap_GrossProfit
Net income	13.0us-gaap_ProfitLoss	7.5us-gaap_ProfitLoss	22.4us-gaap_ProfitLoss	10.0us-gaap_ProfitLoss
Earnings Per Share, Basic	$ 0.68us-gaap_EarningsPerShareBasic	$ 0.39us-gaap_EarningsPerShareBasic	$ 1.18us-gaap_EarningsPerShareBasic	$ 0.52us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted	$ 0.67us-gaap_EarningsPerShareDiluted	$ 0.39us-gaap_EarningsPerShareDiluted	$ 1.16us-gaap_EarningsPerShareDiluted	$ 0.51us-gaap_EarningsPerShareDiluted
G.X. Clarke (INTL FCStone Partners) [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	25.9		25.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	1.5		1.5
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5		1.5
G.X. Clarke (INTL FCStone Partners) [Member]
Escrow Deposit	10.0		10.0
Adjustment Escrow	5.0		5.0
Indemnity Escrow	5.0		5.0
Deferred consideration payable related to acquisitions			5.0
Business Acquisition, Contingent Consideration, Potential Cash Payment, Year One	0.5		0.5
Business Acquisition, Contingent Consideration, Potential Cash Payment Year Three	1.0		1.0
After-tax net income over earn-out period			30.0
INTL FCStone Partners LP [Member]
Gross Profit	10.5	10.4	8.7	22.9
Net income	1.4	1.2	0.7	3.9
Earnings Per Share, Basic		$ 0.07	$ 0.04	$ 0.20
Earnings Per Share, Diluted		$ 0.07	$ 0.04	$ 0.20
INTL FCStone Inc. [Member]
Gross Profit		139.6	302.7	265.0
Net income		$ 8.7	$ 23.1	$ 13.9
Earnings Per Share, Basic		$ 0.46	$ 1.22	$ 0.72
Earnings Per Share, Diluted		$ 0.46	$ 1.20	$ 0.71

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Indemnity escrow for acquisitions

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For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

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Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

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Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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