Acquisitions Acquisitions (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Deferred consideration payable related to acquisitions				$ 5.0	$ 0.0
After-tax net income over year one				5.0
Gross Profit	$ 151.6		$ 118.2	445.6	360.3
Net income	$ 12.2		$ 3.5	$ 34.6	$ 13.5
Earnings Per Share, Basic	$ 0.64		$ 0.19	$ 1.82	$ 0.71
Earnings Per Share, Diluted	$ 0.62		$ 0.18	$ 1.78	$ 0.69
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		$ 8.7	$ 9.8	20.7	$ 32.7
Net income		$ 0.7	$ 0.8	3.0	$ 4.7
Earnings Per Share, Basic		$ 0.04	$ 0.04		$ 0.24
Earnings Per Share, Diluted		$ 0.04	$ 0.04		$ 0.24
INTL FCStone Inc. [Member]
Gross Profit			$ 128.0	454.3	$ 393.0
Net income			$ 4.3	$ 35.3	$ 18.2
Earnings Per Share, Basic			$ 0.23	$ 1.86	$ 0.95
Earnings Per Share, Diluted			$ 0.22	$ 1.82	$ 0.93