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Acquisitions and Disposals (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2013 | Jan. 01, 2015 | Feb. 28, 2013
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Profit | $ 178.7 | $ 151.6 | $ 156.5 | $ 137.5 | $ 130.6 | $ 118.2 | $ 129.2 | $ 112.9 | $ 624.3 | $ 490.9 | $ 468.2 |  |
Net income | $ 21.1 | $ 12.2 | $ 13.0 | $ 9.4 | $ 5.8 | $ 3.5 | $ 7.5 | $ 2.5 | $ 55.7 | $ 19.3 | $ 19.3 |  |
Net basic earnings per share | $ 1.12 | $ 0.64 | $ 0.68 | $ 0.50 | $ 0.30 | $ 0.19 | $ 0.39 | $ 0.13 | $ 2.94 | $ 1.01 | $ 1.01 |  |
Net diluted earnings per share | $ 1.09 | $ 0.62 | $ 0.67 | $ 0.49 | $ 0.29 | $ 0.18 | $ 0.39 | $ 0.12 | $ 2.87 | $ 0.98 | $ 0.97 |  |
Goodwill, Expected Tax Deductible Amount | $ 1.6 |  |  |  | $ 0.3 |  |  |  | $ 1.6 | $ 0.3 | $ 4.7 |  |
Goodwill | 47.5 |  |  |  | $ 47.5 |  |  |  | 47.5 | 47.5 |  |  |
Finite-lived Intangible Assets Acquired |  |  |  |  |  |  |  |  | 1.6 |  |  |  |
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.8
Assets of Disposal Group, Including Discontinued Operation |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.6
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment Realized upon Sale or Liquidation, before Tax |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 0.1 |  |
Deferred consideration payable related to acquisitions |  |  |  |  |  |  |  |  | 5.0 | 0.0 | $ 0.0 |  |
G.X. Clarke (INTL FCStone Partners) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Present Value of Estimated Total Purchase Price | 27.5 |  |  |  |  |  |  |  | 27.5 |  |  |  |
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 |  |  | $ 0.1 |
Forward Insight Commodities Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Present Value of Estimated Total Purchase Price | 0.5 |  |  |  |  |  |  |  | 0.5 |  |  |  |
Tradewire Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Present Value of Estimated Total Purchase Price | 4.4 |  |  |  |  |  |  |  | 4.4 |  |  |  |
Goodwill | 2.8 |  |  |  |  |  |  |  | 2.8 |  |  |  |
Customer Relationships [Member] | Tradewire Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Finite-lived Intangible Assets Acquired |  |  |  |  |  |  |  |  | 2.8 |  |  |  |
Customer Relationships [Member] | Forward Insight Commodities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Finite-lived Intangible Assets Acquired |  |  |  |  |  |  |  |  | 0.5 |  |  |  |
Tradewire Acquisition [Member] | Tradewire Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Consideration Transferred |  |  |  |  |  |  |  |  | 5.6 |  |  |  |
G.X. Clarke (INTL FCStone Partners) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |
After-tax net income over year one |  |  |  |  |  |  |  |  | 5.0 |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Profit |  |  |  | $ 8.7 |  |  |  |  | 31.4 | 41.9 |  |  |
Net income |  |  |  | $ 0.7 |  |  |  |  | 4.3 | $ 5.1 |  |  |
Net basic earnings per share |  |  |  | $ 0.04 |  |  |  |  |  | $ 0.27 |  |  |
Net diluted earnings per share |  |  |  | $ 0.04 |  |  |  |  |  | $ 0.27 |  |  |
INTL FCStone Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Profit |  |  |  |  |  |  |  |  | 633.0 | $ 532.8 |  |  |
Net income |  |  |  |  |  |  |  |  | $ 56.4 | $ 24.4 |  |  |
Net basic earnings per share |  |  |  |  |  |  |  |  | $ 2.98 | $ 1.28 |  |  |
Net diluted earnings per share |  |  |  |  |  |  |  |  | $ 2.91 | $ 1.25 |  |  |

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