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Acquisitions and Disposals (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Jan. 01, 2015
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Gross Profit | $ 205.1 | $ 197.6 | $ 195.8 | $ 185.5 | $ 178.6 | $ 175.0 | $ 166.1 | $ 151.3 |  | $ 784.0 | $ 671.0 | $ 624.3 |
Net income | $ (23.6) | $ 12.7 | $ 11.0 | $ 6.3 | $ 16.8 | $ 14.6 | $ 14.5 | $ 8.8 |  | 6.4 | 54.7 | 55.7 |
Net basic earnings per share | $ (1.27) | $ 0.67 | $ 0.58 | $ 0.34 | $ 0.91 | $ 0.79 | $ 0.77 | $ 0.47 |  |  |  |  |
Net diluted earnings per share | $ (1.27) | $ 0.66 | $ 0.58 | $ 0.34 | $ 0.90 |  | $ 0.76 | $ 0.46 | $ 0.78 |  |  |  |
Business Combination, Consideration Transferred |  |  |  |  |  |  |  |  |  |  | 45.0 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents |  |  |  |  | $ 24.9 |  |  |  |  |  | 24.9 |  |
Business Combination, Acquired Receivable, Fair Value |  |  |  |  | 151.6 |  |  |  |  |  | 151.6 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets Noncurrent |  |  |  |  | 5.7 |  |  |  |  |  | 5.7 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets |  |  |  |  | 4.8 |  |  |  |  |  | 4.8 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities |  |  |  |  | 136.0 |  |  |  |  |  | 136.0 |  |
Gain on acquisition |  |  |  |  | 6.2 | $ 0.0 | $ 0.0 | $ 0.0 |  | 0.0 | 6.2 | 0.0 |
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual |  |  |  |  |  |  |  |  |  |  | 8.6 |  |
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual |  |  |  |  |  |  |  |  |  |  | 0.1 |  |
Goodwill | $ 47.1 |  |  |  | 47.5 |  |  |  |  | 47.1 | 47.5 |  |
Finite-lived Intangible Assets Acquired |  |  |  |  |  |  |  |  |  | 6.0 |  |  |
Deferred consideration payable related to acquisitions |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 | $ 5.0 |
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
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 |  |
Deposit Liability, Current | $ 4.5 |  |  |  |  |  |  |  |  | $ 4.5 |  |  |
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 |  |

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