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Commitments and Contingencies - Contigent Consideration for Acquisitions (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2017 | Mar. 31, 2015 | Mar. 31, 2017 | Mar. 31, 2016 | Sep. 30, 2016
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |
Business Combination, Contingent Consideration, Liability | $ 0.9 |  | $ 0.9 |  | $ 0.8
Change in fair value of contingent consideration for acquisitions | 0.1 |  | 0.0 | $ 0.3 |
G.X. Clarke (INTL FCStone Partners) [Member] |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |
Business Combination, Contingent Consideration, Liability | 0.9 |  | 0.9 |  |
Present Value of Estimated Total Purchase Price | 28.7 |  | 28.7 |  |
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High | $ 1.5 |  | 1.5 |  |
Contingent Consideration [Member] |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |
Change in fair value of contingent consideration for acquisitions |  | $ 0.2 | $ 0.1 | $ 0.3 |

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- Definition Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e7008-128479 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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- Definition Amount of liability recognized arising from contingent consideration in a business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 35 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6911338&loc=d3e6819-128478 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476 + Details
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