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Acquisitions - Gain Capital Holdings Inc (Details) - USD ($) $ / shares in Units, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Feb. 26, 2020 | Sep. 30, 2020 | Sep. 30, 2019
Aggregated Merger Consideration |  |  |
Common stock outstanding (in shares) |  | 19,376,594 | 19,075,360
Gain Capital Holdings Inc [Member] |  |  |
Aggregated Merger Consideration |  |  |
Common stock outstanding (in shares) | 3,600,000 |  |
Gain Capital Holdings Inc [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition, share price (in dollars per share) | $ 6.00 |  |
Aggregated Merger Consideration |  |  |
Aggregate cash consideration | $ 215.0 |  |
Accrual for merger cash consideration | 21.6 |  |
Total merger consideration | $ 236.6 |  |
Business acquisition, cost incurred related to merger |  | $ 5.2 |

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