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v3.19.3.a.u2

Description of Business and Significant Accounting Policies - Accounts, Customers and Other Facts (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Number of different types of foreign currencies | 140 |  |
Number of commercial and institutional clients | 20,000 |  |
Number retail clients | 80,000 |  |
Number of Countries in which Entity Operates | 130 |  |
Non-Trading Technology and Support [Member] |  |  |
Prior Period Reclassification Adjustment |  |  | $ 11.6
communications [Member] |  |  |
Prior Period Reclassification Adjustment |  |  | 5.0
Commission and Clearing Fees |  |  |
Prior Period Reclassification Adjustment |  | $ 35.0 | $ 35.2
Exchange memberships not representing an ownership interest [Member] |  |  |
Memberships in Exchanges Owned | $ 5.8 |  |
Exchange memberships representing an ownership interest [Member] |  |  |
Memberships in Exchanges Owned | $ 5.6 |  |

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