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Description of Business and Significant Accounting Policies (Details) $ / shares in Units, $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2020 USD ($) country client $ / shares shares | Sep. 30, 2019 USD ($) $ / shares shares | Sep. 30, 2018 USD ($) | Jan. 14, 2019
Number of commercial and institutional clients (more than) | client | 32,000 |  |  |
Number retail clients (more than) | client | 330,000 |  |  |
Number of countries in which entity operates (more than) | country | 180 |  |  |
Selling and marketing | $ 12.2 | $ 5.2 | $ 6.2 |
Amount recognized in loss due to inflationary accounting |  | 3.9 | 3.4 |
Net assets and liabilities denominated in argentine pesos | 0.1 | (5.5) |  |
Cash denominated in argentine pesos | 0.1 | 1.4 |  |
Net denominated in argentine pesos | 0.9 | 1.0 |  |
Cash held at banks and money market funds - segregated | 1,907.2 | 743.9 |  |
US government obligations and other securities | 10.6 | 299.8 |  |
Commodities warehouse receipts | 2.4 | 6.2 |  |
Cash and cash equivalents - deposits and receivables | 1,600.0 | 1,600.0 |  |
US government securities and other securities - deposits and receivables | $ 1,949.3 | 603.8 |  |
Acquired finite-lived intangible asset, weighted average useful life | 1 year |  |  |
Fair Value of exchange memberships | $ 6.7 | $ 6.0 |  |
Preferred stock - authorized (in shares) | shares | 1,000,000 | 1,000,000 |  |
Preferred stock - par value (in dollar per share) | $ / shares | $ 0.01 | $ 0.01 |  |
Preferred stock - outstanding (in shares) | shares | 0 | 0 |  |
Cumulative effect of new accounting principle in period of adoption |  | $ 0.7 |  |
Minimum [Member] |  |  |  |
Property plant and equipment, useful life | 3 years |  |  |
Identifiable intangible assets | 5 years |  |  |
Maximum [Member] |  |  |  |
Property plant and equipment, useful life | 10 years |  |  |
Identifiable intangible assets | 20 years |  |  |
Retail [Member] |  |  |  |
Number of countries in which entity operates (more than) | country | 130 |  |  |
Software Development [Member] |  |  |  |
Property plant and equipment, useful life | 3 years |  |  |
Exchange Memberships Representing an Ownership Interest [Member] |  |  |  |
Memberships in exchanges owned | $ 5.6 | 5.6 |  |
Exchange Memberships not Representing an Ownership Interest [Member] |  |  |  |
Memberships in exchanges owned | $ 5.8 | $ 5.8 |  |
Commission and Clearing Fees [Member] |  |  |  |
Prior period reclassification adjustment |  |  | $ 35.0 |
GMP Securities Acquisition [Member] |  |  |  |
Percentage of stock purchased |  |  |  | 100.00%

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