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Description of Business and Significant Accounting Policies (Details) |  | 12 Months Ended
--- | --- | ---
Oct. 01, 2020 USD ($) | Sep. 30, 2021 USD ($) country client $ / shares shares | Sep. 30, 2020 USD ($) $ / shares shares | Sep. 30, 2019 USD ($)
Number of countries in which entity operates (more than) | country |  | 180 |  |
Number of commercial and institutional clients (more than) | client |  | 45,000 |  |
Number retail clients (more than) | client |  | 370,000 |  |
Selling and marketing |  | $ 33,300,000 | $ 12,200,000 | $ 5,200,000
Translation losses |  | (800,000) |  |
Net assets denominated in argentine pesos |  | 400,000 |  |
Net liabilities denominated in argentine pesos |  |  | 100,000 |
Cash denominated in argentine pesos (less than) |  | 100,000 | 100,000 |
Net nonmonetary denominated in Argentine pesos |  | 900,000 | 900,000 |
Cash held at banks and money market funds - segregated |  | 2,260,300,000 | 1,907,200,000 |
US government obligations and other securities |  | 200,000 | 10,600,000 |
Commodities warehouse receipts |  | 13,900,000 | 2,400,000 |
Cash and cash equivalents - deposits and receivables |  | 3,100,000,000 | 1,600,000,000 |
US government securities and other securities - deposits and receivables |  | $ 810,700,000 | 1,949,300,000 |
Acquired finite-lived intangible asset, weighted average useful life |  | 1 year |  |
Goodwill impairment charges |  | $ 0 | 0 | 0
Fair Value of exchange memberships |  | $ 6,800,000 | $ 6,700,000 |
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 |
Retained earnings |  | $ 682,500,000 | $ 572,400,000 |
Income tax expense |  | 37,800,000 | 37,100,000 | 25,900,000
Other expense | Revision of Prior Period, Adjustment |  |  |  |
Selling and marketing |  |  |  | (5,200,000)
Selling and marketing expense | Revision of Prior Period, Adjustment |  |  |  |
Selling and marketing |  |  |  | 5,200,000
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |
Retained earnings | $ (6,200,000) |  |  |
Income tax expense | $ 2,000,000 |  |  |
Exchange Memberships Representing an Ownership Interest |  |  |  |
Memberships in exchanges owned |  | 5,500,000 | 5,600,000 |
Exchange Memberships not Representing an Ownership Interest |  |  |  |
Memberships in exchanges owned |  | $ 6,000,000 | $ 5,800,000 |
Minimum |  |  |  |
Property plant and equipment, useful life |  | 3 years |  |
Identifiable intangible assets |  | 5 years |  |
Maximum |  |  |  |
Property plant and equipment, useful life |  | 10 years |  |
Identifiable intangible assets |  | 20 years |  |
Capitalized software development |  |  |  |
Property plant and equipment, useful life |  | 3 years |  |
ARGENTINA |  |  |  |
Translation losses |  |  |  | $ 3,900,000
Geographic Concentration Risk | Revenue Benchmark | ARGENTINA |  |  |  |
Percentage of operating revenues |  | 1.00% |  |
Retail |  |  |  |
Number of countries in which entity operates (more than) | country |  | 180 |  |

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