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Revenue from Contracts with Clients - Schedule of Principal Gains, Net Related To Income and Expense of Dividends (Details) - Equity securities - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2023 | Mar. 31, 2022 | Mar. 31, 2023 | Mar. 31, 2022
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal Gains, net | $ (0.9) | $ 0.8 | $ 0.1 | $ 9.6
Long |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal Gains, net | 3.2 | 26.3 | 17.4 | 87.5
Short |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal Gains, net | $ (4.1) | $ (25.5) | $ (17.3) | $ (77.9)

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