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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 | 9 Months Ended
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Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal gains, net | $ (2.0) | $ (1.4) | $ (0.7) | $ 1.1
Dividend income on long equity positions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal gains, net | 35.8 | 77.7 | 108.7 | 163.7
Dividend expense on short equity positions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Dividend (loss)/income, net reported within Principal gains, net | $ (37.8) | $ (79.1) | $ (109.4) | $ (162.6)

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