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Revenue from Contracts with Clients - Schedule of Income and Expense (Details) - Equity Positions - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Disaggregation of Revenue [Line Items] |  |  |
Dividend (loss)/income net of dividend expense reported within Principal Gains, net | $ (0.1) | $ 1.1 | $ (1.3)
Long |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Dividend (loss)/income net of dividend expense reported within Principal Gains, net | 142.3 | 192.3 | 32.0
Short |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Dividend (loss)/income net of dividend expense reported within Principal Gains, net | $ (142.4) | $ (191.2) | $ (33.3)

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