Revenue from Contracts with Clients - Schedule of Principal Gains, Net Related To Income and Expense of Dividends (Details) - Equity position - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Dividend (loss)/income net of dividend expense reported within Principal Gains, net	$ (1.3)	$ 8.3	$ 1.4
Long
Disaggregation of Revenue [Line Items]
Dividend (loss)/income net of dividend expense reported within Principal Gains, net	32.0	142.3	282.7
Short
Disaggregation of Revenue [Line Items]
Dividend (loss)/income net of dividend expense reported within Principal Gains, net	$ (33.3)	$ (134.0)	$ (281.3)