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
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Dividend (loss)/income, net reported within Principal Gains, net	$ (1.4)	$ (0.4)	$ (1.3)	$ 9.2
Dividend income on long equity positions
Disaggregation of Revenue [Line Items]
Dividend (loss)/income, net reported within Principal Gains, net	10.2	31.6	27.5	119.1
Dividend expense on short equity positions
Disaggregation of Revenue [Line Items]
Dividend (loss)/income, net reported within Principal Gains, net	$ (11.6)	$ (32.0)	$ (28.8)	$ (109.9)