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)