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, 2022	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	$ 8.3	$ 1.4	$ 0.4
Long
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	142.3	282.7	77.0
Short
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	$ (134.0)	$ (281.3)	$ (76.6)