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
	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	$ 1.0	$ 8.8
Long
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	14.2	61.1
Short
Disaggregation of Revenue [Line Items]
Dividend income net of dividend expense reported within Principal Gains, net	$ (13.2)	$ (52.3)