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, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	$ 1.4	$ 1.0
Dividend income on long equity positions
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	20.2	14.2
Dividend expense on short equity positions
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	$ (18.8)	$ (13.2)