Revenue from Contracts with Clients - Principal Gains, Net Related To Income and Expense of Dividends (Details) - Equity securities - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	$ 2.1	$ 1.1	$ 1.3	$ 2.5
Dividend income on long equity positions
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	37.3	65.8	72.9	86.0
Dividend expense on short equity positions
Disaggregation of Revenue [Line Items]
Dividend income, net reported within Principal Gains, net	$ (35.2)	$ (64.7)	$ (71.6)	$ (83.5)

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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