Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Statement of Financial Position [Abstract]
Cash, securities and other assets segregated under federal and other regulations at fair value	$ 5.8	$ 805.7
Deposits with and receivables from broker-dealers, clearing organizations and counterparties at fair value	4,248.3	2,817.2
Receivables from clients at fair value	(7.9)	(0.5)
Financial instruments owned, securities pledged as collateral that can be sold or repledged	1,466.4	2,372.3
Physical commodities inventory at fair value	386.5	359.8
Accounts payable and other accrued liabilities at fair value	1.5	0.0
Payables to client at fair value	79.8	(1,392.4)
Payables to broker-dealers, clearing organizations and counterparties at fair value	$ 10.2	$ 55.8
Preferred stock, par value (in dollar per share)	$ 0.01	$ 0.01
Preferred stock, authorized (in shares)	1,000,000	1,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollar per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	200,000,000	200,000,000
Common stock, issued (in shares)	23,403,960	22,911,227
Common stock, outstanding (in shares)	20,796,637	20,303,904
Treasury stock, shares (in shares)	2,607,323	2,607,323