Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Jun. 30, 2023	Sep. 30, 2022
Statement of Financial Position [Abstract]
Cash, securities and other assets segregated under federal and other regulations at fair value	$ 4,800,000	$ 805,700,000
Deposits with and receivables from broker-dealers, clearing organizations and counterparties at fair value	3,641,600,000	2,817,200,000
Receivables from clients at fair value	(100,000)	(500,000)
Financial instruments owned, securities pledged as collateral that can be sold or repledged	1,417,700,000	2,372,300,000
Physical commodities inventory at fair value	275,200,000	359,800,000
Accounts payable and other accrued liabilities at fair value	1,400,000	0
Payables to client at fair value	(296,500,000)	(1,392,400,000)
Payables to broker-dealers, clearing organizations and counterparties at fair value	$ 3,200,000	$ 55,800,000
Preferred stock - par value (in dollars 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)	30,000,000	30,000,000
Common stock - issued (in shares)	23,356,948	22,911,227
Common stock - outstanding (in shares)	20,749,625	20,303,904
Treasury stock - shares (in shares)	2,607,323	2,607,323