Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2023	Sep. 30, 2022
Statement of Financial Position [Abstract]
Cash and cash equivalents at fair value	$ 25,800,000	$ 805,700,000
Deposits and receivables from broker dealers and counterparties at fair value	4,655,700,000	2,817,200,000
Receivables from clients at fair value	2,300,000	(500,000)
Collateral that can be sold or repledged	1,593,100,000	2,372,300,000
Physical commodities inventory at fair value	351,600,000	359,800,000
Accounts payable and other accrued liabilities at fair value	1,300,000	0
Client payables at fair value	669,500,000	(1,392,400,000)
Payables to broker dealer at fair value	$ 34,100,000	$ 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)	23,286,021	22,911,227
Common stock - outstanding (in shares)	20,678,698	20,303,904
Treasury stock - shares (in shares)	2,607,323	2,607,323