Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Statement of Financial Position [Abstract]
Cash and cash equivalents, fair value	$ 190.7	$ 14.1
Deposits and receivables from broker dealers and counterparties at fair value	746.8	1,070.6
Receivables from clients at fair value	(15.5)	2.6
Collateral that can be sold or repledged	1,448.7	843.3
Physical commodities inventory at fair value	362.4	359.9
Accounts payable and other accrued liabilities at fair value	1.5	2.8
Client payables at fair value	(546.1)	291.5
Payables to broker dealer at fair value	$ 52.0	$ 12.7
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)	22,775,522	22,431,233
Common stock - outstanding (in shares)	20,168,199	19,823,910
Treasury stock - shares (in shares)	2,607,323	2,607,323