Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Cash and cash equivalents, fair value	$ 20.0	$ 805.7
Deposits and receivables from broker dealers and counterparties at fair value	3,129.6	2,817.2
Receivables from clients at fair value	0.3	(0.5)
Collateral that can be sold or repledged	1,495.8	2,372.3
Physical commodities inventory at fair value	360.3	359.8
Client payables at fair value	(820.4)	(1,392.4)
Payables to broker dealer at fair value	$ 13.5	$ 55.8
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)	23,188,290	22,911,227
Common stock - outstanding (in shares)	20,580,967	20,303,904
Treasury stock - shares (in shares)	2,607,323		2,607,323