Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021
Statement of Financial Position [Abstract]
Cash and cash equivalents, fair value	$ 805.7	$ 14.1
Deposits and receivables from broker dealer at fair value	2,817.2	1,070.6
Receivables from clients at fair value	(0.5)	2.6
Collateral that can be sold or repledged	2,372.3	843.3
Physical commodities inventory at fair value	359.8	359.9
Accounts payable and other accrued liabilities at fair value	0.0	2.8
Client payables at fair value	(1,392.4)	291.5
Payables to broker dealer at fair value	$ 55.8	$ 12.7
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)	30,000,000	30,000,000
Common stock - issued (in shares)	22,911,227	22,431,233
Common stock - outstanding (in shares)	20,303,904	19,823,910
Treasury stock - shares (in shares)	2,607,323	2,607,323