Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020
Statement of Financial Position [Abstract]
Cash and cash equivalents, fair value	$ 96.1	$ 2.6
Deposits and receivables from bds and counterparties at fair value	1,028.8	1,775.8
Receivables from clients at fair value	5.1	1.5
Collateral that can be sold or repledged	778.2	468.6
Physical commodities inventory at fair value	376.3	215.7
Accounts payable and other accrued liabilities at fair value	0.4	1.5
Client payables at fair value	(495.7)	(222.7)
Payables to broker dealer at fair value	$ 12.4	$ 14.7
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)	22,331,856	21,798,551
Common stock - outstanding (in shares)	19,859,899	19,376,594
Treasury stock - shares (in shares)	2,471,957	2,421,957