Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020
Statement of Financial Position [Abstract]
Cash and cash equivalents, fair value	$ 2.0	$ 2.6
Deposits and receivables from bds and counterparties at fair value	1,193.9	1,775.8
Collateral that can be sold or repledged	713.2	468.6
Physical commodities inventory at fair value	299.5	215.7
Accounts pay and other accrued - fair value	1.3	1.5
Payables to broker dealer at fair value	$ 30.6	$ 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)	22,026,627	21,798,551
Common stock - outstanding (in shares)	19,604,670	19,376,594
Treasury stock - shares (in shares)	2,421,957	2,421,957