Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Statement of Financial Position [Abstract]
Securities and other assets segregated, fair value	$ 2.6	$ 306.0
Deposits and receivables from broker dealer at fair value	1,775.8	626.9
Financial instrument owned, at fair value	468.6	478.8
Physical commodities inventory, net at fair value	215.7	151.9
Accounts payable and other accrued at fair value	1.5	1.8
Payables to broker dealer at fair value	$ 14.7	$ 5.6
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)	21,798,551	21,297,317
Common stock - outstanding (in shares)	19,376,594	19,075,360
Treasury stock - shares (in shares)	2,421,957	2,221,957