Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 312.9	$ 643.3
Dep and rec -exch clear org - fair value	232.5	0.0
Dep and rec -b/d - fair value	307.3	517.4
Physical commodities inventory at fair value	164.4	156.9
Collateral that can be sold or repledged	320.9	123.0
Accounts pay and other accrued - fair value	0.0	0.0
Payables to b/d - fair value	$ 12.7	$ 0.0
Preferred stock - par value	$ 0.01	$ 0.01
Preferred stock - authorized	1,000,000	1,000,000
Preferred stock - issued	0	0
Preferred stock - outstanding	0	0
Common stock - par value	$ 0.01	$ 0.01
Common stock - authorized	30,000,000	30,000,000
Common stock - issued	21,154,065	21,030,497
Common stock - outstanding	19,032,108	21,030,497
Treasury stock - shares	2,121,957	2,121,957