Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 16.5	$ 54.5
Dep and rec -exch clear org - fair value	232.5	0.0
Dep and rec -b/d - fair value	53.7	204.7
Physical commodities inventory at fair value	142.7	73.2
Collateral that can be sold or repledged	3.4	19.4
Accounts pay and other accrued - fair value	0.0	1.0
Payables to b/d - fair value	$ 1.1	$ 4.8
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	20,931,720	20,855,243
Common stock - outstanding	18,809,763	18,733,286
Treasury stock - shares	2,121,957	2,121,957