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	$ 715.7	$ 618.8
Dep and rec -exch clear org - fair value	921.0	868.5
Dep and rec -b/d - fair value	4.3	(15.2)
Physical commodities inventory at fair value	129.8	71.2
Collateral that can be sold or repledged	482.4	47.2
Accounts pay and other accrued - fair value	0.9	0.8
Payables to b/d - fair value	$ 6.4	$ 3.5
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	20,740,489	20,557,175
Common stock - outstanding	18,618,532	18,435,218
Treasury stock - shares	2,121,957	2,121,957