Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Jun. 30, 2015	Sep. 30, 2014
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 511.8	$ 15.3
Dep and rec -exch clear org - fair value	1,002.8	1,255.4
Dep and rec -b/d - fair value	(16.2)	(1.1)
Physical commodities inventory at fair value	14.0	0.0
Collateral that can be sold or repledged	205.1	0.0
Accounts pay and other accrued - fair value	3.3	5.5
Payables to b/d - fair value	$ 0.0	$ 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	20,177,924	19,826,635
Common stock - outstanding	19,010,442	18,883,662
Treasury stock - shares	1,167,482	942,973