Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Jun. 30, 2016	Sep. 30, 2015
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 631.1	$ 515.5
Dep and rec -exch clear org - fair value	1,082.0	1,009.4
Dep and rec -b/d - fair value	(14.8)	(52.9)
Physical commodities inventory at fair value	40.6	15.2
Collateral that can be sold or repledged	343.8	170.7
Accounts pay and other accrued - fair value	0.8	3.3
Payables to b/d - fair value	$ 3.5	$ 1.6
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	20,518,964	20,184,556
Common stock - outstanding	18,397,007	18,812,803
Treasury stock - shares	2,121,957	1,371,753