Condensed Consolidated Balance Sheets Parentheticals - USD ($) $ in Millions	Mar. 31, 2016	Sep. 30, 2015
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 570.9	$ 515.5
Dep and rec -exch clear org - fair value	895.7	1,009.4
Dep and rec -b/d - fair value	(16.0)	(52.9)
Physical commodities inventory at fair value	2.8	15.2
Collateral that can be sold or repledged	50.8	170.7
Accounts pay and other accrued - fair value	1.5	3.3
Payables to b/d - fair value	$ 1.9	$ 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	20,460,788	20,184,556
Common stock - outstanding	18,688,729	18,812,803
Treasury stock - shares	1,772,059	1,371,753