Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Assets [Abstract]
Securities and other assets segregated, fair value	$ 515.5	$ 15.3
Dep and rec -exch clear org - fair value	1,009.4	1,255.4
Dep and rec -b/d- fair value	(52.9)	(1.1)
Physical commodities inventory at fair value	15.2	0.0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	170.7	0.0
Accounts pay and other accrued - fair value	3.3	5.5
Payables to b/ds and counterparties at 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	20,184,556	19,826,635
Common stock - outstanding	18,812,803	18,883,662
Treasury stock - shares	1,371,753	942,973