Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Assets [Abstract]
Securities and other assets segregated, fair value	$ 618.8	$ 515.5
Dep and rec -exch clear org - fair value	868.5	1,009.4
Dep and rec -b/d- fair value	(15.2)	(52.9)
Physical commodities inventory at fair value	71.2	15.2
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	47.2	170.7
Accounts pay and other accrued - fair value	0.8	3.3
Payables to b/ds and counterparties at 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	20,557,175	20,184,556
Common stock - outstanding	18,435,218	18,812,803
Treasury stock - shares	2,121,957	1,371,753