Condensed Consolidated Balance Sheets Parentheticals (USD $) In Millions, except Share data, unless otherwise specified	Jun. 30, 2013	Sep. 30, 2012
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 34.7	$ 72.8
Dep and rec -exch clear org - fair value	1,305.9	1,510.0
Dep and rec -b/d - fair value	0.3	(0.7)
Accounts pay and other accrued - fair value	$ 16.1	$ 14.8
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	19,540,217	19,214,219
Common stock - outstanding	19,201,135	18,984,951
Treasury stock - shares	339,082	229,064