Condensed Consolidated Balance Sheets Condensed Consolidated Balance Sheet Parentheticals (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 41.4	$ 72.8
Dep and rec -exch clear org - fair value	1,430.2	1,510.0
Dep and rec -b/d - fair value	4.8	(0.7)
Accounts pay and other accrued - fair value	$ 17.9	$ 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	19,407,303	19,214,219
Common stock - outstanding	19,068,221	18,984,951
Treasury stock - shares	339,082	229,064