Condensed Consolidated Balance Sheets Parentheticals (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013
Balance Sheet Parentheticals [Abstract]
Securities and other assets segregated, fair value	$ 33.5	$ 107.6
Dep and rec -exch clear org - fair value	1,353.5	1,371.7
Dep and rec -b/d - fair value	(5.1)	(13.1)
Accounts payable and other accrued liabilities - contingent consideration, fair value	$ 9.1	$ 9.6
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	19,675,192	19,638,330
Common stock - outstanding	19,182,219	19,209,157
Treasury stock - shares	492,973	429,173