Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 5.7	$ 6.2
Advisory fees receivable, allowance for doubtful accounts	0.3	0.1
Property and equipment, accumulated depreciation	$ 59.1	$ 58.5
Common stock, par value (usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	40,443,286	38,898,163
Common stock, shares outstanding (shares)	29,285,938	28,053,563
Contingent convertible preferred stock, par value (usd per share)	$ 0.01	$ 0.01
Contingent convertible preferred stock, shares authorized (shares)	10,000,000	10,000,000
Contingent convertible preferred stock, shares issued (shares)	1,099,877	1,099,877
Contingent convertible preferred stock, shares outstanding (shares)	0	439,951
Exchangeable shares of subsidiary, shares issued (shares)	257,156	257,156
Exchangeable shares of subsidiary, shares outstanding (shares)	32,804	32,804
Treasury stock, par value (usd per share)	$ 0.01	$ 0.01
Treasury stock, shares (shares)	11,157,348	10,884,600