Consolidated Statements of Financial Condition (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 5.6	$ 6.2
Advisory fees receivable, allowance for doubtful accounts	0.3	0.1
Property and equipment, accumulated depreciation	$ 58.5	$ 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)	39,630,891	38,898,163
Common stock, shares outstanding (shares)	28,479,284	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)	439,951	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)	11,151,607	10,884,600