Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 4.9	$ 4.7
Advisory fees receivable, allowance for doubtful accounts	0.7	0.3
Property and equipment, accumulated depreciation	$ 59.6	$ 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)	41,322,880	40,511,209
Common stock, shares outstanding (shares)	29,578,743	29,325,374
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)	11,744,137	11,185,835