Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 3,819	$ 3,872
Advisory fees receivable, allowance for doubtful accounts	0	300
Property and equipment, accumulated depreciation	$ 44,400	$ 62,100
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)	44,906,164	43,750,170
Common stock, shares outstanding (shares)	21,000,919	27,084,520
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)	23,905,245	16,665,560