Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 9,671	$ 9,769
Fees receivable, allowance for doubtful accounts	300	1,100
Property and equipment, accumulated depreciation	$ 48,900	$ 47,700
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)	48,642,520	46,801,812
Common stock, shares outstanding (shares)	18,963,567	18,355,907
Treasury stock, par value (usd per share)	$ 0.01	$ 0.01
Treasury stock, shares (shares)	29,678,953	28,445,905