Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounts receivable, allowance for doubtful accounts	$ 3,084	$ 1,975
Investments at fair value, cost basis	$ 9,880	$ 80,717
Treasury stock, shares	2,042,911	1,426,115
Class A Common Stock
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	49,457,698	47,031,095
Common stock, shares outstanding	47,414,787	45,604,980
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	10,493,358	10,493,358
Common stock, shares outstanding	10,493,358	10,493,358