Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts receivable, allowance for doubtful accounts	$ 3,900	$ 1,975
Investments at fair value, cost basis	$ 635	$ 80,717
Treasury stock, shares	2,397,950	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	51,629,289	47,031,095
Common stock, shares outstanding	49,231,339	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	10,431,422	10,493,358
Common stock, shares outstanding	10,431,422	10,493,358