Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, allowance for doubtful accounts	$ 1,516	$ 1,433
Investments at fair value, cost basis	$ 144,803	$ 96,351
Treasury stock, shares	1,191,477	707,416
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	46,908,723	34,163,042
Common stock, shares outstanding	45,717,246	33,455,626
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	19,912,230
Common stock, shares outstanding	10,493,358	19,912,230