Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Treasury stock, shares (in shares)	2,699,822	2,617,587
Class A Common Stock
Common stock, par value per share (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized shares (in shares)	600,000,000	600,000,000
Common stock, issued shares (in shares)	100,637,615	96,258,847
Common stock, outstanding shares (in shares)	97,937,793	93,641,260
Class B Common Stock
Common stock, par value per share (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized shares (in shares)	100,000,000	100,000,000
Common stock, issued shares (in shares)	13,317,419	17,667,251
Common stock, outstanding shares (in shares)	13,317,419	17,667,251