Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Treasury stock, shares (in shares)	2,617,587	1,095,048
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)	96,258,847	72,681,218
Common stock, outstanding shares (in shares)	93,641,260	71,586,170
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)	17,667,251	38,002,344
Common stock, outstanding shares (in shares)	17,667,251	38,002,344