Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Common stock, issued shares (in shares)	5,800,000
Treasury stock, shares (in shares)	2,701,162	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)	106,642,335	96,258,847
Common stock, outstanding shares (in shares)	103,941,173	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)	13,117,419	17,667,251
Common stock, outstanding shares (in shares)	13,117,419	17,667,251