Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Stockholders' equity (deficit)
Allowance for doubtful accounts	$ 4,398	$ 2,700
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	20,000,000	20,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Class A common stock
Stockholders' equity (deficit)
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized	250,000,000	250,000,000
Common stock, issued	37,278,690	36,758,233
Common stock, outstanding	37,192,364	36,749,072
Class B common stock
Stockholders' equity (deficit)
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	75,000,000	75,000,000
Common stock, issued	69,066,445	69,066,445
Common stock, outstanding	50,706,629	50,836,629
Class C common stock
Stockholders' equity (deficit)
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	1	1
Common stock, issued	1	1
Common stock, outstanding	1	1