CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Allowance for doubtful accounts receivable	$ 578		$ 576
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Class A Common Stock
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	46,391,000	37,654,000
Common stock, shares outstanding	46,390,804	37,654,000
Class B Common Stock
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	215,000,000
Common stock, shares issued	28,718,000	37,236,000
Common stock, shares outstanding	28,718,409	37,236,000