Condensed consolidated balance sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounts receivable, allowance for bad debts	$ 86	$ 84
Accumulated depreciation	$ 59,029	$ 53,086
Class A Common Stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	84,463,000	83,584,000
Common stock, shares outstanding	84,463,000	83,584,000
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	8,589,000	9,448,000
Common stock, shares outstanding	8,589,000	9,448,000