Condensed consolidated balance sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, allowance for bad debts	$ 20	$ 32
Accumulated depreciation	$ 44,676	$ 36,228
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	87,932,000	87,188,000
Common stock, shares outstanding	87,932,000	87,188,000
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	10,471,000	11,193,000
Common stock, shares outstanding	10,471,000	11,193,000