Consolidated balance sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable, allowance for bad debts	$ 84	$ 32
Class A Common Stock
Common stock, par value (in usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	83,584,000	87,188,000
Common stock, shares outstanding (in shares)	83,584,000	87,188,000
Class B Common Stock
Common stock, par value (in usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	9,448,000	11,193,000
Common stock, shares outstanding (in shares)	9,448,000	11,193,000