CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, allowance for doubtful accounts	$ 2,367	$ 2,590
Class A Common Stock
Common Stock, par value (USD per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (shares)	500,000,000	500,000,000
Common Stock, shares issued (shares)	74,978,424	76,071,145
Common Stock, shares outstanding (shares)	74,978,424	76,071,145
Class B Common Stock
Common Stock, par value (USD per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (shares)	140,000,000	140,000,000
Common Stock, shares issued (shares)	25,670,684	25,670,684
Common Stock, shares outstanding (shares)	25,670,684	25,670,684