Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, allowance for doubtful accounts	$ 2,501	$ 2,327
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Treasury stock, shares	5,591,680	5,390,107
Common Class A
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	24,831,256	25,171,433
Treasury stock, shares	1,900,000
Common Class B
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	33,000,000	33,000,000
Common stock, shares issued	19,720,381	20,800,998
Common stock, shares outstanding	19,720,381	20,800,998