CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Trade receivables, allowance	$ 3,772	$ 3,225
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Treasury stock, shares	48,654	3,157
Common Class A
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	5,429,199	5,256,325
Common Class B
Common stock, shares authorized	60,000,000	60,000,000
Common stock, shares issued	2,428,191	2,443,191