CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jul. 01, 2018	Dec. 31, 2017
Trade receivables, allowance	$ 2,919	$ 3,225
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Treasury stock, shares	43,155	3,157
Common Class A
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	5,385,339	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