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Geographic And Group Information (Geographic Revenue) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jul. 01, 2016
Jun. 30, 2017
Jul. 01, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]        
Total revenue $ 73,570 $ 75,613 $ 144,999 $ 144,391
North America [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Total revenue 55,247 65,013 113,867 124,530
International (Primarily European Countries) [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Total revenue $ 18,323 $ 10,600 $ 31,132 $ 19,861