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Marketable Securities (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jul. 01, 2017
Jun. 30, 2018
Jul. 01, 2017
Marketable Securities [Abstract]        
Investment income $ 178 $ 672 $ (187) $ 1,800
Unrealized loss on equity securities $ 459   $ 1,200