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Marketable Securities (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 24, 2016
Sep. 30, 2017
Sep. 24, 2016
Marketable Securities [Abstract]        
Investment income $ 831 $ 687 $ 2,600 $ 2,000