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Consolidated Statement of Shareholders' Equity (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Oct. 01, 2017
Sep. 30, 2018
Oct. 01, 2017
Statement of Stockholders' Equity [Abstract]        
Tax effect of unrealized gain (loss) on available-for-sale investments $ 24 $ 41 $ (82) $ 191