Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Financial Instruments (Details 1)

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Fair Value of Financial Instruments (Details 1) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 26, 2015
Sep. 27, 2014
Summary of maturities of marketable securities    
Due in less than one year, adjusted cost $ 17,742  
Due after one year but within five years, adjusted cost 17,383  
Due after five years but within ten years, adjusted cost 4,673  
Due after ten years, adjusted cost 3,614  
Equity securities, adjusted cost 1,598  
Mutual Funds, adjusted cost 3,011  
Due in less than one year, fair value 17,589  
Due after one year but within five years, fair value 16,781  
Due after five years but within ten years, fair value 3,956  
Due after ten years, fair value 3,410  
Equity securities, fair value 1,345  
Mutual Funds, fair value 2,515  
Available-for-sale Debt Securities, Amortized Cost Basis 48,021  
Asset measured at, fair value $ 45,596  
Phantom Share Units (PSUs) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years  
Fair Value Liability Recorded for Share Based Awards Related Expense $ 10 $ 26