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FAIR VALUE MEASUREMENTS - Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 26, 2018
Dec. 27, 2017
Dec. 28, 2016
Dec. 30, 2015
Debt Securities, Available-for-sale [Line Items]        
Cost Basis $ 24,750 $ 21,507 $ 11,607 $ 70,849
Cash and cash equivalents fair value 24,750 21,507    
Gross Unrealized Gains 0 2    
Gross Unrealized Losses (122) (76)    
Fair value of marketable securities 62,113 63,036    
Total cost basis 86,985 84,617    
Total fair value 86,863 84,543    
Level 1: | Mutual Fund        
Debt Securities, Available-for-sale [Line Items]        
Cost Basis 62,235 60,985    
Cash and cash equivalents fair value 62,113 60,924    
Gross Unrealized Gains 0      
Gross Unrealized Losses (122) (61)    
Fair value of marketable securities 62,113 60,924    
Level 2: | Corporate debt securities        
Debt Securities, Available-for-sale [Line Items]        
Marketable securities cost Basis 0 2,125    
Gross Unrealized Gains 0 2    
Gross Unrealized Losses 0 (15)    
Fair value of marketable securities 0 2,112    
Cash        
Debt Securities, Available-for-sale [Line Items]        
Cost Basis 19,746 16,138    
Cash and cash equivalents fair value 19,746 16,138    
Money market funds | Level 1:        
Debt Securities, Available-for-sale [Line Items]        
Cost Basis 5,004 5,369    
Cash and cash equivalents fair value $ 5,004 $ 5,369