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Commitments (Details)
 3 Months Ended
Mar. 30, 2018
USD ($)
debt_instrument
Dec. 30, 2017
USD ($)
Dec. 29, 2017
USD ($)
Long-term Purchase Commitment [Line Items]      
Number of standby letters of credit obtained | debt_instrument 2    
Amount drawn under letters of credit $ 0    
Collateral provided     $ 3,000,000
South San Fransisco Lease      
Long-term Purchase Commitment [Line Items]      
Credit limit 500,000 $ 500,000  
Workers Compensation Insurance Policies      
Long-term Purchase Commitment [Line Items]      
Credit limit 500,000   $ 600,000
Standby Letters of Credit      
Long-term Purchase Commitment [Line Items]      
Credit limit $ 1,000,000 $ 1,000,000