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Cash and Investments (Narrative) (Details)
3 Months Ended 12 Months Ended
Dec. 28, 2018
USD ($)
investment
Dec. 28, 2018
USD ($)
investment
Dec. 29, 2017
USD ($)
investment
Dec. 30, 2016
USD ($)
Jan. 01, 2016
USD ($)
Aug. 31, 2016
Investment [Line Items]            
Gain (loss) on sales of investments available-for-sale       $ 0 $ 0  
Number of investments in an unrealized loss position | investment 199 199 134      
Other-than-temporary impairment charges on available-for-sale securities     $ 0 0 $ 0  
Beneficial ownership interest by BlackRock (more than) 10.00% 10.00%        
Fair value of cash and investments managed by BlackRock $ 298,500,000 $ 298,500,000 141,000,000      
Fair value of cash and investments managed by BlackRock in the BlackRock Liquidity Money Market Fund 3,000,000 3,000,000 1,000,000      
Fees for advisory services paid to BlackRock $ 200,000          
Akarna Therapeutics, Ltd.            
Investment [Line Items]            
Realized gain on sale of cost method investment   $ 200,000        
Realized gain on sale of cost method investment     $ 3,000,000 $ 2,500,000    
Cost method investment ownership percentage           9.00%