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Marketable Securities - Available for Sale Securities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 25, 2016
Dec. 27, 2015
Schedule of Available-for-sale Securities [Line Items]      
Net unrealized loss in other comprehensive income $ 2,545 $ 0 $ 0
Short-term AFS securities      
Amortized cost, short-term AFS securities 309,029    
Gross unrealized gains, short-term AFS securities 0    
Gross unrealized losses, short-term AFS securities (440)    
Fair value, short-term AFS securities 308,589    
Long-term AFS securities      
Amortized cost, long-term marketable securities 243,516    
Gross unrealized gains, long-term marketable securities 0    
Gross unrealized losses, long-term marketable securities (2,105)    
Fair value, long-term marketable securities 241,411    
US Treasury Securities [Member]      
Short-term AFS securities      
Amortized cost, short-term AFS securities 70,985    
Gross unrealized gains, short-term AFS securities 0    
Gross unrealized losses, short-term AFS securities (34)    
Fair value, short-term AFS securities 70,951    
Long-term AFS securities      
Amortized cost, long-term marketable securities 53,031    
Gross unrealized gains, long-term marketable securities 0    
Gross unrealized losses, long-term marketable securities (403)    
Fair value, long-term marketable securities 52,628    
Corporate Debt Securities [Member]      
Short-term AFS securities      
Amortized cost, short-term AFS securities 150,334    
Gross unrealized gains, short-term AFS securities 0    
Gross unrealized losses, short-term AFS securities (227)    
Fair value, short-term AFS securities 150,107    
Long-term AFS securities      
Amortized cost, long-term marketable securities 92,687    
Gross unrealized gains, long-term marketable securities 0    
Gross unrealized losses, long-term marketable securities (683)    
Fair value, long-term marketable securities 92,004    
US Government Agencies Securities [Member]      
Short-term AFS securities      
Amortized cost, short-term AFS securities 45,819    
Gross unrealized gains, short-term AFS securities 0    
Gross unrealized losses, short-term AFS securities (179)    
Fair value, short-term AFS securities 45,640    
Long-term AFS securities      
Amortized cost, long-term marketable securities 97,798    
Gross unrealized gains, long-term marketable securities 0    
Gross unrealized losses, long-term marketable securities (1,019)    
Fair value, long-term marketable securities 96,779    
Certificates of Deposit [Member]      
Short-term AFS securities      
Amortized cost, short-term AFS securities 9,300    
Gross unrealized gains, short-term AFS securities 0    
Fair value, short-term AFS securities 9,300    
Commercial Paper [Member]      
Short-term AFS securities      
Amortized cost, short-term AFS securities 32,591    
Gross unrealized gains, short-term AFS securities 0    
Fair value, short-term AFS securities $ 32,591