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Cover Page - shares
 6 Months Ended
Jun. 28, 2019
Jul. 22, 2019
Cover page.    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 28, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Entity Registrant Name EXELIXIS, INC.  
Entity Central Index Key 0000939767  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Current Fiscal Year End Date --01-03  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   302,906,238
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company false  
Title of 12(b) Security Common Stock $.001 Par Value per Share  
Trading Symbol EXEL  
Security Exchange Name NASDAQ  
Entity File Number 000-30235  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 04-3257395  
Entity Address, Address Line One 1851 Harbor Bay Parkway  
Entity Address, City or Town Alameda,  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94502  
City Area Code 650  
Local Phone Number 837-7000  
Document Quarterly Report true  
Document Transition Report false