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Cover Page - USD ($)
 12 Months Ended
Jan. 01, 2021
Feb. 01, 2021
Jul. 03, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Jan. 01, 2021    
Current Fiscal Year End Date --01-01    
Document Transition Report false    
Entity File Number 000-30235    
Entity Registrant Name EXELIXIS, INC.    
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    
Title of 12(b) Security Common Stock $.001 Par Value per Share    
Trading Symbol EXEL    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 6,399,562,142
Entity Common Stock, Shares Outstanding (in shares)   311,989,661  
Documents Incorporated by Reference Certain portions of the registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission pursuant to Regulation 14A, not later than May 3, 2021, in connection with the registrant’s 2021 Annual Meeting of Stockholders are incorporated herein by reference into Part III of this Annual Report on Form 10-K.    
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000939767