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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Mar. 18, 2021
Jun. 30, 2020
Document And Entity Information [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2020    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 000-29959    
Entity Registrant Name CASSAVA SCIENCES INC    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 91-1911336    
Entity Address, Address Line One 7801 N. Capital of Texas Highway    
Entity Address, Address Line Two Suite 260    
Entity Address, City or Town Austin    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 78731    
City Area Code 512    
Local Phone Number 501-2444    
Title of 12(b) Security Common Stock, $0.001 par value    
Trading Symbol sava    
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 Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 71,146,906
Entity Common Stock, Shares Outstanding   39,898,325  
Documents Incorporated by Reference

Portions of the Registrant's Proxy Statement for its 2021 Annual Meeting of Stockholders (the Proxy Statement), to be filed with the U.S. Securities and Exchange Commission, are incorporated by reference to Part III of this Form 10-K Report.

   
Amendment Flag false    
Entity Central Index Key 0001069530