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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Feb. 25, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Trading Symbol ANNX    
Entity Registrant Name ANNEXON, INC.    
Entity Central Index Key 0001528115    
Current Fiscal Year End Date --12-31    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Title of 12(b) Security Common stock, par value $0.001 per share    
Security Exchange Name NASDAQ    
Entity File Number 001-39402    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 27-5414423    
Entity Address, Address Line One 1400 Sierra Point Parkway    
Entity Address, Address Line Two Bldg C    
Entity Address, Address Line Three Suite 200    
Entity Address, City or Town Brisbane    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 94005    
City Area Code 650    
Local Phone Number 822-5500    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   38,563,565  
Entity Public Float     $ 750
ICFR Auditor Attestation Flag true    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Auditor Name KPMG LLP    
Auditor Location San Francisco, CA    
Auditor Firm ID 185    
Documents Incorporated by Reference

Portions of the Registrant’s definitive proxy statement relating to the 2022 Annual Meeting of Stockholders, which will be filed with the Securities and Exchange Commission within 120 days after the end of the Registrant’s fiscal year ended December 31, 2021, are incorporated by reference into Part III of this Annual Report on Form 10-K.