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Document and Entity Information
Nov. 09, 2021
Cover [Abstract]  
Amendment Flag false
Entity Central Index Key 0001528115
Document Type 8-K
Document Period End Date Nov. 09, 2021
Entity Registrant Name ANNEXON, INC.
Entity Incorporation State Country Code DE
Entity File Number 001-39402
Entity Tax Identification Number 27-5414423
Entity Address, Address Line One 180 Kimball Way
Entity Address, Address Line Two Suite 200
Entity Address, City or Town South San Francisco
Entity Address, State or Province CA
Entity Address, Postal Zip Code 94080
City Area Code (650)
Local Phone Number 822-5500
Written Communications false
Soliciting Material false
Pre Commencement Tender Offer false
Pre Commencement Issuer Tender Offer false
Security 12b Title Common Stock, par value $0.001 per share
Trading Symbol ANNX
Security Exchange Name NASDAQ
Entity Emerging Growth Company true
Entity Ex Transition Period false