| Document and Entity Information | Aug. 12, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001528115 |
| Document Type | 8-K |
| Document Period End Date | Aug. 12, 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 |
