Cover Page - shares	9 Months Ended
	Sep. 30, 2024	Nov. 11, 2024
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Sep. 30, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q3
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	Non-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	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		106,593,505
Prefunded Warrant [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		41,283,052