Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 20, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Registrant Name	Avidity Biosciences, Inc.
Entity Central Index Key	0001599901
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		46,897,083
Entity File Number	001-39321
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-1336960
Entity Address, Address Line One	10975 N. Torrey Pines Road
Entity Address, Address Line Two	Suite 150
Entity Address, City or Town	La Jolla
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	92037
City Area Code	858
Local Phone Number	401-7900
Title of 12(b) Security	Common Stock, $0.0001 par value
Trading Symbol	RNA
Security Exchange Name	NASDAQ
Document Quarterly Report	true
Document Transition Report	false