Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding	42,949,562
Entity Public Float		$ 1,310,113,569
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-37553
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-1851754
Entity Address, Address Line One	9804 Medical Center Drive
Entity Address, City or Town	Rockville
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20850
City Area Code	240
Local Phone Number	552-8181
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Specified portions of the registrant’s definitive proxy statement with respect to the registrant’s 2022 Annual Meeting of Stockholders, which is to be filed pursuant to Regulation 14A 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.
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Washington, District of Columbia
Auditor Firm ID	238