Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 02, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol RGNX  
Entity Registrant Name REGENXBIO Inc.  
Entity Central Index Key 0001590877  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity File Number 001-37553  
Entity Tax Identification Number 47-1851754  
Entity Address, Address Line One 9600 Blackwell Road  
Entity Address, Address Line Two Suite 210  
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  
Entity Common Stock, Shares Outstanding   36,806,632
Document Transition Report false  
Document Quarterly Report true  
Entity Interactive Data Current Yes  
Security Exchange Name NASDAQ  
Title of 12(b) Security Common Stock, $0.0001 par value per share  
Entity Incorporation, State or Country Code DE