| Document and Entity Information | Jan. 05, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001590877 |
| Document Type | 8-K |
| Document Period End Date | Jan. 5, 2021 |
| Entity Registrant Name | REGENXBIO INC. |
| Entity Incorporation State Country Code | DE |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.0001 par value per share |
| Trading Symbol | RGNX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
