| Document And Entity Information | May 08, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 08, 2024 |
| Entity Registrant Name | REGENXBIO Inc. |
| Entity Central Index Key | 0001590877 |
| Entity Emerging Growth Company | false |
| 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 |
| Entity Information, Former Legal or Registered Name | N/A |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Trading Symbol | RGNX |
| Security Exchange Name | NASDAQ |
