| Cover Page | Dec. 14, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001733294 |
| Document Period End Date | Dec. 14, 2021 |
| Entity Registrant Name | Generation Bio Co. |
| Trading Symbol | GBIO |
| Entity File Number | 001-39319 |
| Entity Tax Identification Number | 81-4301284 |
| Entity Incorporation, State or Country Code | DE |
| Entity Emerging Growth Company | true |
| Entity Address, Address Line One | 301 Binney Street |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 617 |
| Local Phone Number | 655-7500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Security Exchange Name | NASDAQ |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Entity Ex Transition Period | false |
