| Document and Entity Information Document | Jan. 06, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 6, 2022 |
| Entity Registrant Name | 2seventy bio, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-40791 |
| Entity Tax Identification Number | 86-3658454 |
| Entity Address, Address Line One | 60 Binney Street, |
| Entity Address, City or Town | Cambridge, |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 339 |
| Local Phone Number | 499-9300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Trading Symbol | TSVT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001860782 |
| Amendment Flag | false |
