| Cover Page - shares | 9 Months Ended | |
| Sep. 30, 2023 | Nov. 08, 2023 | |
| Cover [Abstract] | ||
| Document Type | 10-Q/A | |
| Document Quarterly Report | true | |
| Document Period End Date | Sep. 30, 2023 | |
| Document Transition Report | false | |
| Entity File Number | 001-40791 | |
| Entity Registrant Name | 2seventy bio, Inc. | |
| Entity Incorporation, State or Country Code | DE | |
| 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 | |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share | |
| Trading Symbol | TSVT | |
| Security Exchange Name | NASDAQ | |
| Entity Current Reporting Status | Yes | |
| Entity Interactive Data Current | Yes | |
| Entity Filer Category | Non-accelerated Filer | |
| Entity Small Business | true | |
| Entity Emerging Growth Company | true | |
| Entity Ex Transition Period | false | |
| Entity Shell Company | false | |
| Entity Common Stock, Shares Outstanding | 50,624,606 | |
| Amendment Flag | false | |
| Document Fiscal Year Focus | 2023 | |
| Document Fiscal Period Focus | Q3 | |
| Entity Central Index Key | 0001860782 | |
| Current Fiscal Year End Date | --12-31 |
