Cover - USD ($)	12 Months Ended
Cover - USD ($)	Dec. 31, 2024	Apr. 23, 2025	Jun. 30, 2024
Cover [Abstract]
Document Type	10-K/A
Document Annual Report	true
Document Period End Date	Dec. 31, 2024
Current Fiscal Year End Date	--12-31
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	617
Local Phone Number	675-7270
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	TSVT
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 198,251,396
Entity Common Stock, Shares Outstanding		53,228,315
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE None.
Amendment Description	The registrant is filing this Amendment No. 1 to Annual Report on Form 10-K/A, or this Amendment (also referred to herein as this report), to amend the Annual Report on Form 10-K for the fiscal year ended December 31, 2024 (Commission File Number 001-40791), or the Annual Report on Form 10-K, as filed by the registrant with the Securities and Exchange Commission, or the SEC, on March 25, 2025. The sole purpose of this Amendment is to include in Part III the information previously omitted from the Annual Report on Form 10-K in reliance on General Instruction G(3) to Form 10-K. This Amendment hereby amends the cover page, Part III, Items 10 through 14, and Part IV, Item 15 of the Annual Report on Form 10-K. In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended, or the Exchange Act, new certifications by the registrant’s principal executive officer and principal financial officer are filed as exhibits to this Amendment. Because no financial statements have been included in this Amendment, and this Amendment does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted.Except as explicitly set forth herein, this Amendment does not purport to modify or update the disclosures in, or exhibits to, the Annual Report on Form 10-K. This Amendment does not reflect events occurring after the filing of the Annual Report on Form 10-K (i.e., those events occurring after March 25, 2025) or modify or update those disclosures that may be affected by subsequent events. Accordingly, this Amendment should be read in conjunction with the Annual Report on Form 10-K and the registrant’s other filings with the SEC.In this report, unless otherwise indicated or the context otherwise requires, all references to “2seventy bio,” “the registrant,” “the company,” “we,” “us,” and “our” refer to 2seventy bio, Inc.
Amendment Flag	true
Entity Central Index Key	0001860782
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY

2seventy bio, Inc. 10-K/A