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Cover - USD ($)
12 Months Ended
Dec. 31, 2022
Mar. 08, 2023
Jun. 30, 2022
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2022    
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 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 Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 496,765,830
Entity Common Stock, Shares Outstanding   50,189,753  
Documents Incorporated by Reference The registrant intends to file a definitive proxy statement pursuant to Regulation 14A relating to the 2023 Annual Meeting of Stockholders within 120 days of the end of the registrant’s fiscal year ended December 31, 2022. Portions of such definitive proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K to the extent stated herein.    
Amendment Flag false    
Entity Central Index Key 0001860782    
Document Fiscal Year Focus 2022    
Document Fiscal Period Focus FY