Description of the business	12 Months Ended
Description of the business	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the business	Description of the business 2seventy bio, Inc. (the “Company” or “2seventy bio”) is a cell and gene therapy company focused on the research, development, and commercialization of transformative treatments for cancer. The Company’s approach combines its expertise in T cell engineering technology and lentiviral vector gene delivery approaches, experience in research, development, and manufacturing of cell therapies and a suite of technologies that can be selectively deployed to develop highly innovative, targeted cellular therapies for patients with cancer. The Company is advancing multiple preclinical and clinical programs in oncology and, together with BMS, delivering the first FDA-approved CAR T therapy in multiple myeloma, Abecma, to patients in the United States. Please refer to Note 10, Collaborative arrangements and strategic partnerships, for further discussion of the collaboration with BMS. 2seventy bio Securities Corporation is a wholly-owned subsidiary of the Company which was incorporated in Massachusetts on December 13, 2021 and was granted securities corporation status in Massachusetts for the 2021 tax year. 2seventy bio Securities Corporation has no employees. On January 29, 2024, the Company began undertaking a strategic realignment to focus on the development and commercialization of Abecma. In connection with the strategic realignment, the Company entered into an asset purchase agreement with Regeneron Pharmaceuticals, Inc., or Regeneron, to sell to Regeneron substantially all of the assets related to its oncology and autoimmune cell therapy programs. Upon closing of the transaction, which is subject to customary closing conditions, Regeneron will assume all of the ongoing program, infrastructure and personnel costs related to these programs. The transaction is expected to close in the first half of 2024. The Company is subject to risks and uncertainties similar to other companies in the biotechnology industry. There can be no assurance that the Company’s clinical trials will be successfully completed, that it will maintain protection of proprietary technology, or that it will obtain the necessary regulatory approvals for Abecma or any future product candidates. Even following approval, Abecma or any future product candidates may not be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. Additionally, the Company is dependent on key personnel and on third party organizations such as financial institutions, collaborators, and contract manufacturing organizations. The separation from bluebird bio, Inc. In January 2021, bluebird bio, Inc. (“bluebird bio”) announced its plans to separate its oncology portfolio and programs from its severe genetic disease portfolio and programs, and spin off its oncology portfolio and programs into a separate, publicly traded company. In furtherance of this plan, 2seventy bio was incorporated as a Delaware corporation in April 2021, and in September 2021, bluebird bio’s board of directors approved the distribution of all of the issued and outstanding shares of 2seventy bio common stock on the basis of one share of 2seventy bio common stock for every three shares of bluebird bio common stock issued and outstanding on October 19, 2021, the record date for the distribution. As a result of the distribution, which occurred on November 4, 2021, 2seventy bio became an independent, publicly traded company. On November 3, 2021, the Company also entered into a separation agreement with bluebird bio, which is referred to in this annual report as the “Separation Agreement”, as well as various other agreements with bluebird bio, including a tax matters agreement, an employee matters agreement, an intellectual property license agreement, a transition services agreement under which 2seventy bio temporarily receives certain services from bluebird bio, and a second transition services agreement under which 2seventy bio temporarily provides certain services to bluebird bio. These agreements also govern certain of 2seventy bio’s relationships with bluebird bio after the separation. For additional information regarding the Separation Agreement and the other related agreements, refer to Note 14, Related-party transactions. Going concern In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern, the Company evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated and combined financial statements are issued. The Company has incurred losses and has experienced negative operating cash flows for all historical periods presented. During the year ended December 31, 2023, the Company incurred a net loss of $217.6 million and used $166.9 million of cash in operations. The Company expects to continue to generate operating losses and negative operating cash flows for the near future. As of December 31, 2023, the Company had cash, cash equivalents and marketable securities of $221.8 million. Based on the Company’s current operating plans, including with respect to the ongoing commercialization of Abecma and not taking into consideration our strategic realignment and the Asset Sale to Regeneron which is expected to close in the first half of 2024, the Company expects that its cash, cash equivalents and marketable securities, will be sufficient to fund current planned operations for at least the next twelve months from the date of filing this Annual Report on Form 10-K. The Company may, in the future, pursue additional cash resources through a combination of public or private equity offerings, debt financings, collaborations, strategic alliances or licensing arrangements with third parties. This includes the potential sale of shares of the Company’s common stock of up to $150.0 million in gross proceeds under the at-the-market, or the ATM, facility established in November 2022 with Cowen and Company, LLC. No sales of common stock have occurred under this ATM as of the date of this Annual Report on Form 10-K and the Company does not currently have any plans to sell shares under the ATM.