Description of the business	6 Months Ended
Description of the business	Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the business	Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company is a biotechnology company committed to researching, developing and commercializing, following marketing approval, potentially transformative gene therapies for severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide selling, general and administrative support for these operations, including commercial activities in Europe as well as commercial-readiness activities in the United States. The Company’s programs in severe genetic diseases include programs for transfusion-dependent β-thalassemia, or TDT, sickle cell disease, or SCD, and cerebral adrenoleukodystrophy, or CALD. The Company’s programs in oncology are focused on developing novel engineered cell and gene therapies for cancer, including the anti-BCMA CAR T programs for multiple myeloma under the Company’s collaboration arrangement with Bristol-Myers Squibb ("BMS"). Please refer to Note 10, Collaborative arrangements, for further discussion of the Company’s collaboration with BMS. In March 2021, BMS received marketing approval from the U.S. Food and Drug Administration for idecabtagene vicleucel (ide-cel, marketed as ABECMA®) as a treatment of adult patients with relapsed or refractory multiple myeloma after four or more prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. In July 2021, the Company received marketing approval from the European Commission for elivaldogene autotemcel (eli-cel; formerly Lenti-D gene therapy) as a treatment for patients less than 18 years of age with early cerebral adrenoleukodystrophy and without a matched sibling donor. In June 2019, the Company received conditional marketing authorization from the European Commission for betibeglogene autotemcel (beti-cel; formerly LentiGlobin for β-thalassemia gene therapy) as a treatment of patients 12 years and older with TDT who do not have a β0/β0 genotype, for whom hematopoietic stem cell (HSC) transplantation is appropriate but a human leukocyte-matched related HSC donor is not available. In January 2021, the Company announced its intent to separate its severe genetic disease and oncology programs into two separate, independent publicly traded companies, bluebird bio, Inc. and 2seventy bio, Inc., a newly-formed Delaware corporation and wholly-owned subsidiary of the Company prior to the separation. bluebird bio, Inc. intends to retain focus on its severe genetic disease programs, with a focus on the U.S. market. As part of this strategy, bluebird bio, Inc. plans to execute an orderly wind down of its European operations. 2seventy bio, Inc. is expected to focus on the Company's oncology programs. The spin-off transaction is expected to be completed in late 2021 and is anticipated to be tax-free, subject to receipt of a favorable Internal Revenue Service (“IRS”) ruling. 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 financial statements are issued. The Company has incurred losses since inception and to date has financed its operations primarily through the sale of equity securities and, to a lesser extent, through collaboration agreements and grants from governmental agencies and charitable foundations. As of June 30, 2021, the Company had an accumulated deficit of $3.35 billion. During the six months ended June 30, 2021, the Company incurred a loss of $447.5 million and used $349.0 million of cash in operations. The Company expects to continue to generate operating losses and negative operating cash flows for the next few years and will need additional funding to support its planned operating activities through profitability. The transition to profitability is dependent upon the successful development, approval, and commercialization of beti-cel, eli-cel, LentiGlobin for SCD, and its oncology product candidates, and the achievement of a level of revenues adequate to support its cost structure. As of June 30, 2021, the Company had cash, cash equivalents and marketable securities of $941.6 million. The Company expects 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 issuance of these financial statements, although it intends to pursue additional cash resources through public or private equity or debt financings or by establishing additional collaborations with other companies. Management's expectations with respect to its ability to fund current planned operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management's estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any of these strategic or financing opportunities will be executed or executed on favorable terms, and some could be dilutive to existing stockholders. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or commercialize products following marketing approval.