Nature of the Business and Basis of Presentation	12 Months Ended
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a biopharmaceutical company dedicated to developing the first generation of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of December 31, 2017 and December 31, 2016, the Company had an accumulated deficit of $216.9 million and $151.6 million, respectively. The Company’s cash and cash equivalents as of December 31, 2017 and December 31, 2016, includes $1.2 million and $2.4 million of unrestricted cash held by its Russian subsidiary. The future success of the Company is dependent upon its ability to obtain additional capital through issuances of equity and debt securities and from collaboration and grant agreements in order to further the development of its technology and product candidates, and ultimately upon its ability to attain profitable operations. There can be no assurance that the Company will be able to obtain the necessary financing to successfully develop and market its product candidates or attain profitability. Based on the current operating plan, the Company expects that its cash, cash equivalents, short-term investments and short-term restricted cash as of December 31, 2017, will fund its operating expenses and capital expenditure requirements into mid-2019. The Company has based this estimate on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it currently expects. Additionally, the process of testing product candidates in clinical trials is costly, and the timing of progress in these trials is uncertain. Because the Company’s product candidates are in various stages of clinical and preclinical development and the outcome of these efforts is uncertain, the Company cannot estimate the actual amounts necessary to successfully complete the development and commercialization of its product candidates or whether, or when, it may achieve profitability. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2017, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Nature of the Business and Basis of Presentation

Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a biopharmaceutical company dedicated to developing the first generation of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital.

The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities.

The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants.

Liquidity

The Company has incurred losses since inception and negative cash flows from operating activities. As of December 31, 2017 and December 31, 2016, the Company had an accumulated deficit of $216.9 million and $151.6 million, respectively. The Company’s cash and cash equivalents as of December 31, 2017 and December 31, 2016, includes $1.2 million and $2.4 million of unrestricted cash held by its Russian subsidiary. The future success of the Company is dependent upon its ability to obtain additional capital through issuances of equity and debt securities and from collaboration and grant agreements in order to further the development of its technology and product candidates, and ultimately upon its ability to attain profitable operations. There can be no assurance that the Company will be able to obtain the necessary financing to successfully develop and market its product candidates or attain profitability.

Based on the current operating plan, the Company expects that its cash, cash equivalents, short-term investments and short-term restricted cash as of December 31, 2017, will fund its operating expenses and capital expenditure requirements into mid-2019. The Company has based this estimate on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it currently expects. Additionally, the process of testing product candidates in clinical trials is costly, and the timing of progress in these trials is uncertain. Because the Company’s product candidates are in various stages of clinical and preclinical development and the outcome of these efforts is uncertain, the Company cannot estimate the actual amounts necessary to successfully complete the development and commercialization of its product candidates or whether, or when, it may achieve profitability.

Guarantees and Indemnifications

As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2017, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.