Organization and Basis of Presentation	9 Months Ended
Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Basis of Presentation	Organization and Basis of Presentation Sesen Bio, Inc. (the “Company”), formerly Eleven Biotherapeutics, Inc., a Delaware corporation, is a late-stage clinical company developing targeted fusion proteins composed of an anti-cancer antibody fragment tethered to a protein toxin for the treatment of cancer. The Company genetically fuses the cancer-targeting antibody fragment and the cytotoxic protein payload into a single molecule which is produced through the Company's proprietary one-step manufacturing process. The Company targets tumor cell surface antigens that allow for rapid internalization into the targeted cancer cell while also having limited expression on normal cells. The Company has designed its targeted fusion proteins to overcome the fundamental efficacy and safety challenges inherent in existing antibody-drug conjugates ("ADCs"), where a payload is chemically attached to a targeting antibody. Basis of presentation The condensed consolidated financial statements as of September 30, 2018 and December 31, 2017 and for the three and nine months ended September 30, 2018 and 2017 and the related information contained within the notes to the condensed consolidated financial statements are unaudited. The unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting standards applicable to interim financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position as of September 30, 2018, its results of operations for the three and nine months ended September 30, 2018 and 2017 and its cash flows for the nine months ended September 30, 2018 and 2017. The financial data and the other financial information disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. The condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 that was filed with the Securities and Exchange Commission (“SEC”) on April 2, 2018 (the "2017 Form 10-K"). The condensed consolidated financial statements include the accounts of Sesen Bio, Inc., its wholly owned subsidiary, Viventia Bio Inc. ("Viventia"), and its indirect subsidiaries, Viventia Bio USA Inc. and Viventia Biotech (EU) Limited. All inter-company transactions and balances have been eliminated in consolidation. The condensed consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles. The functional currency of Viventia Bio Inc., Viventia Bio USA Inc. and Viventia Biotech (EU) Limited is the U.S. dollar. Liquidity The Company has financed its operations to date primarily through debt and equity offerings and collaboration and licensing arrangements. As of September 30, 2018, the Company had cash and cash equivalents totaling $57.9 million, net working capital of $52.8 million and an accumulated deficit of $179.3 million. During the nine months ended September 30, 2018, the Company raised approximately $50.9 million of net proceeds from the sale of its common stock and common stock purchase warrants and received an additional $7.3 million of proceeds upon the cash exercise of certain of its outstanding common stock purchase warrants. See note 5 to these unaudited condensed consolidated financial statements for additional information regarding the Company’s equity financing activities during the nine months ended September 30, 2018. The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. In order to commercialize its product candidates, the Company needs to complete clinical development and comply with comprehensive regulatory requirements. The Company is subject to a number of risks similar to other late-stage clinical companies, including, but not limited to, successful discovery and development of its product candidates, raising additional capital with favorable terms, development by its competitors of new technological innovations, protection of proprietary technology and market acceptance of the Company’s products. The successful discovery and development of product candidates requires substantial working capital which may not be available to the Company on favorable terms or at all. To date, the Company has no revenue from product sales and management expects continuing operating losses in the future. As of September 30, 2018, the Company had available cash and cash equivalents of $57.9 million, which it believes is sufficient to fund the Company’s current operating plan for at least the next twelve months from the date of this Form 10-Q filing. There can be no assurance, however, that the current operating plan will be achieved in the timeframe anticipated by the Company, or that its cash resources will fund the Company’s operating plan for the period anticipated by the Company. Management expects to seek additional funds through equity or debt financings or through additional collaboration, licensing transactions or other sources. The Company may be unable to obtain equity or debt financings or enter into additional collaboration or licensing transactions and, if necessary, the Company will be required to implement cost reduction strategies.

Organization and Basis of Presentation

Sesen Bio, Inc. (the “Company”), formerly Eleven Biotherapeutics, Inc., a Delaware corporation, is a late-stage clinical company developing targeted fusion proteins composed of an anti-cancer antibody fragment tethered to a protein toxin for the treatment of cancer. The Company genetically fuses the cancer-targeting antibody fragment and the cytotoxic protein payload into a single molecule which is produced through the Company's proprietary one-step manufacturing process. The Company targets tumor cell surface antigens that allow for rapid internalization into the targeted cancer cell while also having limited expression on normal cells. The Company has designed its targeted fusion proteins to overcome the fundamental efficacy and safety challenges inherent in existing antibody-drug conjugates ("ADCs"), where a payload is chemically attached to a targeting antibody.

Basis of presentation

The condensed consolidated financial statements as of September 30, 2018 and December 31, 2017 and for the three and nine months ended September 30, 2018 and 2017 and the related information contained within the notes to the condensed consolidated financial statements are unaudited. The unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting standards applicable to interim financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position as of September 30, 2018, its results of operations for the three and nine months ended September 30, 2018 and 2017 and its cash flows for the nine months ended September 30, 2018 and 2017. The financial data and the other financial information disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. The condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 that was filed with the Securities and Exchange Commission (“SEC”) on April 2, 2018 (the "2017 Form 10-K").

The condensed consolidated financial statements include the accounts of Sesen Bio, Inc., its wholly owned subsidiary, Viventia Bio Inc. ("Viventia"), and its indirect subsidiaries, Viventia Bio USA Inc. and Viventia Biotech (EU) Limited. All inter-company transactions and balances have been eliminated in consolidation. The condensed consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles.

The functional currency of Viventia Bio Inc., Viventia Bio USA Inc. and Viventia Biotech (EU) Limited is the U.S. dollar.

Liquidity

The Company has financed its operations to date primarily through debt and equity offerings and collaboration and licensing arrangements. As of September 30, 2018, the Company had cash and cash equivalents totaling $57.9 million, net working capital of $52.8 million and an accumulated deficit of $179.3 million.

During the nine months ended September 30, 2018, the Company raised approximately $50.9 million of net proceeds from the sale of its common stock and common stock purchase warrants and received an additional $7.3 million of proceeds upon the cash exercise of certain of its outstanding common stock purchase warrants. See note 5 to these unaudited condensed consolidated financial statements for additional information regarding the Company’s equity financing activities during the nine months ended September 30, 2018.

The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. In order to commercialize its product candidates, the Company needs to complete clinical development and comply with comprehensive regulatory requirements. The Company is subject to a number of risks similar to other late-stage clinical companies, including, but not limited to, successful discovery and development of its product candidates, raising additional capital with favorable terms, development by its competitors of new technological innovations, protection of proprietary technology and market acceptance of the Company’s products. The successful discovery and development of product candidates requires substantial working capital which may not be available to the Company on favorable terms or at all.

To date, the Company has no revenue from product sales and management expects continuing operating losses in the future. As of September 30, 2018, the Company had available cash and cash equivalents of $57.9 million, which it believes is sufficient to fund the Company’s current operating plan for at least the next twelve months from the date of this Form 10-Q filing. There can be no assurance, however, that the current operating plan will be achieved in the timeframe anticipated by the Company, or that its cash resources will fund the Company’s operating plan for the period anticipated by the Company. Management expects to seek additional funds through equity or debt financings or through additional collaboration, licensing transactions or other sources. The Company may be unable to obtain equity or debt financings or enter into additional collaboration or licensing transactions and, if necessary, the Company will be required to implement cost reduction strategies.