Organization and Basis of Presentation	3 Months Ended
Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Basis of Presentation	Organization and Basis of Presentation Sesen Bio, Inc. (the “Company”), a Delaware corporation, is a late-stage clinical company developing targeted fusion protein therapeutics ("TFPTs") 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 with limited expression on normal cells. Binding of the target antigen by the TFPT allows for rapid internalization into the targeted cancer cell. 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 March 31, 2019 and December 31, 2018 and for the three months ended March 31, 2019 and 2018 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 March 31, 2019, its results of operations for the three months ended March 31, 2019 and 2018, its statement of shareholders' equity for the three months ended March 31, 2019 and 2018, and its cash flows for the three months ended March 31, 2019 and 2018. The financial data and the other financial information disclosed in these notes to the condensed consolidated financial statements related to the three-month periods are also unaudited. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 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, 2018 that was filed with the Securities and Exchange Commission (“SEC”) on March 1, 2019 (the "2018 Form 10-K"). The condensed consolidated financial statements include the accounts of the Company, 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, 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 private placements of its common stock and preferred stock, and convertible bridge notes, venture debt borrowings, its initial public offering ("IPO"), follow-on public offerings, sales effected in an "at-the-market" offering, and the License Agreement (the "License Agreement") with F. Hoffmann La-Roche Inc. (collectively, "Roche"). As of March 31, 2019, the Company had cash and cash equivalents totaling $42.4 million, net working capital of $38.4 million and an accumulated deficit of $192.5 million. 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 not at all. To date, the Company has no revenue from product sales and management expects continuing operating losses in the future. As of March 31, 2019, the Company had available cash and cash equivalents of $42.4 million, which it does not believe is sufficient to fund the Company’s current operating plan for at least the next twelve months after the date of this Form 10-Q filing. 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. These factors raise substantial doubt about the Company’s ability to continue as a going concern. The accompanying condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the ordinary course of business. The condensed consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty.

Organization and Basis of Presentation

Sesen Bio, Inc. (the “Company”), a Delaware corporation, is a late-stage clinical company developing targeted fusion protein therapeutics ("TFPTs") 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 with limited expression on normal cells. Binding of the target antigen by the TFPT allows for rapid internalization into the targeted cancer cell. 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 March 31, 2019 and December 31, 2018 and for the three months ended March 31, 2019 and 2018 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 March 31, 2019, its results of operations for the three months ended March 31, 2019 and 2018, its statement of shareholders' equity for the three months ended March 31, 2019 and 2018, and its cash flows for the three months ended March 31, 2019 and 2018. The financial data and the other financial information disclosed in these notes to the condensed consolidated financial statements related to the three-month periods are also unaudited. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 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, 2018 that was filed with the Securities and Exchange Commission (“SEC”) on March 1, 2019 (the "2018 Form 10-K").

The condensed consolidated financial statements include the accounts of the Company, 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, 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 private placements of its common stock and preferred stock, and convertible bridge notes, venture debt borrowings, its initial public offering ("IPO"), follow-on public offerings, sales effected in an "at-the-market" offering, and the License Agreement (the "License Agreement") with F. Hoffmann La-Roche Inc. (collectively, "Roche"). As of March 31, 2019, the Company had cash and cash equivalents totaling $42.4 million, net working capital of $38.4 million and an accumulated deficit of $192.5 million.

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 not at all.

To date, the Company has no revenue from product sales and management expects continuing operating losses in the future. As of March 31, 2019, the Company had available cash and cash equivalents of $42.4 million, which it does not believe is sufficient to fund the Company’s current operating plan for at least the next twelve months after the date of this Form 10-Q filing. 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. These factors raise substantial doubt about the Company’s ability to continue as a going concern. The accompanying condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the ordinary course of business. The condensed consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty.