Significant Accounting Policies and Recent Accounting Pronouncements (Policies)	9 Months Ended
Sep. 30, 2018
Accounting Policies [Abstract]
Basis of presentation	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.
Recently adopted accounting standards and Recently issued accounting standards	Recently adopted accounting standards In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, codified as Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers ("ASC 606"), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard was effective on January 1, 2018 and the Company adopted this standard using the modified retrospective approach. As a result of this adoption, no amounts were recorded as a cumulative effect adjustment to accumulated deficit as of January 1, 2018. In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"). This update clarifies the changes to terms or conditions of a share-based payment award that require an entity to apply modification accounting. ASU 2017-09 is effective as of January 1, 2018. The adoption of this guidance did not have an impact on the Company's financial position, results of operations or cash flows. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. Amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 was effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this standard effective January 1, 2018. Upon adoption of ASU 2016-18, the Company applied the retrospective transition method for each period presented and included $10,000 and $20,000 of restricted cash in the beginning-of-period and end-of-period cash, cash equivalents and restricted cash balance, respectively, in the condensed consolidated statement of cash flows for the nine months ended September 30, 2018. A reconciliation of cash, cash equivalents and restricted cash for each period presented is provided in note 10 to these unaudited condensed consolidated financial statements. On December 22, 2017, the Tax Cuts and Jobs Act (the “Act”) was enacted in the United States. The Act reduces the U.S. federal corporate tax rate from 34% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. In December 2017, the SEC issued guidance under Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act directing taxpayers to consider the impact of the U.S. legislation as “provisional” when it does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete its accounting for the change in tax law. As of September 30, 2018, the Company had not yet completed its accounting for all of the tax effects of the enactment of the Act. Recently issued accounting pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) ("ASU 2016-02"). ASU 2016-02 addresses the financial reporting of leasing transactions. Under current guidance for lessees, leases are only included on the balance sheet if certain criteria, classifying the agreement as a capital lease, are met. This update will require the recognition of a right-of-use asset and a corresponding lease liability, discounted to the present value, for all leases that extend beyond 12 months. For operating leases, the asset and liability will be expensed over the lease term on a straight-line basis, with all cash flows included in the operating section of the statement of cash flows. For finance leases, interest on the lease liability will be recognized separately from the amortization of the right-of-use asset in the statement of operations and the repayment of the principal portion of the lease liability will be classified as a financing activity while the interest component will be included in the operating section of the statement of cash flows. This guidance is effective for annual and interim reporting periods beginning after December 15, 2018. Early adoption is permitted. The Company is in the process of implementing a plan for the adoption of FASB ASU No. 2016-02. Through these implementation efforts, the Company has determined that it intends to elect to apply the package of practical expedients, the practical expedient to not apply the recognition requirements of FASB ASU No. 2016-02 to short-term leases, and the practical expedient allowing for an accounting policy election to choose not to separate non-lease components from lease components for certain classes of assets, and instead to account for each non-lease component with its associated lease component as a single lease component. The Company does not intend to elect to apply the hindsight practical expedient and plans on adopting FASB ASU No. 2016-02 on January 1, 2019 using the transition expedient. The Company expects that adoption of this standard will result in the recognition of a lease liability and right-of-use asset for certain operating leases that are currently not recorded on the condensed consolidated balance sheets. Critical accounting policies There have been no material changes to the critical accounting polices recently disclosed in the 2017 Form 10-K other than the adoption of ASU 2014-09 and related updates which are codified as ASC 606. The Company enters into collaboration agreements with strategic partners for the development and commercialization of product candidates which are within the scope of ASC 606. Under these agreements, the Company license rights to certain of the Company’s product candidates and may complete other performance obligations, such as the deliver of drug product or research and development services. The terms of these arrangements typically include payment of non-refundable upfront fees, milestone payments, and royalties on net sales of licensed products and may also contain additional payment provisions. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract, (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contact, (iii) measurement of the transaction price, including the constraint on variable consideration, (iv) allocation of the transaction price to the performance obligations, and (v) recognition of revenue when the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. At the end of each reporting period, the Company re-evaluates the probability of achievement of such milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services.

Recently adopted accounting standards

In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, codified as Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers ("ASC 606"), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard was effective on January 1, 2018 and the Company adopted this standard using the modified retrospective approach. As a result of this adoption, no amounts were recorded as a cumulative effect adjustment to accumulated deficit as of January 1, 2018.

In May 2017, the FASB issued ASU No. 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"). This update clarifies the changes to terms or conditions of a share-based payment award that require an entity to apply modification accounting. ASU 2017-09 is effective as of January 1, 2018. The adoption of this guidance did not have an impact on the Company's financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. Amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 was effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this standard effective January 1, 2018. Upon adoption of ASU 2016-18, the Company applied the retrospective transition method for each period presented and included $10,000 and $20,000 of restricted cash in the beginning-of-period and end-of-period cash, cash equivalents and restricted cash balance, respectively, in the condensed consolidated statement of cash flows for the nine months ended September 30, 2018. A reconciliation of cash, cash equivalents and restricted cash for each period presented is provided in note 10 to these unaudited condensed consolidated financial statements.

On December 22, 2017, the Tax Cuts and Jobs Act (the “Act”) was enacted in the United States. The Act reduces the U.S. federal corporate tax rate from 34% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. In December 2017, the SEC issued guidance under Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act directing taxpayers to consider the impact of the U.S. legislation as “provisional” when it does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete its accounting for the change in tax law. As of September 30, 2018, the Company had not yet completed its accounting for all of the tax effects of the enactment of the Act.

Recently issued accounting pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) ("ASU 2016-02"). ASU 2016-02 addresses the financial reporting of leasing transactions. Under current guidance for lessees, leases are only included on the balance sheet if certain criteria, classifying the agreement as a capital lease, are met. This update will require the recognition of a right-of-use asset and a corresponding lease liability, discounted to the present value, for all leases that extend beyond 12 months. For operating leases, the asset and liability will be expensed over the lease term on a straight-line basis, with all cash flows included in the operating section of the statement of cash flows. For finance leases, interest on the lease liability will be recognized separately from the amortization of the right-of-use asset in the statement of operations and the repayment of the principal portion of the lease liability will be classified as a financing activity while the interest component will be included in the operating section of the statement of cash flows. This guidance is effective for annual and interim reporting periods beginning after December 15, 2018. Early adoption is permitted. The Company is in the process of implementing a plan for the adoption of FASB ASU No. 2016-02. Through these implementation efforts, the Company has determined that it intends to elect to apply the package of practical expedients, the practical expedient to not apply the recognition requirements of FASB ASU No. 2016-02 to short-term leases, and the practical expedient allowing for an accounting policy election to choose not to separate non-lease components from lease components for certain classes of assets, and instead to account for each non-lease component with its associated lease component as a single lease component. The Company does not intend to elect to apply the hindsight practical expedient and plans on adopting FASB ASU No. 2016-02 on January 1, 2019 using the transition expedient. The Company expects that adoption of this standard will result in the recognition of a lease liability and right-of-use asset for certain operating leases that are currently not recorded on the condensed consolidated balance sheets.

Critical accounting policies

There have been no material changes to the critical accounting polices recently disclosed in the 2017 Form 10-K other than the adoption of ASU 2014-09 and related updates which are codified as ASC 606.

The Company enters into collaboration agreements with strategic partners for the development and commercialization of product candidates which are within the scope of ASC 606. Under these agreements, the Company license rights to certain of the Company’s product candidates and may complete other performance obligations, such as the deliver of drug product or research and development services. The terms of these arrangements typically include payment of non-refundable upfront fees, milestone payments, and royalties on net sales of licensed products and may also contain additional payment provisions.

Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract, (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contact, (iii) measurement of the transaction price, including the constraint on variable consideration, (iv) allocation of the transaction price to the performance obligations, and (v) recognition of revenue when the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer.

The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period.

The Company’s contracts include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. At the end of each reporting period, the Company re-evaluates the probability of achievement of such milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue in the period of adjustment.

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements.

The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract.

The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services.