Impact of Recently Issued Accounting Standards	9 Months Ended
Sep. 30, 2017
Accounting Policies [Abstract]
Impact of Recently Issued Accounting Standards	Impact of Recently Issued Accounting Standards The recent accounting pronouncements below may have a significant effect on the Company's financial statements. Recent accounting pronouncements that are not anticipated to have an impact on or are unrelated to the Company's financial condition, results of operations, or related disclosures are not discussed. Accounting Standards Update (“ASU”) No. 2016-09. In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation - Improvements to Employee Share-Based Payment Accounting. The new guidance simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The amendments in this standard were effective for the Company's annual year and first fiscal quarter beginning on January 1, 2017 with early adoption permitted. The Company adopted this guidance as of January 1, 2017 using a modified retrospective transition method. As a result of the adoption of this standard, the Company elected to change its policy from estimating forfeitures to recognizing forfeitures when they occur and as a result recorded an adjustment of $312,000 to accumulated deficit with a corresponding offset to additional paid-in-capital at January 1, 2017. The Company also reversed a deferred tax asset related to the balance of unrecognized excess tax benefits of $1.1 million, with an offsetting adjustment to the valuation allowance. ASU No. 2016-02. In February 2016, the FASB issued ASU No. 2016-02, Leases. Under the new guidance, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: (a) a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and (b) a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The ASU will be effective for the Company beginning January 1, 2019 with early adoption permitted. The Company is currently evaluating the impact of the application of this accounting standard update on its financial statements and related disclosures. ASU No. 2016-01. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The amended guidance requires the Company to measure and record equity investments, except those accounted for under the equity method of accounting that have a readily determinable fair value, at fair value and for the Company to recognize the changes in fair value in its consolidated statements of operations, instead of recognizing unrealized gains and losses through accumulated other comprehensive income (loss), as currently done under the existing guidance. The amended guidance also changes several disclosure requirements for financial instruments, including the methods and significant assumptions used to estimate fair value. The standard is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. The Company will adopt this guidance on January 1, 2018 and will make any adjustments to beginning balances through a cumulative-effect adjustment to accumulated deficit on that date. The Company is currently determining the effects the adoption will have on its financial statements and disclosures. ASU No. 2014-09. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("Topic 606”), which amended the existing accounting standards for revenue recognition, outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. The new standard requires a company to recognize revenue upon transfer of goods or services to a customer at an amount that reflects the expected consideration to be received in exchange for those goods or services. The amended guidance defines a five-step approach for recognizing revenue, which will require a company to use more judgment and make more estimates than under the current guidance. The amended guidance will be effective for the Company starting in the 2018 fiscal year, including interim periods. The new standard allows for two methods of adoption: (a) full retrospective adoption, meaning the standard is applied to all periods presented, or (b) modified retrospective adoption, meaning the cumulative effect of applying the new standard is recognized as an adjustment to the opening retained earnings balance. The Company currently plans on applying the modified retrospective method upon adoption in the first quarter of 2018. The Company is continuing to assess the potential impact that Topic 606 may have on its financial statements and related disclosures with respect to its collaboration agreement with MedImmune. The evaluation of variable consideration, and in particular, milestone payments due from MedImmune will require further judgment to assess whether to include them in the transaction price, which could accelerate revenue recognized under Topic 606 compared to the Milestone Method under Topic 605. However, the Company does not expect this to have a material impact on its financial position and results of operations. The Company is also continuing to assess the potential impact that Topic 606 may have with respect to its various grant agreements, and currently does not expect any impact on its financial position and results of operations.

Impact of Recently Issued Accounting Standards

The recent accounting pronouncements below may have a significant effect on the Company's financial statements. Recent accounting pronouncements that are not anticipated to have an impact on or are unrelated to the Company's financial condition, results of operations, or related disclosures are not discussed.

Accounting Standards Update (“ASU”) No. 2016-09. In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation - Improvements to Employee Share-Based Payment Accounting. The new guidance simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The amendments in this standard were effective for the Company's annual year and first fiscal quarter beginning on January 1, 2017 with early adoption permitted. The Company adopted this guidance as of January 1, 2017 using a modified retrospective transition method. As a result of the adoption of this standard, the Company elected to change its policy from estimating forfeitures to recognizing forfeitures when they occur and as a result recorded an adjustment of $312,000 to accumulated deficit with a corresponding offset to additional paid-in-capital at January 1, 2017. The Company also reversed a deferred tax asset related to the balance of unrecognized excess tax benefits of $1.1 million, with an offsetting adjustment to the valuation allowance.

ASU No. 2016-02. In February 2016, the FASB issued ASU No. 2016-02, Leases. Under the new guidance, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: (a) a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and (b) a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The ASU will be effective for the Company beginning January 1, 2019 with early adoption permitted. The Company is currently evaluating the impact of the application of this accounting standard update on its financial statements and related disclosures.

ASU No. 2016-01. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The amended guidance requires the Company to measure and record equity investments, except those accounted for under the equity method of accounting that have a readily determinable fair value, at fair value and for the Company to recognize the changes in fair value in its consolidated statements of operations, instead of recognizing unrealized gains and losses through accumulated other comprehensive income (loss), as currently done under the existing guidance. The amended guidance also changes several disclosure requirements for financial instruments, including the methods and significant assumptions used to estimate fair value. The standard is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. The Company will adopt this guidance on January 1, 2018 and will make any adjustments to beginning balances through a cumulative-effect adjustment to accumulated deficit on that date. The Company is currently determining the effects the adoption will have on its financial statements and disclosures.

ASU No. 2014-09. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("Topic 606”), which amended the existing accounting standards for revenue recognition, outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. The new standard requires a company to recognize revenue upon transfer of goods or services to a customer at an amount that reflects the expected consideration to be received in exchange for those goods or services. The amended guidance defines a five-step approach for recognizing revenue, which will require a company to use more judgment and make more estimates than under the current guidance. The amended guidance will be effective for the Company starting in the 2018 fiscal year, including interim periods. The new standard allows for two methods of adoption: (a) full retrospective adoption, meaning the standard is applied to all periods presented, or (b) modified retrospective adoption, meaning the cumulative effect of applying the new standard is recognized as an adjustment to the opening retained earnings balance. The Company currently plans on applying the modified retrospective method upon adoption in the first quarter of 2018. The Company is continuing to assess the potential impact that Topic 606 may have on its financial statements and related disclosures with respect to its collaboration agreement with MedImmune. The evaluation of variable consideration, and in particular, milestone payments due from MedImmune will require further judgment to assess whether to include them in the transaction price, which could accelerate revenue recognized under Topic 606 compared to the Milestone Method under Topic 605. However, the Company does not expect this to have a material impact on its financial position and results of operations. The Company is also continuing to assess the potential impact that Topic 606 may have with respect to its various grant agreements, and currently does not expect any impact on its financial position and results of operations.