Summary of Significant Accounting Policies	6 Months Ended
Jun. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates— The accompanying consolidated financial statements have been prepared in accordance with U.S. GAAP. The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates. Accounting Standard Updates Recently Adopted Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. Effective January 1, 2018, the Company adopted the new revenue standards under Topic 606 using the modified retrospective approach. Results for reporting periods beginning after January 1, 2018 are presented under Topic 606, while prior period results are not adjusted and continue to be reported in accordance with the revenue standards under Topic 605. Under Topic 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company’s license and collaboration arrangements with Regeneron Pharmaceuticals, Inc. (“Regeneron”) and Editas Medicine, Inc. (“Editas”) are within the scope of Topic 606. Upon the adoption of Topic 606, the Company recorded a net decrease of $6.1 million to its deferred revenue and opening accumulated deficit as of January 1, 2018 for the cumulative effect of the adoption. The effect of the adoption is summarized for the Company’s license and collaboration agreements as follows: Collaboration Agreement with Regeneron— Under Topic 606, the transaction price at contract inception was determined to be $8.0 million, which was related to the non-refundable upfront payment for license and research services. The arrangement also provides for additional payments to the Company when certain development and regulatory milestones are achieved. Because these milestone payments are not within the control of the Company and are not considered probable of being achieved until the events occur, the Company did not include them in the transaction price at contract inception. The transaction price of $8.0 million at contract inception was allocated to two performance obligations. The Company’s deferred revenue associated with its Regeneron collaboration agreement as of December 31, 2017 under Topic 605 was $6.5 million. As a result of adopting Topic 606, the Company recorded a $6.5 million reduction to its deferred revenue and opening accumulated deficit during the three months ended March 31, 2018 as the performance obligations associated with the Regeneron deferred revenue were satisfied as of January 1, 2018. There was no outstanding deferred revenue associated with Regeneron as of March 31, 2018 or June 30, 2018. Collaboration Agreement with Editas— Under Topic 606, the transaction price at contract inception was determined to be $1.0 million, which was related to the non-refundable upfront payment for license and research services. The arrangement provides for additional payments to the Company when certain development and regulatory milestones are achieved. Because these milestone payments are not within the control of the Company and are not considered probable of being achieved until the events occur, the Company did not include them in the transaction price at contract inception. The transaction price of $1.0 million at contract inception was allocated to a single performance obligation. The Company’s deferred revenue associated with its Editas collaboration agreement as of December 31, 2017 under Topic 605 was $0.5 million. As a result of adopting Topic 606, the Company recorded an increase of $0.4 million to its deferred revenue and opening accumulated deficit during the three months ended March 31, 2018 due to differences in the timing of recognition under Topic 606. During the three and six months ended June 30, 2018, we recognized revenue of $0.5 million and $0.7 million, respectively, associated with Editas. Our deferred revenue balance of $0.8 million as of June 30, 2018 was associated with Editas and is expected to be recognized over a period of four months as the research and development services are performed. Under Topic 605, the Company’s revenue for the three and six months ended June 30, 2018 would have been $0.5 million and 1.1 million, respectively. Recently-Issued and Not Yet Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-2, Leases, which amends the current guidance on leasing activities to provide more transparency and comparability, and requires that all leases be recognized by lessees on their balance sheet as a right-of-use asset and corresponding lease liability, which are currently accounted for as operating leases. The new standard is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2018, with early adoption permitted. This ASU will be effective for the Company in the first quarter of 2019 and must be adopted using a modified retrospective transition approach. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements and related disclosures.

2. Summary of Significant Accounting Policies

Use of Estimates— The accompanying consolidated financial statements have been prepared in accordance with U.S. GAAP. The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.

Accounting Standard Updates Recently Adopted

Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. Effective January 1, 2018, the Company adopted the new revenue standards under Topic 606 using the modified retrospective approach. Results for reporting periods beginning after January 1, 2018 are presented under Topic 606, while prior period results are not adjusted and continue to be reported in accordance with the revenue standards under Topic 605. Under Topic 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

The Company’s license and collaboration arrangements with Regeneron Pharmaceuticals, Inc. (“Regeneron”) and Editas Medicine, Inc. (“Editas”) are within the scope of Topic 606.

Upon the adoption of Topic 606, the Company recorded a net decrease of $6.1 million to its deferred revenue and opening accumulated deficit as of January 1, 2018 for the cumulative effect of the adoption. The effect of the adoption is summarized for the Company’s license and collaboration agreements as follows:

Collaboration Agreement with Regeneron— Under Topic 606, the transaction price at contract inception was determined to be $8.0 million, which was related to the non-refundable upfront payment for license and research services. The arrangement also provides for additional payments to the Company when certain development and regulatory milestones are achieved. Because these milestone payments are not within the control of the Company and are not considered probable of being achieved until the events occur, the Company did not include them in the transaction price at contract inception. The transaction price of $8.0 million at contract inception was allocated to two performance obligations. The Company’s deferred revenue associated with its Regeneron collaboration agreement as of December 31, 2017 under Topic 605 was $6.5 million. As a result of adopting Topic 606, the Company recorded a $6.5 million reduction to its deferred revenue and opening accumulated deficit during the three months ended March 31, 2018 as the performance obligations associated with the Regeneron deferred revenue were satisfied as of January 1, 2018. There was no outstanding deferred revenue associated with Regeneron as of March 31, 2018 or June 30, 2018.

Collaboration Agreement with Editas— Under Topic 606, the transaction price at contract inception was determined to be $1.0 million, which was related to the non-refundable upfront payment for license and research services. The arrangement provides for additional payments to the Company when certain development and regulatory milestones are achieved. Because these milestone payments are not within the control of the Company and are not considered probable of being achieved until the events occur, the Company did not include them in the transaction price at contract inception. The transaction price of $1.0 million at contract inception was allocated to a single performance obligation. The Company’s deferred revenue associated with its Editas collaboration agreement as of December 31, 2017 under Topic 605 was $0.5 million. As a result of adopting Topic 606, the Company recorded an increase of $0.4 million to its deferred revenue and opening accumulated deficit during the three months ended March 31, 2018 due to differences in the timing of recognition under Topic 606.

During the three and six months ended June 30, 2018, we recognized revenue of $0.5 million and $0.7 million, respectively, associated with Editas. Our deferred revenue balance of $0.8 million as of June 30, 2018 was associated with Editas and is expected to be recognized over a period of four months as the research and development services are performed.

Under Topic 605, the Company’s revenue for the three and six months ended June 30, 2018 would have been $0.5 million and 1.1 million, respectively.

Recently-Issued and Not Yet Adopted Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-2, Leases, which amends the current guidance on leasing activities to provide more transparency and comparability, and requires that all leases be recognized by lessees on their balance sheet as a right-of-use asset and corresponding lease liability, which are currently accounted for as operating leases. The new standard is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2018, with early adoption permitted. This ASU will be effective for the Company in the first quarter of 2019 and must be adopted using a modified retrospective transition approach. The Company is currently evaluating the impact of the adoption of this standard on its consolidated financial statements and related disclosures.