Summary of Significant Accounting Policies	6 Months Ended
Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying condensed consolidated financial statements are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments which are necessary to present fairly the Company's financial position and results of operations for the reported periods. These condensed consolidated financial statements have been prepared on a basis substantially consistent with, and should be read in conjunction with, the audited financial statements for the year ended December 31, 2018 and notes thereto, included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on March 27, 2019. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. Certain information and note disclosures normally included in the audited financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. The results of operations for any interim period are not necessarily indicative of the results for the year ending December 31, 2019, or for any future period. The accompanying condensed consolidated financial statements reflect the operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and expenses during the reporting period. Actual results could differ from those estimates. Summary of Significant Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2019 are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, except as noted below with respect to the Company’s lease and marketable securities accounting policies and as noted within the “Recent Accounting Pronouncements – Recently Adopted Accounting Pronouncements” section. Leases The Company determines if an arrangement is, or contains, a lease at inception and then classifies the lease as operating or financing based on the underlying terms and conditions of the contract. Leases with terms greater than one year are initially recognized on the balance sheet as right-of-use assets and lease liabilities based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the incremental borrowing rate, which is the rate incurred to borrow, on a collateralized basis, an amount equal to the lease payments over a similar term and in a similar economic environment. The Company has elected not to recognize leases with terms of one year or less on the balance sheet. Marketable Securities The Company invests excess cash balances in short-term marketable securities. The investments in marketable securities are classified as either held-to-maturity or available-for-sale based on facts and circumstances present at the time of purchase. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expensed over the life of the instrument. Equity securities with readily determinable fair values are also carried at fair value with unrealized gains and losses included in other income (expense), net. Realized gains and losses on both debt and equity securities are determined using the specific identification method and are included in other income (expense), net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss. Recent accounting pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”), under its ASC or other standard setting bodies, and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which set out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). In July 2018, the FASB issued ASU 2018-10, Leases (Topic 842), Codification Improvements, and ASU 2018-11, Leases (Topic 842), Targeted Improvements. ASU 2018-10 clarified certain provisions and corrected unintended applications of the guidance such as the application of implicit rate, lessee reassessment of lease classification, and certain transition adjustments that should be recognized to earnings rather than to stockholders' equity. ASU 2018-11 provided an alternative transition method and practical expedient for separating contract components for the adoption of Topic 842. ASU 2016-02, ASU 2018-10, and ASU 2018-11 (collectively, "the new lease standards") superseded the previous leases standard, ASC 840 Leases. The Company adopted the new lease standards as of January 1, 2019 using the modified retrospective approach to recognize a cumulative-effect adjustment on the effective date and to not adjust financial information and disclosures required under the new lease standards for comparative prior periods. The Company did not elect for the package of practical expedients and assessed all contracts at the transition date. The Company did not utilize the practical expedient which allows the use of hindsight in determining lease term and assessing impairment in right-of-use assets. The Company elected to apply the practical expedient and accounted for each lease component and related non-lease component as one single component. The Company elected not to recognize leases with terms of one year or less on the balance sheet. The adoption of the new lease standards on January 1, 2019 resulted in the initial recognition of right-of-use asset of $2.2 million and operating lease liability of $2.3 million related to the operating lease for the Company’s office and laboratory space in Palo Alto, California on the consolidated balance sheets with no material impact to the consolidated statements of operations, stockholders’ equity or cash flows. Refer to Note 6. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. This update simplifies the accounting for certain financial instruments with down round features, a provision in an equity-linked financial instrument (or embedded feature) that provides a downward adjustment of the current exercise price based on the price of future equity offerings. Down round features are common in warrants, preferred shares, and convertible debt instruments issued by private companies and early-stage public companies. This update requires companies to disregard the down round feature when assessing whether the instrument is indexed to its own stock, for purposes of determining liability or equity classification. ASU 2017-11 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not result in a material impact on its consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of Topic 718 to include all share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 specifies that Topic 718 applies to all share-based payment transactions in which the grantor acquires goods and services to be used or consumed in its own operations by issuing share-based payment awards. ASU 2018-07 also clarifies that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under ASC 606. ASU 2018-07 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not result in a material impact on its consolidated financial statements and related disclosures. New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements, which intends to improve financial reporting by requiring earlier recognition of credit losses on certain financial assets, such as available-for-sale debt securities. The standard is effective for interim and annual periods after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurements, which eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of the FASB’s disclosure framework project. The standard is effective for interim and annual periods beginning after December 15, 2019. The standard specifies certain amendments which should be applied prospectively while all other amendments should be applied retrospectively. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which clarifies the accounting for implementation, set-up, and other upfront costs incurred in cloud computing arrangements. The standard is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2019

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying condensed consolidated financial statements are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments which are necessary to present fairly the Company's financial position and results of operations for the reported periods.

These condensed consolidated financial statements have been prepared on a basis substantially consistent with, and should be read in conjunction with, the audited financial statements for the year ended December 31, 2018 and notes thereto, included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on March 27, 2019. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. Certain information and note disclosures normally included in the audited financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. The results of operations for any interim period are not necessarily indicative of the results for the year ending December 31, 2019, or for any future period.

The accompanying condensed consolidated financial statements reflect the operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and expenses during the reporting period. Actual results could differ from those estimates.

Summary of Significant Accounting Policies

The significant accounting policies used in preparation of these condensed consolidated financial statements for the three and six months ended June 30, 2019 are consistent with those discussed in Note 2 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, except as noted below with respect to the Company’s lease and marketable securities accounting policies and as noted within the “Recent Accounting Pronouncements – Recently Adopted Accounting Pronouncements” section.

Leases

The Company determines if an arrangement is, or contains, a lease at inception and then classifies the lease as operating or financing based on the underlying terms and conditions of the contract. Leases with terms greater than one year are initially recognized on the balance sheet as right-of-use assets and lease liabilities based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the incremental borrowing rate, which is the rate incurred to borrow, on a collateralized basis, an amount equal to the lease payments over a similar term and in a similar economic environment. The Company has elected not to recognize leases with terms of one year or less on the balance sheet.

Marketable Securities

The Company invests excess cash balances in short-term marketable securities. The investments in marketable securities are classified as either held-to-maturity or available-for-sale based on facts and circumstances present at the time of purchase. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expensed over the life of the instrument. Equity securities with readily determinable fair values are also carried at fair value with unrealized gains and losses included in other income (expense), net. Realized gains and losses on both debt and equity securities are determined using the specific identification method and are included in other income (expense), net.

If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss.

Recent accounting pronouncements

From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”), under its ASC or other standard setting bodies, and adopted by the Company as of the specified effective date, unless otherwise discussed below.

Recently Adopted Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which set out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). In July 2018, the FASB issued ASU 2018-10, Leases (Topic 842), Codification Improvements, and ASU 2018-11, Leases (Topic 842), Targeted Improvements. ASU 2018-10 clarified certain provisions and corrected unintended applications of the guidance such as the application of implicit rate, lessee reassessment of lease classification, and certain transition adjustments that should be recognized to earnings rather than to stockholders' equity. ASU 2018-11 provided an alternative transition method and practical expedient for separating contract components for the adoption of Topic 842. ASU 2016-02, ASU 2018-10, and ASU 2018-11 (collectively, "the new lease standards") superseded the previous leases standard, ASC 840 Leases.

The Company adopted the new lease standards as of January 1, 2019 using the modified retrospective approach to recognize a cumulative-effect adjustment on the effective date and to not adjust financial information and disclosures required under the new lease standards for comparative prior periods. The Company did not elect for the package of practical expedients and assessed all contracts at the transition date. The Company did not utilize the practical expedient which allows the use of hindsight in determining lease term and assessing impairment in right-of-use assets. The Company elected to apply the practical expedient and accounted for each lease component and related non-lease component as one single component. The Company elected not to recognize leases with terms of one year or less on the balance sheet.

The adoption of the new lease standards on January 1, 2019 resulted in the initial recognition of right-of-use asset of $2.2 million and operating lease liability of $2.3 million related to the operating lease for the Company’s office and laboratory space in Palo Alto, California on the consolidated balance sheets with no material impact to the consolidated statements of operations, stockholders’ equity or cash flows. Refer to Note 6.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. This update simplifies the accounting for certain financial instruments with down round features, a provision in an equity-linked financial instrument (or embedded feature) that provides a downward adjustment of the current exercise price based on the price of future equity offerings. Down round features are common in warrants, preferred shares, and convertible debt instruments issued by private companies and early-stage public companies. This update requires companies to disregard the down round feature when assessing whether the instrument is indexed to its own stock, for purposes of determining liability or equity classification. ASU 2017-11 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not result in a material impact on its consolidated financial statements and related disclosures.

In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of Topic 718 to include all share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 specifies that Topic 718 applies to all share-based payment transactions in which the grantor acquires goods and services to be used or consumed in its own operations by issuing share-based payment awards. ASU 2018-07 also clarifies that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under ASC 606. ASU 2018-07 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not result in a material impact on its consolidated financial statements and related disclosures.

New Accounting Pronouncements Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements, which intends to improve financial reporting by requiring earlier recognition of credit losses on certain financial assets, such as available-for-sale debt securities. The standard is effective for interim and annual periods after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurements, which eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of the FASB’s disclosure framework project. The standard is effective for interim and annual periods beginning after December 15, 2019. The standard specifies certain amendments which should be applied prospectively while all other amendments should be applied retrospectively. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which clarifies the accounting for implementation, set-up, and other upfront costs incurred in cloud computing arrangements. The standard is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.