Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) for interim financial information and pursuant to Form 10-Q and Article 10 of Regulation S-X of the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the Company’s opinion, all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation of the results of operations and cash flows for the periods presented have been included. Operating results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the year ending December 31, 2018 or for any future period. The balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. The accompanying condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2017 included in the Company’s prospectus dated March 14, 2018 filed with the SEC pursuant to Rule 424(b)(4) under the Securities Act of 1993, as amended (the Prospectus). There have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2018 as compared to the significant accounting policies described in the Company’s audited financial statements for the year ended December 31, 2017 included in the Prospectus. From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB), or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the JOBS Act), the Company meets the definition of an emerging growth company and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. The Company will remain an emerging growth company until the earliest of (1) the last day of its first fiscal year (a) following the fifth anniversary of the completion of its initial public offering, (b) in which the Company has a total annual gross revenue of at least $1.07 billion, or (c) in which the Company is deemed to be a large accelerated filer, which means the market value of the common stock that is held by non-affiliates exceeds $700.0 million of the prior June 30th and (2) the date on which the Company has issued more than $1.0 billion in non-convertible debt securities during the prior three-year period.
Principles of Consolidation	Principles of Consolidation During 2017, the Company established a wholly-owned subsidiary in Australia. The condensed consolidated financial statements include the Company’s accounts and those of its wholly-owned subsidiary. All intercompany accounts, transactions and balances have been eliminated.
Reverse Stock Split	Reverse Stock Split On March 9, 2018, the Company effected a reverse split of all shares of its common and preferred stock at a ratio of 1-for-3.96 (the Reverse Split). The par values and the authorized shares of the common and preferred stock were not adjusted as a result of the Reverse Split. All references to shares of common stock outstanding, average number of shares outstanding and per share amounts in these condensed consolidated financial statements and notes to the condensed consolidated financial statements have been adjusted within the condensed consolidated financial statements, on a retroactive basis, to reflect the Reverse Split.
Use of Estimates	Use of Estimates The preparation of the Company’s condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Estimates were used to determine the fair value of common stock prior to the IPO and are used to determine stock-based awards and other issuances, accruals for research and development costs, useful lives of long-lived assets, and uncertain tax positions. Actual results could differ materially from the Company’s estimates.
Cash Equivalents, Short-Term and Long-Term Investments	Cash Equivalents, Short-Term and Long-Term Investments Cash equivalents include marketable securities having an original maturity of three months or less at the time of purchase. Short-term investments have maturities of greater than three months at the time of purchase. Long-term investments have maturities greater than 12 months at the time of purchase. Collectively, cash equivalents, short-term and long-term investments are considered available-for-sale and are recorded at fair value. Unrealized gains and losses are recorded in accumulated other comprehensive loss. Realized gains and losses are included in interest and other income, net in the condensed consolidated statements of operations and comprehensive loss.
Restricted Cash	Restricted Cash Restricted cash at September 30, 2018 and December 31, 2017 comprises cash balances primarily held as security in connection with the Company’s facility lease agreement and is included in long-term assets in the condensed consolidated balance sheets.
Concentration of Credit Risk	Concentration of Credit Risk Cash equivalents, short-term and long-term investments are financial instruments that potentially subject the Company to concentrations of credit risk. The Company invests in money market funds, treasury bills and notes, government bonds, commercial paper and corporate notes. The Company limits its credit risk associated with cash equivalents, short-term and long-term investments by placing them with banks and institutions it believes are highly credit worthy and in highly rated investments.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs, consisting of legal, accounting, filing and other fees directly related to the Company’s IPO, are capitalized. The deferred offering costs, which totaled $3.6 million, were reclassified to additional paid-in capital upon the effectiveness of the IPO in March 2018. As of December 31, 2017, $1.3 million of deferred offering costs were capitalized and included in other long-term assets in the condensed consolidated balance sheet.
Revenue Recognition	Revenue Recognition The Company generates revenue from its option and license agreement for the development and commercialization of its product candidates. Option and license agreements may include non-refundable upfront research and development fees, option fees to obtain development and commercialization licenses for the Company’s products, milestone payments based on achievement of defined development, regulatory and sales targets, and royalties on sales of commercialized products. To date, the Company has not recognized revenue from sales of its product candidates. The Company recognizes revenue when all four of the following criteria have been met: (i) collectability is reasonably assured; (ii) delivery has occurred or services have been rendered; (iii) persuasive evidence of an arrangement exists; and (iv) the fee is fixed or determinable. Revenue under option and license arrangements is recognized based on evaluation of the performance obligations of the contract. Collectability is assessed based on evaluation of payment criteria as stated in the contract as well as the creditworthiness of the customer. Determination of whether delivery has occurred, or services rendered are based on management’s evaluation of the performance obligations as stated in the contract and progress made against those obligations. Evidence of arrangement is deemed to exist upon execution of the contract. Fees are considered fixed and determinable when the amount payable to the Company is no longer subject to any acceptance, refund rights or other contingencies that would alter the fixed nature of the fees charged for the deliverables. Option and license agreements may contain multiple elements as evaluated under Accounting Standards Codification (ASC) 605-25, Revenue Recognition- Multiple-Element Arrangements, including agreements to provide research and development services, participation in development and/or steering committees, manufacturing services, sharing of know-how and other information, and grants of licenses to develop and commercialize product candidates. Each deliverable under the agreement is evaluated to determine whether it qualifies as a separate unit of accounting based on whether the deliverable has standalone value to the customer. The arrangement’s consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the following hierarchy: (i) vendor-specific objective evidence of the fair value of the deliverable, if it exists; (ii) third-party evidence of selling price, if vendor-specific objective evidence is not available; or (iii) the best estimate of selling price if neither vendor-specific objective evidence or third-party evidence is available. A delivered item or items that do not qualify as a separate unit of accounting within the arrangement are combined with the other applicable undelivered items within the arrangement. The allocation of arrangement consideration and the recognition of revenue is then determined for those combined deliverables as a single unit of accounting. For a combined unit of accounting, non-refundable upfront fees are recognized as performance obligations related to the final deliverable are completed. In the case of research and development services, performance would generally be expected to be ratable over the estimated performance period unless the Company determines there is a discernible pattern of performance other than straight-line, in which case the Company uses a proportionate performance method to recognize the revenue over the estimated performance period. Amounts received in advance of performance are recorded as deferred revenue. If any of the initial deliverables are determined to have standalone value separate from the research and development services, then the allocated consideration is recorded as revenue when those items are delivered. Options to acquire development and commercialization licenses of the Company’s products are evaluated to determine if they are substantive. Fees for substantive options are recognized as revenue when the option is exercised by the collaboration partner and the Company has completed the deliverables that are associated with exercise of such option. Option and license agreements may also contain milestone payments that become due upon the achievement of certain milestones. The Company applies ASC 605-28, Revenue Recognition—Milestone Method. Under the milestone method, payments that are contingent upon achievement of a substantive milestone are recognized in the period in which the milestone is achieved. Milestones are defined as an event that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones subject to this guidance. Further, for the milestone to be considered substantive, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables, and the consideration must be commensurate with the Company’s performance to achieve the milestone. Non-substantive milestone payments are recognized as revenue over the estimated period of any remaining performance obligations.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred. Research and development expenses consist primarily of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical and clinical studies and research services, laboratory supplies and equipment maintenance, product licenses, and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical and clinical studies and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments associated with licensing agreements to acquire exclusive licenses to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate commercial use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered.
Recently Issued Accounting Standards or Updates Not Yet Effective	Recently Issued Accounting Standards or Updates Not Yet Effective In January 2016, the FASB issued ASU No. 2016-01 (Subtopic 825-10), Financial Instruments (ASU 2016-01). ASU 2016-01 requires management to measure equity investments at fair value with changes in fair value recognized in net income or loss. ASU 2016-01 will impact the disclosure and presentation of financial assets and liabilities. ASU 2016-01 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-01 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is not permitted. The Company is currently in the process of evaluating the impact of adoption of this standard on its consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18 (Topic 230), Restricted Cash, Statement of Cash Flows (ASU 2016-18). ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash and, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, 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 condensed consolidated statement of cash flows. ASU 2016-18 was effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-18 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delayed the effective date of ASU 2014-09 by one year. ASU 2014-09, as amended, became effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2014-09 will be effective for the Company for the year ended December 31, 2019, and all interim periods thereafter. Early adoption is permitted. ASU 2014-09 also permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The core principle of ASU 2014-09 is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. The Company is still in the process of evaluating the effect that this guidance will have on revenue recognition for its option and license agreement with Taiho Pharmaceutical Co., Ltd. (Taiho), specifically as it pertains to the non-refundable, non-creditable upfront cash payments to the Company totaling $35.0 million, an option exercise payment of $3.0 million and all other future contingent payments the Company may become entitled to. The Company expects its evaluation to be completed during 2018. In February 2016, the FASB issued ASU No. 2016-02 (Topic 842), Leases (ASU 2016-02). ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. For public entities, ASU 2016-02 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-02 is effective for the Company for the year ended December 31, 2020, and all interim periods thereafter. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its condensed consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07 (Topic 718), Compensation – Stock Compensation (ASU 2018-07). ASU 2018-07 requires an entity to expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. For public entities, ASU 2018-07 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-07 is effective for the Company for the year ended December 31, 2020, and all interim periods thereafter. Early adoption is permitted. The Company has not yet determined the potential effects of this ASU on its condensed consolidated financial statements. In August 2018, the FASB issued ASU No.2018-13 (Topic 820), Fair Value Measurement. ASU 2018-13 modifies the disclosure requirements on fair value measurement in Topic 820. For public entities, ASU 2018-013 is effective for fiscal years beginning after December 15, 2019. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2018-13 is effective for the Company for the year ended December 31, 2020, and all interim periods thereafter. Early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s consolidated financial statements.