Denali Therapeutics (DNLI) 10-Q 8 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 02, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Denali Therapeutics Inc.
Entity Central Index Key	1,714,899
Trading Symbol	DNLI
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	false
Entity Common Stock, Shares Outstanding (shares)		94,957,092

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 43,651	$ 218,375
Short-term marketable securities	331,307	187,851
Prepaid expenses and other current assets	8,445	3,381
Total current assets	383,403	409,607
Long-term marketable securities	142,173	60,750
Property and equipment, net	16,245	14,923
Other non-current assets	2,654	1,441
Total assets	544,475	486,721
Current liabilities:
Accounts payable	3,354	2,716
Accrued liabilities	7,393	5,364
Accrued compensation	4,847	5,166
Contract liability	12,658	0
Deferred rent	3,227	855
Other current liabilities	138	63
Total current liabilities	31,617	14,164
Contract liability, less current portion	44,452	0
Deferred rent, less current portion	7,103	6,294
Other non-current liabilities	124	467
Total liabilities	83,296	20,925
Commitments and contingencies (Note 7)
Stockholders' equity:
Convertible preferred stock, $0.01 par value; 40,000,000 shares authorized as of September 30, 2018 and December 31, 2017; 0 shares issued and outstanding as of September 30, 2018 and December 31, 2017	0	0
Common stock, $0.01 par value; 400,000,000 shares authorized as of September 30, 2018 and December 31, 2017; 93,973,747 shares and 87,480,362 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	1,266	1,201
Additional paid-in capital	766,799	656,660
Accumulated other comprehensive loss	(1,416)	(368)
Accumulated deficit	(305,470)	(191,697)
Total stockholders' equity	461,179	465,796
Total liabilities and stockholders’ equity	$ 544,475	$ 486,721

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (usd per share)	$ 0.01	$ 0.01
Convertible preferred stock, shares authorized (in shares)	40,000,000	40,000,000
Convertible preferred stock, shares issued (in shares)	0	0
Convertible preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common stock, shares issued (in shares)	93,973,747	87,480,362
Common stock, shares outstanding (in shares)	93,973,747	87,480,362

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Collaboration revenue	$ 1,195	$ 0	$ 3,484	$ 0
Operating expenses:
Research and development	30,321	18,515	103,274	55,989
General and administrative	8,838	3,773	21,304	10,611
Total operating expenses	39,159	22,288	124,578	66,600
Loss from operations	(37,964)	(22,288)	(121,094)	(66,600)
Interest and other income, net	2,593	444	7,321	1,302
Net loss	(35,371)	(21,844)	(113,773)	(65,298)
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities, net of tax	77	140	(1,048)	136
Comprehensive loss	$ (35,294)	$ (21,704)	$ (114,821)	$ (65,162)
Net loss per share, basic and diluted (usd per share)	$ (0.38)	$ (2.14)	$ (1.24)	$ (6.77)
Weighted average number of shares outstanding, basic and diluted (in shares)	93,665,231	10,231,036	92,056,812	9,643,686

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (113,773)	$ (65,298)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,036	2,275
Stock–based compensation expense	13,145	2,952
Net amortization of premiums and discounts on marketable securities	(1,864)	899
Gain (loss) on disposal of property and equipment	(36)	1
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(5,379)	(3)
Accounts payable	1,235	318
Accrued and other current liabilities	2,639	884
Deferred rent	(1,182)	0
Contract liability	57,110	0
Other non-current liabilities	0	(327)
Net cash used in operating activities	(43,069)	(58,299)
Investing activities
Purchase of marketable securities	(400,637)	(46,651)
Purchase of property and equipment	(1,956)	(1,804)
Maturities and sales of marketable securities	176,574	102,438
Net cash (used in) provided by investing activities	(226,019)	53,983
Financing activities
Issuance of common stock in connection with collaboration agreement	94,406	0
Proceeds from exercise of awards under equity incentive plans	2,309	732
Net cash provided by financing activities	95,329	732
Net decrease in cash, cash equivalents and restricted cash	(173,759)	(3,584)
Cash, cash equivalents and restricted cash at beginning of period	218,910	40,388
Cash, cash equivalents and restricted cash at end of period	45,151	36,804
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	4,364	0
Property and equipment purchases accrued but not yet paid	37	78
Deferred IPO costs accrued but not yet paid	0	1,136
Common Stock [Member]
Financing activities
Payments of issuance costs related to issuance of stock	(1,342)	0
Preferred Stock [Member]
Financing activities
Payments of issuance costs related to issuance of stock	$ (44)	$ 0

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies Organization and Description of Business Denali Therapeutics Inc. ("Denali" or the “Company”) is a biopharmaceutical company, incorporated in Delaware, that discovers and develops therapeutics to defeat neurodegenerative diseases. The Company is headquartered in South San Francisco, California. 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 (“U.S. GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of SEC Regulation S-X for interim financial information. These unaudited condensed consolidated financial statements and notes should be read in conjunction with the audited consolidated financial statements and notes thereto contained in the Annual Report on Form 10-K for the year ended December 31, 2017 , as filed with the Securities and Exchange Commission on March 19, 2018 (the "2017 Annual Report on Form 10-K"). The condensed consolidated Balance Sheet as of December 31, 2017 was derived from the audited annual consolidated financial statements as of the period then ended. Certain information and footnote disclosures typically included in the Company's annual consolidated financial statements have been condensed or omitted. The accompanying unaudited condensed consolidated financial statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of the interim periods presented. All such adjustments are of a normal recurring nature except for the impacts of adopting new accounting standards discussed below. These interim financial results are not necessarily indicative of results expected for the full fiscal year or for any subsequent interim period. During the three and nine months ended September 30, 2018 , except as discussed below in the sections titled Derivatives and Hedging Activities, Revenue Recognition, and Recently Adopted Accounting Standards, there were no material changes to the Company's significant accounting and financial reporting policies from those reflected in the 2017 Annual Report on Form 10-K. For further information with regard to the Company’s Significant Accounting Policies, please refer to Note 1, "Significant Accounting Policies," to the Company’s Consolidated Financial Statements included in the 2017 Annual Report on Form 10-K. Initial Public Offering On December 7, 2017, the Company's Registration Statement on Form S-1 was declared effective by the SEC for Denali's initial public offering ("IPO") of common stock. In connection with the IPO, the Company sold an aggregate of 15,972,221 shares of common stock, including 2,083,333 shares sold pursuant to the underwriters’ full exercise of their option to purchase additional shares, at a price to the public of $18.00 per share. The aggregate net proceeds received by the Company from the offering, net of underwriting discounts and commissions and offering expenses, were $264.3 million . Upon the closing of the IPO, all then-outstanding shares of Company convertible preferred stock converted into 60,365,020 shares of common stock. The related carrying value of $378.6 million was reclassified to common stock and additional paid-in capital. Principles of Consolidation These unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated on consolidation. For the Company and its subsidiary, the functional currency has been determined to be U.S. dollars. Monetary assets and liabilities denominated in foreign currency are remeasured at period-end exchange rates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured at historical rates. Foreign currency transaction gains and losses resulting from remeasurement are recognized in interest and other income, net in the condensed consolidated statements of operations and comprehensive loss. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires the Company to make certain estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements, as well as the reported amounts of expenses during the reporting period. Actual results could differ from those estimates, and such differences could be material to the condensed consolidated financial position and statements of operations and comprehensive loss. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities and forward foreign currency exchange contracts. Substantially all of the Company’s cash and cash equivalents are deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents and marketable securities and issuers of marketable securities to the extent recorded on the consolidated balance sheets. As of September 30, 2018 and December 31, 2017 , the Company had no off-balance sheet concentrations of credit risk. The Company is exposed to counterparty credit risk on all of its derivative financial instruments. The Company has established and maintains strict counterparty credit guidelines and enters into hedges only with financial institutions that are investment grade or better to minimize the Company’s exposure to potential defaults. The Company does not require collateral to be pledged under these agreements. The Company is subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability. Segments The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources. Restricted Cash The Company’s restricted cash consists of the letter of credit for the Company’s headquarters building lease, and is included within other non-current assets on the accompanying condensed consolidated balance sheets. Derivatives and Hedging Activities The Company accounts for its derivative instruments as either assets or liabilities on the condensed consolidated balance sheet and measures them at fair value. Derivatives are adjusted to fair value through Interest and other income, net in the condensed consolidated statements of operations and comprehensive loss. Revenue Recognition License and Collaboration Revenues The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements (“ASC 808”) to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of Topic 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. The accounting treatment pursuant to Topic 606 is outlined below. The terms of licensing and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and royalties on net sales of licensed products. Each of these payments results in license, collaboration and other revenues, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenues. The core principle of Topic 606 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following 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 contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. Amounts received prior to satisfying the revenue recognition criteria are recorded as contract liabilities in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company's consolidated balance sheets. If the Company expects to have an unconditional right to receive the consideration in the next twelve months, this will be classified in current assets. A net contract asset or liability is presented for each contract with a customer. At contract inception, the Company assesses the goods or services promised in a contract with a customer and identifies those distinct goods and services that represent a performance obligation. A promised good or service may not be identified as a performance obligation if it is immaterial in the context of the contract with the customer, if it is not separately identifiable from other promises in the contract (either because it is not capable of being separated or because it is not separable in the context of the contract), or if the performance obligation does not provide the customer with a material right. The Company considers the terms of the contract and its customary business practices to determine the transaction price. The transaction price is the amount of consideration to which the Company expects to be entitled in exchange for transferring promised goods or services to a customer. The consideration promised in a contract with a customer may include fixed amounts, variable amounts, or both. Variable consideration will only be included in the transaction price when it is not considered constrained, which is when it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur. If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling prices. The relative selling price for each deliverable is estimated using objective evidence if it is available. If objective evidence is not available, the Company uses its best estimate of the selling price for the deliverable. Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset, which for a service is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the good or service promised to the customer. After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the transaction price is allocated to the performance obligations on the same basis as at contract inception. Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented. Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update ("ASU") No. 2016-02, Leases (Topic 842) , which supersedes the guidance in former ASC 840, Leases. The FASB issued further updates to this guidance in July 2018 through ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. The standard is effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and is required to be adopted using a modified retrospective approach. The Company plans to adopt this standard on January 1, 2019. ASU 2016-02 is expected to impact the Company’s consolidated financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. Management is continuing to evaluate the impact the adoption of ASU 2016-02 will have on the Company’s financial position, results of operations and related disclosures. Management expects that the adoption of this standard will result in the recognition of an asset for the right to use a leased facility on the Company’s balance sheet, as well as the recognition of a liability for the lease payments remaining on the lease. While the balance sheet presentation is expected to change, management still does not expect a material change to the condensed consolidated statements of operations and comprehensive loss or cash flows. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The standard adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company is currently assessing the impact of this standard on its consolidated financial statements. Recently Adopted Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014–09, Revenue from Contracts with Customers (Topic 606) , which amends the existing accounting standards for revenue recognition. The FASB issued further updates to this guidance through ASU 2016-12 Narrow-Scope Improvements and Practical Expedients, ASU 2016-10 Identifying Performance Obligations and Licensing and ASU 2016-08 Principal Versus Agent Considerations (Reporting Revenue Gross Versus Net) . The new standard is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This standard was adopted on January 1, 2018 using a full retrospective application. There was no impact to the consolidated financial statements upon adoption of ASU 2014-09 as the Company had not recognized any revenue through December 31, 2017. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The purpose of ASU 2016-18 is to clarify the guidance for and presentation of restricted cash in the statement of cash flows. The amendment requires beginning-of-period and end-of-period total amounts shown on the statement of cash flows to include cash and cash equivalents as well as restricted cash and restricted cash equivalents. This standard was adopted on January 1, 2018. Accordingly, the condensed consolidated statements of cash flows and Note 3 "Cash and Marketable Securities" have been updated to reconcile cash, cash equivalents and restricted cash for all periods presented. In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting , which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. This standard was adopted as of January 1, 2018 and will be applied prospectively to any award modified after the adoption date. In June 2018, the FASB issued ASU No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . ASU 2018-07 is intended to reduce the cost and complexity and to improve financial reporting for non-employee share-based payments. ASU 2018-07 expands the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to non-employees for goods or services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity-Based Payments to Non-Employees . The Company elected to early adopt this standard effective September 30, 2018. The new guidance will be applied to all new equity-classified share-based payment awards issued to non-employees after the date of adoption. In addition, for all previously issued equity-classified share-based payment awards to non-employees for which a measurement date was not established by the adoption date, these awards were remeasured at fair value as of the adoption date and will no longer be remeasured. The future expense for these share-based payment awards to non-employees will be based on the fair value as of the adoption date. The adoption of this standard will not result in any other changes to the condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Assets and liabilities measured at fair value at each balance sheet date are as follows (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 27,530 $ — $ — $ 27,530 Short-term marketable securities: U.S. government treasuries 181,248 — — 181,248 U.S. government agency securities — 92,096 — 92,096 Corporate debt securities — 55,012 — 55,012 Commercial paper — 2,951 — 2,951 Long-term marketable securities: U.S. government treasuries 75,829 — — 75,829 U.S. government agency securities — 11,840 — 11,840 Corporate debt securities — 54,504 — 54,504 Foreign currency derivative contracts — 4 — 4 Total $ 284,607 $ 216,407 $ — $ 501,014 Liabilities: Foreign currency derivative contracts $ — $ 123 $ — $ 123 Total $ — $ 123 $ — $ 123 December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 212,868 $ — $ — $ 212,868 Short-term marketable securities: U.S. government treasuries 42,587 — — 42,587 U.S. government agency securities — 106,139 — 106,139 Corporate debt securities — 39,125 — 39,125 Long-term marketable securities: U.S. government treasuries 39,848 — — 39,848 U.S. government agency securities — 19,911 — 19,911 Corporate debt securities — 991 — 991 Total $ 295,303 $ 166,166 $ — $ 461,469 The carrying amounts of accounts payable and accrued liabilities approximate their fair values due to their short-term maturities. The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly. There were no transfers of assets or liabilities between the fair value measurement levels during the three and nine months ended September 30, 2018 or 2017 .

Cash and Marketable Securities

Cash and Marketable Securities	9 Months Ended
	Sep. 30, 2018
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Marketable Securities	Cash and Marketable Securities Cash, cash equivalents and restricted cash A reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets to the amount reported within the condensed consolidated statements of cash flows is shown in the table below (in thousands): September 30, 2018 December 31, 2017 September 30, 2017 Cash and cash equivalents $ 43,651 $ 218,375 $ 36,269 Restricted cash included within prepaid expenses and other current assets — 84 84 Restricted cash included within other non-current assets 1,500 451 451 Total cash, cash equivalents, and restricted cash $ 45,151 $ 218,910 $ 36,804 Marketable Securities All marketable securities were considered available-for-sale at September 30, 2018 and December 31, 2017 . The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands): September 30, 2018 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 181,610 $ — $ (362 ) $ 181,248 U.S. government agency securities 92,330 — (234 ) 92,096 Corporate debt securities 55,136 — (124 ) 55,012 Commercial paper 2,951 — — 2,951 Total short-term marketable securities 332,027 — (720 ) 331,307 Long-term marketable securities: U.S. government treasuries 76,226 — (397 ) 75,829 U.S. government agency securities 11,919 — (78 ) 11,841 Corporate debt securities 54,723 — (220 ) 54,503 Total long-term marketable securities 142,868 — (695 ) 142,173 Total $ 474,895 $ — $ (1,415 ) $ 473,480 December 31, 2017 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 42,614 $ — $ (27 ) $ 42,587 U.S. government agency securities 106,368 — (229 ) 106,139 Corporate debt securities 39,197 — (72 ) 39,125 Total short-term marketable securities 188,179 — (328 ) 187,851 Long-term marketable securities: U.S. government treasuries 39,868 — (20 ) 39,848 U.S. government agency securities 19,931 — (20 ) 19,911 Corporate debt securities 991 — — 991 Total long-term marketable securities 60,790 — (40 ) 60,750 Total $ 248,969 $ — $ (368 ) $ 248,601 As of September 30, 2018 and December 31, 2017 , certain of the Company’s marketable securities were in an unrealized loss position. The Company determined that it had the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery, thus there has been no recognition of any other-than-temporary impairment for the three and nine months ended September 30, 2018 and 2017 . All marketable securities with unrealized losses as of each balance sheet date have been in a loss position for less than twelve months or the loss is not material. All of the Company’s marketable securities have an effective maturity of less than two years.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments Foreign Currency Exchange Rate Exposure The Company uses forward foreign currency exchange contracts to hedge certain operational exposures resulting from potential changes in foreign currency exchange rates. Such exposures result from portions of the Company’s forecasted cash flows being denominated in currencies other than the U.S. dollar, primarily the Euro and British Pound. The derivative instruments the Company uses to hedge this exposure are not designated as cash flow hedges, and as a result, changes in their fair value are recorded in Interest and other income, net , on the Company's condensed consolidated statements of operations and comprehensive loss. The fair values of forward foreign currency exchange contracts are estimated using current exchange rates and interest rates and take into consideration the current creditworthiness of the counterparties. Information regarding the specific instruments used by the Company to hedge its exposure to foreign currency exchange rate fluctuations is provided below. The Company did not have foreign currency exchange contracts prior to June 2018. The following table summarizes the Company’s forward foreign currency exchange contracts outstanding as of September 30, 2018 (notional amounts in thousands): Foreign Exchange Contracts Number of Contracts Aggregate Notional (1) Amount in Foreign Currency Maturity Euros 24 3,884 Oct. 2018 - Aug. 2019 British Pounds 19 1,625 Oct. 2018 - Aug. 2019 Swiss Francs 16 466 Oct. 2018 - Aug. 2019 Total 59 _________________________________________________ (1) The notional amount represents the net amount of foreign currency that will be received upon maturity of the forward contracts. The maximum length of time over which the Company is hedging its exposure to changes in exchange rates is through August 2019. The derivative liability balance of $0.1 million is recorded in Other current liabilities and the derivative asset balance of $3,667 is recorded in Prepaid assets and other current assets on the condensed consolidated balance sheet as of September 30, 2018 . The net loss associated with the Company's derivative instruments of $0.1 million is recognized in Interest and other income, net on the condensed consolidated statement of operations and comprehensive loss for the three and nine months ended September 30, 2018 .

Acquisition

Acquisition	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Acquisition	Acquisition In August 2016, the Company entered into a License and Collaboration Agreement (“F-star Collaboration Agreement”) with F-star Gamma Limited (“F-star Gamma”), F-star Biotechnologische Forschungs-Und Entwicklungsges M.B.H ("F-star GmbH") and F-star Biotechnology Limited ("F-Star Ltd") (collectively, “F-star”) to leverage F-star’s modular antibody technology and the Company’s expertise in the development of therapies for neurodegenerative diseases. In connection with the entry into the F-star Collaboration Agreement, the Company also purchased an option for an upfront option fee of $0.5 million (the “buy-out-option”), to acquire all of the outstanding shares of F-star Gamma pursuant to a pre-negotiated buy-out option agreement (the “Option Agreement”). On May 30, 2018, the Company exercised such buy-out option and entered into a Share Purchase Agreement (the “Purchase Agreement”) with the shareholders of F-star Gamma and Shareholder Representative Services LLC, pursuant to which the Company acquired all of the outstanding shares of F-star Gamma (the “Acquisition”). As a result of the Acquisition, F-star Gamma has become a wholly-owned subsidiary of the Company and the Company has changed the entity’s name to Denali BBB Holding Limited. In addition, the Company became a direct licensee of certain intellectual property of F-star Ltd (by way of the Company’s assumption of F-star Gamma’s license agreement with F-star Ltd, dated August 24, 2016, (the “F-star Gamma License”)). The Company has made initial exercise payments under the Purchase Agreement and the F-star Gamma License in the aggregate, of $18.0 million , less the estimated net liabilities of F-star Gamma, which is approximately $0.2 million . In addition, the Company is required to make future contingent payments, to F-star Ltd and the former shareholders of F-star Gamma, up to a maximum amount of $447.0 million in the aggregate upon the achievement of certain defined preclinical, clinical, regulatory and commercial milestones. The amount of the contingent payments varies based on whether F-star delivers an Fcab (constant Fc-domains with antigen-binding activity) that meets pre-defined criteria and whether the Fcab has been identified solely by the Company or solely by F-star or jointly by the Company and F-star. Under the terms of the original F-star Collaboration Agreement, the Company could nominate up to three Fcab targets (“Accepted Fcab Targets”) within the first three years of the date of the F-star Collaboration Agreement. Upon entering into the F-star Collaboration Agreement, the Company had selected transferrin receptor (“TfR”) as the first Accepted Fcab Target and paid F-star Gamma an upfront fee of $5.5 million , which included selection of the first Accepted Fcab Target. In May 2018, the Company exercised its right to nominate two additional Fcab Targets and identified a second Accepted fcab Target. The Company is obligated to make a one-time payment for the two additional Accepted Fcab Targets of, in the aggregate, $6.0 million and has extended the time period for its selection of the third Accepted Fcab Target until approximately the fourth anniversary of the date of the original F-Star Collaboration Agreement. The Company is also responsible for certain research costs incurred by F-star Ltd in conducting activities under each agreed development plan, for up to 24 months. These research costs for the agreed TfR development plan will be up to $2.1 million . The Company concluded that the assets acquired and liabilities assumed upon the exercise of the Option Agreement did not meet the accounting definition of a business, and as such, the acquisition was accounted for as an asset purchase. The Company recorded the upfront purchase price less estimated net liabilities acquired of $17.8 million in research and development expense in the accompanying condensed consolidated statement of operations and comprehensive loss in the nine months ended September 30, 2018 since it represented consideration for in-process research and development with no future alternative use. The upfront option fee of $0.5 million previously included within other non-current assets was also included in research and development expense during the nine months ended September 30, 2018 . As this transaction was accounted for as an asset purchase rather than a business combination, no amounts were recognized on the acquisition date relating to the contingent consideration. Contingent consideration will be recognized in research and development expense as incurred. The Company recognized $0.3 million and $0.8 million of research and development expense related to the funding of F-star Gamma research costs during the three and nine months ended September 30, 2018 , respectively, and $0.3 million and $0.8 million during the three and nine months ended September 30, 2017 , respectively.

License and Collaboration Agree

License and Collaboration Agreements	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License and Collaboration Agreements	License and Collaboration Agreements Takeda On January 3, 2018, the Company entered into a Collaboration and Option Agreement ("Takeda Collaboration Agreement") with Takeda Pharmaceutical Company Limited ("Takeda"), pursuant to which the Company granted Takeda an option in respect of three programs to develop and commercialize, jointly with the Company, certain biologic products that are enabled by Denali's BBB delivery technology and intended for the treatment of neurodegenerative disorders. The three programs are Denali’s ATV:BACE1/Tau and ATV: TREM2 programs, as well as a third identified discovery stage program. The Takeda Collaboration Agreement became effective on February 12, 2018 when the requirements of the Hart-Scott-Rodino Antitrust Improvements Act of 1976 were satisfied. Under the Takeda Collaboration Agreement and unless otherwise agreed jointly between both parties, Denali will be responsible, at its cost, for conducting activities relating to pre-IND development of biologic products directed to the three identified targets and enabled by its BBB delivery technology targeting transferrin receptor during the applicable research period. The period through which the option can be exercised continues for each target until the first biologic product directed to the relevant target is IND-ready or about five years after selection of the target, whichever is earlier. The Takeda Collaboration Agreement provided that Takeda pay a $40.0 million upfront payment, and up to an aggregate of $25.0 million with respect to each program directed to a target and based upon the achievement of certain preclinical milestone events, up to $75.0 million in total. The upfront payment of $40.0 million was received in February 2018 , as well as the first preclinical milestone payment of $5.0 million related to one of the programs. If Takeda exercises its option with respect to a particular target, then Takeda will have the right to develop and commercialize, jointly with the Company, a specified number of biologic products enabled by its BBB delivery technology that were developed during the research period and which are directed to the relevant target, and the Company will grant to Takeda a co-exclusive license under the intellectual property the Company controls related to those biologic products. Takeda is obligated to pay Denali a $5.0 million option fee for each target for which Takeda exercises its option, up to $15.0 million in total. In addition, Takeda may be obligated to pay Denali up to an aggregate of $707.5 million upon achievement of certain clinical and regulatory milestone events if Takeda exercises its option for all three collaboration programs. Takeda may also be obligated to pay Denali up to $75.0 million per biologic product upon achievement of a certain sales-based milestone, or an aggregate of $225.0 million if one biologic product from each program achieves the milestone. If Takeda exercises its option for a particular target, Denali and Takeda will share equally the development and commercialization costs, and, if applicable, the profits, for each collaboration program. However, for each collaboration program, the Company may elect not to continue sharing development and commercialization costs, or Takeda may elect to terminate Denali's cost-profit sharing rights and obligations if, following notice from Takeda and a cure period, the Company fails to satisfy its cost sharing obligations with respect to the relevant collaboration program. After such an election by the Company or termination by Takeda becomes effective, Denali will no longer be obligated to share in the development and commercialization costs for the relevant collaboration program, and will not share in any profits from that collaboration program. Instead the Company will be entitled to receive tiered royalties. The royalty rates will be in the low- to mid-teen percentages on net sales, or low- to high-teen percentages on net sales if certain co-funding thresholds have been met at the time of the Company's election to opt out of co-development or Takeda’s termination of Denali's cost-profit sharing rights and obligations, and, in each case, these royalty rates will be subject to certain reductions specified in the Takeda Collaboration Agreement. Takeda will pay these royalties for each biologic product included in the relevant collaboration program, on a country-by-country basis, until the latest of (i) the expiration of certain patents covering the relevant biologic product, (ii) the expiration of all regulatory exclusivity for that biologic product, and (iii) an agreed period of time after the first commercial sale of that biologic product in the applicable country, unless biosimilar competition in excess of a significant level specified in the Takeda Collaboration Agreement occurs earlier, in which case Takeda’s royalty obligations in the applicable country would terminate. For each collaboration program for which costs and profits are shared with Takeda, Denali will lead the conduct of clinical activities for each indication through the first Phase 2 trial with a clinical outcomes-based efficacy endpoints, and Takeda will lead the conduct of all subsequent clinical activities for that indication. Further, Denali and Takeda will jointly commercialize biologic products included in the relevant collaboration program in the United States and China. Unless Denali has opted out of cost-sharing for two collaboration programs, it has the right to lead commercialization activities in the United States for one collaboration program and Takeda will lead commercialization activities in the United States for all collaboration programs for which Denali does not lead commercialization activities. Further, Takeda will lead commercialization activities in China and will solely conduct commercialization activities in all other countries. The Company has the right to lead all manufacturing activities for all collaboration programs for which the parties are sharing costs and profits. Each party may terminate the Takeda Collaboration Agreement in its entirety, or with respect to a particular collaboration program, as applicable, if the other party remains in material breach of the Takeda Collaboration Agreement following a cure period to remedy the material breach. Takeda may terminate the Takeda Collaboration Agreement in its entirety or with respect to any particular collaboration program, for convenience and after giving a specified amount of prior notice, but Takeda may not do so for a certain period of time after the Effective Date of the Takeda Collaboration Agreement. Takeda may also terminate the Takeda Collaboration Agreement with respect to any collaboration program if the joint steering committee ("JSC") established under the Takeda Collaboration Agreement unanimously agrees that a material safety event has occurred with respect to the applicable collaboration program. Denali may terminate the Takeda Collaboration Agreement with respect to a particular collaboration program if Takeda fails to conduct material development and commercial activities for a specified period of time with respect to a collaboration program, unless Takeda cures such failure within a certain period of time. Denali and Takeda may each terminate the Takeda Collaboration Agreement in its entirety if the other party is declared insolvent or in similar financial distress or if, subject to a specified cure period, the other party challenges any patents licensed to it under the Takeda Collaboration Agreement. Pursuant to the terms of the Takeda Collaboration Agreement, the Company entered into a common stock purchase agreement (the "Stock Purchase Agreement") with Takeda on January 3, 2018 , pursuant to which Takeda purchased 4,214,559 shares of Denali’s common stock (the "Shares") for an aggregate purchase price of $110.0 million . The sale of the Shares closed on February 23, 2018 . The fair market value of the common stock sold to Takeda was $94.4 million , based on the closing stock price of $22.40 on the date of issuance, resulting in a $15.6 million premium paid to the Company above the fair value of the Company's common stock which was credited to contract liability in our condensed consolidated balance sheet. The Company believes that the Takeda Collaboration Agreement is a collaboration arrangement as defined in ASC 808, Collaborative Agreements . Further, during the research period, the Company believes that the arrangement is a contract with a customer as defined in ASC 606, Revenue From Contracts With Customers. The Takeda Collaboration Agreement and the Stock Purchase Agreement are being accounted for as one arrangement because they were entered into at the same time with interrelated financial terms. The Company identified performance obligations during the research period consisting of the license, the development options, and JSC participation together with the research services for each collaboration program. The license rights, JSC involvement, option and research services are considered to be a single performance obligation for each program since the research services are highly interrelated with the option and JSC involvement and will significantly modify the license. The performance obligations under each of the three programs are separate since the activities and risks under the programs are distinct. The Company has determined that all other goods or services which are contingent upon Takeda exercising its option for each program are not considered performance obligations at the inception of the arrangement. The transaction price at inception included fixed consideration consisting of the upfront fee of $40.0 million , the $15.6 million premium on the sale of common stock, and the first preclinical milestone payment of $5.0 million . It also included variable consideration of $26.0 million relating to future milestones that are not constrained. The amount of variable consideration was estimated using the most likely amount method. In October 2018, the Company confirmed the first preclinical milestone was met for the third identified discovery stage program, triggering a milestone payment of $5.0 million , which is expected to be received in November 2018. The remaining $44.0 million of preclinical milestones were considered constrained at the inception of the arrangement since the Company could not conclude it is probable that a significant reversal in the amount recognized will not occur. Additionally, cost and profit sharing income, and the development and commercial milestones as outlined above, have not been considered given Takeda has not exercised its options for the development and commercial phases for each program. There was no change in the transaction price from inception through September 30, 2018 . This will be reassessed at each reporting period. The transaction price has been ascribed in its entirety to the three performance obligations identified in the research term of the Takeda Collaboration Agreement. Revenue is recognized when, or as, the Company satisfies its performance obligations by transferring the promised services to Takeda. Revenue will be recognized over time using the input method, based on costs incurred to perform the research services, since the level of costs incurred over time is thought to best reflect the transfer of services to Takeda. A contract liability of $57.1 million is recorded on the balance sheet at September 30, 2018 , which relates to the three performance obligations identified, with such amounts to be recognized over the period of the pre-IND research services, which is expected to be several years. Revenue recognized relating to future milestone payments of approximately $1.2 million , for which the Company concluded that it is probable that a significant reversal in the amount recognized will not occur, is presented net of contract liability on the balance sheet. Significant changes in the net contract liability balance during the period are as follows (in thousands): Contract liability Balance at January 1, 2018 $ — Increases due to cash received, excluding amounts recognized as revenue during the period 58,329 Decreases due to revenue recognized in the period for which cash has not been received (1,219 ) Balance at September 30, 2018 $ 57,110 There are no receivables or net contract assets as of September 30, 2018 associated with this arrangement. In assessing this arrangement, management was required to exercise considerable judgment in estimating revenue to be recognized. Management applied judgment in determining the separate performance obligations in the research period, estimating variable consideration, and estimating total future costs when using the input method. Genentech In June 2016, the Company entered into an Exclusive License Agreement with Genentech, Inc. (“Genentech”). The agreement gives the Company access to Genentech’s LRRK2 small molecule program for Parkinson’s disease. Under the agreement, Genentech granted the Company (i) an exclusive, worldwide, sublicenseable license under Genentech’s rights to certain patents and patent applications directed to small molecule compounds which bind to and inhibit LRRK2 and (ii) a non-exclusive, worldwide, sublicenseable license to certain related know-how, in each case, to develop and commercialize certain compounds and licensed products incorporating any such compound. The Company is obligated to use commercially reasonable efforts during the first three years of the agreement to research, develop and commercialize at least one licensed product. As consideration, the Company paid an upfront fee of $8.5 million and a technology transfer fee of $1.5 million , both of which were recognized as research and development expense for the year ended December 31, 2016 . The Company may owe Genentech milestone payments upon the achievement of certain development, regulatory, and commercial milestones, up to a maximum of $315.0 million in the aggregate, as well as royalties on net sales of licensed products ranging from low to high single-digit percentages, with the exact royalty rate dependent on various factors, including (i) whether the compound incorporated in the relevant licensed product is a Genentech-provided compound or a compound acquired or developed by the Company, (ii) the date a compound was first discovered, derived or optimized by the Company, (iii) the existence of patent rights covering the relevant licensed product in the relevant country, (iv) the existence of orphan drug exclusivity covering a licensed product that is a Genentech-provided compound and (v) the level of annual net sales of the relevant licensed product. The Company also has the right to credit a certain amount of third-party royalty and milestone payments against royalty and milestone payments owed to Genentech, up to a maximum reduction of fifty percent. The Company’s royalty payment obligations will expire on a country-by-country and licensed product-by-licensed product basis upon the later of (a) ten years after the first commercial sale of such licensed product in such country and (b) the expiration of the last valid claim of a licensed patent covering such licensed product in such country. Genentech may terminate the agreement if the Company challenges any of the patent rights licensed to the Company by Genentech, or if the Company materially breaches the agreement, subject to specified notice and cure provisions, or enters into bankruptcy or insolvency proceedings. If Genentech terminates the agreement for the Company’s material breach, bankruptcy or insolvency after the Company has made a milestone payment to Genentech, then the Company is obligated to grant to Genentech an exclusive right of first negotiation with respect to certain of the Company’s patents, know-how and regulatory filings directed to Genentech-provided compounds. The Company does not have the right to terminate the agreement without cause, but may terminate the agreement for Genentech’s material breach, subject to specified notice and cure provisions. Unless earlier terminated, the agreement with Genentech will continue in effect until all of the Company’s royalty and milestone payment obligations to Genentech expire. Following expiration of the agreement, the Company will retain the licenses under the intellectual property Genentech licensed to the Company on a non-exclusive, royalty-free basis. The first clinical milestone of $2.5 million became due in June 2017 upon first patient dosing in the Phase 1 clinical trial for DNL201. The full amount was recorded as research and development expense in the nine months ended September 30, 2017 .

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Lease Obligations In September 2015, the Company entered into a non-cancelable operating lease for its corporate headquarters comprising 38,109 of rentable square feet in a building in South San Francisco (the "Headquarters Lease"). The Headquarters Lease commenced on August 1, 2016 with a lease term of eight years . The Headquarters Lease provides for monthly base rent amounts escalating over the term of the lease. In addition, the Headquarters Lease provided a tenant improvement allowance (“TIA”) of up to $7.4 million , of which $1.9 million was to be repaid to the landlord in the form of additional monthly rent with interest applied. This additional monthly rent commenced in November 2016 when the entire TIA was utilized, and resulted in an increase of base rent of $0.4 million per year over the eight -year lease term. On May 2, 2018 , the Company entered into an amendment to the Headquarters Lease (the "Headquarters Lease Amendment") to relocate and expand its headquarters to 148,020 rentable square feet in a to-be-constructed building located in South San Francisco, California (the "New Premises"). The Headquarters Lease Amendment has a contractual term of ten years from the legal commencement date, which is the later of February 1, 2019 or the date that the premises are ready for occupancy. For accounting purposes, the lease commencement date was determined to be August 1, 2018, which was the date at which the Company obtained control over the property. The Company has an option to extend the lease term for a period of ten years by giving the landlord written notice of the election to exercise the option at least nine months, but not more than twelve months, prior to the expiration of the Headquarters Lease Amendment lease term. Under the terms of the Headquarters Lease, the Company was required to pay a security deposit of $0.5 million , which was increased to $1.5 million under the Headquarters Lease Amendment. This is recorded as other non-current assets in the accompanying condensed consolidated balance sheets. The Headquarters Lease Amendment provides for monthly base rent amounts escalating over the term of the lease. In addition, the Headquarters Lease Amendment provides a TIA of up to $25.9 million , of which $4.4 million , if utilized, would be repaid to the landlord in the form of additional monthly rent. The Company will also be required to pay its share of operating expenses for the New Premises. The total $7.4 million TIA under the 2015 Headquarters Lease was recorded as leasehold improvements and deferred rent liability on the condensed consolidated balance sheet under the Headquarters Lease. The Company is amortizing the deferred rent liability as a reduction of rent expense and the leasehold improvement through an increase of depreciation expense of leasehold improvements ratably over the remaining period of expected use. The portion of the TIA utilized to date under the Headquarters Lease Amendment was $4.4 million and has been recorded as leasehold improvements and deferred rent liability on the condensed consolidated balance sheet as of September 30, 2018 . The Company will amortize the deferred rent liability as a reduction of rent expense and the leasehold improvement through an increase of depreciation expense of leasehold improvements ratably over the period of expected use, which is expected to commence in April 2019. The Company recognizes rent expense on a straight-line basis over the non-cancelable lease term and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Where leases contain escalation clauses, rent abatements, and/or concessions such as rent holidays and landlord or tenant incentives or allowances, the Company applies them in the determination of straight-line rent expense over the lease term. The Company records tenant improvement allowances as deferred rent and associated expenditures as leasehold improvements that are being amortized over the shorter of their estimated useful life or the term of the lease. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably assured at lease inception. As of September 30, 2018 , the future minimum lease payments under the Headquarters Lease and subsequently the Headquarters Lease Amendment are as follows (in thousands): Year Ended December 31: 2018 (three months) $ 658 2019 4,941 2020 9,097 2021 9,716 2022 10,056 2023 and later 71,290 $ 105,758 Rent expense excluding amortization of leasehold improvements was $2.4 million and $3.7 million for the three and nine months ended September 30, 2018 , and $0.4 million and $1.6 million for the three and nine months ended September 30, 2017 , respectively. Indemnification In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to vendors, lessors, business partners, board members, officers, and other parties with respect to certain matters, including, but not limited to, losses arising out of breach of such agreements, services to be provided by the Company, negligence or willful misconduct of the Company, violations of law by the Company, or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with directors and certain officers and employees that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. No demands have been made upon the Company to provide indemnification under such agreements, and thus, there are no claims that the Company is aware of that could have a material effect on the Company’s balance sheet, statements of comprehensive loss, or statements of cash flows. Commitments Effective September 2017, the Company entered into a Development and Manufacturing Services Agreement as amended (“DMSA”) with Lonza Sales AG (“Lonza”) for the development and manufacture of biologic products. Under the DMSA, the Company will execute purchase orders based on project plans authorizing Lonza to provide development and manufacturing services with respect to certain of the Company's antibody and enzyme products, and will pay for the services provided and batches delivered in accordance with the DMSA and project plan. Unless earlier terminated, the Lonza agreement will expire on September 6, 2022. As of September 30, 2018 , the Company had several purchase orders for biological product development and manufacturing costs totaling $13.0 million . The activities under these purchase orders are expected to be completed by April 2024 . During the three and nine months ended September 30, 2018 , the Company incurred costs of $1.4 million and $2.6 million , respectively, and made payments of $1.3 million and $2.0 million , respectively, for the development and manufacturing services rendered under the agreement. As of September 30, 2018 , the Company had total non-refundable purchase commitments of $7.7 million under the DMSA.

Stock-Based Awards

Stock-Based Awards	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Awards	Stock-Based Awards 2017 Equity Incentive Plan In December 2017, the Company adopted the 2017 Equity Incentive Plan (the “2017 Plan”), which initially reserved 6,379,238 shares for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Awards granted under the 2017 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value on the day of grant. Options granted typically vest over a four -year period but may be granted with different vesting terms. 2015 Stock Incentive Plan In May 2015, the Company adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which as amended, reserved 8,325,000 shares for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Awards granted under the 2015 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value on the day of grant. For all stock options granted between August 2015 and February 2016 with an exercise price of $0.68 , a deemed fair value of $1.20 per share was used in calculating stock-based compensation expense, which was determined using management hindsight. Options granted typically vest over a four -year p eriod but may be granted with different vesting terms. Upon adoption of the 2017 Plan, no new awards or grants are permitted under the 2015 Plan, and the 169,238 shares that were then unissued and available for future award under the 2015 Plan became available under the 2017 Plan. The 2015 Plan will continue to govern restricted stock awards and option awards previously granted thereunder. As of September 30, 2018 , there were 2,659,306 shares available for the Company to grant under the 2017 Plan. Stock Option Activity The following table summarizes option award activity under the 2017 Plan and the 2015 Plan: Number of Options Weighted- Exercise Price Weighted- remaining contractual life (years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2017 6,689,479 $ 4.08 8.37 $ 77,317 Options granted 3,591,734 20.55 Options exercised (489,483 ) 2.57 Options forfeited (238,542 ) 10.36 Balance at September 30, 2018 9,553,188 $ 10.18 8.39 $ 110,407 Options vested and expected to vest at September 30, 2018 7,808,456 $ 12.31 8.72 $ 73,663 Options exercisable at September 30, 2018 1,779,017 $ 3.78 7.84 $ 31,946 Aggregate intrinsic value represents the difference between the Company’s fair value of its common stock and the exercise price of outstanding options. The total intrinsic value of options exercised was $2.6 million and $7.3 million for the three and nine months ended September 30, 2018 , and $1.2 million and $3.5 million for the three and nine months ended September 30, 2017 , respectively. During the three and nine months ended September 30, 2018 , the weighted-average grant-date fair value of the vested options was $4.01 and $3.29 per share, respectively. During the three and nine months ended September 30, 2017 , the weighted-average grant-date fair value of the vested options was $2.43 and $2.08 per share, respectively. The weighted-average grant date fair value of all options granted during the three and nine months ended September 30, 2018 was $11.01 and $15.05 per share, respectively. The weighted-average grant date fair value of all options granted during the three and nine months ended September 30, 2017 was $7.08 and $4.84 per share, respectively. Stock Options Granted to Employees with Service-Based Vesting The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 6.08 6.08 5.50 - 6.08 6.08 Volatility 81.9% - 84.8% 86.8% - 88.1% 80.0% - 87.5% 86.8% - 91.3% Risk-free interest rate 2.8% - 2.9% 1.8% - 1.9% 2.6% - 2.9% 1.8% - 2.3% Dividend yield — — — — Expected Term: The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). Expected Volatility: The Company uses an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have sufficient trading history for its common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Risk-Free Interest Rate: The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant. Expected Dividend: The Company has not paid and does not anticipate paying any dividends in the near future. Therefore, the expected dividend yield was zero . Early Exercise of Stock Options The Company permits early exercise of certain stock options prior to vesting by certain directors and officers. Any shares issued pursuant to unvested options are restricted and subject to repurchase by the Company until the conditions for vesting are met. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported in stockholders’ equity once those shares vest. Upon termination of employment of an option holder, the Company has the right to repurchase, at the original purchase price, any unvested restricted shares. A total of $31,874 and $0.4 million was reclassified from other non-current liabilities to stockholders' equity during the three and nine months ended September 30, 2018 , respectively, related to vesting of early exercised options. A total of $31,874 and $0.2 million was reclassified from other non-current liabilities to stockholders' equity during the three and nine months ended September 30, 2017 , respectively, related to vesting of early exercised options. Unvested early exercised options of $0.1 million and $0.5 million remained in other non-current liabilities as of September 30, 2018 and December 31, 2017 , respectively. Performance and Market Contingent Stock Options Granted to Employees In August and November 2015, the Board of Directors granted performance- and market- contingent options to purchase 1,619,738 shares and 125,000 shares of the Company's common stock, respectively, to members of the senior management team. These awards have an exercise price of $0.68 per share. These awards have two separate market triggers for vesting based upon either (i) the successful achievement of stepped target closing prices on a national securities exchange for 90 consecutive trading days later than 180 days after the Company’s initial public offering for its common stock, or (ii) stepped target prices for a change in control transaction. By definition, the market condition in these awards can only be achieved after the performance condition of a liquidity event has been achieved. As such, the requisite service period is based on the estimated period over which the market condition can be achieved. When a performance goal is deemed to be probable of achievement, time-based vesting and recognition of stock-based compensation expense commences. In the event any the milestones are not achieved by the specified timelines, such award will terminate and no longer be exercisable with respect to that portion of the shares. The maximum potential expense associated with the performance- and market- contingent awards is $6.2 million ( $5.8 million and $0.4 million of general and administrative and research and development expense, respectively) if all of the performance and market conditions are achieved as stated in the option agreement. Management evaluates whether the achievement of performance- and market- contingent awards is probable at each reporting date. Management has concluded that the achievement of these awards is not probable through September 30, 2018 . Accordingly, no stock-based compensation expense has been recognized in the three and nine months ended September 30, 2018 and 2017 related to these awards. The Company uses a lattice model with a Monte Carlo simulation to value stock options with performance and market conditions. This valuation methodology utilizes the estimated fair value of the Company’s common stock on grant date and several key assumptions, including expected volatility of the Company’s stock price based on comparable public companies, risk-free rates of return and expected dividend yield. Stock Options Granted to Non-Employees with Service-Based Vesting Valuation Assumptions Stock-based compensation related to stock options granted to non-employees is recognized as the stock options are earned. Prior to the adoption of ASU 2018-07 during the third quarter of 2018, the unvested options granted to non-employees were revalued using the Company's estimate of fair value on each reporting date. Subsequent to the adoption of ASU 2018-07, existing stock options granted to non-employees will no longer be revalued, and the estimated fair value of new stock options granted to non-employees will be calculated on the date of grant and not remeasured, similar to stock options granted to employees. The estimated fair value of the stock options granted to non-employees has been calculated using the Black-Scholes option-pricing model with the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 7.14 - 9.36 7.75 - 8.35 7.14 - 9.86 7.75 - 9.45 Volatility 89.4% - 90.5% 86.8% - 90.5% 88.9% - 103.1% 86.8% - 98.0% Risk-free interest rate 3.0% 2.2% 2.7% - 3.0% 2.2% - 2.4% Dividend yield — — — — Restricted Stock Activity Under the 2017 Plan, the Company may grant restricted stock awards ("RSAs"), which represent restricted shares of common stock issued upon the date of grant in which the recipient's rights in the stock are restricted until the shares are vested, and restricted stock units ("RSUs"), which represent a commitment to issue shares of common stock in the future upon vesting. The fair value of restricted stock underlying the RSAs and RSUs is determined based on the closing price of the Company's common stock on the date of grant. Aggregated information regarding RSAs and RSUs granted under the Plan for the nine months ended September 30, 2018 is summarized below: Share Awards & Units Weighted-Average Fair Value at Date of Grant per Share Unvested at December 31, 2017 2,293,788 $ 0.18 Granted 149,658 15.92 Vested (1,532,994 ) 0.18 Forfeited — — Unvested at September 30, 2018 910,452 $ 2.76 Vested and expected to vest –September 30, 2018 910,452 $ 2.76 At September 30, 2018 , there was $2.4 million of total unrecognized compensation cost related to unvested RSAs and RSUs, all of which is expected to be recognized over a remaining weighted-average vesting period of 0.7 years . Employee Stock Purchase Plan In December 2017, the Company adopted the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which initially reserved 1,000,000 shares of the Company's common stock for employee purchases under terms and provisions established by the Board of Directors. Under the 2017 ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the exercise date. The 2017 ESPP provides for consecutive, overlapping 12-month offering periods. The offering periods are scheduled to start on the first trading day on or after May 31 or November 30 of each year, except for the first offering period which commenced on December 8, 2017, the first trading day after the effective date of the Company’s registration statement. Contributions under the 2017 ESPP are limited to a maximum of 15% of an employee's eligible compensation. The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions: Nine Months Ended September 30, 2018 Expected term (in years) 0.50 - 1.00 Volatility 63.2% - 63.7% Risk-free interest rate 2.1% Dividend yield — The Company did not issue any new stock purchase rights under the 2017 ESPP during the three months ended September 30, 2018 . Stock-Based Compensation Expense The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 2,907 $ 725 $ 7,179 $ 1,947 General and administrative 2,603 403 5,966 1,005 Total $ 5,510 $ 1,128 $ 13,145 $ 2,952 As of September 30, 2018 and December 31, 2017 , total unamortized stock-based compensation expense related to unvested stock-based awards that are expected to vest was $60.2 million and $17.7 million , respectively. The weighted-average periods over which such stock-based compensation expense will be recognized are approximately 3.2 years and 3.2 years , respectively. The Company recorded stock-based compensation expense for options issued to non-employees of $0.2 million and $0.6 million for the three and nine months ended September 30, 2018 , respectively, and $0.2 million and $0.5 million for the three and nine months ended September 30, 2017 , respectively.

Net Loss and Net Loss Per Share

Net Loss and Net Loss Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss and Net Loss Per Share	Net Loss and Net Loss Per Share The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (35,371 ) $ (21,844 ) $ (113,773 ) $ (65,298 ) Denominator: Weighted average common shares outstanding 93,665,231 10,231,036 92,056,812 9,643,686 Net loss per share, basic and diluted $ (0.38 ) $ (2.14 ) $ (1.24 ) $ (6.77 ) Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share for all periods as the inclusion of all potential shares of common stock outstanding would have been anti-dilutive. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Three and Nine Months Ended September 30, 2018 2017 Series A-1 convertible preferred stock — 46,114,423 Series A-2 convertible preferred stock — 4,361,527 Series B-1 convertible preferred stock — 8,124,365 Options issued and outstanding and ESPP shares issuable and outstanding 9,644,444 6,179,687 Restricted shares subject to future vesting 910,452 2,701,059 Early exercised common stock subject to future vesting 182,299 416,669 Shares to be issued under Incro acquisition agreement — 81,164 Total 10,737,195 67,978,894

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Event	Subsequent Events Sanofi On October 29, 2018, the Company entered into a Collaboration and License Agreement ("Collaboration Agreement") with Genzyme Corporation, a wholly owned subsidiary of Sanofi S.A. ("Sanofi") to develop and commercialize therapeutic products to treat neurological and systemic inflammatory diseases by targeting receptor interacting serine/theronine-protein kinase 1 ("RIPK1"). The two most advanced RIPK1 Inhibitors in the collaboration are DNL747, a potent and selective CNS Product that was discovered by Denali and is currently in Phase 1 testing in healthy volunteers, and DNL758, a Peripheral Product discovered by Denali for which IND-enabling studies have been completed. The Collaboration Agreement will become effective when the requirements of the Hart-Scott-Rodino Antitrust Improvements Act of 1976 have been satisfied. Under the terms of the Collaboration Agreement, Sanofi will make an upfront cash payment to Denali of $125.0 million , with future development and commercial milestone payments that could exceed $1 billion . Sanofi and Denali will share commercial profits and losses from DNL747 in the U.S. and China equally, while Denali will receive a royalty from Sanofi for other territories for DNL747 and worldwide for DNL758. Phase 1b and 2 clinical development costs for DNL747 will be fully funded by Sanofi for Multiple Sclerosis, ALS, and other neurological indications, except for Alzheimer’s disease, which will be funded by Denali. Phase 3 trials for all neurological indications will be jointly funded by Sanofi ( 70% ) and Denali ( 30% ). Sanofi will fully fund the clinical development costs for DNL758 in systemic inflammatory diseases. Lonza Effective October 2018, the Company executed the second project plan under the Development and Manufacturing Services Agreement with Lonza for approximately $10.5 million , which covers activities that are expected to take place through December 2024. Sublease Agreement On October 18, 2018, the Company entered into a sublease agreement ("Sublease Agreement") to sublease approximately 36,835 rentable square feet of space in its New Premises at 161 Oyster Point in South San Francisco, California. The Sublease Agreement has a term of five years from the commencement date, which is the legal commencement date of the Headquarters Lease Amendment, and provides for the Company to receive monthly base rent amounts escalating over the term of the lease, totaling approximately $14.8 million over the term of the Sublease Agreement.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Organization and Description of Business	Organization and Description of Business Denali Therapeutics Inc. ("Denali" or the “Company”) is a biopharmaceutical company, incorporated in Delaware, that discovers and develops therapeutics to defeat neurodegenerative diseases. The Company is headquartered in South San Francisco, California.
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 (“U.S. GAAP”) for interim financial information and the instructions to Form 10-Q and Article 10 of SEC Regulation S-X for interim financial information. These unaudited condensed consolidated financial statements and notes should be read in conjunction with the audited consolidated financial statements and notes thereto contained in the Annual Report on Form 10-K for the year ended December 31, 2017 , as filed with the Securities and Exchange Commission on March 19, 2018 (the "2017 Annual Report on Form 10-K"). The condensed consolidated Balance Sheet as of December 31, 2017 was derived from the audited annual consolidated financial statements as of the period then ended. Certain information and footnote disclosures typically included in the Company's annual consolidated financial statements have been condensed or omitted. The accompanying unaudited condensed consolidated financial statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of the interim periods presented. All such adjustments are of a normal recurring nature except for the impacts of adopting new accounting standards discussed below. These interim financial results are not necessarily indicative of results expected for the full fiscal year or for any subsequent interim period. During the three and nine months ended September 30, 2018 , except as discussed below in the sections titled Derivatives and Hedging Activities, Revenue Recognition, and Recently Adopted Accounting Standards, there were no material changes to the Company's significant accounting and financial reporting policies from those reflected in the 2017 Annual Report on Form 10-K. For further information with regard to the Company’s Significant Accounting Policies, please refer to Note 1, "Significant Accounting Policies," to the Company’s Consolidated Financial Statements included in the 2017 Annual Report on Form 10-K.
Initial Public Offering	Initial Public Offering On December 7, 2017, the Company's Registration Statement on Form S-1 was declared effective by the SEC for Denali's initial public offering ("IPO") of common stock. In connection with the IPO, the Company sold an aggregate of 15,972,221 shares of common stock, including 2,083,333 shares sold pursuant to the underwriters’ full exercise of their option to purchase additional shares, at a price to the public of $18.00 per share. The aggregate net proceeds received by the Company from the offering, net of underwriting discounts and commissions and offering expenses, were $264.3 million . Upon the closing of the IPO, all then-outstanding shares of Company convertible preferred stock converted into 60,365,020 shares of common stock. The related carrying value of $378.6 million was reclassified to common stock and additional paid-in capital.
Principles of Consolidation	Principles of Consolidation These unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated on consolidation. For the Company and its subsidiary, the functional currency has been determined to be U.S. dollars. Monetary assets and liabilities denominated in foreign currency are remeasured at period-end exchange rates. Non-monetary assets and liabilities denominated in foreign currencies are remeasured at historical rates. Foreign currency transaction gains and losses resulting from remeasurement are recognized in interest and other income, net in the condensed consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires the Company to make certain estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements, as well as the reported amounts of expenses during the reporting period. Actual results could differ from those estimates, and such differences could be material to the condensed consolidated financial position and statements of operations and comprehensive loss.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities and forward foreign currency exchange contracts. Substantially all of the Company’s cash and cash equivalents are deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents and marketable securities and issuers of marketable securities to the extent recorded on the consolidated balance sheets. As of September 30, 2018 and December 31, 2017 , the Company had no off-balance sheet concentrations of credit risk. The Company is exposed to counterparty credit risk on all of its derivative financial instruments. The Company has established and maintains strict counterparty credit guidelines and enters into hedges only with financial institutions that are investment grade or better to minimize the Company’s exposure to potential defaults. The Company does not require collateral to be pledged under these agreements. The Company is subject to a number of risks similar to other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under any license or collaboration agreements, and the need to secure and maintain adequate manufacturing arrangements with third parties. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability.
Segments	Segments The Company has one operating segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources.
Restricted Cash	Restricted Cash The Company’s restricted cash consists of the letter of credit for the Company’s headquarters building lease, and is included within other non-current assets on the accompanying condensed consolidated balance sheets.
Derivatives and Hedging Activities	Derivatives and Hedging Activities The Company accounts for its derivative instruments as either assets or liabilities on the condensed consolidated balance sheet and measures them at fair value. Derivatives are adjusted to fair value through Interest and other income, net in the condensed consolidated statements of operations and comprehensive loss.
Revenue Recognition	Revenue Recognition License and Collaboration Revenues The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements (“ASC 808”) to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of Topic 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. The accounting treatment pursuant to Topic 606 is outlined below. The terms of licensing and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and royalties on net sales of licensed products. Each of these payments results in license, collaboration and other revenues, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenues. The core principle of Topic 606 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following 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 contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. Amounts received prior to satisfying the revenue recognition criteria are recorded as contract liabilities in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company's consolidated balance sheets. If the Company expects to have an unconditional right to receive the consideration in the next twelve months, this will be classified in current assets. A net contract asset or liability is presented for each contract with a customer. At contract inception, the Company assesses the goods or services promised in a contract with a customer and identifies those distinct goods and services that represent a performance obligation. A promised good or service may not be identified as a performance obligation if it is immaterial in the context of the contract with the customer, if it is not separately identifiable from other promises in the contract (either because it is not capable of being separated or because it is not separable in the context of the contract), or if the performance obligation does not provide the customer with a material right. The Company considers the terms of the contract and its customary business practices to determine the transaction price. The transaction price is the amount of consideration to which the Company expects to be entitled in exchange for transferring promised goods or services to a customer. The consideration promised in a contract with a customer may include fixed amounts, variable amounts, or both. Variable consideration will only be included in the transaction price when it is not considered constrained, which is when it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur. If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling prices. The relative selling price for each deliverable is estimated using objective evidence if it is available. If objective evidence is not available, the Company uses its best estimate of the selling price for the deliverable. Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset, which for a service is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the good or service promised to the customer. After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the transaction price is allocated to the performance obligations on the same basis as at contract inception. Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations.
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented.
Recently Issued and Recently Adopted Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update ("ASU") No. 2016-02, Leases (Topic 842) , which supersedes the guidance in former ASC 840, Leases. The FASB issued further updates to this guidance in July 2018 through ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. The standard is effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and is required to be adopted using a modified retrospective approach. The Company plans to adopt this standard on January 1, 2019. ASU 2016-02 is expected to impact the Company’s consolidated financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. Management is continuing to evaluate the impact the adoption of ASU 2016-02 will have on the Company’s financial position, results of operations and related disclosures. Management expects that the adoption of this standard will result in the recognition of an asset for the right to use a leased facility on the Company’s balance sheet, as well as the recognition of a liability for the lease payments remaining on the lease. While the balance sheet presentation is expected to change, management still does not expect a material change to the condensed consolidated statements of operations and comprehensive loss or cash flows. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The standard adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company is currently assessing the impact of this standard on its consolidated financial statements. Recently Adopted Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014–09, Revenue from Contracts with Customers (Topic 606) , which amends the existing accounting standards for revenue recognition. The FASB issued further updates to this guidance through ASU 2016-12 Narrow-Scope Improvements and Practical Expedients, ASU 2016-10 Identifying Performance Obligations and Licensing and ASU 2016-08 Principal Versus Agent Considerations (Reporting Revenue Gross Versus Net) . The new standard is based on principles that govern the recognition of revenue at an amount to which an entity expects to be entitled when products are transferred to customers. This standard was adopted on January 1, 2018 using a full retrospective application. There was no impact to the consolidated financial statements upon adoption of ASU 2014-09 as the Company had not recognized any revenue through December 31, 2017. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The purpose of ASU 2016-18 is to clarify the guidance for and presentation of restricted cash in the statement of cash flows. The amendment requires beginning-of-period and end-of-period total amounts shown on the statement of cash flows to include cash and cash equivalents as well as restricted cash and restricted cash equivalents. This standard was adopted on January 1, 2018. Accordingly, the condensed consolidated statements of cash flows and Note 3 "Cash and Marketable Securities" have been updated to reconcile cash, cash equivalents and restricted cash for all periods presented. In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting , which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. This standard was adopted as of January 1, 2018 and will be applied prospectively to any award modified after the adoption date. In June 2018, the FASB issued ASU No. 2018-07, Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . ASU 2018-07 is intended to reduce the cost and complexity and to improve financial reporting for non-employee share-based payments. ASU 2018-07 expands the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to non-employees for goods or services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity-Based Payments to Non-Employees . The Company elected to early adopt this standard effective September 30, 2018. The new guidance will be applied to all new equity-classified share-based payment awards issued to non-employees after the date of adoption. In addition, for all previously issued equity-classified share-based payment awards to non-employees for which a measurement date was not established by the adoption date, these awards were remeasured at fair value as of the adoption date and will no longer be remeasured. The future expense for these share-based payment awards to non-employees will be based on the fair value as of the adoption date. The adoption of this standard will not result in any other changes to the condensed consolidated financial statements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Assets Measured at Fair Value	Assets and liabilities measured at fair value at each balance sheet date are as follows (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 27,530 $ — $ — $ 27,530 Short-term marketable securities: U.S. government treasuries 181,248 — — 181,248 U.S. government agency securities — 92,096 — 92,096 Corporate debt securities — 55,012 — 55,012 Commercial paper — 2,951 — 2,951 Long-term marketable securities: U.S. government treasuries 75,829 — — 75,829 U.S. government agency securities — 11,840 — 11,840 Corporate debt securities — 54,504 — 54,504 Foreign currency derivative contracts — 4 — 4 Total $ 284,607 $ 216,407 $ — $ 501,014 Liabilities: Foreign currency derivative contracts $ — $ 123 $ — $ 123 Total $ — $ 123 $ — $ 123 December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 212,868 $ — $ — $ 212,868 Short-term marketable securities: U.S. government treasuries 42,587 — — 42,587 U.S. government agency securities — 106,139 — 106,139 Corporate debt securities — 39,125 — 39,125 Long-term marketable securities: U.S. government treasuries 39,848 — — 39,848 U.S. government agency securities — 19,911 — 19,911 Corporate debt securities — 991 — 991 Total $ 295,303 $ 166,166 $ — $ 461,469

Cash and Marketable Securities

Cash and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Schedule of Restricted Cash and Cash Equivalents	A reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets to the amount reported within the condensed consolidated statements of cash flows is shown in the table below (in thousands): September 30, 2018 December 31, 2017 September 30, 2017 Cash and cash equivalents $ 43,651 $ 218,375 $ 36,269 Restricted cash included within prepaid expenses and other current assets — 84 84 Restricted cash included within other non-current assets 1,500 451 451 Total cash, cash equivalents, and restricted cash $ 45,151 $ 218,910 $ 36,804
Summary of Available for Sale Securities	The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands): September 30, 2018 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 181,610 $ — $ (362 ) $ 181,248 U.S. government agency securities 92,330 — (234 ) 92,096 Corporate debt securities 55,136 — (124 ) 55,012 Commercial paper 2,951 — — 2,951 Total short-term marketable securities 332,027 — (720 ) 331,307 Long-term marketable securities: U.S. government treasuries 76,226 — (397 ) 75,829 U.S. government agency securities 11,919 — (78 ) 11,841 Corporate debt securities 54,723 — (220 ) 54,503 Total long-term marketable securities 142,868 — (695 ) 142,173 Total $ 474,895 $ — $ (1,415 ) $ 473,480 December 31, 2017 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 42,614 $ — $ (27 ) $ 42,587 U.S. government agency securities 106,368 — (229 ) 106,139 Corporate debt securities 39,197 — (72 ) 39,125 Total short-term marketable securities 188,179 — (328 ) 187,851 Long-term marketable securities: U.S. government treasuries 39,868 — (20 ) 39,848 U.S. government agency securities 19,931 — (20 ) 19,911 Corporate debt securities 991 — — 991 Total long-term marketable securities 60,790 — (40 ) 60,750 Total $ 248,969 $ — $ (368 ) $ 248,601

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Summary of Forward Foreign Currency Exchange Purchase Contracts Outstanding	The following table summarizes the Company’s forward foreign currency exchange contracts outstanding as of September 30, 2018 (notional amounts in thousands): Foreign Exchange Contracts Number of Contracts Aggregate Notional (1) Amount in Foreign Currency Maturity Euros 24 3,884 Oct. 2018 - Aug. 2019 British Pounds 19 1,625 Oct. 2018 - Aug. 2019 Swiss Francs 16 466 Oct. 2018 - Aug. 2019 Total 59 _________________________________________________ (1) The notional amount represents the net amount of foreign currency that will be received upon maturity of the forward contracts

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Changes in the Net Contract Liability Balance	Significant changes in the net contract liability balance during the period are as follows (in thousands): Contract liability Balance at January 1, 2018 $ — Increases due to cash received, excluding amounts recognized as revenue during the period 58,329 Decreases due to revenue recognized in the period for which cash has not been received (1,219 ) Balance at September 30, 2018 $ 57,110

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Summary of Company's Future Minimum Lease Commitments	As of September 30, 2018 , the future minimum lease payments under the Headquarters Lease and subsequently the Headquarters Lease Amendment are as follows (in thousands): Year Ended December 31: 2018 (three months) $ 658 2019 4,941 2020 9,097 2021 9,716 2022 10,056 2023 and later 71,290 $ 105,758

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes option award activity under the 2017 Plan and the 2015 Plan: Number of Options Weighted- Exercise Price Weighted- remaining contractual life (years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2017 6,689,479 $ 4.08 8.37 $ 77,317 Options granted 3,591,734 20.55 Options exercised (489,483 ) 2.57 Options forfeited (238,542 ) 10.36 Balance at September 30, 2018 9,553,188 $ 10.18 8.39 $ 110,407 Options vested and expected to vest at September 30, 2018 7,808,456 $ 12.31 8.72 $ 73,663 Options exercisable at September 30, 2018 1,779,017 $ 3.78 7.84 $ 31,946
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted	Stock Options Granted to Non-Employees with Service-Based Vesting Valuation Assumptions Stock-based compensation related to stock options granted to non-employees is recognized as the stock options are earned. Prior to the adoption of ASU 2018-07 during the third quarter of 2018, the unvested options granted to non-employees were revalued using the Company's estimate of fair value on each reporting date. Subsequent to the adoption of ASU 2018-07, existing stock options granted to non-employees will no longer be revalued, and the estimated fair value of new stock options granted to non-employees will be calculated on the date of grant and not remeasured, similar to stock options granted to employees. The estimated fair value of the stock options granted to non-employees has been calculated using the Black-Scholes option-pricing model with the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 7.14 - 9.36 7.75 - 8.35 7.14 - 9.86 7.75 - 9.45 Volatility 89.4% - 90.5% 86.8% - 90.5% 88.9% - 103.1% 86.8% - 98.0% Risk-free interest rate 3.0% 2.2% 2.7% - 3.0% 2.2% - 2.4% Dividend yield — — — — Stock Options Granted to Employees with Service-Based Vesting The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 6.08 6.08 5.50 - 6.08 6.08 Volatility 81.9% - 84.8% 86.8% - 88.1% 80.0% - 87.5% 86.8% - 91.3% Risk-free interest rate 2.8% - 2.9% 1.8% - 1.9% 2.6% - 2.9% 1.8% - 2.3% Dividend yield — — — —
Restricted Stock Activity	Aggregated information regarding RSAs and RSUs granted under the Plan for the nine months ended September 30, 2018 is summarized below: Share Awards & Units Weighted-Average Fair Value at Date of Grant per Share Unvested at December 31, 2017 2,293,788 $ 0.18 Granted 149,658 15.92 Vested (1,532,994 ) 0.18 Forfeited — — Unvested at September 30, 2018 910,452 $ 2.76 Vested and expected to vest –September 30, 2018 910,452 $ 2.76
Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted in ESPP	The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions: Nine Months Ended September 30, 2018 Expected term (in years) 0.50 - 1.00 Volatility 63.2% - 63.7% Risk-free interest rate 2.1% Dividend yield —
Summary of Stock-Based Compensation Expense	The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 2,907 $ 725 $ 7,179 $ 1,947 General and administrative 2,603 403 5,966 1,005 Total $ 5,510 $ 1,128 $ 13,145 $ 2,952

Net Loss and Net Loss Per Sha_2

Net Loss and Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Summary of Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss $ (35,371 ) $ (21,844 ) $ (113,773 ) $ (65,298 ) Denominator: Weighted average common shares outstanding 93,665,231 10,231,036 92,056,812 9,643,686 Net loss per share, basic and diluted $ (0.38 ) $ (2.14 ) $ (1.24 ) $ (6.77 )
Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations	Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: Three and Nine Months Ended September 30, 2018 2017 Series A-1 convertible preferred stock — 46,114,423 Series A-2 convertible preferred stock — 4,361,527 Series B-1 convertible preferred stock — 8,124,365 Options issued and outstanding and ESPP shares issuable and outstanding 9,644,444 6,179,687 Restricted shares subject to future vesting 910,452 2,701,059 Early exercised common stock subject to future vesting 182,299 416,669 Shares to be issued under Incro acquisition agreement — 81,164 Total 10,737,195 67,978,894

Significant Accounting Polici_3

Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	Dec. 07, 2017USD ($)$ / sharesshares	Sep. 30, 2018Segment
Significant Accounting Policies [Line Items]
Number of operating segments | Segment		1
IPO [Member]
Significant Accounting Policies [Line Items]
Shares sold during initial public offering (shares)	15,972,221
Share price (usd per share) | $ / shares	$ 18
Proceeds from issuance of common stock, net of underwriting discounts and commissions and offering expenses | $	$ 264.3
Convertible preferred stock converted into shares of common stock (shares)	60,365,020
Amount reclassified to common stock and additional paid-in capital | $	$ 378.6
Over-Allotment Option [Member]
Significant Accounting Policies [Line Items]
Shares sold during initial public offering (shares)	2,083,333

Fair Value Measurements - Asset

Fair Value Measurements - Assets Measured at Fair Value (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 473,480,000	$ 248,601,000
Asset - foreign currency derivative contracts	3,667
Total asset fair value measurements	501,014,000	461,469,000
Liability - foreign currency derivative contracts	123,000
Total liability fair value measurements	123,000
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,530,000	212,868,000
Short-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	181,248,000	42,587,000
Short-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	92,096,000	106,139,000
Short-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	55,012,000	39,125,000
Short-term [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	2,951,000
Long-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	75,829,000	39,848,000
Long-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	11,840,000	19,911,000
Long-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	54,504,000	991,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	0
Total asset fair value measurements	284,607,000	295,303,000
Liability - foreign currency derivative contracts	0
Total liability fair value measurements	0
Level 1 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,530,000	212,868,000
Level 1 [Member] | Short-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	181,248,000	42,587,000
Level 1 [Member] | Short-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 [Member] | Short-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 [Member] | Short-term [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0
Level 1 [Member] | Long-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	75,829,000	39,848,000
Level 1 [Member] | Long-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 [Member] | Long-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	4,000
Total asset fair value measurements	216,407,000	166,166,000
Liability - foreign currency derivative contracts	123,000
Total liability fair value measurements	123,000
Level 2 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 [Member] | Short-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2 [Member] | Short-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	92,096,000	106,139,000
Level 2 [Member] | Short-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	55,012,000	39,125,000
Level 2 [Member] | Short-term [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	2,951,000
Level 2 [Member] | Long-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2 [Member] | Long-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	11,840,000	19,911,000
Level 2 [Member] | Long-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	54,504,000	991,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset - foreign currency derivative contracts	0
Total asset fair value measurements	0	0
Liability - foreign currency derivative contracts	0
Total liability fair value measurements	0
Level 3 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3 [Member] | Short-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 [Member] | Short-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 [Member] | Short-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 [Member] | Short-term [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0
Level 3 [Member] | Long-term [Member] | U.S. Government Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 [Member] | Long-term [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 [Member] | Long-term [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Transfers of assets between the fair value measurement levels, level 1 to level 2	$ 0	$ 0
Transfers of assets between the fair value measurement levels, level 2 to level 1	0	0
Transfers of assets between the fair value measurement levels, level 1 to level 3	0	0
Transfers of assets between the fair value measurement levels, level 3 to level 1	0	0
Transfers of assets between the fair value measurement levels, level 2 to level 3	0	0
Transfers of assets between the fair value measurement levels, level 3 to level 2	0	0
Transfers of liabilities between the fair value measurement levels, level 1 to level 2	0	0
Transfers of liabilities between the fair value measurement levels, level 2 to level 1	0	0
Transfers of liabilities between the fair value measurement levels, level 1 to level 3	0	0
Transfers of liabilities between the fair value measurement levels, level 3 to level 1	0	0
Transfers of liabilities between the fair value measurement levels, level 2 to level 3	0	0
Transfers of liabilities between the fair value measurement levels, level 3 to level 2	$ 0	$ 0

Cash and Marketable Securitie_2

Cash and Marketable Securities - Schedule of Restricted Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 43,651	$ 218,375	$ 36,269
Restricted cash included within prepaid expenses and other current assets	0	84	84
Restricted cash included within other non-current assets	1,500	451	451
Total cash, cash equivalents, and restricted cash	$ 45,151	$ 218,910	$ 36,804	$ 40,388

Cash and Marketable Securitie_3

Cash and Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 474,895	$ 248,969
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(1,415)	(368)
Aggregate Fair Value	473,480	248,601
Short Term Marketable Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	332,027	188,179
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(720)	(328)
Aggregate Fair Value	331,307	187,851
Short Term Marketable Securities [Member] | U.S. Government Treasuries [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	181,610	42,614
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(362)	(27)
Aggregate Fair Value	181,248	42,587
Short Term Marketable Securities [Member] | U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	92,330	106,368
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(234)	(229)
Aggregate Fair Value	92,096	106,139
Short Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	55,136	39,197
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(124)	(72)
Aggregate Fair Value	55,012	39,125
Short Term Marketable Securities [Member] | Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,951
Unrealized Holding Gains	0
Unrealized Holding Losses	0
Aggregate Fair Value	2,951
Long Term Marketable Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	142,868	60,790
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(695)	(40)
Aggregate Fair Value	142,173	60,750
Long Term Marketable Securities [Member] | U.S. Government Treasuries [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	76,226	39,868
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(397)	(20)
Aggregate Fair Value	75,829	39,848
Long Term Marketable Securities [Member] | U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,919	19,931
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(78)	(20)
Aggregate Fair Value	11,841	19,911
Long Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,723	991
Unrealized Holding Gains	0	0
Unrealized Holding Losses	(220)	0
Aggregate Fair Value	$ 54,503	$ 991

Cash and Marketable Securitie_4

Cash and Marketable Securities Cash and Marketable Securities - Marketable Securities Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Other-than-temporary impairment	$ 0	$ 0	$ 0	$ 0
Effective maturity (less than)	2 years		2 years

Derivative Financial Instrume_3

Derivative Financial Instruments - Summary of Forward Foreign Currency Exchange Purchase Contracts Outstanding (Details) - 9 months ended Sep. 30, 2018 € in Thousands, £ in Thousands, SFr in Thousands	USD ($)	GBP (£)derivative_instrument	USD ($)derivative_instrument	EUR (€)derivative_instrument	CHF (SFr)derivative_instrument
Derivative [Line Items]
Derivative liability | $			$ 123,000
Derivative asset | $			$ 3,667
Net loss on derivative instruments | $	$ 100,000
Designated as Hedging Instrument [Member] | Foreign Exchange Contract - Euros [Member]
Derivative [Line Items]
Number of Contracts		24	24	24	24
Aggregate Notional Amount in Foreign Currency | €				€ 3,884
Designated as Hedging Instrument [Member] | Foreign Exchange Contract - British Pounds [Member]
Derivative [Line Items]
Number of Contracts		19	19	19	19
Aggregate Notional Amount in Foreign Currency | £		£ 1,625
Designated as Hedging Instrument [Member] | Foreign Exchange Contract - Swiss Francs [Member]
Derivative [Line Items]
Number of Contracts		16	16	16	16
Aggregate Notional Amount in Foreign Currency | SFr					SFr 466
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Number of Contracts		59	59	59	59

Acquisition (Details)

Acquisition (Details)	May 30, 2018USD ($)Targets	Aug. 24, 2016USD ($)Targets	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense			$ 30,321,000	$ 18,515,000	$ 103,274,000	$ 55,989,000
Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront option fee					500,000
Estimated net liabilities	$ 200,000
Number of Fcab targets | Targets		3
Fcab target period		3 years
Upfront fee paid		$ 5,500,000
Additional Fcab targets | Targets	2
Additional Fcab target obligation	$ 6,000,000
Maximum development plan period for research costs		24 months
Research and development expense			$ 300,000	$ 300,000	800,000	$ 800,000
Upfront purchase price less estimated net liabilities acquired					$ 17,800,000
Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member] | Exercise of buy-out option [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial option exercise payments	18,000,000
Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense		$ 2,100,000
Collaborative Arrangement With F-Star And Acquisition Of F-Star Gamma [Member] | Maximum [Member] | Exercise of buy-out option [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon the achievement of milestones	$ 447,000,000

License and Collaboration Agr_3

License and Collaboration Agreements - Takeda - Additional Information (Detail) - Collaborative Arrangement [Member] - Takeda Pharmaceutical Company Limited [Member]	Feb. 23, 2018USD ($)performance_obligation$ / sharesshares	Jan. 03, 2018USD ($)Program	Sep. 30, 2018USD ($)	Oct. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of programs to develop and commercialize | Program		3
Target option period		5 years
Upfront payment	$ 40,000,000	$ 40,000,000
Preclinical milestone payment received	5,000,000
Option fee		$ 5,000,000
Number of collaboration programs with cost-sharing, minimum | Program		2
Variable consideration relating to future milestones	26,000,000
Remaining preclinical milestones, cost and profit sharing income, and the development and commercial milestones	$ 44,000,000
Number of performance obligations | performance_obligation	3
Contract liability			$ 57,110,000		$ 0
Revenue recognized			1,219,000
Receivables or net contract assets associated with arrangement			$ 0
United States [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of collaboration programs the Company will lead | Program		1
Share Purchase Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common stock (in shares) | shares	4,214,559
Purchase price	$ 110,000,000
Fair market value of common stock	$ 94,400,000
Closing stock price (usd per share) | $ / shares	$ 22.40
Premium on sale of common stock	$ 15,600,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preclinical milestone payments per program		$ 25,000,000
Total aggregate payments due upon achievement of certain preclinical milestone events		75,000,000
Aggregated option exercise fee		15,000,000
Total aggregate payments due upon achievement of certain clinical and regulatory milestone events		707,500,000
Milestone payments per biologic product upon achievement of a certain sales-based milestone		75,000,000
Milestone payments upon achievement of biologic product from each program		$ 225,000,000
Subsequent Event [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preclinical milestone payment earned not yet received				$ 5,000,000

License and Collaboration Agr_4

License and Collaboration Agreements - Summary of Significant Changes in the Net Contract Liability Balance (Detail) - Collaborative Arrangement [Member] - Takeda Pharmaceutical Company Limited [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Change In Contract with Customer, Liability [Roll Forward]
Balance at January 1, 2018	$ 0
Increases due to cash received, excluding amounts recognized as revenue during the period	58,329
Decreases due to revenue recognized in the period for which cash has not been received	(1,219)
Balance at September 30, 2018	$ 57,110

License and Collaboration Agr_5

License and Collaboration Agreements - Genentech - Additional Information (Detail) - Genentech Inc [Member] - USD ($)	Jun. 17, 2016	Sep. 30, 2018	Dec. 31, 2016
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 315,000,000
License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of maximum reduction as credit for third-party royalty and milestone payments	50.00%
Royalty payment obligations, period	10 years
License Agreement [Member] | Research and Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement, obligation period to use commercially reasonable efforts to research, develop and commercialize at least one licensed product	3 years
Upfront fee paid			$ 8,500,000
Technology transfer fee			$ 1,500,000
First clinical milestone		$ 2,500,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	May 02, 2018USD ($)ft²	Aug. 01, 2016USD ($)ft²	Nov. 30, 2016USD ($)	Aug. 31, 2016USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Mar. 31, 2018USD ($)
Loss Contingencies [Line Items]
Rent expense					$ 2,400,000	$ 400,000	$ 3,700,000	$ 1,600,000
Headquarters Lease [Member]
Loss Contingencies [Line Items]
Area under lease | ft²	148,020	38,109
Increase in base rent			$ 400,000
Lease period	10 years	8 years
Lease renewal option term	10 years
Security deposit	$ 1,500,000								$ 500,000
Landlord Funded Tenant Improvements [Member] | Headquarters Lease [Member]
Loss Contingencies [Line Items]
Tenant improvement allowance repayable in rent	$ 4,400,000			$ 1,900,000			4,400,000
DMSA [Member]
Loss Contingencies [Line Items]
Purchase order executed					13,000,000		13,000,000
Costs incurred					1,400,000		2,600,000
Payments for development and manufacturing services					1,300,000		2,000,000
Non-refundable purchase commitments					$ 7,700,000		$ 7,700,000
Minimum [Member] | Headquarters Lease [Member]
Loss Contingencies [Line Items]
Lease renewal notice period	9 months
Maximum [Member] | Headquarters Lease [Member]
Loss Contingencies [Line Items]
Lease renewal notice period	12 years
Maximum [Member] | Landlord Funded Tenant Improvements [Member] | Headquarters Lease [Member]
Loss Contingencies [Line Items]
Tenant improvements	$ 25,900,000	$ 7,400,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Company's Future Minimum Lease Commitments (Detail) - Headquarters Lease [Member] $ in Thousands	Sep. 30, 2018USD ($)
Loss Contingencies [Line Items]
2018 (three months)	$ 658
2,019	4,941
2,020	9,097
2,021	9,716
2,022	10,056
2023 and later	71,290
Total	$ 105,758

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		4 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2017	Nov. 30, 2015	Aug. 31, 2015	May 31, 2015	Sep. 30, 2018	Sep. 30, 2017	Nov. 30, 2015	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value of option (usd per share)					$ 11.01	$ 7.08		$ 15.05	$ 4.84
Aggregate intrinsic value of options exercised					$ 2,600,000	$ 1,200,000		$ 7,300,000	$ 3,500,000
Weighted average grant date fair value of options vested (usd per share)					$ 4.01	$ 2.43		$ 3.29	$ 2.08
Performance and market contingent stock options granted (in shares)								3,591,734
Exercise price (usd per share)								$ 2.57
Share based compensation cost recognized					$ 5,510,000	$ 1,128,000		$ 13,145,000	$ 2,952,000
Expected weighted average period								3 years 1 month 25 days		3 years 73 days
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized					2,603,000	403,000		$ 5,966,000	1,005,000
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized					2,907,000	725,000		$ 7,179,000	1,947,000
Performance and Market Contingent Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance and market contingent stock options granted (in shares)		125,000	1,619,738
Exercise price (usd per share)							$ 0.68
Vesting trigger, number of consecutive trading days								90 days
Vesting trigger, number of days after IPO								180 days
Share based compensation cost recognized					0	0		$ 0	0
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock- based compensation expense related to unvested stock options					$ 2,400,000			$ 2,400,000
Expected weighted average period								8 months 4 days
2017 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)	1,000,000									1,000,000
Common stock purchase discounted rate for employees	85.00%
Maximum employee contribution to ESPP, percent of base compensation	15.00%									15.00%
Stock issued during period (in shares)					0
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock- based compensation expense related to unvested stock options	$ 17,700,000				$ 60,200,000			$ 60,200,000		$ 17,700,000
Non-Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized					200,000	200,000		600,000	500,000
Early Exercised Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reclassification of liability to stockholders' equity					$ 31,874	$ 31,874		400,000	$ 200,000
Unvested early exercised options in non-current liabilities								$ 100,000		$ 500,000
2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)	6,379,238									6,379,238
Number of shares available for grant (in shares)					2,659,306			2,659,306
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				8,325,000
Number of shares available for grant (in shares)	0									0
2015 Stock Incentive Plan [Member] | All Shares Granted At $ 0.68 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				4 years
Estimated fair value of option (usd per share)				$ 1.20
Exercise price (usd per share)				$ 0.68
Shares Transferred As Available For Issuance From 2015 Plan to 2017 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	169,238									169,238
Maximum [Member] | Performance and Market Contingent Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized								$ 6,200,000
Maximum [Member] | Performance and Market Contingent Stock Options [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized								5,800,000
Maximum [Member] | Performance and Market Contingent Stock Options [Member] | Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized								$ 400,000
Maximum [Member] | 2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
Award vesting period	4 years
Maximum [Member] | 2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period				10 years
Minimum [Member] | 2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date	100.00%
Minimum [Member] | 2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date				100.00%

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Number of Options
Number of options, beginning balance (in shares)	6,689,479
Number of options, granted (in shares)	3,591,734
Number of options, exercised (in shares)	(489,483)
Number of options, forfeited (in shares)	(238,542)
Number of options, ending balance (in shares)	9,553,188	6,689,479
Number of options, vested and expected to vest (in shares)	7,808,456
Number of options, exercisable (in shares)	1,779,017
Weighted- Average Exercise Price
Weighted average exercise price, beginning balance (usd per share)	$ 4.08
Weighted average exercise price, granted (usd per share)	20.55
Weighted average exercise price, exercised (usd per share)	2.57
Weighted average exercise price, forfeited (usd per share)	10.36
Weighted average exercise price, ending balance (usd per share)	10.18	$ 4.08
Weighted average exercise price, vested and expected to vest (usd per share)	12.31
Weighted average exercise price, exercisable (usd per share)	$ 3.78
Stock Option Activity, Additional Disclosures
Weighted average remaining contractual life	8 years 4 months 20 days	8 years 4 months 12 days
Weighted average remaining contractual life, vested and expected to vest	8 years 8 months 19 days
Weighted average remaining contractual life, exercisable	7 years 10 months 2 days
Aggregate intrinsic value, beginning balance	$ 77,317
Aggregate intrinsic value, ending balance	110,407	$ 77,317
Aggregate intrinsic value, vested and expected to vest	73,663
Aggregate intrinsic value	$ 31,946

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of Assumptions Used for Estimating the Fair Value of Stock Options Granted (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days		6 years 29 days
Volatility, minimum	81.90%	86.80%	80.00%	86.80%
Volatility, maximum	84.80%	88.10%	87.50%	91.30%
Risk-free interest rate, minimum	2.80%	1.80%	2.60%	1.80%
Risk-free interest rate, maximum	2.90%	1.90%	2.90%	2.30%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			6 years 29 days

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Non Employees Assumptions Used for Estimating the Fair Value of Stock Options Granted (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days		6 years 29 days
Volatility, minimum	81.90%	86.80%	80.00%	86.80%
Volatility, maximum	84.80%	88.10%	87.50%	91.30%
Risk-free interest rate, minimum	2.80%	1.80%	2.60%	1.80%
Risk-free interest rate, maximum	2.90%	1.90%	2.90%	2.30%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Non- Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	89.40%	86.80%	88.90%	86.80%
Volatility, maximum	90.50%	90.50%	103.10%	98.00%
Risk-free interest rate	3.00%	2.20%
Risk-free interest rate, minimum			2.70%	2.20%
Risk-free interest rate, maximum			3.00%	2.40%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			5 years 6 months
Minimum [Member] | Non- Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	7 years 1 month 20 days	7 years 9 months	7 years 1 month 20 days	7 years 9 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			6 years 29 days
Maximum [Member] | Non- Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	9 years 4 months 9 days	8 years 4 months 6 days	9 years 10 months 9 days	9 years 5 months 12 days

Stock-Based Awards - Summary _4

Stock-Based Awards - Summary of Restricted Stock Activity (Detail) - Restricted Stock [Member]	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share Awards & Units
Shares, unvested, beginning balance (in shares) | shares	2,293,788
Shares, granted (in shares) | shares	149,658
Shares, vested (in shares) | shares	(1,532,994)
Shares, forfeited (in shares) | shares	0
Shares, unvested, ending balance (in shares) | shares	910,452
Shares, vested and expected to vest (in shares) | shares	910,452
Weighted-Average Fair Value at Date of Grant per Share
Weighted-average fair value at date of grant per share, unvested, beginning balance (usd per share) | $ / shares	$ 0.18
Weighted-average fair value at date of grant per share, granted (usd per share) | $ / shares	15.92
Weighted-average fair value at date of grant per share, vested (usd per share) | $ / shares	0.18
Weighted-average fair value at date of grant per share, forfeited (usd per share) | $ / shares	0
Weighted-average fair value at date of grant per share, unvested, ending balance (usd per share) | $ / shares	2.76
Weighted-average fair value at date of grant per share, vested and expected to vest (usd per share) | $ / shares	$ 2.76

Stock-Based Awards - Summary _5

Stock-Based Awards - Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted in ESPP (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days		6 years 29 days
Volatility, minimum	81.90%	86.80%	80.00%	86.80%
Volatility, maximum	84.80%	88.10%	87.50%	91.30%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			6 years 29 days
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum			63.20%
Volatility, maximum			63.70%
Risk-free interest rate			2.10%
Dividend yield			0.00%
Employee Stock Purchase Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			6 months
Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)			1 year

Stock-Based Awards - Summary _6

Stock-Based Awards - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 5,510	$ 1,128	$ 13,145	$ 2,952
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	2,907	725	7,179	1,947
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 2,603	$ 403	$ 5,966	$ 1,005

Net Loss and Net Loss Per Sha_3

Net Loss and Net Loss Per Share - Summary of Computation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss	$ (35,371)	$ (21,844)	$ (113,773)	$ (65,298)
Denominator:
Weighted average common shares outstanding (in shares)	93,665,231	10,231,036	92,056,812	9,643,686
Net loss per share, basic and diluted (usd per share)	$ (0.38)	$ (2.14)	$ (1.24)	$ (6.77)

Net Loss and Net Loss Per Sha_4

Net Loss and Net Loss Per Share - Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	10,737,195	67,978,894	10,737,195	67,978,894
Series A-1 Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	46,114,423	0	46,114,423
Series A- 2 Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	4,361,527	0	4,361,527
Series B-1 Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	8,124,365	0	8,124,365
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	9,644,444	6,179,687	9,644,444	6,179,687
Restricted Shares Subject to Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	910,452	2,701,059	910,452	2,701,059
Early Exercised Common Stock Subject to Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	182,299	416,669	182,299	416,669
Shares to Be Issued Under Incro Acquisition Agreement [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	81,164	0	81,164

Subsequent Event - Sanofi (Deta

Subsequent Event - Sanofi (Details) - Subsequent Event [Member] - Collaborative Arrangement [Member] $ in Millions	Oct. 29, 2018USD ($)
Sanofi [Member]
Subsequent Event [Line Items]
Upfront payment	$ 125
Development and commercial milestone payments	$ 1,000
Funded percentage	70.00%
Denali [Member]
Subsequent Event [Line Items]
Funded percentage	30.00%

Subsequent Event - Lonza (Detai

Subsequent Event - Lonza (Details) - DMSA [Member] - USD ($) $ in Millions	Oct. 31, 2018	Sep. 30, 2018
Subsequent Event [Line Items]
Purchase order executed		$ 13
Subsequent Event [Member] | Lonza [Member]
Subsequent Event [Line Items]
Purchase order executed	$ 10.5

Subsequent Event - Sublease Agr

Subsequent Event - Sublease Agreement Narrative (Details) - New Premisis Sublease Agreement [Member] - Subsequent Event [Member] $ in Millions	Oct. 18, 2018USD ($)ft²
Subsequent Event [Line Items]
Rentable square feet | ft²	36,835
Sublease term	5 years
Total sublease rent amount | $	$ 14.8