Denali Therapeutics (DNLI) 10-Q 8 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 01, 2019
Document And Entity Information [Abstract]
Entity Registrant Name	Denali Therapeutics Inc.
Entity Central Index Key	0001714899
Trading Symbol	DNLI
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	false
Entity Common Stock, Shares Outstanding (shares)		95,478,391

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 39,661	$ 77,123
Short-term marketable securities	445,820	387,174
Prepaid expenses and other current assets	14,052	16,539
Total current assets	499,533	480,836
Long-term marketable securities	97,554	147,881
Property and equipment, net	36,955	25,162
Operating lease right-of-use asset	34,407
Other non-current assets	8,147	8,105
Total assets	676,596	661,984
Current liabilities:
Accounts payable	3,662	1,891
Accrued liabilities	12,114	8,520
Accrued compensation	3,213	9,952
Contract liabilities	23,148	11,427
Other current liabilities	1,107	996
Total current liabilities	43,244	32,786
Contract liabilities, less current portion	44,852	57,350
Operating lease liability, less current portion	71,412
Deferred rent, less current portion		24,532
Other non-current liabilities	440	471
Total liabilities	159,948	115,139
Commitments and contingencies
Convertible preferred stock, $0.01 par value; 40,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 0 shares issued and outstanding as of March 31, 2019 and December 31, 2018	0	0
Stockholders' equity:
Common stock, $0.01 par value; 400,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 95,257,705 shares and 94,662,435 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively	1,279	1,273
Additional paid-in capital	781,966	774,158
Accumulated other comprehensive income (loss)	332	(649)
Accumulated deficit	(266,929)	(227,937)
Total stockholders' equity	516,648	546,845
Total liabilities and stockholders’ equity	$ 676,596	$ 661,984

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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)	95,257,705	94,662,435
Common stock, shares outstanding (in shares)	95,257,705	94,662,435

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 4,205	$ 641
Operating expenses:
Research and development	37,403	20,819
General and administrative	9,310	5,570
Total operating expenses	46,713	26,389
Loss from operations	(42,508)	(25,748)
Interest and other income, net	3,516	2,070
Net loss	(38,992)	(23,678)
Other comprehensive income (loss):
Net unrealized income (loss) on marketable securities, net of tax	981	(919)
Comprehensive loss	$ (38,011)	$ (24,597)
Net loss per share, basic and diluted (usd per share)	$ (0.41)	$ (0.26)
Weighted average number of shares outstanding, basic and diluted (in shares)	94,984,503	89,560,576

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2017		87,480,362
Beginning balance at Dec. 31, 2017	$ 465,796	$ 1,201	$ 656,660	$ (368)	$ (191,697)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Takeda Collaboration Agreement (in shares)		4,214,559
Issuance of common stock in connection with the Takeda Collaboration Agreement	94,406	$ 42	94,364
Issuances under equity incentive plans (in shares)		81,735
Issuances under equity incentive plans	219	$ 1	218
Vesting of early exercised common stock (in shares)		93,749
Vesting of early exercised common stock	279	$ 1	278
Vesting of restricted stock awards (in shares)		718,584
Vesting of restricted stock awards	0	$ 7	(7)
Stock-based compensation	2,925		2,925
Net loss	(23,678)				(23,678)
Other comprehensive income (loss)	(919)			(919)
Common stock, shares outstanding ending balance (in shares) at Mar. 31, 2018		92,588,989
Ending balance at Mar. 31, 2018	$ 539,028	$ 1,252	754,438	(1,287)	(215,375)
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2018	94,662,435	94,662,435
Beginning balance at Dec. 31, 2018	$ 546,845	$ 1,273	774,158	(649)	(227,937)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		199,503
Issuances under equity incentive plans	908	$ 2	906
Vesting of early exercised common stock (in shares)		46,874
Vesting of early exercised common stock	32	$ 1	31
Vesting of restricted stock awards (in shares)		348,893
Vesting of restricted stock awards	0	$ 3	(3)
Stock-based compensation	6,874		6,874
Net loss	(38,992)				(38,992)
Other comprehensive income (loss)	$ 981			981
Common stock, shares outstanding ending balance (in shares) at Mar. 31, 2019	95,257,705	95,257,705
Ending balance at Mar. 31, 2019	$ 516,648	$ 1,279	$ 781,966	$ 332	$ (266,929)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net loss	$ (38,992)	$ (23,678)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,403	854
Stock–based compensation expense	6,874	2,925
Net amortization of premiums and (discounts) on marketable securities	(1,317)	(387)
Non-cash rent expense	1,265
Non-cash rent expense		(225)
Gain on disposal of property and equipment	0	(44)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	2,849	(1,613)
Accounts payable	1,965	(951)
Accrued and other current liabilities	(4,593)	(3,257)
Contract liabilities	(777)	59,953
Net cash (used in) provided by operating activities	(30,323)	33,577
Investing activities
Purchases of marketable securities	(109,810)	(328,036)
Purchases of property and equipment	(2,028)	(537)
Maturities and sales of marketable securities	103,791	27,299
Net cash used in investing activities	(8,047)	(301,274)
Financing activities
Issuance of common stock in connection with the Takeda Collaboration Agreement	0	94,406
Proceeds from exercise of awards under equity incentive plans	908	219
Net cash provided by financing activities	908	93,239
Net decrease in cash, cash equivalents and restricted cash	(37,462)	(174,458)
Cash, cash equivalents and restricted cash at beginning of period	78,623	218,910
Cash, cash equivalents and restricted cash at end of period	41,161	44,452
Supplemental disclosures of cash flow information
Tenant improvements provided by the landlord	11,341	0
Property and equipment purchases accrued but not yet paid	1,569	246
Common Stock
Financing activities
Payments of issuance costs related to issuance of stock	0	(1,342)
Preferred Stock
Financing activities
Payments of issuance costs related to issuance of stock	$ 0	$ (44)

Significant Accounting Policies

Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
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, 2018 , as filed with the Securities and Exchange Commission on March 12, 2019 (the "2018 Annual Report on Form 10-K"). The condensed consolidated Balance Sheet as of December 31, 2018 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 months ended March 31, 2019, except as discussed below in the sections titled Leases 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 2018 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 2018 Annual Report on Form 10-K. 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 March 31, 2019 and December 31, 2018, 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. Leases The Company adopted Accounting Standards Update ("ASU") No. 2016-02, Leases as of January 1, 2019. A determination is made as to whether an arrangement is a lease at inception. A right-of-use (“ROU”) asset an d operating lease liability is recognized for identified operating leases in the condensed consolidated balance sheets. ROU assets represent the Company’s right to use the underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the lease commencement date based on the present value of lease payments due over the lease term, with the ROU assets adjusted for lease incentives received. When determining the present value of lease payments, the Company uses its incremental borrowing rate (“IBR”) on the date of lease commencement, or the rate implicit in the lease, if known. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably certain at lease inception. Leases with an initial term of 12 months or less are not recorded on the balance sheet, unless they include an option to purchase the underlying asset that the Company is reasonably certain to exercise. The Company recognizes lease expenses on a straight-line basis over the lease term. The Company has leases with lease and non-lease components, which the Company has elected to account for as a single lease component. Revenue Recognition License and Collaboration Revenue 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 revenue, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenue. The core principle of Topic 606 is to recognize revenue 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 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 ("SSP") . The relative SSP for each deliverable is estimated using external sourced evidence if it is available. If external sourced evidence is not available, the Company uses its best estimate of the SSP 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 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 SSP of identified performance obligations, which may include forecasted revenue, 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 Adopted Accounting Pronouncement In February 2016, the Financial Accounting Standards Board (“FASB”) issued 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, C odification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements , in December 2018 through ASU 2018-20, Leases (Topic 842): Narrow-Scope Improvements for Lessors and in March 2019 through ASU 2019-01 Leases (Topic 842): Codification 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. 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 has adopted this standard as of January 1, 2019 applying the optional transition method such that it is not required to adjust prior period presentations. ASU 2016-02 has impacted the Company’s condensed consolidated balance sheet as of March 31, 2019 as the Company has certain operating lease arrangements for which the Company is the lessee and one future operating lease arrangement for which the Company is the lessor. The Company has no financing leases. Management has elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows us to carryforward the historical lease classification. The impact of adoption of the standard is that the Company has recognized a net ROU asset of $46.1 million and operating lease liability of approximately $71.3 million, as of January 1, 2019. The standard did not have a material impact on the Company’s condensed consolidated statements of operations and comprehensive loss and stockholders' equity.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
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): March 31, 2019 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 24,930 $ — $ — $ 24,930 Short-term marketable securities: U.S. government treasuries 287,681 — — 287,681 U.S. government agency securities — 51,328 — 51,328 Corporate debt securities — 69,678 — 69,678 Commercial paper — 37,133 — 37,133 Long-term marketable securities: U.S. government treasuries 81,526 — — 81,526 Corporate debt securities — 16,028 — 16,028 Foreign currency derivative contracts — 26 — 26 Total $ 394,137 $ 174,193 $ — $ 568,330 Liabilities: Foreign currency derivative contracts $ — $ 159 $ — $ 159 Total $ — $ 159 $ — $ 159 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 42,225 $ — $ — $ 42,225 U.S. government treasuries 1,499 — — 1,499 Commercial paper — 9,979 — 9,979 Short-term marketable securities: U.S. government treasuries 219,754 — — 219,754 U.S. government agency securities — 73,151 — 73,151 Corporate debt securities — 71,675 — 71,675 Commercial paper — 22,594 — 22,594 Long-term marketable securities: U.S. government treasuries 117,131 — — 117,131 U.S. government agency securities — 1,977 — 1,977 Corporate debt securities — 28,773 — 28,773 Foreign currency derivative contracts — 14 — 14 Total $ 380,609 $ 208,163 $ — $ 588,772 Liabilities: Foreign currency derivative contracts $ — $ 182 $ — $ 182 Total $ — $ 182 $ — $ 182 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 months ended March 31, 2019 or 2018.

Cash and Marketable Securities

Cash and Marketable Securities	3 Months Ended
	Mar. 31, 2019
Cash and 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): March 31, 2019 December 31, 2018 March 31, 2018 Cash and cash equivalents $ 39,661 $ 77,123 $ 44,001 Restricted cash included within other non-current assets 1,500 1,500 451 Total cash, cash equivalents, and restricted cash $ 41,161 $ 78,623 $ 44,452 Marketable Securities All marketable securities were considered available-for-sale at March 31, 2019 and December 31, 2018. 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): March 31, 2019 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 287,665 $ 172 $ (156) $ 287,681 U.S. government agency securities 51,404 4 (80) 51,328 Corporate debt securities 69,764 7 (93) 69,678 Commercial paper 37,133 — — 37,133 Total short-term marketable securities 445,966 183 (329) 445,820 Long-term marketable securities: U.S. government treasuries 81,105 421 — 81,526 Corporate debt securities 15,971 57 — 16,028 Total long-term marketable securities 97,076 478 — 97,554 Total $ 543,042 $ 661 $ (329) $ 543,374 December 31, 2018 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 220,081 $ 29 $ (356) $ 219,754 U.S. government agency securities 73,373 — (222) 73,151 Corporate debt securities 71,940 1 (266) 71,675 Commercial paper 22,594 — — 22,594 Total short-term marketable securities 387,988 30 (844) 387,174 Long-term marketable securities: U.S. government treasuries 116,878 329 (76) 117,131 U.S. government agency securities 1,975 2 — 1,977 Corporate debt securities 28,864 8 (99) 28,773 Total long-term marketable securities 147,717 339 (175) 147,881 Total $ 535,705 $ 369 $ (1,019) $ 535,055 As of March 31, 2019 and December 31, 2018, some of the Company’s marketable securities were in an unrealized loss position. At each balance sheet date, the Company determined that it did have 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 in the three months ended March 31, 2019 and 2018. 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	3 Months Ended
	Mar. 31, 2019
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 no t have foreign currency exchange contracts prior to June 2018. The following table summarizes the Company’s forward foreign currency exchange contracts outstanding as of March 31, 2019 (notional amounts in thousands): Foreign Exchange Contracts Number of Contracts Aggregate Notional (1) Amount in Foreign Currency Maturity Euros 22 2,039 Apr. 2019 - Jan. 2020 British Pounds 27 3,982 Apr. 2019 - Feb. 2020 Swiss Francs 24 939 Apr. 2019 - Feb. 2020 Total 73 _________________________________________________ (1) The notional amount represents the net amount of foreign currency that will be received upon maturity of the forward contracts. The derivative liability balance of $0.2 million is recorded in Other current liabilities and the derivative asset balance of $25,675 is recorded in Prepaid assets and other current assets on the condensed consolidated balance sheet as of March 31, 2019. The net gain associated with the Company's derivative instruments of $19,287 is recognized in Interest and other income, net on the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2019.

Acquisition

Acquisition	3 Months Ended
	Mar. 31, 2019
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- s tar 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”). I n May 2018, the Company exercised the Option Agreement 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 F cab Target. The Company made a one-time payment of $6.0 million in aggregate for the two additional Accepted Fcab Targets 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- s tar Collaboration Agreement. 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 , with $18.3 million recorded in research and development expense in the year ended December 31, 2018 . Further, since t his transaction was accounted for as an asset purchase rather than a business combination, the Company did not recognize any contingent consideration on the acquisition date . Contingent consideration is expected to be recognized in research and development expense in the future as incurred. The Company was and continues to be responsible for certain research costs incurred by F-star Ltd in conducting activities under each agreed development plan, for up to 24 months. The first of the agreed development plans was completed during the year ended December 31, 2018, and the second commenced during the three months ended March 31, 2019. The Company recognized $0.2 million and $0.3 million of research and development expense related to the funding of F-star Ltd activities under these development plans during the three months ended March 31, 2019 and March 31, 2018, respectively

Collaboration Agreements

Collaboration Agreements	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreements	Collaboration Agreements Sanofi In October 2018, the Company entered into a Collaboration and License Agreement ("Sanofi Collaboration Agreement") with Genzyme Corporation, a wholly owned subsidiary of Sanofi S.A. ("Sanofi") pursuant to which certain small molecule CNS and peripheral RIPK1 inhibitors contributed by Sanofi and by Denali will be developed and commercialized. The Sanofi Collaboration Agreement became effective in November 2018 when the HSR requirements were satisfied upon which Sanofi paid the Company an upfront payment of $125.0 million. Under the Sanofi Collaboration Agreement, Denali is eligible to receive milestone payments from Sanofi up to approximately $1.1 billion upon achievement of certain clinical, regulatory and sales milestone events. Such milestone payments include $600.0 million in clinical and regulatory milestone payments for CNS Products and $495.0 million in clinical, regulatory and commercial milestone payments for Peripheral Products, as defined. Denali will share profits and losses equally with Sanofi for CNS Products sold in the United States and China, and receive royalties on net sales for CNS Products sold outside of the United States and China and for Peripheral Products sold worldwide, each as further described below. RIPK1 Inhibitors contributed by Sanofi and developed and commercialized under the Sanofi Collaboration Agreement will be subject to lower milestone and royalty payments to Denali compared to RIPK1 Inhibitors contributed by Denali. Denali and Sanofi will jointly develop CNS Products pursuant to a global development plan. The Company will be responsible, at its own cost, for conducting Phase 1 and Phase 2 trials for CNS Products for Alzheimer’s disease and any activities required to support such clinical trials and specific for Alzheimer’s disease. Denali will also conduct, at Sanofi’s cost, a Phase 1b trial for the lead CNS penetrant RIPK1 inhibitor, DNL747, for ALS. Sanofi will be responsible, at its cost, for all other Phase 1 and Phase 2 trials for CNS Products, including for multiple sclerosis. Sanofi will lead the conduct of all Phase 3 and later stage development trials for CNS Products, with Sanofi and Denali funding 70% and 30% of such costs, respectively. The Company will have the ability to opt out of the cost-profit sharing provisions of the Sanofi Collaboration Agreement, as further described below. Sanofi will lead commercialization activities globally for CNS Products. The Company may elect to conduct certain co-commercialization activities outside of MS with respect to each CNS Product in the United States and/or China, provided that the cost-profit sharing provisions of the Sanofi Collaboration Agreement for the relevant CNS Product are still in effect, as further described below. The Company may opt out of the cost-profit sharing provisions of the Sanofi Collaboration Agreement for CNS Products in the United States and China on a CNS Product and country basis. Sanofi may also terminate Denali's cost-profit sharing provisions of the Sanofi Collaboration Agreement in its entirety if, following notice from Sanofi and a cure period, the Company fails to satisfy its cost-sharing obligations. After such an opt out by the Company or termination by Sanofi, Denali will no longer be obligated to share in the development and commercialization costs for the applicable CNS Products and Denali will not share in the applicable profits from such CNS Products. Instead, the Company will be entitled to receive tiered royalties on net sales of the applicable CNS Products in the relevant country (or countries). The royalty rates are percentages in the low double digits to mid-teens, but may increase to the mid-teens to low-twenties percentages for all countries in which Sanofi is paying royalties on the applicable CNS Products, if the Company has met certain co-funding thresholds at the time of its election or Sanofi’s termination of the Company's cost-profit sharing rights and obligations. Sanofi will be responsible, at its cost, for conducting activities relating to the development and commercialization of all Peripheral Products. Sanofi will lead commercialization activities globally for Peripheral Products. Denali will be entitled to receive tiered royalties in the low- to mid- teen percentages on net sales of Peripheral Products. The Company identified the following distinct performance obligations associated with the Sanofi Collaboration Agreement upon inception: the CNS program license, the Peripheral program license, the Phase 1 and Phase 2 trials for CNS Products for Alzheimer’s disease ("Alzheimer's Disease Services"), and the Phase 1b trial for DNL747 for ALS and associated activities ("Retained Activities"). The Company believes that the Sanofi Collaboration Agreement is a collaboration arrangement as defined in ASC 808, Collaborative Agreements. The Company also believes that Sanofi meets the definition of a customer as defined in ASC 606, Revenue From Contracts With Customers for three of the performance obligations identified at inception, but does not meet the definition of a customer for the Alzheimer's Disease Services. Further, Sanofi does not meet the definition of a customer for all Phase 3 and later stage development trials for CNS Products led by Sanofi for which Denali will fund 30% of total costs. Since ASC 808 does not address recognition and measurement, the Company looked to other accounting literature for guidance where the performance obligation does not fall under ASC 606, and determined that for the Alzheimer's Disease Services, the guidance in ASC 606 should be analogized for the recognition, measurement and reporting of this performance obligation, and for the cost sharing provisions, the Company determined that the guidance in ASC 730, Research and Development should be applied. The transaction price at inception included upfront fixed consideration of $125.0 million. All potential future milestones and other payments were considered constrained at the inception of the Sanofi Collaboration Agreement since the Company could not conclude it is probable that a significant reversal in the amount recognized will not occur. The transaction price increased by $5.8 million from inception through March 31, 2019 as amounts due for costs incurred related to the Retained Activities were no longer constrained. The respective standalone value for each of the performance obligations has been determined by applying the SSP method and the transaction price allocated based on the relative SSP method with revenue recognition timing to be determined either by delivery or the provision of services. The Company used an adjusted market assessment approach to estimate the selling price for the program licenses, and an expected cost plus margin approach for estimating the Alzheimer’s Disease Services, and the Retained Activities. The program licenses and existing know-how were delivered on the effective date of the Sanofi Collaboration Agreement. The Alzheimer’s Disease Services and the Retained Activities are expected to be delivered over time as the services are performed. For the Alzheimer's Disease Services, revenue will be recognized over time using the input method, based on costs incurred to perform the services, since the level of costs incurred over time is thought to best reflect the transfer of services to Sanofi. For the Retained Activities, revenue will be recognized over time using the output method, based on amounts invoiced to Sanofi, since this is believed to directly correlate to the value of the services performed. A contract liability of $3.8 million and $3.9 million is recorded on the condensed consolidated balance sheet as of March 31, 2019 and December 31, 2018, respectively, which relates to the portion of the Alzheimer's Disease Services performance obligation yet to be satisfied, with such amounts to be recognized over the estimated period of the services, which is expected to be several years. There was a receivable of $3.4 million and $2.3 million at March 31, 2019 and December 31, 2018, respectively, associated with the Sanofi Collaboration Agreement. In assessing the Sanofi Collaboration Agreement, management is required to exercise considerable judgment in estimating revenue to be recognized. Managem ent applies judgment in determining the separate performance obligations, in estimating the selling price, in determining when control was transferred to Sanofi for the licenses, and in estimating total future costs when using the input method. Through March 31, 2019, Denali has not achieved any milestones and has no product sales recorded under the Sanofi Collaboration Agreement. Takeda I n January 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 to develop and commercialize, jointly with the Company, certain biologic products that are enabled by Denali's blood-brain barrier (" BBB ") delivery technology and intended for the treatment of neurodegenerative disorders. The programs were Denali’s ATV:BACE1/Tau and ATV: TREM2 programs, as well as a third identified discovery stage program. The Takeda Collaboration Agreement became effective i n February 2018 when the requirements of the Hart-Scott-Rodino Antitrust Improvements Act of 1976 were satisfied. In February 2019, the agreement was amended to replace ATV:BACE1/Tau with ATV:Tau. The amendment did not have a material impact to the condensed consolidated financial statements. 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 TfR 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 approximately five years after selection of the target, whichever is earlier. Under t he Takeda Collaboration Agreement , Takeda pa id a $40.0 million upfront payment , and may pay 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 . 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 th is 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, 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 the Company's 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 Takeda Collaboration Agreement . 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 were 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 second program, triggering a milestone payment of $5.0 million, which was received in November 2018. In December 2018, the Company confirmed the first preclinical milestone was met for the third program, triggering a milestone payment of $5.0 million, which was received in February 2019. The remaining $44.0 million of preclinical milestones were considered constrained at the inception of the Takeda Collaboration Agreement 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 any program. There was no change in the transaction price from inception through March 31, 2019. 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. There were no material changes in estimates during the three months ended March 31, 2019. A contract liability of $64.2 million and $64.9 million is recorded on the condensed consolidated balance sheet as of March 31, 2019 and December 31, 2018, respectively , which relate to the three performance obligations identified, with such amounts to be recognized over the estimated period of the pre-IND research services, which is expected to be several years. Revenue recognized relating to future milestone payments of approximately $1.4 million, for which the Company concluded that it is probable that a significant reversal in the amount recognized will not occur, is presented net in the contract liability on the condensed consolidated balance sheet. In assessing the Takeda Collaboration Agreement , management is required to exercise considerable judgment in estimating revenue to be recognized. Management applie s judgment in determining the separate performance obligations in the research period, estimating variable consideration, and estimating total future costs when using the input method. Through March 31, 2019, Denali has recognized $15.0 million in milestones from Takeda and has not recorded any product sales under the Takeda Collaboration Agreement. There was no receivable as of March 31, 2019, and a receivable of $5.0 million as of December 31, 2018, related to the Takeda Collaboration Agreement. Collaboration Revenue Revenue disaggregated by collaboration agreement and performance obligation is as follows (in thousands): Three Months Ended March 31, 2019 2018 Takeda Collaboration Agreement (1) $ 682 $ 641 Sanofi Collaboration Agreement Alzheimer's Disease Services (1) 94 — Retained Activities 3,429 — Total Sanofi Collaboration Revenue 3,523 — Total Collaboration Revenue $ 4,205 $ 641 _________________________________________________ ( 1)

License Agreements

License Agreements	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License Agreements	License Agreements 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, sublicensable 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, sublicensable 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. To date, the Company has paid an upfront fee of $8.5 million and a technology transfer fee of $1.5 million, both of which were recorded as research and development expense in the year ended December 31, 2016, and the first clinical milestone of $2.5 million which was recorded as research and development expense in the year ended December 31, 2017. 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 its payments of third-party royalty and milestones against royalty and milestones 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.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
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 former corporate headquarters comprising 38,109 of rentable square feet in a building in South San Francisco ("Former Headquarters Lease"). The Former Headquarters Lease commenced on August 1, 2016 with a lease term of eight years. The Former Headquarters Lease provided for monthly base rent amounts escalating over the term of the lease. In addition, the Former Headquarters Lease provided both a tenant improvement allowance (“TIA”) of up to $7.4 million, of which $1.9 million would be repaid to the landlord in the form of additional monthly rent with interest applied. In May 2018, the Company entered into an amendment to the Former Headquarters Lease (the "Headquarters Lease Amendment") to relocate and expand its headquarters to 148,020 rentable square feet in a building 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 was April 1, 2019 when the building was 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. The Company did not believe this renewal was reasonably certain at lease inception. 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 will be repaid to the landlord in the form of additional monthly rent. The Company will also be required to pay the operating expenses for the New Premises, such as taxes and insurance, which will be treated as variable lease payments. All of the TIA under the Headquarters Lease Amendment has been utilized as of March 31, 2019, and as such the $25.9 million and has been recorded as leasehold improvements assets and an offset to the lease ROU asset on the condensed consolidated balance sheet as of March 31, 2019. As of March 31, 2019, the carrying value of the ROU asset was $34.4 million and is separately stated on the condensed consolidated balance sheet. The related current and non-current liabilities as of March 31, 2019 were $0.8 million and $71.4 million, respectively. The current and non-current lease liabilities are included in Other current liabilities and Operating lease liability, less current portion, respectively, in the condensed consolidated balance sheet. Management was required to exercise judgment in applying the requirements of ASC 842, including the determination as to whether certain contracts contain a lease and for the Headquarters Lease Amendment, the discount rate used to determine the measurement of the lease liability. The discount rate used was 9.0%, which was management’s estimate of the Company’s incremental borrowing rate since the rate implicit in the lease was not known. To estimate the Company’s incremental borrowing rate, management considered observable debt yields of comparable market instruments, as well as benchmarks within the Headquarters Lease Amendment that may be indicative of the rate implicit in the lease. In October 2018, the Company entered into a sublease agreement ("Sublease Agreement") to sublease approximately 36,835 rentable square feet of space in its New Premises. The Sublease Agreement has a term of five years from the commencement date of April 12, 2019, and provides for the Company to receive monthly base rent amounts escalating over the term of the lease. The Company will also pass through the sublessee’s portion of the operating expenses for the New Premises, such as taxes and insurance, which will be treated as variable sublease income. Total operating lease costs, including variable and short term lease costs, were $2.3 million for the three months ended March 31, 2019. Rent expense including amortization of leasehold improvements was $0.4 million for the three months ended March 31, 2018. Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, we use our incremental borrowing rate based on the information available at the lease commencement date. As of March 31, 2019, the weighted average remaining lease term is 10 years and the weighted average discount rate used to determine the operating lease liability was 9.0%. Cash paid for amounts included in the measurement of lease liabilities for the three months ended March 31, 2019 was $0.7 million and was included in Net cash (used in) provided by operating activities in our condensed Consolidated Statements of Cash Flows. The following table reconciles the undiscounted cash flows for the first five years and total of the remaining years to the operating lease liabilities recorded in the condensed consolidated balance sheet as of March 31, 2019 (in thousands): Year Ending December 31: 2019 $ 4,785 2020 9,767 2021 10,386 2022 10,726 2023 11,078 2024 and later 64,399 Total undiscounted lease payments 111,141 Present value adjustment (38,979) Net operating lease liabilities $ 72,162 The following table details the future undiscounted cash inflows relating to the Company's sublease as of March 31, 2019 (in thousands): Year Ending December 31: 2019 $ 2,003 2020 2,842 2021 2,925 2022 3,009 2023 3,096 2024 and later 875 Total undiscounted sublease receipts $ 14,750 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 March 31, 2019 and December 31, 2018 , the Company had non-cancellable purchase orders for biological product development and manufacturing costs totaling $25.4 million and $24.7 million respectively . The activities under these purchase orders are expected to be completed by November 2024. During the three months ended March 31, 2019 and 2018 , the Company incurred costs of $3.6 million and $0.1 million, respectively, and made payments of $2.6 million and $0.1 million, respectively, for the development and manufacturing services rendered under the agreement. As of March 31, 2019 and December 31, 2018 , the Company had total non-refundable purchase commitments of $12.3 million and $14.0 million, respectively, under the DMSA.

Stock-Based Awards

Stock-Based Awards	3 Months Ended
	Mar. 31, 2019
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 approximately 6.4 million shares of common stock 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. The 2017 Plan provides that the number of shares reserved and available for issuance under the 2017 Plan will automatically increase each January 1, beginning on January 1, 2019, by the least of (i) 10.0 million shares, (ii) 5% of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the administrator of the 2017 Plan. In January 2019, the number of common stock available for issuance under the 2017 Plan was increased by approximately 4.8 million as a result of this automatic increase provision. 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 of the Company's common stock on the day of grant. Options granted typically vest over a four 2015 Stock Incentive Plan In May 2015, the Company adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which as amended, reserved approximately 8.3 million shares of common stock 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 of the Company's common stock 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 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 approximately 0.2 million 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 March 31, 2019 there were approximately 4.1 million 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- Average Exercise Price Weighted- Average remaining contractual life (years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2018 9,612,652 $ 10.49 8.20 $ 97,804 Options granted 2,745,750 11.60 Options exercised (199,503) 4.55 Options forfeited (133,273) 16.11 Balance at March 31, 2019 12,025,626 $ 12.29 8.39 $ 131,473 Options vested and expected to vest at March 31, 2019 10,280,894 $ 14.26 8.73 $ 92,147 Options exercisable at March 31, 2019 2,797,873 $ 8.74 7.76 $ 40,509 Aggregate intrinsic value represents the difference between the Company’s estimated fair value of its common stock and the exercise price of outstanding options. The total intrinsic value of options exercised was $3.3 million and $1.3 million for the three months ended March 31, 2019, an d 2018 , respectively. During the three months ended March 31, 2019 and 2018 , the weighted-average grant-date fair value of the vested options was $11.82 and $2.90 per share, respectively. The weighted-average grant date fair value of all options granted during the three months ended March 31, 2019 and 2018 was $11.60 and $16.36 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 March 31, 2019 2018 Expected term (in years) 6.08 6.08 Volatility 69.3% - 69.4% 86.0% - 87.5% Risk-free interest rate 2.5% - 2.6% 2.6% - 2.7% 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. Performance and Market Contingent Stock Options Granted to Employees In August and November 2015, the Board of Directors granted approximately 1.6 million and 0.1 million shares of performance- and market- contingent awards to members of the senior management team , respectively . 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 of the milestones are not achieved by the specified timelines, such vesting 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. Through March 31, 2019 , the Company continues to believe that the achievement of the requisite performance conditions is not probable and, as a result, no compensation cost has been recognized for 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. No stock options were granted to non-employees during the three months ended March 31, 2019. 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 market price of the Company's common stock on the date of grant. Aggregated information regarding RSAs and RSUs granted under the Plan for the three months ended March 31, 2019 is summarized below: Share Awards & Units Weighted-Average Fair Value at Date of Grant per Share Unvested at December 31, 2018 503,243 $ 4.86 Granted 376,359 18.27 Vested and released (348,893) 0.18 Forfeited (6,350) 19.04 Unvested at March 31, 2019 524,359 $ 17.47 Expected to vest - March 31, 2019 524,359 $ 17.47 Employee Stock Purchase Plan In December 2017, the Company adopted the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which initially reserved $1.0 million shares of common stock for employee purchases under terms and provisions established by the Board of Directors. The 2017 ESPP provides that the number of shares reserved and available for issuance under the 2017 ESPP will automatically increase each January 1, beginning on January 1, 2019, by the least of (i) 2.0 million shares, (ii) 1% of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the administrator of the 2017 Plan. In January 2019, the number of common stock available for issuance under the 2017 ESPP was increased by approximately 1.0 million shares as a result of this automatic increase provision. 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 . Contributions under the 2017 ESPP are limited to a maximum of 15% of an employee's eligible compensation. During the three months ended March 31, 2018 and 2018, no shares of common stock were issued under the 2017 ESPP. Stock-Based Compensation Expense The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 3,982 $ 1,686 General and administrative 2,892 1,239 Total $ 6,874 $ 2,925 As of March 31, 2019, total unamortized stock-based compensation expense related to unvested employee and non-employee awards that are expected to vest was $88.0 million. The weighted-average periods over which such stock-based compensation expense will be recognized is approximately 3.2 years.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share 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: March 31, 2019 2018 Options issued and outstanding and ESPP shares issuable 12,202,568 9,243,112 Restricted shares subject to future vesting 524,359 1,575,204 Early exercised common stock subject to future vesting 88,550 276,047 Total 12,815,477 11,094,363

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
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, 2018 , as filed with the Securities and Exchange Commission on March 12, 2019 (the "2018 Annual Report on Form 10-K"). The condensed consolidated Balance Sheet as of December 31, 2018 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 months ended March 31, 2019, except as discussed below in the sections titled Leases 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 2018 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 2018 Annual Report on Form 10-K.
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 March 31, 2019 and December 31, 2018, 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.
Leases	Leases The Company adopted Accounting Standards Update ("ASU") No. 2016-02, Leases as of January 1, 2019. A determination is made as to whether an arrangement is a lease at inception. A right-of-use (“ROU”) asset an d operating lease liability is recognized for identified operating leases in the condensed consolidated balance sheets. ROU assets represent the Company’s right to use the underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the lease commencement date based on the present value of lease payments due over the lease term, with the ROU assets adjusted for lease incentives received. When determining the present value of lease payments, the Company uses its incremental borrowing rate (“IBR”) on the date of lease commencement, or the rate implicit in the lease, if known. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably certain at lease inception.
Revenue Recognition	Revenue Recognition License and Collaboration Revenue 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 revenue, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenue. The core principle of Topic 606 is to recognize revenue 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 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 ("SSP") . The relative SSP for each deliverable is estimated using external sourced evidence if it is available. If external sourced evidence is not available, the Company uses its best estimate of the SSP 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 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 SSP of identified performance obligations, which may include forecasted revenue, 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 Adopted Accounting Pronouncement In February 2016, the Financial Accounting Standards Board (“FASB”) issued 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, C odification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements , in December 2018 through ASU 2018-20, Leases (Topic 842): Narrow-Scope Improvements for Lessors and in March 2019 through ASU 2019-01 Leases (Topic 842): Codification 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. 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 has adopted this standard as of January 1, 2019 applying the optional transition method such that it is not required to adjust prior period presentations. ASU 2016-02 has impacted the Company’s condensed consolidated balance sheet as of March 31, 2019 as the Company has certain operating lease arrangements for which the Company is the lessee and one future operating lease arrangement for which the Company is the lessor. The Company has no financing leases. Management has elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows us to carryforward the historical lease classification. The impact of adoption of the standard is that the Company has recognized a net ROU asset of $46.1 million and operating lease liability of approximately $71.3 million, as of January 1, 2019. The standard did not have a material impact on the Company’s condensed consolidated statements of operations and comprehensive loss and stockholders' equity.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
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): March 31, 2019 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 24,930 $ — $ — $ 24,930 Short-term marketable securities: U.S. government treasuries 287,681 — — 287,681 U.S. government agency securities — 51,328 — 51,328 Corporate debt securities — 69,678 — 69,678 Commercial paper — 37,133 — 37,133 Long-term marketable securities: U.S. government treasuries 81,526 — — 81,526 Corporate debt securities — 16,028 — 16,028 Foreign currency derivative contracts — 26 — 26 Total $ 394,137 $ 174,193 $ — $ 568,330 Liabilities: Foreign currency derivative contracts $ — $ 159 $ — $ 159 Total $ — $ 159 $ — $ 159 December 31, 2018 Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 42,225 $ — $ — $ 42,225 U.S. government treasuries 1,499 — — 1,499 Commercial paper — 9,979 — 9,979 Short-term marketable securities: U.S. government treasuries 219,754 — — 219,754 U.S. government agency securities — 73,151 — 73,151 Corporate debt securities — 71,675 — 71,675 Commercial paper — 22,594 — 22,594 Long-term marketable securities: U.S. government treasuries 117,131 — — 117,131 U.S. government agency securities — 1,977 — 1,977 Corporate debt securities — 28,773 — 28,773 Foreign currency derivative contracts — 14 — 14 Total $ 380,609 $ 208,163 $ — $ 588,772 Liabilities: Foreign currency derivative contracts $ — $ 182 $ — $ 182 Total $ — $ 182 $ — $ 182

Cash and Marketable Securities

Cash and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2019
Cash and Cash Equivalents [Abstract]
Schedule of 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): March 31, 2019 December 31, 2018 March 31, 2018 Cash and cash equivalents $ 39,661 $ 77,123 $ 44,001 Restricted cash included within other non-current assets 1,500 1,500 451 Total cash, cash equivalents, and restricted cash $ 41,161 $ 78,623 $ 44,452
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): March 31, 2019 December 31, 2018 March 31, 2018 Cash and cash equivalents $ 39,661 $ 77,123 $ 44,001 Restricted cash included within other non-current assets 1,500 1,500 451 Total cash, cash equivalents, and restricted cash $ 41,161 $ 78,623 $ 44,452
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): March 31, 2019 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 287,665 $ 172 $ (156) $ 287,681 U.S. government agency securities 51,404 4 (80) 51,328 Corporate debt securities 69,764 7 (93) 69,678 Commercial paper 37,133 — — 37,133 Total short-term marketable securities 445,966 183 (329) 445,820 Long-term marketable securities: U.S. government treasuries 81,105 421 — 81,526 Corporate debt securities 15,971 57 — 16,028 Total long-term marketable securities 97,076 478 — 97,554 Total $ 543,042 $ 661 $ (329) $ 543,374 December 31, 2018 Amortized Cost Unrealized Holding Gains Unrealized Holding Losses Aggregate Fair Value Short-term marketable securities: U.S. government treasuries $ 220,081 $ 29 $ (356) $ 219,754 U.S. government agency securities 73,373 — (222) 73,151 Corporate debt securities 71,940 1 (266) 71,675 Commercial paper 22,594 — — 22,594 Total short-term marketable securities 387,988 30 (844) 387,174 Long-term marketable securities: U.S. government treasuries 116,878 329 (76) 117,131 U.S. government agency securities 1,975 2 — 1,977 Corporate debt securities 28,864 8 (99) 28,773 Total long-term marketable securities 147,717 339 (175) 147,881 Total $ 535,705 $ 369 $ (1,019) $ 535,055

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2019
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 March 31, 2019 (notional amounts in thousands): Foreign Exchange Contracts Number of Contracts Aggregate Notional (1) Amount in Foreign Currency Maturity Euros 22 2,039 Apr. 2019 - Jan. 2020 British Pounds 27 3,982 Apr. 2019 - Feb. 2020 Swiss Francs 24 939 Apr. 2019 - Feb. 2020 Total 73 _________________________________________________ (1)

Collaboration Agreements (Table

Collaboration Agreements (Tables)	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Collaboration Revenue	Revenue disaggregated by collaboration agreement and performance obligation is as follows (in thousands): Three Months Ended March 31, 2019 2018 Takeda Collaboration Agreement (1) $ 682 $ 641 Sanofi Collaboration Agreement Alzheimer's Disease Services (1) 94 — Retained Activities 3,429 — Total Sanofi Collaboration Revenue 3,523 — Total Collaboration Revenue $ 4,205 $ 641 _________________________________________________ ( 1)

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Operating Lease Maturity, Payments	The following table reconciles the undiscounted cash flows for the first five years and total of the remaining years to the operating lease liabilities recorded in the condensed consolidated balance sheet as of March 31, 2019 (in thousands): Year Ending December 31: 2019 $ 4,785 2020 9,767 2021 10,386 2022 10,726 2023 11,078 2024 and later 64,399 Total undiscounted lease payments 111,141 Present value adjustment (38,979) Net operating lease liabilities $ 72,162
Operating Lease Maturity, Receivable	The following table details the future undiscounted cash inflows relating to the Company's sublease as of March 31, 2019 (in thousands): Year Ending December 31: 2019 $ 2,003 2020 2,842 2021 2,925 2022 3,009 2023 3,096 2024 and later 875 Total undiscounted sublease receipts $ 14,750

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	3 Months Ended
	Mar. 31, 2019
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- Average Exercise Price Weighted- Average remaining contractual life (years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2018 9,612,652 $ 10.49 8.20 $ 97,804 Options granted 2,745,750 11.60 Options exercised (199,503) 4.55 Options forfeited (133,273) 16.11 Balance at March 31, 2019 12,025,626 $ 12.29 8.39 $ 131,473 Options vested and expected to vest at March 31, 2019 10,280,894 $ 14.26 8.73 $ 92,147 Options exercisable at March 31, 2019 2,797,873 $ 8.74 7.76 $ 40,509
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted	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 March 31, 2019 2018 Expected term (in years) 6.08 6.08 Volatility 69.3% - 69.4% 86.0% - 87.5% Risk-free interest rate 2.5% - 2.6% 2.6% - 2.7% Dividend yield — —
Restricted Stock Activity	Aggregated information regarding RSAs and RSUs granted under the Plan for the three months ended March 31, 2019 is summarized below: Share Awards & Units Weighted-Average Fair Value at Date of Grant per Share Unvested at December 31, 2018 503,243 $ 4.86 Granted 376,359 18.27 Vested and released (348,893) 0.18 Forfeited (6,350) 19.04 Unvested at March 31, 2019 524,359 $ 17.47 Expected to vest - March 31, 2019 524,359 $ 17.47
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 March 31, 2019 2018 Research and development $ 3,982 $ 1,686 General and administrative 2,892 1,239 Total $ 6,874 $ 2,925

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
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: March 31, 2019 2018 Options issued and outstanding and ESPP shares issuable 12,202,568 9,243,112 Restricted shares subject to future vesting 524,359 1,575,204 Early exercised common stock subject to future vesting 88,550 276,047 Total 12,815,477 11,094,363

Significant Accounting Polici_3

Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)contractSegment	Jan. 01, 2019USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Number of financing leases | contract	0
Operating lease right-of-use asset	$ 34,407
Net operating lease liabilities	$ 72,162
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Operating lease right-of-use asset		$ 46,100
Net operating lease liabilities		$ 71,300

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 543,374,000	$ 535,055,000
Derivative asset	25,675	14,000
Total asset fair value measurements	568,330,000	588,772,000
Liability - foreign currency derivative contracts	159,000	182,000
Total liability fair value measurements	159,000	182,000
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	24,930,000	42,225,000
U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		1,499,000
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		9,979,000
Short-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	287,681,000	219,754,000
Short-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	51,328,000	73,151,000
Short-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	69,678,000	71,675,000
Short-term marketable securities: | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	37,133,000	22,594,000
Long-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	81,526,000	117,131,000
Long-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		1,977,000
Long-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	16,028,000	28,773,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Total asset fair value measurements	394,137,000	380,609,000
Liability - foreign currency derivative contracts	0	0
Total liability fair value measurements	0	0
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	24,930,000	42,225,000
Level 1 | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		1,499,000
Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Level 1 | Short-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	287,681,000	219,754,000
Level 1 | Short-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 | Short-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 | Short-term marketable securities: | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 1 | Long-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	81,526,000	117,131,000
Level 1 | Long-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		0
Level 1 | Long-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	26,000	14,000
Total asset fair value measurements	174,193,000	208,163,000
Liability - foreign currency derivative contracts	159,000	182,000
Total liability fair value measurements	159,000	182,000
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 2 | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		9,979,000
Level 2 | Short-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2 | Short-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	51,328,000	73,151,000
Level 2 | Short-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	69,678,000	71,675,000
Level 2 | Short-term marketable securities: | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	37,133,000	22,594,000
Level 2 | Long-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 2 | Long-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		1,977,000
Level 2 | Long-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	16,028,000	28,773,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Total asset fair value measurements	0	0
Liability - foreign currency derivative contracts	0	0
Total liability fair value measurements	0	0
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3 | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Level 3 | Short-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 | Short-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 | Short-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 | Short-term marketable securities: | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 | Long-term marketable securities: | U.S. government treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Level 3 | Long-term marketable securities: | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		0
Level 3 | Long-term marketable securities: | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 0	$ 0

Cash and Marketable Securitie_2

Cash and Marketable Securities - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 39,661	$ 77,123	$ 44,001
Restricted cash included within other non-current assets	1,500	1,500	451
Total cash, cash equivalents, and restricted cash	$ 41,161	$ 78,623	$ 44,452	$ 218,910

Cash and Marketable Securitie_3

Cash and Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 543,042	$ 535,705
Unrealized Holding Gains	661	369
Unrealized Holding Losses	(329)	(1,019)
Marketable securities	543,374	535,055
Short-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	445,966	387,988
Unrealized Holding Gains	183	30
Unrealized Holding Losses	(329)	(844)
Marketable securities	445,820	387,174
Short-term marketable securities: | U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	287,665	220,081
Unrealized Holding Gains	172	29
Unrealized Holding Losses	(156)	(356)
Marketable securities	287,681	219,754
Short-term marketable securities: | U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,404	73,373
Unrealized Holding Gains	4	0
Unrealized Holding Losses	(80)	(222)
Marketable securities	51,328	73,151
Short-term marketable securities: | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	69,764	71,940
Unrealized Holding Gains	7	1
Unrealized Holding Losses	(93)	(266)
Marketable securities	69,678	71,675
Short-term marketable securities: | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,133	22,594
Unrealized Holding Gains	0	0
Unrealized Holding Losses	0	0
Marketable securities	37,133	22,594
Long-term marketable securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	97,076	147,717
Unrealized Holding Gains	478	339
Unrealized Holding Losses	0	(175)
Marketable securities	97,554	147,881
Long-term marketable securities: | U.S. government treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81,105	116,878
Unrealized Holding Gains	421	329
Unrealized Holding Losses	0	(76)
Marketable securities	81,526	117,131
Long-term marketable securities: | U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,975
Unrealized Holding Gains		2
Unrealized Holding Losses		0
Marketable securities		1,977
Long-term marketable securities: | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,971	28,864
Unrealized Holding Gains	57	8
Unrealized Holding Losses	0	(99)
Marketable securities	$ 16,028	$ 28,773

Cash and Marketable Securitie_4

Cash and Marketable Securities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash and Cash Equivalents [Abstract]
Other-than-temporary impairment	$ 0	$ 0
Effective maturity (less than)	2 years

Derivative Financial Instrume_3

Derivative Financial Instruments - Summary of Forward Foreign Currency Exchange Contracts Outstanding (Details) € in Thousands, £ in Thousands, SFr in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)	Mar. 31, 2019GBP (£)derivative_instrument	Mar. 31, 2019USD ($)derivative_instrument	Mar. 31, 2019CHF (SFr)derivative_instrument	Mar. 31, 2019EUR (€)derivative_instrument	Dec. 31, 2018USD ($)	Jun. 30, 2018derivative_instrument
Derivative [Line Items]
Derivative liability | $			$ 159,000			$ 182,000
Asset - foreign currency derivative contracts | $			$ 25,675			$ 14,000
Net gain on derivative instruments | $	$ 19,287
Euros | Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts		22	22	22	22
Aggregate Notional Amount in Foreign Currency | €					€ 2,039
British Pounds | Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts		27	27	27	27
Aggregate Notional Amount in Foreign Currency | £		£ 3,982
Swiss Francs | Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts		24	24	24	24
Aggregate Notional Amount in Foreign Currency | SFr				SFr 939
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative [Line Items]
Number of Contracts		73	73	73	73		0

Acquisition (Details)

Acquisition (Details) $ in Thousands	May 30, 2018USD ($)derivative_instrument	Aug. 24, 2016USD ($)derivative_instrument	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	May 31, 2018USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development			$ 37,403	$ 20,819
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront option fee					$ 500
Estimated net liabilities	$ 200
Number of Fcab targets | derivative_instrument		3
Fcab target period		3 years
Upfront fee paid		$ 5,500
Additional Fcab targets | derivative_instrument	2
Additional Fcab target obligation	$ 6,000
Research and development expense					$ 18,300
Contingent consideration recognized						$ 0
Maximum development plan period for research costs		24 months
Research and development			$ 200	$ 300
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Exercise of buy-out option
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial option exercise payments	18,000
Collaborative Arrangement with F-Star and Acquisition of F-Star Gamma | Maximum | Exercise of buy-out option
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent payments upon the achievement of milestones	$ 447,000

Collaboration Agreements - Sano

Collaboration Agreements - Sanofi (Details) - USD ($) $ in Thousands	Nov. 20, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Oct. 29, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue		$ 4,205	$ 641
Collaborative Arrangement | Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment	$ 125,000
Milestone payments upon achievement of certain clinical, regulatory and sales milestone events	1,100,000
Funded percentage						70.00%
Increase in transaction price		5,800		$ 5,800
Receivable		$ 3,400		3,400	$ 2,300
Milestones recognized				0
Collaboration revenue				$ 0
Collaborative Arrangement | Sanofi | CNS Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical and regulatory milestone payments	600,000
Collaborative Arrangement | Sanofi | Peripheral Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical, regulatory and commercial milestone payments	$ 495,000
Collaborative Arrangement | Denali
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Funded percentage						30.00%

Collaboration Agreements - Sa_2

Collaboration Agreements - Sanofi Performance Obligation (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Alzheimer's Disease Services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation	$ 3.8	$ 3.9

Collaboration Agreements - Take

Collaboration Agreements - Takeda (Details) $ / shares in Units, $ in Thousands	Feb. 23, 2018USD ($)program$ / sharesshares	Jan. 03, 2018USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Oct. 31, 2018USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales			$ 4,205	$ 641
Collaborative Arrangement | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Target option period		5 years
Upfront payment	$ 40,000	$ 40,000
Preclinical milestone payment received	5,000
Option fee		5,000
Variable consideration relating to future milestones	26,000
Preclinical milestone payment earned not yet received			0		$ 5,000	$ 5,000
Remaining preclinical milestones, cost and profit sharing income, and the development and commercial milestones	$ 44,000
Number of performance obligations | program	3
Contract liability			64,200		$ 64,900
Revenue recognized			1,400
Milestones recognized			15,000
Collaborative Arrangement | Takeda Pharmaceutical Company Limited | Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales			$ 0
Collaborative Arrangement | Share Purchase Agreement | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common stock (in shares) | shares	4,214,559
Purchase price	$ 110,000
Fair market value of common stock	$ 94,400
Closing stock price (usd per share) | $ / shares	$ 22.40
Premium on sale of common stock	$ 15,600
Collaborative Arrangement | Maximum | Takeda Pharmaceutical Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preclinical milestone payments per program		25,000
Total aggregate payments due upon achievement of certain preclinical milestone events		75,000
Aggregated option exercise fee		15,000
Total aggregate payments due upon achievement of certain clinical and regulatory milestone events		707,500
Milestone payments per biologic product upon achievement of a certain sales-based milestone		75,000
Milestone payments upon achievement of biologic product from each program		$ 225,000

Collaboration Agreements - Summ

Collaboration Agreements - Summary of Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 4,205	$ 641
Deferred revenue		0
Takeda Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	682	641
Sanofi Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	3,523	0
Sanofi Collaboration Agreement | Alzheimer's Disease Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	94	0
Sanofi Collaboration Agreement | Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue	$ 3,429	$ 0

License Agreements (Details)

License Agreements (Details) - Genentech Inc - USD ($) $ in Millions	Jun. 17, 2016	Dec. 31, 2017	Dec. 31, 2016
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 315
License Agreement
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 | Research and Development
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			$ 8.5
Technology transfer fee			$ 1.5
First clinical milestone		$ 2.5

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Thousands	May 02, 2018USD ($)ft²	Aug. 01, 2016USD ($)ft²	Aug. 31, 2016USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Oct. 31, 2018ft²
Loss Contingencies [Line Items]
Operating lease right-of-use asset				$ 34,407
Operating lease liability, current				800
Operating lease liability, less current portion				$ 71,412
Discount rate (as a percentage)				9.00%
Operating lease costs				$ 2,300
Rent expense					$ 400
Weighted-average remaining lease term (years)				10 years
Weighted-average discount rate (as a percentage)				9.00%
Operating cash flows from operating leases				$ 700
Headquarters Lease
Loss Contingencies [Line Items]
Area under lease | ft²	148,020	38,109
Lease period	10 years	8 years
Lease renewal option term	10 years
New Premises Sublease Agreement
Loss Contingencies [Line Items]
Rentable square feet | ft²							36,835
Sublease term							5 years
Landlord Funded Tenant Improvements | Headquarters Lease
Loss Contingencies [Line Items]
Tenant improvement allowance repayable in rent	$ 4,400		$ 1,900
DMSA
Loss Contingencies [Line Items]
Purchase order executed				25,400		$ 24,700
Costs incurred				3,600	100
Payments for development and manufacturing services				2,600	$ 100
Non-refundable purchase commitments				$ 12,300		$ 14,000
Minimum | Headquarters Lease
Loss Contingencies [Line Items]
Lease renewal notice period	9 months
Maximum
Loss Contingencies [Line Items]
Leasehold improvements	$ 25,900
Maximum | Headquarters Lease
Loss Contingencies [Line Items]
Lease renewal notice period	12 months
Maximum | Landlord Funded Tenant Improvements | Headquarters Lease
Loss Contingencies [Line Items]
Tenant improvements		$ 7,400

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Company's Future Minimum Lease Commitments (Details) $ in Thousands	Mar. 31, 2019USD ($)
Operating Lease Liabilities, Payments Due [Abstract]
2019	$ 4,785
2020	9,767
2021	10,386
2022	10,726
2023	11,078
2024 and later	64,399
Total undiscounted lease payments	111,141
Present value adjustment	(38,979)
Net operating lease liabilities	$ 72,162

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Company's Future Minimum Lease Receivables (Details) $ in Thousands	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 2,003
2020	2,842
2021	2,925
2022	3,009
2023	3,096
2024 and later	875
Total undiscounted sublease receipts	$ 14,750

Stock-Based Awards - Narrative

Stock-Based Awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended
	Dec. 31, 2017	Nov. 30, 2015	Aug. 31, 2015	May 31, 2015	Mar. 31, 2019	Mar. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value of option (usd per share)					$ 11.60	$ 16.36
Aggregate intrinsic value of options exercised					$ 3,300	$ 1,300
Weighted average grant date fair value of options vested (usd per share)					$ 11.82	$ 2.90
Number of options, granted (in shares)					2,745,750
Exercise price (usd per share)					$ 4.55
Share based compensation cost recognized					$ 6,874	$ 2,925
Expected weighted average period					3 years 2 months 12 days
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized					$ 2,892	1,239
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost recognized					$ 3,982	$ 1,686
Performance and Market Contingent Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options, granted (in shares)		100,000	1,600,000
Exercise price (usd per share)		$ 0.68	$ 0.68
Vesting trigger, number of consecutive trading days					90 days
Vesting trigger, number of days after IPO					180 days
Compensation cost			$ 0
2017 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)	1,000,000
Number of additional shares allowable under the plan (in shares)	2,000,000						1,000,000
Percent of outstanding shares	1.00%
Common stock purchase discounted rate for employees	85.00%
Maximum employee contribution to ESPP, percent of base compensation	15.00%
Stock issued during period (in shares)					0	0
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock- based compensation expense related to unvested stock options					$ 88,000
Non-Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)					0
Number of options, granted (in shares)					0
2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)								6,400,000
Number of additional shares allowable under the plan (in shares)	10,000,000						4,800,000
Percent of outstanding shares	5.00%
Number of shares available for grant (in shares)					4,100,000
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)				8,300,000
Number of shares available for grant (in shares)					0
2015 Stock Incentive Plan | All Shares Granted At $ 0.68
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)								200,000
Maximum | Performance and Market Contingent Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized					$ 6,200
Maximum | Performance and Market Contingent Stock Options | General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized					5,800
Maximum | Performance and Market Contingent Stock Options | Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation cost not recognized					$ 400
Maximum | 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
Maximum | 2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period				10 years
Minimum | 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date	100.00%
Award vesting period	4 years
Minimum | 2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of option price of estimated fair value on grant date				100.00%
Award vesting period	4 years

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Options
Number of options, beginning balance (in shares)	9,612,652
Number of options, granted (in shares)	2,745,750
Number of options, exercised (in shares)	(199,503)
Number of options, forfeited (in shares)	(133,273)
Number of options, ending balance (in shares)	12,025,626	9,612,652
Number of options, vested and expected to vest (in shares)	10,280,894
Number of options, exercisable (in shares)	2,797,873
Weighted- Average Exercise Price
Weighted average exercise price, beginning balance (usd per share)	$ 10.49
Weighted average exercise price, granted (usd per share)	11.60
Weighted average exercise price, exercised (usd per share)	4.55
Weighted average exercise price, forfeited (usd per share)	16.11
Weighted average exercise price, ending balance (usd per share)	12.29	$ 10.49
Weighted average exercise price, vested and expected to vest (usd per share)	14.26
Weighted average exercise price, exercisable (usd per share)	$ 8.74
Stock Option Activity, Additional Disclosures
Weighted average remaining contractual life	8 years 4 months 20 days	8 years 2 months 12 days
Weighted average remaining contractual life, vested and expected to vest	8 years 8 months 23 days
Weighted average remaining contractual life, exercisable	7 years 9 months 3 days
Aggregate intrinsic value, beginning balance	$ 97,804
Aggregate intrinsic value, ending balance	131,473	$ 97,804
Aggregate intrinsic value, vested and expected to vest	92,147
Aggregate intrinsic value	$ 40,509

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of Assumptions Used for Estimating the Fair Value of Stock Options Granted (Details)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days
Volatility, minimum	69.30%	86.00%
Volatility, maximum	69.40%	87.50%
Risk-free interest rate, minimum	2.50%	2.60%
Risk-free interest rate, maximum	2.60%	2.70%
Dividend yield	0.00%	0.00%

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Restricted Stock Activity (Details) - Restricted Stock	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Share Awards & Units
Shares, unvested, beginning balance (in shares) | shares	503,243
Shares, granted (in shares) | shares	376,359
Shares, vested (in shares) | shares	(348,893)
Shares, forfeited (in shares) | shares	(6,350)
Shares, unvested, ending balance (in shares) | shares	524,359
Shares, vested and expected to vest (in shares) | shares	524,359
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	$ 4.86
Weighted-average fair value at date of grant per share, granted (usd per share) | $ / shares	18.27
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	19.04
Weighted-average fair value at date of grant per share, unvested, ending balance (usd per share) | $ / shares	17.47
Weighted-average fair value at date of grant per share, vested and expected to vest (usd per share) | $ / shares	$ 17.47

Stock-Based Awards - Summary _4

Stock-Based Awards - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 6,874	$ 2,925
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	3,982	1,686
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 2,892	$ 1,239

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Dilutive Securities Not Included in Diluted Per Share Calculations (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	12,815,477	11,094,363
Options issued and outstanding and ESPP shares issuable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	12,202,568	9,243,112
Restricted shares subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	524,359	1,575,204
Early exercised common stock subject to future vesting
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	88,550	276,047