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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2019	Mar. 06, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	Unity Biotechnology, Inc.
Entity Central Index Key	0001463361
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	false
Entity Interactive Data Current	Yes
Entity Common Stock, Shares Outstanding		47,613,143
Entity Public Float			$ 276,736,207
Entity File Number	001-38470
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-4726035
Entity Address, Address Line One	285 East Grand Ave.
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-1192
Document Annual Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock, par value $0.0001
Security Exchange Name	NASDAQ
Trading Symbol	UBX
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant’s Definitive Proxy Statement relating to the 2020 Annual Meeting of Shareholders, scheduled to be held on June18, 2020, are incorporated by reference into Part III of this Report. Such proxy statement will be filed with the Securities and Exchange Commission within 120 days of the end of the fiscal year covered by this Annual Report on Form 10-K.

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 37,473	$ 15,399
Short-term marketable securities	84,508	155,736
Strategic investments	5,507
Prepaid expenses and other current assets	1,999	1,830
Total current assets	129,487	172,965
Property and equipment, net	16,636	6,238
Long-term marketable securities	3,025
Restricted cash	1,446	550
Other long-term assets	627	1,622
Total assets	151,221	181,375
Current liabilities:
Accounts payable	5,185	4,847
Accrued compensation	5,905	3,791
Accrued and other current liabilities	4,995	4,990
Settlement liability		2,059
Contingent consideration liability	1,131	895
Total current liabilities	17,216	16,582
Deferred rent, net of current portion	13,298	2,467
Contingent consideration liability, net of current portion		1,588
Other non-current liabilities		45
Total liabilities	30,514	20,682
Commitments and contingencies (Note 7)
Stockholders’ equity (deficit):
Common stock, $0.0001 par value; 300,000,000 shares authorized as of December 31, 2019 and 2018; 47,227,065 and 42,414,294 shares issued and outstanding as of December 31, 2019 and 2018, respectively	5	4
Additional paid-in capital	366,695	324,663
Related party promissory notes for purchase of common stock	(210)	(201)
Employee promissory notes for purchase of common stock	(418)	(400)
Accumulated other comprehensive loss	90	(95)
Accumulated deficit	(245,455)	(163,278)
Total stockholders’ equity	120,707	160,693
Total liabilities and stockholders’ equity	$ 151,221	$ 181,375

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Feb. 28, 2018
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	10,000,000	10,000,000
Convertible preferred stock, shares issued	0	0
Convertible preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000	140,000,000	122,000,000
Common stock, shares issued	47,227,065	42,414,294
Common stock, shares outstanding	47,227,065	42,414,294

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Contribution revenue			$ 1,382
Operating expenses:
Research and development	$ 70,957	$ 58,907	37,373
General and administrative	20,046	16,016	9,617
Change in fair value of contingent consideration	(1,352)	4,542
Total operating expenses	89,651	79,465	46,990
Loss from operations	(89,651)	(79,465)	(45,608)
Interest income	3,289	3,312	1,055
Other income (expense), net	4,185	(245)	(103)
Net loss	(82,177)	(76,398)	(44,656)
Other comprehensive loss
Unrealized gain (loss) on marketable debt securities, net of tax	185	9	(104)
Comprehensive loss	$ (81,992)	$ (76,389)	$ (44,760)
Net loss per share, basic and diluted	$ (1.88)	$ (2.70)	$ (13.97)
Weighted average number of shares used in computing net loss per share, basic and diluted	43,624,807	28,269,907	3,197,516

Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Series A-1, A-2, B and C convertible preferred Stock	ATM Equity offering program	Common Stock	Common Stock ATM Equity offering program	Additional Paid-In Capital	Additional Paid-In Capital ATM Equity offering program	Related Party Promissory Notes for Purchase of Commmon Stock	Employee Promissory Notes for Purchase of Commmon Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ (41,536)						$ 1		$ 889		$ (202)			$ (42,224)
Beginning balance, (in shares) at Dec. 31, 2016		24,620,615
Beginning balance at Dec. 31, 2016		$ 131,089
Beginning balance, (in shares) at Dec. 31, 2016							4,303,538
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)			3,539,109
Issuance of convertible preferred stock per share for cash, net of issuance costs			$ 42,867
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	8								8
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options (in shares)							43,727
Vesting of early exercised options	97								97
Issuance of restricted stock							625,931
Common stock granted/ issued to third parties	44								44
Common stock granted/ issued to third parties (in shares)							12,711
Stock-based compensation	3,034								3,034
Repurchase of early exercised shares of common stock (in Shares)							(155,518)
Unrealized loss on marketable securities, net of tax	(104)												$ (104)
Net loss	(44,656)													(44,656)
Ending balance at Dec. 31, 2017	(83,113)						$ 1		4,072		(202)		(104)	(86,880)
Ending balance, (in shares) at Dec. 31, 2017		28,159,724
Ending balance at Dec. 31, 2017		$ 173,956
Ending balance (in shares) at Dec. 31, 2017							4,830,389
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)				3,913,425
Issuance of convertible preferred stock per share for cash, net of issuance costs				$ 59,881
Conversion of Series A-1, A-2, B and C convertible preferredstock to common stock	233,839						$ 2		233,837
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock (in Shares)					(32,073,149)
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock					$ (233,837)
Conversion of Series A-1, A-2, B and C convertible preferred stock to common stock (in Shares)							32,073,149
Vesting of early exercised options	584								584
Common stock granted/ issued to third parties	75,852						$ 1		75,851
Common stock granted/ issued to third parties (in shares)							5,000,000
Issuance of common stock upon exercise of warrants and stock options,net of amount related to early exercised options	374								374
Issuance of common stock upon exercise of warrants and stock options,net of amount related to early exercised options (in shares)							510,756
Stock-based compensation	9,441								9,441
Receipt of promissory note from related party for purchase of common stock	(390)										(390)
Receipt of promissory note from employee for purchase of common stock	(400)											$ (400)
Repayment of promissory note from related party	895								504		391
Unrealized loss on marketable securities, net of tax	9												9
Net loss	(76,398)													(76,398)
Ending balance at Dec. 31, 2018	$ 160,693						$ 4		324,663		(201)	(400)	(95)	(163,278)
Ending balance, (in shares) at Dec. 31, 2018	0
Ending balance (in shares) at Dec. 31, 2018	42,414,294						42,414,294
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options	$ 840								840
Issuance of common stock upon exercise of stock options, net of amount related to early exercised options (in shares)	505,226						505,226
Vesting of early exercised options	$ 647								647
Common stock granted/ issued to third parties	2,995					$ 26,086		$ 1	3,022	$ 26,085	(9)	(18)
Common stock granted/ issued to third parties (in shares)							253,334	3,974,908
Stock-based compensation	10,852								10,852
Issuance of common stock under employee stock purchase plan (“ESPP”)	586								586
Issuance of common stock under employee stock purchase plan (“ESPP”) (in Shares)							83,584
Receipt of promissory note from related party for purchase of common stock	(27)
Repurchase of early exercised shares of common stock (in Shares)							(4,281)
Unrealized loss on marketable securities, net of tax	185												185
Net loss	(82,177)													(82,177)
Ending balance at Dec. 31, 2019	$ 120,707						$ 5		$ 366,695		$ (210)	$ (418)	$ 90	$ (245,455)
Ending balance, (in shares) at Dec. 31, 2019	0
Ending balance (in shares) at Dec. 31, 2019	47,227,065						47,227,065

Statements of Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Amount of early exercised options	$ 1,212	$ 5
Issuance cost	$ 9,149
Series B Convertible Preferred Stock
Issuance price per share		$ 12.125
Issuance cost		$ 43
Series C Convertible Preferred Stock
Issuance price per share	$ 15.3317
Issuance cost	$ 119

Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (82,177)	$ (76,398)	$ (44,656)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,663	2,180	1,304
Net accretion and amortization of premium and discounts on marketable securities	(1,151)	(955)	182
Stock-based compensation	10,852	9,441	3,034
Loss on disposal of property and equipment		45	15
Common stock granted to third party	965		44
Change in fair value of strategic investments	(4,507)
Accretion of tenant improvement allowance	(1,275)	(605)	(605)
Change in fair value of contingent consideration for license agreements	(1,352)	4,542
Changes in operating assets and liabilities:
Contribution receivable		1,382	(1,382)
Prepaid expenses and other current assets	(169)	(842)	(746)
Other long-term assets	(31)	(604)	23
Accounts payable	(227)	2,228	1,198
Accrued compensation	2,114	1,610	1,607
Accrued liabilities and other current liabilities	(587)	1,446	1,258
Deferred rent, net of current portion	2,461	(93)	366
Net cash used in operating activities	(72,421)	(56,623)	(38,358)
Investing activities
Purchase of marketable securities	(119,270)	(204,086)	(134,465)
Maturities of marketable securities	188,809	133,644	49,849
Purchase of investment in stock		(500)
Purchase of property and equipment	(1,586)	(1,264)	(1,689)
Net cash provided by (used in) investing activities	67,953	(72,206)	(86,305)
Financing activities
Proceeds from issuance of common stock under ATM offering program, net of issuance costs	26,085
Proceeds from issuance of convertible preferred stock, net of issuance costs		59,881	42,867
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	840	374	(37)
Proceeds from issuance of common stock under ESPP	586
Proceeds from initial public offering, net of issuance costs		79,055
Payment of initial public offering costs		(3,201)
Proceeds from repayment of recourse notes		895
Payments made on capital lease obligations	(73)	(74)	(55)
Net cash provided by financing activities	27,438	136,930	42,775
Net increase (decrease) in cash, cash equivalents and restricted cash	22,970	8,101	(81,888)
Cash, cash equivalents and restricted cash at beginning of year	15,949	7,848	89,736
Cash, cash equivalents and restricted cash at end of year	38,919	15,949	7,848
Supplemental Disclosures of Non-Cash Investing and Financing Activities
Property and equipment included in accounts payable	565	241	314
Property and equipment acquired under capital leases			243
Lessor funded lease incentives included in property and equipment	10,651		$ 3,881
Receipt of promissory note for purchase of common stock		400
Receipt of promissory note from related party for purchase of common stock	$ 27	$ 390

Organization

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1. Organization

Description of Business

Unity Biotechnology, Inc. (the “Company”) is a biotechnology company engaged in the research and development of therapeutics to extend human healthspan. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel. The Company’s headquarters are located in South San Francisco, California and was incorporated in the State of Delaware in 2009.

Need for Additional Capital

The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $245.5 million as of December 31, 2019. During the year ended December 31, 2019, the Company incurred a net loss of $82.2 million and used $72.4 million of cash in operating activities. To date, none of the Company’s drug candidates have been approved for sale. The Company has not generated any revenue from contracts with customers and does not expect positive cash flows from operations in the foreseeable future. The Company has financed its operations primarily through private placements of preferred stock and promissory notes and more recently through our IPO and proceeds from our ATM Offering Program, and will continue to be dependent upon equity and/or debt financing until we are able to generate positive cash flows from our operations. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from contracts with customers.

The Company had cash, cash equivalents and marketable securities of $125.0 million as of December 31, 2019. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of 12 months following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Reverse Stock Split

On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements has been adjusted to reflect the Reverse Split.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of Securities and Exchange Commission (“SEC”) for reporting.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, contingent consideration liability, common stock valuation, and stock-based compensation. Actual results could differ from such estimates or assumptions.

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.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value.

The Company has issued letters of credit under its lease agreements which have been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows (in thousands).

	December 31,
	2019		2018		2017
Cash and cash equivalents	$	37,473	$	15,399	$	7,298
Restricted cash		1,446		550		550
Total cash, cash equivalents, and restricted cash	$	38,919	$	15,949	$	7,848

Marketable Securities

The Company generally invests its excess cash in investment grade, short to intermediate-term, fixed income securities. Such investments are considered available-for-sale debt securities, and reported at fair value with unrealized gains and losses included as a component of stockholders’ deficit. Marketable securities with original maturities of greater than 90 days from the date of purchase but less than one year from the balance sheet date that are available to be converted into cash to fund current operations are classified as short-term, while marketable securities with maturities in one year or beyond one year from the balance sheet date are classified as long-term. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income (expense), net. The cost of securities sold is determined using the specific identification method.

The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and management’s strategy and intentions for holding the marketable security. To date, the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value.

Strategic Investments

The Company may make investments in strategic partners. The Company does not intend to have a controlling interest or significant influence when it makes these strategic investments. Investments in equity securities of strategic partners with readily determinable fair values are measured using quoted market prices in with changes recorded through other income (expense), net in the statement of operations. The Company currently holds a non-controlling equity investment in the common stock of a Hong-Kong based clinical-stage biopharmaceutical company called Ascentage Pharma Group International (“Ascentage International”), which was acquired in connection with certain license agreements (see Note 5, “License Agreements”). In October 2019, Ascentage International completed an initial public offering of shares of its common stock on the Hong Kong stock exchange at HK$34.20 (approximately USD $4.36) per share. The Company is subject to a lock-up agreement with Ascentage International that precludes the Company from selling shares prior to April 2020.

Fair Value Measurements

The Company’s financial instruments during the periods presented consist of cash and cash equivalents, restricted cash, marketable securities, strategic investments, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities and contingent consideration liabilities. Fair value estimates of these instruments are made at a specific point in time, based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of judgment.

Concentrations of Risk

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash and marketable securities. Substantially all of the Company’s cash and cash equivalents and restricted cash is 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 contribution receivable is unsecured and is concentrated with one third-party organization, and accordingly the Company may be exposed to credit risk. To date, the Company has not experienced any loss related to its contributions receivable.

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, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of December 31, 2019, the Company had no off-balance sheet concentrations of credit risk.

The Company is also exposed to market risk in its strategic investments. As of December 31, 2019, the Company held an investment in common stock which is publicly traded.

The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials.

Contribution Revenue

The Company recognizes contribution revenue related to the receipt of cash from third-party resource providers not considered to be customers and where the transfer of assets is not an exchange transaction or financing of research and development. Contribution revenue and related receivables are recognized for conditional contributions as the conditions related to the contribution are relieved.

In July 2017, the Company entered an arrangement with a third-party organization under which the Company would be provided with up to $1.5 million of funding for the performance of certain research and development activities during the 90-day period following the arrangement in pursuit of the third-party organization’s philanthropic mission. All conditions related to this contribution were met during 2017 and the Company recognized $1.4 million under this arrangement, which was recorded as contribution revenue in the statement of operations.

Research and Development Expenses and Accruals

Costs related to research, design and development of drug candidates are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses for personnel contributing to research and development activities, laboratory supplies, outside services, licenses acquired to be used in research and development, manufacturing of clinical material, pre-clinical testing and consultants and allocated overhead, including rent, equipment, depreciation and utilities. Research and development costs are expensed as incurred unless there is an alternative future use in other research and development projects. Payments made prior to the receipt of goods or services to be used in research and development are deferred and recognized as expense in the period in which the related goods are received or services are rendered. Such payments are evaluated for current or long-term classification based on when they will be realized.

As part of the process of preparing our financial statements, we are required to estimate expenses resulting from our obligations under contracts with vendors and consultants and clinical site agreements in connection with conducting clinical trials. The financial terms of these contracts are subject to negotiations which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided under such contracts. Our objective is to reflect the appropriate expenses in our financial statements by matching those expenses with the period in which services and efforts are expended. We account for these expenses according to the progress of the production of clinical trial materials or based on progression of the clinical trial, as measured by patient progression and the timing of various aspects of the trial. We determine accrual estimates by taking into account discussion with applicable personnel and outside service providers as to the progress or state of consummation of goods and services, or the services completed. During the course of a clinical trial, we adjust the rate of expense recognition if actual results differ from our estimates. We make estimates of accrued expenses as of each balance sheet date in our financial statements based on the facts and circumstances known at that time. Our clinical trial accrual is dependent in part upon the timely and accurate reporting of contract research organizations, contract manufacturers and other third-party vendors. Although we do not expect our estimates to be materially different from amounts actually incurred, our understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in our reporting changes in estimates in any particular period. Adjustments to prior period estimates have not been material for the years ended December 31, 2019 and 2018.

We have and may continue to enter into license agreements to access and utilize certain technology. We evaluate if the license agreement is an acquisition of an asset or a business. To date none of our license agreements have been considered to be an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements may also include contingent consideration in the form of cash and additional issuances of our common stock.

Contingent Consideration Liability

The Company has entered into and may continue to enter into, license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date none of the Company’s license agreements have been considered an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements also include contingent consideration in the form of additional issuances of the Company’s common stock based on the achievement of certain milestones. For asset acquisitions, the Company assesses on a continuous basis whether such contingent consideration meets the definition of a derivative and can or cannot be classified within stockholders’ equity, until such time that equity classification criteria are met or the milestones expire. The derivative related to this contingent consideration is measured at fair value as of each balance sheet date with the related change in fair value being reflected in operating expenses. Upon a reassessment event that results in the contingent consideration no longer meeting the definition of a derivative and/or meeting equity classification criteria, the final change in fair value of the instrument is recorded within operating expenses and the liability is reclassified into stockholders’ equity.

Variable Interest Entities

The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event.

Property and Equipment, Net

Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, generally three years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the lease. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to expense as incurred and costs of improvement are capitalized.

Impairment of Long-Lived Assets

The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying value of the assets, the Company reduces the carrying amount of the assets through an impairment charge, to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. No impairment losses have been recorded for the periods presented.

Leases

The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances to fund leasehold improvements and rent holidays, and are recognized as reductions to rental expense on a straight-line basis over the term of the lease. Lessor funded leasehold improvement incentives not yet received are recorded in prepaid expense and other current assets on the balance sheet. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease and begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the rent expense recognized. The Company recognizes a liability for costs that will continue to be incurred under a lease contract for its remaining term without economic benefit at its fair value when the entity ceases using the right conveyed by the contract, which is it when the space is completely vacated.

The Company entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations is included in accrued and other liabilities and the noncurrent capital lease obligations is included in other noncurrent liabilities in the balance sheet.

Stock-Based Compensation

The Company measures employee and director stock-based compensation expense for all stock-based awards based on their grant date fair value. For stock-based awards with service conditions only, stock-based compensation expense is recognized over the requisite service period using the straight-line method. For awards with performance conditions, the Company evaluates the probability of achieving performance condition at each reporting date. The Company begins to recognize stock-based compensation expense using an accelerated attribution method when it is deemed probable that the performance condition will be met. Forfeitures are recognized as they occur.

The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock option awards that do not contain market conditions. The Black-Scholes option-pricing model requires assumptions to be made related to the expected term of an award, expected dividends, expected volatility and risk-free rate. The Company uses the lattice models to estimate the fair value of stock option awards that contain market conditions. Lattice models require the use of subjective and complex assumptions which determine the fair value of such awards including price volatility of the underlying stock and derived service periods.

The Company recognizes stock-based compensation expense for stock options granted to non-employees based on the estimated fair value of the award as it is more readily measurable than the fair value of the services received. The fair value of stock options granted to non-employees is estimated at grant date and re-measured at each reporting period using the Black-Scholes option-pricing model until the awards vest and the resulting change in value, if any, is recognized in the statements of operations.

Income Taxes

The Company uses the asset and liability method of accounting for income taxes, in which deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be reversed. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established if it is more likely than not that all or a portion of the deferred tax asset will not be realized.

The Company’s tax positions are subject to income tax audits. The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. The tax benefit recognized is measured as the largest amount of benefit which is more likely than not to be realized upon settlement with the taxing authority. The Company recognizes interest accrued and penalties related to unrecognized tax benefits in its tax provision. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate, as well as the related net interest and penalties.

Net Loss per Common Share

Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive. The calculation of diluted earnings (loss) per share also requires that, to the extent the presumed issuance of additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses on the Company’s marketable securities.

Recently Adopted Accounting Pronouncements

In January 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. This guidance makes amendments to the classification and measurement of financial instruments and revises the accounting related to: (1) the classification and measurement of investments in equity securities (except for investments accounted for under the equity method of accounting); and (2) the presentation of certain fair value changes for financial liabilities measured at fair value. In addition, the update also amends certain disclosure requirements associated with the fair value of financial instruments. The guidance is effective for the Company for annual periods beginning in 2019 and interim periods beginning in 2020. Early adoptions of certain amendments within the update are permitted. The Company adopted this guidance during the first quarter of fiscal year 2019. The adoption of this guidance did not have a material impact on the Company's financial statements other than to the treatment of its strategic investment in Ascentage International subsequent to their initial public offering.

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. This ASU clarifies the definition of a business when evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance is effective for the Company for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company adopted this ASU during the first quarter of fiscal year 2019. The adoption of this ASU did not have a significant impact on its financial statements and related disclosures.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments). This guidance addresses specific cash flow issues with the objective of reducing the diversity in practice for the treatment of these issues. The areas identified include: debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance policies; distributions received from equity method investees; beneficial interests in securitization transactions; and application of the predominance principle with respect to separately identifiable cash flows. The guidance will generally be applied retrospectively and is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company adopted this ASU during the first quarter of fiscal year 2019. The adoption of this ASU did not have a significant impact on its financial statements and related disclosures.

Recently Issued Accounting Pronouncements Not Yet Adopted

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (ASU 2018-18), which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under the guidance for contracts with customers (Topic 606) when the collaborative arrangement participant is a customer in the context of a unit of account. The standard is effective for interim and annual periods beginning after December 15, 2020 , with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. The Company is still finalizing its analysis and evaluating the impact adopting this new accounting standard will have on its financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact of adoption on its financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of this new standard to have a significant impact on its disclosures.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. The amendments in this ASU expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. This new guidance is effective for the Company in fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company will adopt this standard on January 1, 2020 and does not currently expect it will have a material impact on its financial statements.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, as clarified in subsequent amendments. ASU 2016-13 changes the impairment model for certain financial instruments. The new model is a forward-looking expected loss model and will apply to financial assets subject to credit losses and measured at amortized cost and certain off-balance sheet credit exposures. This includes loans, held-to-maturity debt securities, loan commitments, financial guarantees and net investments in leases, as well as trade receivables. For available-for-sale debt securities with unrealized losses, credit losses will be measured in a manner similar to today, except that the losses will be recognized as allowances rather than reductions in the amortized cost of the securities. In October 2019, the FASB voted to delay the effective date of this standard. Topic 326 will be effective for the Company for fiscal years beginning after December 15, 2022. Early adoption is permitted. The Company is currently assessing the effect that this ASU will have on its financial position, results of operations, and disclosures.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), and related amendments which supersedes the guidance in former ASC 840, Leases. The new standard, as amended by subsequent ASUs on the Topic, requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. On November 15, 2019, the FASB issued ASU 2019-10 to delay the effective date of this standard. Topic 842 is now effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021. Early adoption is permitted. The Company expects to adopt this standard on January 1, 2020 using the modified retrospective approach with a cumulative effect adjustment to accumulated deficit at the beginning of the period of adoption. The Company also expects to adopt certain practical expedients provided by Topic 842. Topic 842 is expected to impact the Company’s financial statements as the Company has certain operating lease arrangements for which the Company is the lessee. As permitted by the standard, the Company will elect the transition practical expedient package, which among other things, allows the carryforward of historical lease classifications. The adoption of this accounting standard update is also expected to impact the Company’s financial statement disclosures. While the Company is finalizing its evaluation of the impact of adopting this accounting standard update on its financial statements and related disclosures, the Company expects to recognize on its balance sheet for associated leases a new right of use (“ROU”) assets ranging from $22.0 million to $27.0 million and lease liability ranging from $37.0 million to $42.0 million, with the difference between ROU assets and lease liability attributed to the elimination of remaining unamortized lease incentive obligations, and deferred rent balances. The adoption of this standard are also expected to impact the Company’s financial statement disclosures.

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows:

•

Level 1: Quoted prices in active markets for identical instruments

•

Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments)

•

Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments)

The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments.

The fair value of the Company’s cost method investment was measured when it was deemed to be other-than-temporarily impaired until the nature of the underlying investment changed to be an equity security with a readily determinable fair value which is measured at fair value on a recurring basis.

The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	December 31, 2019
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	29,377	$	29,377	$	—	$	—
U.S. and foreign commercial paper		4,999		—		4,999		—
U.S government debt securities		2,550		—		2,550		—
Total cash equivalents		36,926		29,377		7,549		—
Short-term marketable securities:
U.S. treasuries		15,063		—		15,063		—
U.S. and foreign commercial paper		11,972		—		11,972		—
U.S. and foreign corporate debt securities		8,755		—		8,755		—
U.S. government debt securities		48,718		—		48,718		—
Total short-term marketable securities		84,508		—		84,508		—
Strategic investments
Foreign equity securities		5,507		5,507		—		—
Total strategic investments		5,507		5,507		—		—
Long-term marketable securities
U.S. treasuries		3,025		—		3,025		—
Total long-term marketable securities		3,025		—		3,025		—
Total assets subject to fair value measurements on a recurring basis	$	129,966	$	34,884	$	95,082	$	—
Liabilities:
Contingent consideration liability	$	1,131	$	—	$	—	$	1,131
Total liabilities subject to fair value measurements on a recurring basis	$	1,131	$	—	$	—	$	1,131

	December 31, 2018
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	14,131	$	14,131	$	—	$	—
Total cash equivalents		14,131		14,131		—		—
Short-term marketable securities:
U.S. treasuries		34,121		—		34,121		—
U.S. and foreign commercial paper		10,635		—		10,635		—
U.S. and foreign corporate debt securities		26,533		—		26,533		—
Asset-backed securities		2,748		—		2,748		—
U.S. government debt securities		81,699		—		81,699		—
Total short-term marketable securities		155,736		—		155,736		—
Total assets subject to fair value measurements on a recurring basis	$	169,867	$	14,131	$	155,736	$	—
Liabilities:
Contingent consideration liability	$	2,483	$	—	$	—	$	2,483
Total liabilities subject to fair value measurements on a recurring basis	$	2,483	$	—	$	—	$	2,483

The Company estimates the fair value of its money market funds, U.S. and foreign commercial paper, U.S. and foreign corporate debt securities, asset-backed securities, U.S. treasuries, U.S. government debt securities and foreign debt securities by taking into consideration valuations obtained from third-party pricing services. 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, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs.

The fair value of the contingent consideration liability includes inputs not observable in the market and thus represents a Level 3 measurement. The Company has recorded a contingent consideration liability related to three agreements with a clinical-stage biopharmaceutical company (see Note 5, “License Agreements”). As of December 31, 2019, these Commercial Agreements included contingent consideration of up to an aggregate of 533,336 additional shares of common stock to be issued in specified portions to Ascentage Pharma and an academic institution from which Ascentage Pharma had previously in-licensed the underlying technology based on achievement of certain specified preclinical and clinical development and sales milestone events. The probability of achieving the defined milestone events under the Commercial Agreements is estimated on a quarterly basis by the Company’s management using a probability-weighted valuation approach model which reflects the probability and timing of future issuances of shares. Total contingent consideration may change significantly as preclinical and clinical development related to the compounds covered by the Commercial Agreements progresses and additional data is obtained, impacting the Company’s assumptions regarding probabilities of and timing for successful achievement of the related milestone events. For example, significant increases in the estimated probability of achieving a milestone would result in a significant higher fair value measurement while significant decreases in the estimated probability of achieving a milestone would result in a significantly lower fair value measurement. The potential outstanding contingent consideration value results in shares to be issued ranging from zero, if none of the milestones are achieved, to a maximum of $4.6 million (using the Company’s stock price as of December 31, 2019). As of December 31, 2019, and December 31, 2018, none of the commercial milestones had been achieved and no royalties were due from the sales of licensed products.

As of December 31, 2018, the Company determined that the net settlement criteria of the definition of a derivative had been met for 133,333 shares of common stock to the third parties and recorded a settlement liability of $2.0 million. The Company issued 106,667 of these shares in January 2019 and the remaining 26,667 shares in March 2019. The settlement liability recorded at December 31, 2018 was reclassified to stockholders’ equity upon the issuance of these shares. The Company recorded a contingent consideration liability of $1.1 million at December 31, 2019 related to additional potential shares subject to the achievement of certain specified clinical development and sales milestone events under the agreements. The following table provides a reconciliation of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands):

	Amount
Balance at December 31, 2018	$	2,483
Additions		—
Settlements		—
Change in fair value		(1,352	)
Balance at December 31, 2019	$	1,131

The Company holds an equity investment in a clinical-stage biopharmaceutical company. The equity interest represents an insignificant level of ownership in the investee and has been recorded within strategic investments on the Company’s balance sheet (see Note 5, “License Agreements”). In October 2019, the investee completed an initial public offering of common stock on the Hong Kong stock exchange at HK$34.20 (approximately USD $4.36) per share. Following the initial public offering, the underlying investment changed to be an equity security with a readily determinable fair value which is measured at fair value on a recurring basis based on quoted stock price available on the Hong Kong Stock Exchange, which are considered observable inputs (Level 1). The investment was $5.5 million as of December 31, 2019 and is included in strategic investments. The investment was $1.0 million as of December 31, 2018 and is included in other long-term assets. The change in fair value of this investment for the twelve months ended December 31, 2019 and December 31, 2018 was $4.5 million and zero, respectively.

There were no transfers between the hierarchies during the years ended December 31, 2019 and 2018.

See Note 4, ‘Investments,” for further information regarding the carrying value of the Company's financial instruments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Investments

12 Months Ended

Dec. 31, 2019

Investments Debt And Equity Securities [Abstract]

Investments

4. Investments

Marketable Securities

Marketable debt securities, which are classified as available-for-sale, consisted of the following (in thousands):

	December 31, 2019
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	29,377	$	—	$	—		$	29,377
U.S. and foreign commercial paper		4,999		—		—			4,999
U.S. government debt securities		2,550		—		—			2,550
Total cash equivalents		36,926		—		—			36,926
Short-term marketable securities:
U.S. and foreign commercial paper		11,965		7		—			11,972
U.S. and foreign corporate debt securities		8,748		8		(1	)		8,755
U.S. government debt securities		48,647		71		—			48,718
U.S. treasuries		15,057		6		—			15,063
Total short-term marketable securities		84,417		92		(1	)		84,508
Long-term marketable securities
U.S. treasuries		3,025		—		—			3,025
Total long-term marketable securities		3,025		—		—			3,025
Total marketable securities	$	124,368	$	92	$	(1	)	$	124,459

	December 31, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	14,131	$	—	$	—		$	14,131
Total cash equivalents		14,131		—		—			14,131
Short-term marketable securities:
U.S. and foreign commercial paper		10,638		—		(3	)		10,635
U.S. and foreign corporate debt securities		26,552		2		(21	)		26,533
Asset-backed securities		2,750		—		(2	)		2,748
U.S. government debt securities		81,755		1		(57	)		81,699
U.S. treasuries		34,136		1		(16	)		34,121
Total short-term marketable securities		155,831		4		(99	)		155,736
Total marketable securities	$	169,962	$	4	$	(99	)	$	169,867

At December 31, 2019, the remaining contractual maturities of available-for-sale debt securities were less than two years. There have been no significant realized gains or losses on available-for-sale debt securities for the periods presented. Available-for-sale debt securities that were in a continuous loss position but were not deemed to be other than temporarily impaired were immaterial at both December 31, 2019 and 2018. The Company does not intend to and believes it is not more likely than not that it will be required to sell these debt securities before their maturities.

See Note 3, “Fair Value Measurements,” for further information regarding the fair value of the Company's financial instruments.

License Agreements

12 Months Ended

Dec. 31, 2019

Research And Development [Abstract]

License Agreements

5. License Agreements

License and Compound Library and Option Agreement and Related License Agreements with Ascentage

The Company is a party to three agreements with Ascentage Pharma Group Corp. Limited, a clinical-stage biopharmaceutical company based in Hong Kong, China (“Ascentage Pharma”): (a) a compound library and option agreement executed in February 2016 granting the Company the right to identify and take licenses to research, develop, and seek and obtain marketing approval for library compounds for the treatment of indications outside of oncology, (b) an initial license agreement executed in February 2016 granting the Company rights to an initial licensed compound, and (c) a second license agreement executed in January 2019 granting the Company rights to a second licensed compound (collectively, the “Commercial Agreements”). As of December 31, 2019, as part of these agreements, the Company had issued 640,002 shares of common stock to Ascentage Pharma and 160,000 shares of common stock to an academic institution from whom Ascentage Pharma had previously licensed the technology.

The Commercial Agreements referenced above include cash payments of up to $70.3 million as well as remaining equity payments of up to an aggregate 533,336 additional shares of common stock, in each case to be issued based on the Company’s achievement of certain specified clinical development and sales milestone events. The milestones include the filing of an investigational drug application, the commencement of clinical studies, Food and Drug Administration and/or European Medicines Agency approval, and a net sales threshold. The license agreement also includes tiered royalties in the low-single digits based on sales of licensed products.

In December 2018, the Company elected to advance the second compound into formal preclinical development which gave rise to an obligation under the compound library and option agreement to issue 133,334 shares of common stock to Ascentage Pharma and the academic institution. These shares were issued to Ascentage Pharma in January 2019 and the academic institution in March 2019. In connection with the additional shares of common stock that the Company may be obligated to issue under the Commercial Agreements upon achievement of the specified milestones events, the Company recorded a contingent consideration liability of $1.1 million at December 31, 2019 and $2.5 million at December 31, 2018. The $1.1 million contingent consideration liability was recorded as a current liability based on the latest estimates for milestone achievements. To date, no royalties were due from the sales of licensed products.

In April 2016, in connection with the Commercial Agreements the Company purchased an interest in an affiliate of Ascentage Pharma for an aggregate purchase price of $0.5 million. In May 2018, this interest was exchanged for an interest in a newly formed affiliate of Ascentage Pharma called Ascentage Pharma Group International (“Ascentage International”) as part of a reorganization of those entities. The Company also invested an additional $0.5 million in Ascentage International in May 2018 which was recorded within other long-term assets on the Company’s balance sheet as of December 31, 2018.

In October 2019, Ascentage International completed an initial public offering of shares of its common stock on the Hong Kong stock exchange at HK$34.20 (approximately USD $4.36) per share. In connection with Ascentage International’s initial public offering, the Company’s interest converted into shares of common stock of Ascentage International. The Company determined that its investment in Ascentage International met the definition of an equity security with a readily determinable fair value which is measured at fair value on a recurring basis based on quoted stock price available on the Hong Kong Stock Exchange. The Company is subject to a lock-up agreement with Ascentage International that precludes the Company from selling shares prior to April 2020. The fair value of the Company’s investment in Ascentage International as of December 31, 2019 was $5.5 million, which was included in strategic investments. The fair value investment of the Company’s in Ascentage International as of December 31, 2018 was $1.0 million, which was included in other long-term assets.

The Company agreed to provide funding to Ascentage Pharma for research and development work performed at a cost of up to $2.0 million through February 2020. The research and development expense under the research services agreement was $0.5 million and $0.5 million for the years ended December 31, 2019 and 2018.

Under the consolidation guidance, the Company had previously determined that Ascentage Pharma was a VIE and the Company did not have the power to direct the activities that most significantly affect the economic performance of this entity. As such, the Company was not the primary beneficiary and consolidation is not required. Upon completion of its initial public offering of common stock on the Hong Kong stock exchange, the Company determined that Ascentage Pharma no longer meets the definition of a VIE. As of December 31, 2019 and 2018, the Company has not provided financial, or other, support to Ascentage Pharma that was not contractually required.

Other License Agreements with Research Institutions

In May 2019, the Company entered into a license agreement with The Regents of the University of California on behalf its San Francisco campus (collectively, “UCSF”) which provides the Company the rights to certain patents and related know-how to make, use, sell, offer for sale and import certain products and practice certain methods for use in the development of human therapeutics, which excludes the provision of services to third parties for consideration of any kind. The license to the Company is subject to UCSF’s reserved rights under the licensed intellectual property for educational and non-commercial research purposes and a requirement to substantially manufacture any licensed products in the United States. The Company is obligated to use diligent efforts to develop and obtain regulatory approval for at least one product commercialized pursuant to the agreement, and must meet certain regulatory and development milestones. In June 2019, as part of this license agreement, the Company issued 120,000 shares of its common stock to UCSF. In addition, the Company is obligated to pay an annual license maintenance fee and may be obligated to make milestone payments or issue up to an additional 34,000 shares of its common stock upon the occurrence of specified development events, up to aggregate milestone payments of $13.6 million for each product licensed under the agreement, and upon commercialization, to make royalty payments in the low single digit percentages (subject to a specified minimum annual royalty) based on net sales of products commercialized pursuant to the agreement. None of these events had occurred and no milestone payments or royalty payments had been recognized as of December 31, 2019. The upfront issuance of 120,000 shares of the Company’s common stock was valued at $1.0 million at the time of issuance and recorded as additional paid-in capital upon issuance in June 2019.

The Company has also entered into license agreements with various research institutions which have provided the Company with rights to patents, and in certain cases, research “know-how” and proprietary research tools to research, develop and commercialize drug candidates. In addition to upfront consideration paid to these various research institutions in either cash or shares of the Company’s common stock, the Company may be obligated to make milestone payments, payable in cash and/or the issuance of shares of the Company’s common stock upon achievement of certain specified clinical development and/or sales events. The contingent consideration liability considered to be a derivative associated with the potential issuance of common stock related to these license agreements was not significant at December 31, 2019 or 2018. To date, none of these events has occurred and no contingent consideration, milestone or royalty payments have been recognized.

Balance Sheet Components

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

6. Balance Sheet Components

Property and Equipment, Net

Property and equipment, net, consists of the following (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	5,219		$	4,162
Computer equipment		472			247
Furniture and fixtures		825			113
Leasehold improvements		16,436			5,366
Total property and equipment		22,952			9,888
Less: accumulated depreciation and amortization		(6,316	)		(3,650	)
Total property and equipment, net	$	16,636		$	6,238

Depreciation expense related to property and equipment was $2.7 million, $2.2 million and $1.3 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Accrued and Other Current Liabilities

Accrued and other current liabilities consist of the following (in thousands):

	December 31,
	2019		2018
Accrued research and development	$	2,214	$	1,837
Deferred rent, current portion		1,849		783
Liability related to early exercise shares		237		885
Accrued other		695		1,485
	$	4,995	$	4,990

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

7. Commitments and Contingencies

Operating Lease

In February 2019, the Company entered into a lease agreement for new office and laboratory space in South San Francisco, California. The term of the lease agreement commenced in May 2019. The lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional eight years at the then market rental rates as determined pursuant to the lease agreement. The total base rent payment escalates annually based on a fixed percentage beginning from the 13th month of the lease agreement. The Company will also be responsible for the operating expenses and tax expenses allocated to the building, and the operating expenses and tax expenses attributable to the common areas. Pursuant to the lease agreement, the landlord provided the Company with a tenant improvement allowance of up to $7.8 million and will finance up to $2.9 million for additional tenant improvements subject to repayment provisions as described in the lease agreement. The value of this tenant improvement allowance of $10.7 million was included in deferred rent and leasehold improvements on the balance sheet at December 31, 2019. In connection with the execution of the lease agreement, the Company delivered a letter of credit of approximately $0.9 million to the landlord.

In May 2016, the Company executed a non-cancellable lease agreement for office and laboratory space in Brisbane, California which commenced in May 2016 and continues through October 2022. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional four years by giving the landlord written notice of the election to exercise the option at least fifteen months prior to the original expiration of the lease term. The lease provides for monthly base rent amounts escalating over the term of the lease and the lessor provided the Company a $3.9 million tenant improvement allowance to complete the laboratory and office renovation which was recorded as deferred rent liability and leasehold improvements within property and equipment, net. In May 2017, the Company entered into an amendment to expand the leased space and received a three-month rent holiday for the expanded space.

As of December 31, 2019, the Company’s future minimum payments under the noncancelable operating leases were as follows (in thousands):

	Amount
2020	$	5,373
2021		6,230
2022		5,859
2023		4,386
2024 and later		29,272
Total future minimum lease payments	$	51,120

Rent expense was $4.5 million, $1.8 million and $2.0 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Indemnifications

The Company indemnifies each of its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with the Company’s amended and restated certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity.

The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable the Company to recover a portion of any future amounts paid. The Company believes that the fair value of these potential indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Related Party Transactions

12 Months Ended

Dec. 31, 2019

Related-Party Transactions

8. Related-Party Transactions

Recourse Notes

In December 2015, April 2016, and July 2016, the Company issued three full-recourse promissory notes to two executive officers for an aggregate principal amount of $0.2 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 667,253 shares of the Company’s common stock, in aggregate. All of these related party full-recourse notes were repaid on April 4, 2018 in accordance with the terms of such notes.

In October 2017, the Company issued two promissory notes to an executive officer for $1.6 million and $0.5 million, each with an interest rate of 1.85% per annum. The aggregate principal amount of $2.1 million was used to purchase 625,084 shares of restricted stock. The promissory notes were considered to be non-recourse in substance and accordingly, the shares sold subject to such promissory notes are considered to be an option for accounting purposes. In April 2018, the Company’s board of directors approved the forgiveness of all outstanding principal and accrued interest of the $1.6 million non-recourse promissory note. The non-recourse promissory note outstanding of $0.5 million was repaid on April 4, 2018 in accordance with the terms of the note. The forgiveness of the promissory note was accounted for as a modification of a share-based payment. The Company recorded an incremental charge of $1.5 million related to the modification for the year ended December 31, 2018.

In January 2018, the Company issued full-recourse promissory notes to an executive and an executive officer of the Company for an aggregate principal amount of $0.4 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 114,406 shares of the Company’s common stock. The full recourse note of $0.2 million for the executive officer was repaid on April 4, 2018 in accordance with the terms of the note. In December 2019, the full recourse note to an executive was deemed satisfied and superseded by a new full recourse promissory note agreement with a principal amount of $0.2 million and an interest rate of 1.51% per annum.

Financing Activities

During the year ended December 31, 2018, the Company issued convertible preferred stock for total proceeds of $3.0 million to shareholders who are considered to be related parties.

Common and Preferred Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Common and Preferred Stock

9. Common and Preferred Stock

The Company has 10,000,000 shares of convertible preferred stock authorized for issuance, par value of $0.0001 per share. As of December 31, 2019 and 2018, no shares of preferred stock were issued and outstanding. In connection with the Company’s IPO, all outstanding shares of convertible preferred stock were automatically converted into 32.1 million shares of common stock.

The Company has 300,000,000 shares of common stock authorized for issuance, par value of $0.0001 per share. Holders of the Company’s common stock are entitled to one vote per share. As of December 31, 2019 and 2018, there were 47,227,065 and 42,414,294 shares of common stock issued and outstanding.

Sale of Common and Preferred Stock

In March 2017, the Company issued an aggregate of 659,821 shares of Series B convertible preferred stock at $12.125 per share for gross proceeds of $8.0 million.

In June 2017, the Company closed the second and final tranche of its Series B convertible preferred stock round of financing by selling an aggregate of 2,879,288 shares of Series B convertible preferred stock at $12.125 per share for gross proceeds of $34.9 million.

In March 2018, the Company amended and restated its certificate of incorporation to, among other things, (i) increase its authorized shares of common stock from 122,000,000 to 140,000,000 shares, (ii) increase its authorized shares of preferred stock from 91,739,149 to 103,283,818 shares, of which 11,544,669 shares were designated as Series C convertible preferred stock, and (iii) set forth the rights, preferences and privileges of the Series C convertible preferred stock. In March 2018, the Company sold 3,590,573 shares of Series C convertible preferred stock at $15.3317 per share for net proceeds of $54.9 million and in April 2018, the Company sold an additional 322,852 shares of Series C convertible preferred stock $15.3317 per share for net proceeds of $5.0 million.

On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $75.9 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $9.1 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of the Company’s convertible preferred stock warrants were converted into warrants to purchase shares of common stock.

In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.

In June 2019, the Company filed a Registration Statement on Form S-3 (the “Shelf Registration Statement”), covering the offering of up to $250.0 million of common stock, preferred stock, debt securities, warrants and units. The Shelf Registration Statement included a prospectus covering the offering, issuance and sale of up to $75.0 million of the Company’s common stock from time to time through an “at-the-market” offering under the Securities Act of 1933, as amended (the “ATM Offering Program”). The SEC declared the Shelf Registration Statement effective on June 6, 2019.

In June 2019, the Company also entered into a sales agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, from time to time, with aggregate gross sales proceeds of up to $75.0 million, through the ATM Offering Program under which Cowen will act as its sales agent Cowen is entitled to compensation for its services equal to up to 3.0% of the gross proceeds of any shares of common stock sold through Cowen under the Sales Agreement. In addition, the Company has agreed to reimburse a portion of the expenses of Cowen in connection with the offering up to a maximum of $0.1 million. During the year ended December 31, 2019, the Company issued and sold 3,974,908 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $26.1 million, after deducting commissions and other offering expenses of $1.4 million.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

Equity Incentive Plans

In March 2018, the Company’s board of directors adopted the Company’s 2018 Incentive Award Plan (the “2018 Plan”). The 2018 Plan was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 Plan initially reserved 4,289,936 shares for the issuance of stock options as well as any automatic annual increases in the number of shares of common stock reserved for future issuance under the 2018 Plan. Awards granted under the 2018 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. Unvested options not exercised at the time of an employee’s termination of employment are added back to the 2018 Plan.

Following the Company’s IPO and in connection with the effectiveness of the 2018 Plan, the 2013 Equity Incentive Plan (the “2013 Plan”) terminated and no further awards will be granted under that plan. All outstanding awards under the 2013 Plan will continue to be governed by their existing terms and the shares that remained outstanding for issuance under the 2013 Plan were transferred into the 2018 Plan. As of December 31, 2019, there was an aggregate 6,846,928 shares of common stock authorized for issuance under the 2018 Plan.

Prior to its termination, the 2013 Plan provided for the granting of incentive stock options (“ISOs”), non-statutory stock options (“NSOs”) and restricted shares to employees, directors, and consultants at the discretion of management and the Board of Directors. The exercise price of an ISO and NSO shall not be less than 100% of the estimated fair value of the shares on the date of grant, and the exercise price of an ISO and NSO granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant. For awards granted between September 2017 and February 2018 with an exercise price of $3.42, a deemed fair value ranging from $3.95 to $8.47 per share was used in calculating stock-based compensation expense, which was determined using management hindsight. Options granted under the 2013 Plan expire no later than 10 years from the date of grant and generally vest over a four-year period but may be granted with different vesting terms. Unvested options not exercised at the time of an employee’s termination of employment are added back to the 2018 Plan.

Under the 2013 Plan, the Company permitted early exercise of certain stock options prior to vesting. These unvested shares are subject to repurchase by the Company at the original issuance price in the event the optionee’s employment is terminated either voluntarily or involuntarily. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported as a liability and reclassified into additional paid-in capital as the shares vest.

Stock Options and Restricted Stock Units (RSUs) Activity

A summary of the Company’s stock option activity under the 2013 and 2018 Plan is as follows:

	Shares Available for Grant			Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
									(in Years)		(in thousands)
Balance at December 31, 2018		3,027,802			5,500,531		$	6.75
Shares Added		2,357,131			—			—
Granted		(2,935,629	)		2,582,912			8.78
Exercised		—			(505,226	)		1.66
Canceled		467,016			(671,319	)		9.47
Balance at December 31, 2019		2,916,320			6,906,898		$	7.62		8.20	$	12,297
Vested and exercisable at December 31, 2019					2,574,677		$	6.21		7.36	$	7,269
Vested and expected to vest at December 31, 2019					6,906,898		$	7.62		8.20	$	12,297

The total intrinsic value of options exercised was $5.8 million, $1.5 million and $0.1 million for the years ended December 31, 2019, 2018 and 2017, respectively. The weighted-average estimated fair value of stock options granted was $7.12, $13.20 and $3.40 for the years ended December 31, 2019, 2018 and 2017, respectively.

The aggregate intrinsic value of options exercisable was $7.3 million and $21.9 million as of December 31, 2019 and 2018, respectively.

The following table summarizes the Company’s RSU activity for the year ended December 31, 2019.

	Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2018		—		$	—
Granted		352,717		$	9.00
Forfeited		(26,830	)	$	9.00
Unvested at December 31, 2019		325,887		$	9.00

As of December 31, 2019, the total stock-based compensation cost related to options and RSUs granted but not yet amortized was $28.1 million and will be recognized over a weighted-average period of approximately 3.5 years. The total grant-date fair value of stock options granted to employees that vested during the years ended December 31, 2019 and 2018 was approximately $14.2 million and $3.5 million, respectively.

Stock Options Granted to Employees with Service-Based Vesting

The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions:

	Year Ended December 31,
	2019		2018		2017
Expected dividend yield		—		—		—
Expected term of options (in years)	6.1		6.1		5.6–6.7
Risk-free interest rate	1.59%-2.27%		2.6%-3.0%		1.8%–2.2%
Expected stock price volatility	99.4%-111.3%		87.4%-92.6%		77.0%–82.0%

The valuation assumptions were determined as follows:

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) as the Company has concluded that its stock option exercise history does not provide a reasonable basis upon which to estimate expected term.

Expected Volatility—The Company used 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 a sufficient historical trading history for its own 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 Dividends—The Company has never paid any dividends and does not plan to pay dividends in the foreseeable future. Therefore, the expected dividend yield is zero.

Performance Contingent Stock Options Granted to Employees

During the year ended December 31, 2018, the Board of Directors granted performance contingent stock option awards exercisable for 53,575 shares, to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42 which was based on the fair market value on the grant date, as determined by the Board of Directors, and vest upon the successful achievement of one or more specified performance goals.

The total estimated fair value of these awards was $0.4 million at the date of grant and was estimated using a Black-Scholes option-pricing model using the same assumptions as the stock options granted to employees with service-based vesting conditions.

As of December 31, 2018, there were 329,499 total performance contingent stock option awards outstanding with a total grant date fair value of $0.7 million. During the year ended December 31, 2019, the Company determined that the achievement of the requisite performance conditions was probable and, as a result, compensation cost of $0.7 million was recognized for these awards.

Performance and Market Contingent Stock Options Granted to Employees

During the year ended December 31, 2018, the Board of Directors granted performance and market contingent stock option awards exercisable for 160,727 shares of common stock to certain members of the Company’s executive team. These awards had a weighted average exercise price of $3.42, which was based on the fair market value on the grant date, as determined by the Board of Directors. The total estimated grant-date fair value of these options was $1.0 million. Key assumptions in the valuation model included expected volatility, a risk-free interest rate, expected dividend yield, and an expected term unique to the terms of these awards.

Under the performance and market contingent awards, 53,575 of the shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share. The remaining 107,152 shares have three separate market triggers for vesting based upon (i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with minimum aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) either an initial public offering or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price.

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, which for liquidity events is generally upon achievement, time-based vesting and recognition of stock-based compensation expense commence.

As of December 31, 2019 and 2018, there were 454,584 performance and market contingent stock option awards outstanding with a grant date total fair value of $1.5 million, respectively. As of December 31, 2019 and 2018, the Company determined that the achievement of the requisite performance conditions was not probable and, as a result, no compensation cost was recognized for these awards.

Stock-Based Compensation for Nonemployees

The Company has granted options to purchase shares of common stock to consultants in exchange for services performed. During the year ended December 31, 2018, the Company granted options to purchase an aggregate of 20,337 shares (of which an aggregate of 169,491 were issued outside of the 2018 and 2013 Plans) of the Company’s common stock with a weighted average exercise price of $6.19 per share.

The fair value of stock options granted to nonemployees was estimated on the date of grant using the Black-Scholes option pricing model. The valuation assumptions used were substantially consistent with the assumption used to value the employee options with the exception of the expected term which was based on the contractual term of the award. During the years ended December 31, 2019 and 2018, stock-based compensation expense recognized related to nonemployee options was $0.4 million and $1.2 million, respectively.

Restricted Stock

A summary of the Company’s restricted stock activity for the year ended December 31, 2019 is as follows:

	Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2018		359,229		$	4.57
Vested		(359,229	)	$	4.57
Unvested at December 31, 2019		—		$	—

In October 2017, the Company and an executive officer entered into two restricted stock agreements whereby the executive officer purchased an aggregate of 625,084 shares of restricted stock of which 146,113 shares vested immediately, 119,742 shares vest on January 1, 2018 and 359,229 shares vest on January 1, 2019. As discussed in Note 8, Related-Party Transactions, the purchase of the restricted stock was through the issuance of promissory notes which were considered to be non-recourse in substance and accordingly, considered an option for accounting purposes. The Company measured compensation cost for this option based on its fair value on the grant date using the Black-Scholes option pricing model considering an expected term commensurate with the expected timing to a liquidity event which would trigger repayment of these promissory notes and an exercise price consistent with the repayment term of the promissory notes. The Company recognized compensation cost over the requisite service period with an offsetting credit to additional paid-in capital. The shares of restricted stock have only been included in the shares issued and outstanding as such shares are legally issued.

2018 Employee Stock Purchase Plan

In March 2018, the Company’s board of directors adopted the Company’s 2018 Employee Stock Purchase Plan (the “2018 ESPP”). The 2018 ESPP was approved by the Company’s stockholders in April 2018 and became effective on May 2, 2018. The 2018 ESPP reserved 536,242 shares of common stock for issuance pursuant to future awards, as well as any automatic increases in the number of shares of our common stock reserved for future issuance under this plan.

Under the 2018 ESPP, employees are offered the option to purchase the Company’s common stock at a discount during the offering periods, at semi-annual intervals, with their accumulated payroll deductions. The option purchase price will be 85% of the lower of the closing trading price per share at the beginning of the offering period or at the purchase date. The 2018 ESPP provides for consecutive offering periods and eligible employees may elect to withhold up to 15% of their compensation through payroll deductions during the offering period for the purchase of stock. The maximum number of shares that may be purchased by any one participant is limited to 15,000 shares in each offering period and $25,000 in fair market value during any calendar year per the Internal Revenue Code limits. The first offering period commenced on September 16, 2018.

Stock-Based Compensation Expense

The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, and costs associated with the Company’s 2018 ESPP included in the Company’s statement of operations (in thousands):

	Year Ended December 31,
	2019		2018		2017
Research and development	$	4,979	$	6,043	$	1,695
General and administrative		5,873		3,398		1,339
Total	$	10,852	$	9,441	$	3,034

Warrants

12 Months Ended

Dec. 31, 2019

Warrants And Rights Note Disclosure [Abstract]

Warrants

11. Warrants

In June 2013, the Company granted warrants to its then Chief Executive Officer (“CEO”), considered to be a related party, to purchase 192,823 shares of Series A-1 convertible preferred stock with an exercise price of $0.65 per share and 190,226 shares of Series A-2 convertible preferred stock at a price of $0.66 per share as compensation. In January 2015, the Company granted warrants to the aforementioned CEO to purchase an aggregate of 380,452 shares of Series A-2 convertible preferred stock with an exercise price of $0.66 per share as compensation. Upon the completion of the IPO, these warrants converted to common stock warrants. These warrants were exercisable beginning on January 1, 2018 and expired on the earlier of (i) December 31, 2018, (ii) December 31 of the year in which a change of control occurs or (iii) December 31 of the year in which the holder terminates service. All of the vested warrants expired unexercised on December 31, 2018.

In October 2013, the Company granted warrants to a nonemployee to purchase an aggregate of 96,610 shares of common stock with an exercise price of $0.18 per share of which 9,425 warrants vested immediately. During April 2018 the nonemployee exercised the vested shares and the remaining unvested warrants expired on May 3, 2018 upon the closing of the IPO.

Net Loss per Common Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Common Share

12. Net Loss per Common Share

The calculation of diluted earnings (loss) per share also requires that, to the extent the presumed issuance of additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants, shares subject to the 2018 ESPP and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive.

A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts):

	December 31,
	2019			2018			2017
	(in thousands, except share and per share amounts)
Numerator:
Net loss	$	(82,177	)	$	(76,398	)	$	(44,656	)
Denominator:
Weighted average number of shares outstanding—basic and diluted		43,624,807			28,269,907			3,197,516
Net loss per share—basic and diluted	$	(1.88	)	$	(2.70	)	$	(13.97	)

Since the Company was in a loss position for all periods presented, basic net loss per common share is the same as diluted net loss per common share as the inclusion of all potential common shares 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:

	Year Ended December 31,
	2019		2018		2017
Convertible preferred stock		—		—		28,159,724
Options to purchase common stock		6,906,898		5,500,531		4,365,694
Early exercised common stock subject to future vesting		146,915		704,028		831,439
Restricted stock accounted for as options		—		359,228		625,084
RSUs		325,887		—		—
Warrants to purchase convertible preferred stock		—		—		763,501
Warrants to purchase common stock		—		—		96,610
Shares subject to the 2018 ESPP		47,597		27,622		—
Total		7,427,297		6,591,409		34,842,052

Up to 640,218 shares may be contingently issued, if certain performance conditions are met under the Company’s in-licensing agreements.

Defined Contribution Plan

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

Defined Contribution Plan

13. Defined Contribution Plan

The Company sponsors a 401(k) Plan that stipulates that eligible employees can elect to contribute to the 401(k) Plan, subject to certain limitations, on a pretax basis. The Company does not match any employee contributions. In January 2019, the Company began to match 4% of employees’ salary.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

14. Income Taxes

The Company has incurred net operating losses for all the periods presented. The Company has not reflected the benefit of any such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets. All losses to date have been incurred domestically as the Company has no international operations or subsidiaries.

No provision for U.S. income taxes exists due to tax losses incurred in all periods presented. All losses incurred were U.S. based.

The effective tax rate for the years ended December 31, 2019, 2018 and 2017 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The effective tax rate of the Company’s provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2019				2018				2017
Taxes at the U.S. statutory income tax rate		21.0		%		21.0		%		34.0		%
State tax, net of federal benefit		(2.2	)			0.9				—
Other		(0.9	)			(0.1	)			(2.2	)
Stock-based compensation		(0.5	)			0.3				—
Research and development tax credits		(0.2	)			1.0				—
Reduction to state net operating losses		(3.9	)			—				—
Change in valuation allowance		(13.3	)			(23.1	)			(13.3	)
Change in income tax rate due to Tax Act		—				—				(18.5	)
Total provision for income taxes		—		%		—		%		—		%

On December 22, 2017, the U.S. federal government enacted the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act contains, among other things, significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21% for tax years beginning after December 31, 2017, limitation of the deduction for net operating losses to 80% of current year taxable income and elimination of net operating loss carrybacks, implementing a territorial tax system, and requiring a mandatory one-time tax on U.S. owned undistributed foreign earnings and profits known as the transition tax.

Pursuant to SAB 118, an entity may select between one of three scenarios to determine a reasonable estimate arising from the Tax Act. The scenarios are (i) a final estimate which effectively closes the measurement window; (ii) a reasonable estimate leaving the measurement window open for future revisions; and (iii) no estimate as the law is still being analyzed. The Company was able to provide a reasonable estimate for the revaluation of deferred taxes. As such, the Company recorded a $8.3 million reduction in deferred tax assets for the revaluation of deferred taxes in 2017 which was offset by a corresponding decrease to the Company’s full valuation allowance. The ultimate impact of the Act did not differ materially from provision amounts recorded. Adjustments, if any, would not have impacted the statement of operations and comprehensive loss due to the full valuation allowance on the Company’s deferred tax assets

Deferred income taxes reflect the net tax effects of (a) temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and (b) operating losses and tax credit carryforwards.

The tax effects of significant items comprising the Company’s deferred tax assets are as follows:

	December 31,
	2019			2018
	(in thousands)
Deferred tax assets:
Federal and state operating loss carryforwards	$	40,435		$	29,926
Research and development tax credits		3,436			3,865
Stock-based compensation		3,514			1,839
Accruals and other		1,473			1,040
Contingent consideration		670			954
Charitable contributions		253			253
Total deferred tax assets		49,781			37,877
Deferred tax liabilities:
Unrealized gain on equity investment		(947	)		—
Total deferred tax liabilities		(947	)		—
Valuation allowance		(48,834	)		(37,877	)
Net deferred tax assets	$	—		$	—

The tax benefit of net operating losses, temporary differences and credit carryforwards should be recorded as an asset to the extent that management assesses that their realization is "more likely than not." Realization of the future tax benefits is dependent on the Company's ability to generate sufficient taxable income within the carryforward period. Because of the Company's recent history of operating losses, management believes that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, has provided a valuation allowance.

Realization of the net deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which is uncertain. Based on the weight of available positive and negative objective evidence, management believes it more likely than not that the Company’s deferred tax assets are not realizable. Accordingly, the net deferred tax assets have been fully offset by a valuation allowance. The valuation allowance increased by $11.0 million, $17.6 million and $9.4 million during the years ended December 31, 2019, 2018 and 2017, respectively.

Net operating losses and tax credit carryforwards as of December 31, 2019 are as follows:

	Amount		Expiration Years
Net operating losses, federal (post December 31, 2017)	$	137,097	Do Not Expire
Net operating losses, federal (pre January 1, 2018)		64,136	2030 - 2037
Net operating losses, state		26,123	2030 - 2036
Research and development tax credits, federal		5,931	2031 - 2039
Research and development tax credits, state		5,216	Indefinite

Federal and state laws impose restrictions on the utilization of net operating loss carryforwards and R&D credit carryforwards in the event of a change in ownership of the Company, which constitutes an 'ownership change' as defined by Internal Revenue Code Section 382 and 383. The Company experienced an ownership change in the past that impacts the availability of its net operating losses and tax credits. The amounts indicated in the above tables reflect the reduction of net operating losses and credit carryforwards as a result of previous ownership changes that the Company experienced. Should there be additional ownership changes in the future, the Company's ability to utilize existing carryforwards could be substantially restricted.

The Company determines its uncertain tax positions based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings is more likely than not to be sustained upon examination by the relevant income tax authorities.

As of December 31, 2019, the Company has no accrued interest or penalties related to uncertain tax positions.

The following table summarizes the activity related to our unrecognized tax benefits:

	December 31,
	2019			2018
	(in thousands)
Gross unrecognized tax benefits at January 1	$	3,714		$	3,065
Additions for tax positions taken in the current year		6,221			753
Reductions for tax positions taken in the prior year		(173	)		(104	)
Gross unrecognized tax benefits at December 31	$	9,762		$	3,714

If recognized, none of the unrecognized tax benefits as of December 31, 2019 and 2018 would reduce the annual effective tax rate, primarily due to corresponding adjustments to the valuation allowance. The Company will recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. As of December 31, 2019 and 2018, no liability has been recorded for potential interest or penalties. The Company does not expect the unrecognized tax benefits to change significantly over the next 12 months.

The Company files income tax returns in the U.S. federal jurisdiction and California, Colorado and Delaware. The Company is not currently under audit by the Internal Revenue Service or other similar state or local authorities. All tax years remain open to examination by major taxing jurisdictions to which the Company is subject.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

15. Selected Quarterly Financial Data (Unaudited)

The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2019 and 2018 and have been prepared in accordance with GAAP for interim financial reporting (in thousands, except for per share data):

	Quarter
Year Ended December 31, 2019	First			Second			Third			Fourth
Loss from operations	$	(19,737	)	$	(24,470	)	$	(22,354	)	$	(23,090	)
Net loss	$	(18,767	)	$	(23,673	)	$	(21,710	)	$	(18,027	)
Net loss per common share, basic and diluted	$	(0.44	)	$	(0.56	)	$	(0.51	)	$	(0.39	)

	Quarter
Year Ended December 31, 2018	First			Second			Third			Fourth
Loss from operations	$	(16,482	)	$	(20,798	)	$	(19,377	)	$	(22,808	)
Net loss	$	(16,133	)	$	(20,002	)	$	(18,346	)	$	(21,917	)
Net loss per common share, basic and diluted	$	(4.69	)	$	(0.76	)	$	(0.45	)	$	(0.53	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Need for Additional Capital

Reverse Stock Split

Basis of Presentation

Use of Estimates

Segments

Cash, Cash Equivalents and Restricted Cash

Marketable Securities

Strategic Investments

Fair Value Measurements

•

Level 1: Quoted prices in active markets for identical instruments

•

Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments)

•

Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments)

Concentration of Risk

Concentrations of Risk

The Company is also exposed to market risk in its strategic investments. As of December 31, 2019, the Company held an investment in common stock which is publicly traded.

Contribution Revenue

Research and Development Expenses and Accruals

Contingent Consideration Liability

Variable Interest Entities

Property and Equipment, Net

Impairment of Long-Lived Assets

Leases

Stock-Based Compensation

Income Taxes

Net Loss per Common Share

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses on the Company’s marketable securities.

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

	December 31,
	2019		2018		2017
Cash and cash equivalents	$	37,473	$	15,399	$	7,298
Restricted cash		1,446		550		550
Total cash, cash equivalents, and restricted cash	$	38,919	$	15,949	$	7,848

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements

The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	December 31, 2019
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	29,377	$	29,377	$	—	$	—
U.S. and foreign commercial paper		4,999		—		4,999		—
U.S government debt securities		2,550		—		2,550		—
Total cash equivalents		36,926		29,377		7,549		—
Short-term marketable securities:
U.S. treasuries		15,063		—		15,063		—
U.S. and foreign commercial paper		11,972		—		11,972		—
U.S. and foreign corporate debt securities		8,755		—		8,755		—
U.S. government debt securities		48,718		—		48,718		—
Total short-term marketable securities		84,508		—		84,508		—
Strategic investments
Foreign equity securities		5,507		5,507		—		—
Total strategic investments		5,507		5,507		—		—
Long-term marketable securities
U.S. treasuries		3,025		—		3,025		—
Total long-term marketable securities		3,025		—		3,025		—
Total assets subject to fair value measurements on a recurring basis	$	129,966	$	34,884	$	95,082	$	—
Liabilities:
Contingent consideration liability	$	1,131	$	—	$	—	$	1,131
Total liabilities subject to fair value measurements on a recurring basis	$	1,131	$	—	$	—	$	1,131

	December 31, 2018
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	14,131	$	14,131	$	—	$	—
Total cash equivalents		14,131		14,131		—		—
Short-term marketable securities:
U.S. treasuries		34,121		—		34,121		—
U.S. and foreign commercial paper		10,635		—		10,635		—
U.S. and foreign corporate debt securities		26,533		—		26,533		—
Asset-backed securities		2,748		—		2,748		—
U.S. government debt securities		81,699		—		81,699		—
Total short-term marketable securities		155,736		—		155,736		—
Total assets subject to fair value measurements on a recurring basis	$	169,867	$	14,131	$	155,736	$	—
Liabilities:
Contingent consideration liability	$	2,483	$	—	$	—	$	2,483
Total liabilities subject to fair value measurements on a recurring basis	$	2,483	$	—	$	—	$	2,483

Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs

	Amount
Balance at December 31, 2018	$	2,483
Additions		—
Settlements		—
Change in fair value		(1,352	)
Balance at December 31, 2019	$	1,131

Investments (Tables)

12 Months Ended

Dec. 31, 2019

Investments Debt And Equity Securities [Abstract]

Summary of Marketable Debt Securities Classified as Available-for-Sale

Marketable debt securities, which are classified as available-for-sale, consisted of the following (in thousands):

	December 31, 2019
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	29,377	$	—	$	—		$	29,377
U.S. and foreign commercial paper		4,999		—		—			4,999
U.S. government debt securities		2,550		—		—			2,550
Total cash equivalents		36,926		—		—			36,926
Short-term marketable securities:
U.S. and foreign commercial paper		11,965		7		—			11,972
U.S. and foreign corporate debt securities		8,748		8		(1	)		8,755
U.S. government debt securities		48,647		71		—			48,718
U.S. treasuries		15,057		6		—			15,063
Total short-term marketable securities		84,417		92		(1	)		84,508
Long-term marketable securities
U.S. treasuries		3,025		—		—			3,025
Total long-term marketable securities		3,025		—		—			3,025
Total marketable securities	$	124,368	$	92	$	(1	)	$	124,459

	December 31, 2018
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	14,131	$	—	$	—		$	14,131
Total cash equivalents		14,131		—		—			14,131
Short-term marketable securities:
U.S. and foreign commercial paper		10,638		—		(3	)		10,635
U.S. and foreign corporate debt securities		26,552		2		(21	)		26,533
Asset-backed securities		2,750		—		(2	)		2,748
U.S. government debt securities		81,755		1		(57	)		81,699
U.S. treasuries		34,136		1		(16	)		34,121
Total short-term marketable securities		155,831		4		(99	)		155,736
Total marketable securities	$	169,962	$	4	$	(99	)	$	169,867

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net, consists of the following (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	5,219		$	4,162
Computer equipment		472			247
Furniture and fixtures		825			113
Leasehold improvements		16,436			5,366
Total property and equipment		22,952			9,888
Less: accumulated depreciation and amortization		(6,316	)		(3,650	)
Total property and equipment, net	$	16,636		$	6,238

Schedule of Accrued and Other Current Liabilities

Accrued and other current liabilities consist of the following (in thousands):

	December 31,
	2019		2018
Accrued research and development	$	2,214	$	1,837
Deferred rent, current portion		1,849		783
Liability related to early exercise shares		237		885
Accrued other		695		1,485
	$	4,995	$	4,990

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease

As of December 31, 2019, the Company’s future minimum payments under the noncancelable operating leases were as follows (in thousands):

	Amount
2020	$	5,373
2021		6,230
2022		5,859
2023		4,386
2024 and later		29,272
Total future minimum lease payments	$	51,120

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

A summary of the Company’s stock option activity under the 2013 and 2018 Plan is as follows:

	Shares Available for Grant			Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
									(in Years)		(in thousands)
Balance at December 31, 2018		3,027,802			5,500,531		$	6.75
Shares Added		2,357,131			—			—
Granted		(2,935,629	)		2,582,912			8.78
Exercised		—			(505,226	)		1.66
Canceled		467,016			(671,319	)		9.47
Balance at December 31, 2019		2,916,320			6,906,898		$	7.62		8.20	$	12,297
Vested and exercisable at December 31, 2019					2,574,677		$	6.21		7.36	$	7,269
Vested and expected to vest at December 31, 2019					6,906,898		$	7.62		8.20	$	12,297

Summary of Restricted Stock Units Activity

The following table summarizes the Company’s RSU activity for the year ended December 31, 2019.

	Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2018		—		$	—
Granted		352,717		$	9.00
Forfeited		(26,830	)	$	9.00
Unvested at December 31, 2019		325,887		$	9.00

Summary of Valuation Assumption to Estimate Fair Value of Stock Options

The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions:

	Year Ended December 31,
	2019		2018		2017
Expected dividend yield		—		—		—
Expected term of options (in years)	6.1		6.1		5.6–6.7
Risk-free interest rate	1.59%-2.27%		2.6%-3.0%		1.8%–2.2%
Expected stock price volatility	99.4%-111.3%		87.4%-92.6%		77.0%–82.0%

Summary of Restricted Stock Activity

A summary of the Company’s restricted stock activity for the year ended December 31, 2019 is as follows:

	Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2018		359,229		$	4.57
Vested		(359,229	)	$	4.57
Unvested at December 31, 2019		—		$	—

Summary of Stock-based Compensation Expense

	Year Ended December 31,
	2019		2018		2017
Research and development	$	4,979	$	6,043	$	1,695
General and administrative		5,873		3,398		1,339
Total	$	10,852	$	9,441	$	3,034

Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share

A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts):

	December 31,
	2019			2018			2017
	(in thousands, except share and per share amounts)
Numerator:
Net loss	$	(82,177	)	$	(76,398	)	$	(44,656	)
Denominator:
Weighted average number of shares outstanding—basic and diluted		43,624,807			28,269,907			3,197,516
Net loss per share—basic and diluted	$	(1.88	)	$	(2.70	)	$	(13.97	)

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share

Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:

	Year Ended December 31,
	2019		2018		2017
Convertible preferred stock		—		—		28,159,724
Options to purchase common stock		6,906,898		5,500,531		4,365,694
Early exercised common stock subject to future vesting		146,915		704,028		831,439
Restricted stock accounted for as options		—		359,228		625,084
RSUs		325,887		—		—
Warrants to purchase convertible preferred stock		—		—		763,501
Warrants to purchase common stock		—		—		96,610
Shares subject to the 2018 ESPP		47,597		27,622		—
Total		7,427,297		6,591,409		34,842,052

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Effective Tax Rate of Provision for Income Taxes Differs From Federal Statutory Rate

The effective tax rate of the Company’s provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2019				2018				2017
Taxes at the U.S. statutory income tax rate		21.0		%		21.0		%		34.0		%
State tax, net of federal benefit		(2.2	)			0.9				—
Other		(0.9	)			(0.1	)			(2.2	)
Stock-based compensation		(0.5	)			0.3				—
Research and development tax credits		(0.2	)			1.0				—
Reduction to state net operating losses		(3.9	)			—				—
Change in valuation allowance		(13.3	)			(23.1	)			(13.3	)
Change in income tax rate due to Tax Act		—				—				(18.5	)
Total provision for income taxes		—		%		—		%		—		%

Schedule of Components of Deferred Tax Assets

The tax effects of significant items comprising the Company’s deferred tax assets are as follows:

	December 31,
	2019			2018
	(in thousands)
Deferred tax assets:
Federal and state operating loss carryforwards	$	40,435		$	29,926
Research and development tax credits		3,436			3,865
Stock-based compensation		3,514			1,839
Accruals and other		1,473			1,040
Contingent consideration		670			954
Charitable contributions		253			253
Total deferred tax assets		49,781			37,877
Deferred tax liabilities:
Unrealized gain on equity investment		(947	)		—
Total deferred tax liabilities		(947	)		—
Valuation allowance		(48,834	)		(37,877	)
Net deferred tax assets	$	—		$	—

Summary of Net Operating Losses and Tax Credit Carryforwards

Net operating losses and tax credit carryforwards as of December 31, 2019 are as follows:

	Amount		Expiration Years
Net operating losses, federal (post December 31, 2017)	$	137,097	Do Not Expire
Net operating losses, federal (pre January 1, 2018)		64,136	2030 - 2037
Net operating losses, state		26,123	2030 - 2036
Research and development tax credits, federal		5,931	2031 - 2039
Research and development tax credits, state		5,216	Indefinite

Schedule of Reconciliation of Unrecognized Tax Benefits

The following table summarizes the activity related to our unrecognized tax benefits:

	December 31,
	2019			2018
	(in thousands)
Gross unrecognized tax benefits at January 1	$	3,714		$	3,065
Additions for tax positions taken in the current year		6,221			753
Reductions for tax positions taken in the prior year		(173	)		(104	)
Gross unrecognized tax benefits at December 31	$	9,762		$	3,714

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information

	Quarter
Year Ended December 31, 2019	First			Second			Third			Fourth
Loss from operations	$	(19,737	)	$	(24,470	)	$	(22,354	)	$	(23,090	)
Net loss	$	(18,767	)	$	(23,673	)	$	(21,710	)	$	(18,027	)
Net loss per common share, basic and diluted	$	(0.44	)	$	(0.56	)	$	(0.51	)	$	(0.39	)

	Quarter
Year Ended December 31, 2018	First			Second			Third			Fourth
Loss from operations	$	(16,482	)	$	(20,798	)	$	(19,377	)	$	(22,808	)
Net loss	$	(16,133	)	$	(20,002	)	$	(18,346	)	$	(21,917	)
Net loss per common share, basic and diluted	$	(4.69	)	$	(0.76	)	$	(0.45	)	$	(0.53	)

Organization - Additional Information (Details) $ in Thousands		3 Months Ended								12 Months Ended
	Apr. 20, 2018	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization [Line Items]
Net loss		$ (18,027)	$ (21,710)	$ (23,673)	$ (18,767)	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (82,177)	$ (76,398)	$ (44,656)
Cash used in operations										(72,421)	(56,623)	$ (38,358)
Accumulated deficit		(245,455)				$ (163,278)				(245,455)	$ (163,278)
Cash, cash equivalents and marketable securities		$ 125,000								$ 125,000
Reverse split of shares of common and convertible preferred stock, conversion ratio	0.33898
Reverse split of shares of common and convertible preferred stock, effective date	Apr. 20, 2018
Amended and Restated Certificate of Incorporation
Organization [Line Items]
Reverse split of shares of common and convertible preferred stock	1-for-2.95 reverse split

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2017 USD ($)	Oct. 31, 2019 $ / shares	Oct. 31, 2019 $ / shares	May 07, 2018 $ / shares	Jul. 30, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of segments \| Segment	1
Off-balance sheet concentrations of credit risk description	As of December 31, 2019, the Company had no off-balance sheet concentrations of credit risk.
Off-balance sheet concentrations of credit risk	$ 0
Recognition of revenue		$ 1,382,000
Capital lease term	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Funding provided for the research and development activities						$ 1,500,000
Maximum \| Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right of use assets	$ 27,000,000
Operating lease liability	42,000,000
Minimum \| Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right of use assets	22,000,000
Operating lease liability	$ 37,000,000
IPO
Summary Of Significant Accounting Policies [Line Items]
Issuance price per share \| $ / shares					$ 17.00
IPO \| License and Compound Library and Option Agreement \| Common Stock
Summary Of Significant Accounting Policies [Line Items]
Issuance price per share \| (per share)			$ 4.36	$ 34.20

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 37,473	$ 15,399	$ 7,298
Restricted cash	1,446	550	550
Total cash, cash equivalents, and restricted cash	$ 38,919	$ 15,949	$ 7,848	$ 89,736

Fair Value Measurements - Summary of Financial Assets Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Short-term marketable securities	$ 84,508	$ 155,736
Strategic investments	5,507
Long-term marketable securities	3,025
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	36,926	14,131
Short-term marketable securities	84,508	155,736
Strategic investments	5,507
Long-term marketable securities	3,025
Total assets subject to fair value measurements on a recurring basis	129,966	169,867
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,131	2,483
Fair Value, Measurements, Recurring \| Level 1
Assets:
Cash equivalents	29,377	14,131
Strategic investments	5,507
Total assets subject to fair value measurements on a recurring basis	34,884	14,131
Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents	7,549
Short-term marketable securities	84,508	155,736
Long-term marketable securities	3,025
Total assets subject to fair value measurements on a recurring basis	95,082	155,736
Fair Value, Measurements, Recurring \| Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,131	2,483
Fair Value, Measurements, Recurring \| Contingent Consideration Liability
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,131	2,483
Fair Value, Measurements, Recurring \| Contingent Consideration Liability \| Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,131	2,483
Money market funds \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	29,377	14,131
Money market funds \| Fair Value, Measurements, Recurring \| Level 1
Assets:
Cash equivalents	29,377	14,131
U.S. and Foreign Commercial Paper \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	4,999
Short-term marketable securities	11,972	10,635
U.S. and Foreign Commercial Paper \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents	4,999
Short-term marketable securities	11,972	10,635
US Government Debt Securities \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	2,550
Short-term marketable securities	48,718	81,699
US Government Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents	2,550
Short-term marketable securities	48,718	81,699
U.S. Treasuries \| Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	15,063	34,121
Long-term marketable securities	3,025
U.S. Treasuries \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Short-term marketable securities	15,063	34,121
Long-term marketable securities	3,025
U.S. and Foreign Corporate Debt Securities \| Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	8,755	26,533
U.S. and Foreign Corporate Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Short-term marketable securities	8,755	26,533
Asset-backed Securities \| Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities		2,748
Asset-backed Securities \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Short-term marketable securities		$ 2,748
Foreign Equity Securities \| Fair Value, Measurements, Recurring
Assets:
Strategic investments	5,507
Foreign Equity Securities \| Fair Value, Measurements, Recurring \| Level 1
Assets:
Strategic investments	$ 5,507

Fair Value Measurements - Additional Information (Details)		1 Months Ended			12 Months Ended
	May 07, 2018 $ / shares shares	Mar. 31, 2019 shares	Jan. 31, 2019 shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 shares	Oct. 31, 2019 $ / shares	Oct. 31, 2019 $ / shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Settlement liability				$ 2,000,000		$ 2,000,000
Strategic investments					$ 5,507,000
Fair value assets hierarchy levels transfers amount					$ 0	$ 0
Common Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares) \| shares					253,334	5,000,000	12,711
IPO
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares) \| shares	5,000,000
Issuance price per share \| $ / shares	$ 17.00
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities fair value disclosure					$ 1,100,000
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration additional common stock issued \| shares					533,336
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Contingent Consideration \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maximum amount of potential contingent consideration value					$ 4,600,000
Compound Library And Option Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common shares expected to be issued \| shares				133,334		133,333
Strategic investments					5,500,000
Compound Library And Option Agreement \| Other Long-Term Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment at cost				$ 1,000,000		$ 1,000,000
Compound Library And Option Agreement \| Academic Institution
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares) \| shares		26,667
Compound Library And Option Agreement \| Clinical Stage Biopharmaceutical Company
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares) \| shares			106,667
License and Compound Library and Option Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of investment					$ 4,500,000	$ 0
License and Compound Library and Option Agreement \| IPO \| Common Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance price per share \| (per share)								$ 4.36	$ 34.20

Fair Value Measurements - Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December 31, 2018	$ 2,483
Change in fair value	(1,352)
Balance at December 31, 2019	$ 1,131

Investments - Summary of Marketable Debt Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 124,368	$ 169,962
Unrealized Gains	92	4
Unrealized Losses	(1)	(99)
Fair Value	124,459	169,867
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	36,926	14,131
Fair Value	36,926	14,131
Cash Equivalents \| Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	29,377	14,131
Fair Value	29,377	14,131
Cash Equivalents \| U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	4,999
Fair Value	4,999
Cash Equivalents \| US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	2,550
Fair Value	2,550
Short-Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	84,417	155,831
Unrealized Gains	92	4
Unrealized Losses	(1)	(99)
Fair Value	84,508	155,736
Short-Term Marketable Securities \| U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	11,965	10,638
Unrealized Gains	7
Unrealized Losses		(3)
Fair Value	11,972	10,635
Short-Term Marketable Securities \| US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	48,647	81,755
Unrealized Gains	71	1
Unrealized Losses		(57)
Fair Value	48,718	81,699
Short-Term Marketable Securities \| U.S. and Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	8,748	26,552
Unrealized Gains	8	2
Unrealized Losses	(1)	(21)
Fair Value	8,755	26,533
Short-Term Marketable Securities \| U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	15,057	34,136
Unrealized Gains	6	1
Unrealized Losses		(16)
Fair Value	15,063	34,121
Short-Term Marketable Securities \| Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,750
Unrealized Losses		(2)
Fair Value		$ 2,748
Long Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	3,025
Fair Value	3,025
Long Term Marketable Securities \| U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	3,025
Fair Value	$ 3,025

Investments - Additional Information (Details)	12 Months Ended
Investments - Additional Information (Details)	Dec. 31, 2019 USD ($)
Investments Debt And Equity Securities [Abstract]
Long-term available-for sale debt securities maximum remaining contractual maturity	2 years
Realized gains or losses on available-for-sale debt securities	$ 0

License Agreements - Additional Information (Details)			1 Months Ended				12 Months Ended
License Agreements - Additional Information (Details)	Jun. 03, 2019 USD ($)	May 07, 2018 $ / shares shares	Jun. 30, 2019 USD ($) shares	Jan. 31, 2019 shares	Dec. 31, 2018 USD ($) shares	Apr. 30, 2016 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Oct. 31, 2019 $ / shares	Oct. 31, 2019 $ / shares	May 31, 2018 USD ($)	Jul. 30, 2017 USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares					42,414,294		47,227,065	42,414,294
Compound library and option agreement execution month and year							2016-02
Strategic investments							$ 5,507,000
Research and development expense							70,957,000	$ 58,907,000	$ 37,373,000
Upfront consideration							$ 2,995,000	$ 75,852,000	$ 44,000
Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock \| shares							253,334	5,000,000	12,711
Upfront consideration								$ 1,000
Additional Paid-In Capital
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront consideration							$ 3,022,000	75,851,000	$ 44,000
IPO
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance price per share \| $ / shares		$ 17.00
Issuance of common stock \| shares		5,000,000
Licensed Products
Research And Development Arrangement Contract To Perform For Others [Line Items]
Royalties due from sales							$ 0
Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement													$ 1,500,000
Upfront consideration	$ 250,000,000
Initial License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year							2016-02
Second License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year							2019-01
Commercial Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement in period							2020-02
Research and development expense							$ 500,000	$ 500,000
Commercial Agreements \| Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares							640,002
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments							$ 70,300,000
Contingent consideration additional common stock issued \| shares							533,336
Funding provided for the research and development under the agreement							$ 2,000,000
Commercial Agreements \| Academic Institution
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares							160,000
Commercial Agreements \| Affiliate of Clinical-Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Aggregate purchase price of interest						$ 500,000
Additional amount invested												$ 500,000
Compound Library And Option Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued \| shares					133,334			133,333
Contingent consideration liability					$ 2,500,000		$ 1,100,000	$ 2,500,000
Strategic investments							5,500,000
Compound Library And Option Agreement \| Other Long-Term Assets
Research And Development Arrangement Contract To Perform For Others [Line Items]
Investment in fair value					1,000,000			1,000,000
Compound Library And Option Agreement \| Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock \| shares				106,667
License and Compound Library and Option Agreement \| IPO \| Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance price per share \| (per share)										$ 4.36	$ 34.20
Other Licensing Agreements with Research Institutions
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration, milestone or royalty payments					$ 0		0	$ 0
Other Licensing Agreements with Research Institutions \| UCSF
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued \| shares			34,000
Contingent consideration, milestone or royalty payments							$ 0
Milestone payments			$ 13,600,000
Other Licensing Agreements with Research Institutions \| UCSF \| Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock \| shares			120,000
Other Licensing Agreements with Research Institutions \| UCSF \| Additional Paid-In Capital
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront consideration			$ 1,000,000

Balance Sheet Components - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 22,952	$ 9,888
Less: accumulated depreciation and amortization	(6,316)	(3,650)
Total property and equipment, net	16,636	6,238
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,219	4,162
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	472	247
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	825	113
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 16,436	$ 5,366

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2.7	$ 2.2	$ 1.3

Balance Sheet Components - Schedule of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued research and development	$ 2,214	$ 1,837
Deferred rent, current portion	1,849	783
Liability related to early exercise shares	237	885
Accrued other	695	1,485
Accrued and other current liabilities	$ 4,995	$ 4,990

Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
South San Francisco, California
Commitments And Contingencies [Line Items]
Lessee, operating lease, description	The term of the lease agreement commenced in May 2019
Lease, term	10 years
Operating lease, renewal term	8 years
Letter of credit, lease agreement	$ 900,000
South San Francisco, California \| Deferred Rent and Leasehold Improvements
Commitments And Contingencies [Line Items]
Tenant improvement allowance	$ 10,700,000
Brisbane, California
Commitments And Contingencies [Line Items]
Operating lease, renewal term	4 years
Tenant improvement allowance	$ 3,900,000
Operating lease, expiration date	Oct. 31, 2022
Operating leases rent holiday period for expanded space	3 months
Rent expense, operating lease	$ 4,500,000	$ 1,800,000	$ 2,000,000
Maximum \| South San Francisco, California
Commitments And Contingencies [Line Items]
Tenant improvement allowance	7,800,000
Operating lease, Right-of-use Asset	$ 2,900,000
Minimum \| Brisbane, California
Commitments And Contingencies [Line Items]
Operating lease option exercise notice period	15 months

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 5,373
2021	6,230
2022	5,859
2023	4,386
2024 and later	29,272
Total future minimum lease payments	$ 51,120

Related-Party Transactions - Additional Information (Details) $ in Thousands		1 Months Ended							12 Months Ended
	Apr. 04, 2018 USD ($)	Dec. 31, 2019 USD ($) PromissoryNote Executive	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($) shares	Oct. 31, 2017 USD ($) Promissory shares	Jul. 31, 2016 USD ($) shares	Apr. 30, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Number of promissory notes \| PromissoryNote		3
Number of executive officers \| Executive		2
Proceeds from convertible preferred stock									$ 59,881	$ 42,867
Executive Officer
Related Party Transaction [Line Items]
Due from related parties					$ 2,100
Restricted stock purchased by related party, shares \| shares					625,084
Forgiveness of promissory note accounted as modification of share based payment									1,500
Stockholders \| Financing Activities
Related Party Transaction [Line Items]
Proceeds from convertible preferred stock									$ 3,000
Promissory Note \| Executive Officer
Related Party Transaction [Line Items]
Number of promissory notes \| Promissory					2
Due from related parties		$ 200		$ 400		$ 200	$ 200	$ 200
Interest rate		1.51%		2.50%		2.50%	2.50%	2.50%
Promissory Note \| Common Stock \| Executive Officer
Related Party Transaction [Line Items]
Early exercise options in aggregate \| shares				114,406		667,253	667,253	667,253
Promissory Note One \| Executive Officer
Related Party Transaction [Line Items]
Due from related parties					$ 1,600
Interest rate					1.85%
Promissory Note Two \| Executive Officer
Related Party Transaction [Line Items]
Due from related parties					$ 500
Interest rate					1.85%
Non-Recourse Promissory Note \| Executive Officer
Related Party Transaction [Line Items]
Outstanding principal and accrued interest, forgiveness			$ 1,600
Repayment of promissory notes	$ 500
Full-recourse Notes \| Executive Officer
Related Party Transaction [Line Items]
Repayment of promissory notes	$ 200

Common and Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended				12 Months Ended
	Jun. 03, 2019	May 07, 2018	Apr. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 28, 2018	Dec. 31, 2016
Class Of Stock [Line Items]
Convertible preferred stock, shares authorized							10,000,000	10,000,000
Number of stock, par value per share							$ 0.0001	$ 0.0001
Number of stock, shares issued							0	0
Number of stock, shares outstanding							0	0
Common stock, shares authorized				140,000,000			300,000,000	300,000,000		122,000,000
Common stock, par value per share							$ 0.0001	$ 0.0001
Common stock, shares issued							47,227,065	42,414,294
Common stock, shares outstanding							47,227,065	42,414,294
Preferred stock, shares authorized				103,283,818						91,739,149
Net proceeds from sale of convertible preferred stock								$ 59,881	$ 42,867
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs								$ 79,055
Common stock, par value							$ 0.0001	$ 0.0001
Shares issued offering value							$ 2,995	$ 75,852	$ 44
Proceeds from sale of common stock							$ 26,085
Maximum
Class Of Stock [Line Items]
Shares issued offering value	$ 250,000
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued					2,879,288	659,821
Shares price per share					$ 12.125	$ 12.125
Gross proceeds from issuance of convertible preferred stock					$ 34,900	$ 8,000
Issuance price per share									$ 12.125
Series C Convertible Preferred Stock
Class Of Stock [Line Items]
Number of stock, shares issued			322,852	3,590,573
Shares price per share			$ 15.3317	$ 15.3317
Preferred stock, shares designated				11,544,669
Net proceeds from sale of convertible preferred stock			$ 5,000	$ 54,900
Issuance price per share								$ 15.3317
Common Stock
Class Of Stock [Line Items]
Common shares issued upon conversion of preferred shares								32,073,149
Common stock, shares outstanding							47,227,065	42,414,294	4,830,389		4,303,538
Issuance of common stock							253,334	5,000,000	12,711
Shares issued offering value								$ 1
IPO
Class Of Stock [Line Items]
Common stock, shares authorized		300,000,000
Common stock, par value per share		$ 0.0001
Preferred stock, shares authorized		10,000,000
Issuance of common stock		5,000,000
Issuance price per share		$ 17.00
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs		$ 75,900
Approximate underwriting discounts commissions and offering related transaction costs		$ 9,100
Common stock, par value		$ 0.0001
Preferred stock, par value		$ 0.0001
IPO \| Common Stock
Class Of Stock [Line Items]
Common shares issued upon conversion of preferred shares		32,073,149					32,100,000
ATM Equity offering program
Class Of Stock [Line Items]
Shares issued offering value							$ 26,086
ATM Equity offering program \| Sales Agreement \| Cowen
Class Of Stock [Line Items]
Issuance of common stock							3,974,908
Proceeds from sale of common stock							$ 26,100
Payments for commissions and other offering expenses							$ 1,400
ATM Equity offering program \| Sales Agreement \| Cowen \| Maximum
Class Of Stock [Line Items]
Proceeds from sale of common stock	$ 75,000
Percentage of gross sales proceeds of common stock payable as compensation	3.00%
Reimbursement of expenses	$ 100
ATM Equity offering program \| Common Stock
Class Of Stock [Line Items]
Issuance of common stock							3,974,908
Shares issued offering value							$ 1
ATM Equity offering program \| Common Stock \| Maximum
Class Of Stock [Line Items]
Shares issued offering value	$ 75,000

Stock-Based Compensation - Additional Information (Details)	1 Months Ended			6 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) shares	Oct. 31, 2017 Agreement shares	Jun. 30, 2013	Feb. 28, 2018 $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares authorized	140,000,000			122,000,000	300,000,000	300,000,000
Intrinsic value of options exercised \| $					$ 5,800,000	$ 1,500,000	$ 100,000
Weighted-average estimated fair value of stock options granted \| $ / shares					$ 7.12	$ 13.20	$ 3.40
Exercisable, aggregate intrinsic value \| $					$ 7,269,000	$ 21,900,000
Stock option awards outstanding					6,906,898	5,500,531
Stock option awards, grant date fair value \| $					$ 12,297,000
Options granted in period to purchase an aggregate of shares					2,582,912
Weighted-Average Exercise Price, Options granted \| $ / shares					$ 8.78
Stock-based compensation expense \| $					$ 10,852,000	$ 9,441,000	$ 3,034,000
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	536,242
Option purchase date price percentage lower of closing trading price per share	85.00%
Employees subscription rate during the offering period	15.00%
Maximum number of shares may be purchased by an employee	15,000
Maximum fair market value of shares may be purchased by an employee \| $	$ 25,000
Non Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted in period to purchase an aggregate of shares						20,337
Aggregate of shares issued outside the plan						169,491
Weighted-Average Exercise Price, Options granted \| $ / shares						$ 6.19
Stock-based compensation expense \| $					400,000	$ 1,200,000
Stock Option and RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation cost not yet amortized \| $					$ 28,100,000
Weighted-average period for recognition					3 years 6 months
Total grant-date fair value of stock options vested \| $					$ 14,200,000	$ 3,500,000
Performance Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of awards granted \| $					400,000
Stock option awards outstanding						329,499
Stock option awards, grant date fair value \| $						$ 700,000
Compensation cost recognized \| $					$ 700,000
Performance Contingent Stock Options Granted to Employees \| Executive Team
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals						53,575
Weighted average exercise price \| $ / shares						$ 3.42
Performance and Market Contingent Stock Options Granted to Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards outstanding					454,584	454,584
Stock option awards, grant date fair value \| $					$ 1,500,000	$ 1,500,000
Compensation cost recognized \| $					$ 0	$ 0
Performance and Market Contingent Stock Options Granted to Employees \| Vesting 1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals					53,575
Vesting rights					(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum price per share, (ii) a change in control with aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) an initial public offering that becomes effective at a minimum specified price per share.
Trailing period for Initial public offering that becomes effective or achievement with a minimum market capitalization					30 days
Performance and Market Contingent Stock Options Granted to Employees \| Share-based Payment Arrangement, Tranche Two
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option awards exercisable upon the achievement of performance goals					107,152
Vesting rights					(i) the closing of a financing where the Company sells shares of its equity securities to institutional investors at a minimum pre-money valuation, (ii) a change in control with minimum aggregate proceeds payable for the Company’s common stock at a minimum price per share, or (iii) either an initial public offering or an achievement of a minimum market capitalization, as measured by a trailing 30 day volume-weighted average price.
Performance and Market Contingent Stock Options Granted to Employees \| Executive Team
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average exercise price \| $ / shares						$ 3.42
Estimated fair value of awards granted \| $						$ 1,000,000
Stock option awards exercisable						160,727
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested immediately					359,229
Restricted Stock \| Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock agreements \| Agreement		2
Shares purchased		625,084
Shares vested immediately		146,113
Restricted Stock \| Executive Officer \| Vesting 1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested immediately		119,742
Restricted Stock \| Executive Officer \| Share-based Payment Arrangement, Tranche Two
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested immediately		359,229
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	4,289,936
Vesting period, options	4 years
Common stock, shares authorized					6,846,928
2018 Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options	10 years
2018 Incentive Award Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant	100.00%
2013 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, options			4 years
Exercise price \| $ / shares				$ 3.42
Exercise price range, lower range limit \| $ / shares				3.95
Exercise price range, upper range limit \| $ / shares				$ 8.47
2013 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period from the date of grant, options			10 years
2013 Equity Incentive Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price as a percentage of estimated fair value of the shares on the date of grant			100.00%
Exercise price as a percentage of estimated grant date fair value of shares for a 10% shareholder			110.00%

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Shares Available for Grant
Shares Available for Grant, Beginning Balance	3,027,802
Shares Available for Grant, Shares Added	2,357,131
Shares Available for Grant, Granted	(2,935,629)
Shares Available for Grant, Canceled	467,016
Shares Available for Grant, Ending Balance	2,916,320	3,027,802
Outstanding Options
Outstanding options, Beginning Balance	5,500,531
Outstanding options, Granted	2,582,912
Outstanding options, Exercised	(505,226)
Outstanding options, Canceled	(671,319)
Outstanding options, Ending Balance	6,906,898	5,500,531
Outstanding options, Vested and exercisable at December 31, 2019	2,574,677
Outstanding options. Vested and expected to vest at December 31, 2019	6,906,898
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 6.75
Weighted-Average Exercise Price, Options granted	8.78
Weighted-Average Exercise Price, Options exercised	1.66
Weighted-Average Exercise Price, Options canceled	9.47
Weighted-Average Exercise Price, Ending Balance	7.62	$ 6.75
Weighted-Average Exercise Price, Vested exercisable as of December 31, 2019	6.21
Weighted-Average Exercise Price, Options vested and expected to vest as of December 31, 2019	$ 7.62
Weighted-Average Remaining Contract Term
Balance at December 31, 2019	8 years 2 months 12 days
Exercisable, weighted average remaining contractual term (Year)	7 years 4 months 9 days
Options vested or expected to vest at end of period, weighted average remaining contractual term (Year)	8 years 2 months 12 days
Aggregate Intrinsic Value
Outstanding end of period, aggregate intrinsic value	$ 12,297
Exercisable, aggregate intrinsic value	7,269	$ 21,900
Options vested or expected to vest at end of period, aggregate intrinsic value	$ 12,297

Stock-Based Compensation - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs)	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Granted \| shares	352,717
Shares, Forfeited \| shares	(26,830)
Shares, Unvested at December 31, 2019 \| shares	325,887
Weighted Average Grant Date Fair Value, Granted \| $ / shares	$ 9.00
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	9.00
Weighted Average Grant Date Fair Value, Unvested at December 31, 2019 \| $ / shares	$ 9.00

Stock-Based Compensation - Summary of Valuation Assumption to Estimate Fair Value of Stock Options (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, Minimum	1.59%	2.60%	1.80%
Risk-free interest rate, Maximum	2.27%	3.00%	2.20%
Expected stock price volatility, Minimum	99.40%	87.40%	77.00%
Expected stock price volatility, Maximum	111.30%	92.60%	82.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term of options (in years)			5 years 7 months 6 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term of options (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 8 months 12 days

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested at December 31, 2018 \| shares	359,229
Shares, Vested \| shares	(359,229)
Weighted Average Grant Date Fair Value, Unvested at December 31, 2018 \| $ / shares	$ 4.57
Weighted Average Grant Date Fair Value, Vested \| $ / shares	$ 4.57

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 10,852	$ 9,441	$ 3,034
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	4,979	6,043	1,695
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 5,873	$ 3,398	$ 1,339

Warrants - Additional Information (Details) - $ / shares	Oct. 31, 2013	Dec. 31, 2019	Jan. 31, 2015	Jun. 30, 2013
Non Employee \| Common Stock
Class Of Warrant Or Right [Line Items]
Warrants granted	96,610
Warrants granted, exercise price	$ 0.18
Unvested warrants expiration date	May 03, 2018
Warrants vested	9,425
Series A-1 Convertible Preferred Stock \| Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Warrants granted				192,823
Warrants granted, exercise price				$ 0.65
Series A-2 Convertible Preferred Stock \| Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Warrants granted			380,452	190,226
Warrants granted, exercise price			$ 0.66	$ 0.66
Convertible Preferred Stock \| Chief Executive Officer
Class Of Warrant Or Right [Line Items]
Unvested warrants expiration date		Dec. 31, 2018

Net Loss per Common Share - Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (18,027)	$ (21,710)	$ (23,673)	$ (18,767)	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (82,177)	$ (76,398)	$ (44,656)
Denominator:
Weighted average number of shares outstanding—basic and diluted									43,624,807	28,269,907	3,197,516
Net loss per share, basic and diluted	$ (0.39)	$ (0.51)	$ (0.56)	$ (0.44)	$ (0.53)	$ (0.45)	$ (0.76)	$ (4.69)	$ (1.88)	$ (2.70)	$ (13.97)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Common Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	7,427,297	6,591,409	34,842,052
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			28,159,724
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	6,906,898	5,500,531	4,365,694
Early Exercised Common Stock Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	146,915	704,028	831,439
Restricted Stock Accounted For as Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		359,228	625,084
RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	325,887
Warrants to Purchase Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			763,501
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			96,610
Shares Subject to the 2018 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	47,597	27,622

Net Loss per Common Share - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 shares
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares contingently issued	640,218

Defined Contribution Plan - Additional Information (Details) - 401(k) Plan	1 Months Ended
	Jan. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Percent of employer matching contribution of employees' salary	4.00%
Defined Contribution Plan, Sponsor Location [Extensible List]	country:US

Income Taxes - Schedule of Effective Tax Rate of Provision for Income Taxes Differs From Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Taxes at the U.S. statutory income tax rate	21.00%	21.00%	34.00%
State tax, net of federal benefit	(2.20%)	0.90%
Other	(0.90%)	(0.10%)	(2.20%)
Stock-based compensation	(0.50%)	0.30%
Research and development tax credits	(0.20%)	1.00%
Reduction to state net operating losses	(3.90%)
Change in valuation allowance	(13.30%)	(23.10%)	(13.30%)
Change in income tax rate due to Tax Act			(18.50%)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Federal corporate income tax rate	21.00%	21.00%	34.00%
Percentage of net operating loss carry forward deductible from current year taxable income	80.00%
Decrease in deferred tax valuation allowance			$ 8,300,000
Increase in the valuation allowance	$ 11,000,000	$ 17,600,000	$ 9,400,000
Accrued interest or penalties related to uncertain tax position	0
Unrecognized tax benefits that would impact effective tax rate	0	0
Liability recorded for potential interest or penalties	$ 0	$ 0
Maximum
Income Taxes [Line Items]
Federal corporate income tax rate			35.00%

Income Taxes - Schedule of Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Federal and state operating loss carryforwards	$ 40,435	$ 29,926
Research and development tax credits	3,436	3,865
Stock-based compensation	3,514	1,839
Accruals and other	1,473	1,040
Contingent consideration	670	954
Charitable contributions	253	253
Total deferred tax assets	49,781	37,877
Deferred tax liabilities:
Unrealized gain on equity investment	(947)
Total deferred tax liabilities	(947)
Valuation allowance	$ (48,834)	$ (37,877)

Income Taxes - Summary of Net Operating Losses and Tax Credit Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2017
Federal
Income Taxes [Line Items]
Net operating losses	$ 137,097	$ 64,136
Operating losses carryforwards expiration years	Do Not Expire	2030 - 2037
Federal \| Research and Development Tax Credits
Income Taxes [Line Items]
Research and development tax credits	$ 5,931
Research and development tax credits carryforwards expiration years	2031 - 2039
State
Income Taxes [Line Items]
Net operating losses	$ 26,123
Operating losses carryforwards expiration years	2030 - 2036
State \| Research and Development Tax Credits
Income Taxes [Line Items]
Research and development tax credits	$ 5,216
Research and development tax credits carryforwards expiration years	Indefinite

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits at January 1	$ 3,714	$ 3,065
Additions for tax positions taken in the current year	6,221	753
Reductions for tax positions taken in the prior year	(173)	(104)
Gross unrecognized tax benefits at December 31	$ 9,762	$ 3,714

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (23,090)	$ (22,354)	$ (24,470)	$ (19,737)	$ (22,808)	$ (19,377)	$ (20,798)	$ (16,482)	$ (89,651)	$ (79,465)	$ (45,608)
Net loss	$ (18,027)	$ (21,710)	$ (23,673)	$ (18,767)	$ (21,917)	$ (18,346)	$ (20,002)	$ (16,133)	$ (82,177)	$ (76,398)	$ (44,656)
Net loss per share, basic and diluted	$ (0.39)	$ (0.51)	$ (0.56)	$ (0.44)	$ (0.53)	$ (0.45)	$ (0.76)	$ (4.69)	$ (1.88)	$ (2.70)	$ (13.97)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details