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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Mar. 02, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2019
Trading Symbol	KOD
Entity Registrant Name	KODIAK SCIENCES INC.
Entity Central Index Key	0001468748
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity File Number	001-38682
Entity Tax Identification Number	27-0476525
Entity Address, Address Line One	2631 Hanover Street
Entity Address, City or Town	Palo Alto
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94304
City Area Code	650
Local Phone Number	281-0850
Entity Incorporation, State or Country Code	DE
Entity Interactive Data Current	Yes
Document Annual Report	true
Document Transition Report	false
Title of 12(b) Security	Common stock, par value $0.0001
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		44,420,028
Entity Public Float			$ 273.7
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive Proxy Statement relating to the 2020 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. The proxy statement will be filed with the Securities and Exchange Commission within 120 days of the registrant’s fiscal year ended December 31, 2019.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 211,797	$ 88,254
Marketable securities	124,684
Prepaid expenses and other current assets	2,749	2,195
Total current assets	339,230	90,449
Marketable securities	11,696
Restricted cash	140	140
Property and equipment, net	996	1,097
Operating lease right-of-use asset	1,790
Other assets	5,014	503
Total assets	358,866	92,189
Current liabilities:
Accounts payable	2,619	1,050
Accrued and other current liabilities	8,658	3,776
Operating lease liability	434
Total current liabilities	11,711	4,826
Operating lease liability, net of current portion	1,501
Other liabilities	295	530
Total liabilities	13,507	5,356
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized; 0 shares issued and outstanding at December 31, 2019 and 2018
Common stock, $0.0001 par value, 490,000,000 shares authorized at December 31, 2019 and 2018; 44,413,404 and 36,829,857 shares issued and outstanding at December 31, 2019 and 2018, respectively	5	4
Additional paid-in capital	503,475	197,595
Accumulated other comprehensive income	10
Accumulated deficit	(158,131)	(110,766)
Total stockholders’ equity	345,359	86,833
Total liabilities and stockholders’ equity	$ 358,866	$ 92,189

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred Stock, shares issued	0	0
Preferred Stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	490,000,000	490,000,000
Common stock, shares issued	44,413,404	36,829,857
Common stock, shares outstanding	44,413,404	36,829,857

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses
Research and development	$ 37,506	$ 18,793	$ 22,022
General and administrative	11,684	7,581	3,499
Total operating expenses	49,190	26,374	25,521
Loss from operations	(49,190)	(26,374)	(25,521)
Interest income	1,568	617	29
Interest expense (includes $0, $3,030 and $914 attributable to related parties for the years ended December 31, 2019, 2018 and 2017, respectively)	(8)	(5,519)	(1,185)
Other income (expense), net (includes $0, $2,736 and $1,008 attributable to related parties for the years ended December 31, 2019, 2018 and 2017, respectively)	265	(4,688)	(1,259)
Loss on extinguishment of debt (includes $1,587 attributable to related parties for the year ended December 31, 2018)		(5,479)
Net loss	$ (47,365)	$ (41,443)	$ (27,936)
Net loss per common share, basic and diluted	$ (1.25)	$ (2.77)	$ (3.72)
Weighted-average common shares outstanding used in computing net loss per common share, basic and diluted	37,853,616	14,976,515	7,515,336
Other comprehensive income
Change in unrealized gains related to available-for-sale debt securities, net of tax	$ 10
Total other comprehensive income	10
Comprehensive loss	$ (47,355)	$ (41,443)	$ (27,936)

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) (unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Interest expense attributable to related parties	$ 0	$ 3,030	$ 914
Other income (expense) attributable to related parties	$ 0	2,736	$ 1,008
Loss on extinguishment of debt attributable to related parties		$ 1,587

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ (41,083)		$ 1	$ 303		$ (41,387)
Redeemable convertible preferred stock, shares outstanding, beginning balance at Dec. 31, 2016		12,385,154
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2016		$ 50,017
Beginning balance, shares at Dec. 31, 2016			7,930,831
Issuance of common stock upon exercise of stock options	3			3
Issuance of common stock upon exercise of stock options, shares			5,603
Vesting of early exercised stock options	3			3
Stock-based compensation expense	275			275
Net loss	(27,936)					(27,936)
Ending Balance at Dec. 31, 2017	(68,738)		$ 1	584		(69,323)
Redeemable convertible preferred stock, shares outstanding, ending balance at Dec. 31, 2017		12,385,154
Redeemable convertible preferred stock, ending balance at Dec. 31, 2017		$ 50,017
Ending balance, shares at Dec. 31, 2017			7,936,434
Issuance of common stock upon exercise of stock options	49			49
Issuance of common stock upon exercise of stock options, shares			47,800
Issuance of restricted stock awards, shares			27,500
Conversion of redeemable convertible preferred stock into common stock	50,017		$ 1	50,016
Conversion of redeemable convertible preferred stock into common stock, shares		(12,385,154)
Conversion of redeemable convertible preferred stock into common stock		$ (50,017)
Conversion of redeemable convertible preferred stock into common stock, shares			12,385,154
Conversion of redeemable convertible preferred stock warrants into common stock warrants	5,000			5,000
Issuance of common stock upon exercise of common stock warrants, shares			100,000
Conversion of 2017 and 2018 convertible notes into common stock	55,733		$ 1	55,732
Conversion of 2017 and 2018 convertible notes into common stock, shares			6,932,969
Issuance of common stock upon initial public offering/follow-on offering, net of issuance cost	83,459		$ 1	83,458
Issuance of common stock upon initial public offering / follow-on offering, net of issuance cost, shares			9,400,000
Stock-based compensation expense	2,756			2,756
Net loss	(41,443)					(41,443)
Ending Balance at Dec. 31, 2018	86,833		$ 4	197,595		(110,766)
Ending balance, shares at Dec. 31, 2018			36,829,857
Issuance of common stock upon exercise of stock options	2,287			2,287
Issuance of common stock upon exercise of stock options, shares			662,079
Vesting of restricted stock units, net of taxes withheld	(132)			(132)
Vesting of restricted stock units, net of taxes withheld, shares			21,467
Issuance of common stock upon exercise of common stock warrants, shares			1
Issuance of common stock upon initial public offering/follow-on offering, net of issuance cost	297,616		$ 1	297,615
Issuance of common stock upon initial public offering / follow-on offering, net of issuance cost, shares			6,900,000
Stock-based compensation expense	6,110			6,110
Other comprehensive income	10				$ 10
Net loss	(47,365)					(47,365)
Ending Balance at Dec. 31, 2019	$ 345,359		$ 5	$ 503,475	$ 10	$ (158,131)
Ending balance, shares at Dec. 31, 2019			44,413,404

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Issuance costs	$ 19,784	$ 10,542

Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
Consolidated Statements of Cash Flows $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Cash flows from operating activities
Net loss	$ (47,365)	$ (41,443)	$ (27,936)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	538	490	549
Non-cash interest expense and amortization of debt discount and issuance cost	0	5,482	1,161
Change in fair value of redeemable convertible preferred stock warrant liability	0	2,700	1,260
Change in fair value of derivative instrument	0	1,988	0
Extinguishment of debt		5,479
Stock-based compensation	6,110	2,665	275
Amortization (accretion) of premium (discount) on marketable securities	(241)
Amortization of operating lease right-of-use asset	373
Changes in assets and liabilities:
Prepaid expense and other current assets	(168)	(1,995)	368
Other assets	(4,511)	0	8
Accounts payable	1,569	(2,323)	2,282
Accrued and other current liabilities	4,932	(2,105)	4,330
Operating lease liability	(383)
Other liabilities	0	31	48
Net cash used in operating activities	(39,146)	(29,031)	(17,655)
Cash flows from investing activities
Purchase of property and equipment	(437)	(78)	(209)
Deposits on property and equipment		(503)
Purchases of marketable securities	(150,961)
Maturities of marketable securities	14,400
Net cash used in investing activities	(136,998)	(581)	(209)
Cash flows from financing activities
Proceeds from issuance of common stock, net of offering costs	297,616	83,755
Proceeds from issuance of common stock upon option exercise	2,287	49	3
Payments for restricted stock units, net of taxes withheld	(132)
Proceeds from issuance of convertible notes (includes $9,560 and $8,000 from related parties for the years ended December 31, 2018 and 2017, respectively)	0	33,000	10,000
Debt issuance cost	0	(140)	(181)
Principal payments of capital lease	(48)	(108)	(97)
Principal payments of tenant improvement allowance payable	(36)	(85)	(88)
Net cash provided by financing activities	299,687	116,471	9,637
Net increase (decrease) in cash, cash equivalents and restricted cash	123,543	86,859	(8,227)
Cash, cash equivalents and restricted cash, at beginning of year	88,394	1,535	9,762
Cash, cash equivalents and restricted cash, at end of year	211,937	88,394	1,535
Reconciliation of cash, cash equivalents and restricted cash to consolidated balance sheets
Cash and cash equivalents	211,797	88,254	1,395
Restricted cash	140	140	140
Cash, cash equivalents and restricted cash, at end of year	211,937	88,394	1,535
Supplemental cash flow information:
Cash paid for interest	8	19	24
Supplemental disclosures of non-cash investing and financing information:
Operating lease right-of-use asset obtained in exchange for operating lease liability	2,163
Unpaid offering costs	459	205
Offering costs paid in restricted stock awards		91
Derivative instrument related to convertible notes		6,603
Redeemable convertible preferred stock warrant issued in connection with convertible notes	0	0	1,040
Acquisition of equipment through capital lease	$ 0	$ 0	$ 73

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Cash Flows [Abstract]
Proceeds from issuance of convertible notes	$ 9,560	$ 8,000

The Company

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

The Company

1. The Company

Kodiak Sciences Inc. (the “Company”) is a clinical stage biopharmaceutical company specializing in novel therapeutics to treat high-prevalence ophthalmic diseases. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel.

Initial Public Offering

In October 2018, the Company sold and issued 9,000,000 shares of common stock at a price to the public of $10.00 per share for gross proceeds of $90.0 million. In November 2018, the Company sold and issued an additional 400,000 shares of common stock at $10.00 per share to the underwriters of the initial public offering (“IPO”) following the partial exercise of their over-allotment option for gross proceeds of $4.0 million. The aggregate net proceeds to the Company from the IPO, inclusive of the partial over-allotment option exercise, were $83.5 million after deducting underwriting discounts and commissions and other offering costs.

Upon the closing of the IPO, all convertible preferred shares then outstanding automatically converted into 12,385,154 shares of common stock, 500,000 redeemable convertible preferred stock warrants automatically converted into common stock warrants and 100,000 of such warrants were exercised immediately following the closing of the IPO. The 2017 convertible notes converted into 2,637,292 shares of common stock and the 2018 convertible notes converted into 4,295,677 shares of common stock upon closing of the IPO. In connection with the IPO, the Company amended and restated its certificate of incorporation and bylaws.

Follow-On Offering

In December 2019, the Company sold and issued 6,900,000 shares of common stock, including the underwriters’ full exercise of their over-allotment option, at a price to the public of $46.00 per share for gross proceeds of $317.4 million. The aggregate net proceeds to the Company from the follow-on offering were $297.6 million after deducting underwriting discounts and commissions and other offering costs.

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

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“US GAAP”).

Reclassification

Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.

Principles of Consolidation

The consolidated financial statements include the Company’s accounts and the accounts of Kodiak Sciences Financing Corporation and Kodiak Sciences China, the Company’s direct wholly owned subsidiaries, incorporated in the United States and Cayman Islands, respectively, and Kodiak Sciences GmbH, the Company’s indirect wholly owned subsidiary, incorporated in Switzerland. All intercompany accounts and transactions have been eliminated. The functional and reporting currency of the Company and its subsidiaries is the U.S. dollar. The aggregate foreign currency transaction loss included in determining net loss was less than $0.1 million, $0.3 million and less than $0.1 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Segments

The Company operates and manages its business as one reportable and operating segment, which is the business of research and development of drugs for ophthalmic diseases. The chief operating decision maker reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and expenses during the reporting period. Such estimates include, but are not limited to, the accrual for research and development expenses, the valuation of deferred tax assets, useful lives of property and equipment, the measurement of right-of-use assets and lease liabilities, stock-based compensation, and the valuation of common shares, convertible debt, derivatives and redeemable convertible preferred stock warrants prior to the Company’s IPO. Actual results could differ from those estimates.

Risk and Uncertainties

The Company’s future results of operations involve a number of risks and uncertainties common to clinical-stage companies in the biotechnology industry. The Company’s product candidates are in development and the Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of any of the Company’s product candidates that receive regulatory approval, competition from new technological innovations, substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals, contract manufacturer and research organizations, and other suppliers.

Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that any of the Company’s product candidates will receive the necessary approvals. If the Company is denied approval, approval is delayed or the Company is unable to maintain approvals, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales.

The Company expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical trials and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be on terms acceptable by the Company.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and marketable securities. As of December 31, 2019 and 2018, cash, cash equivalents and marketable securities were invested primarily in money market funds, overnight repurchase agreements, U.S. treasury securities, commercial paper and corporate notes through highly rated financial institutions. Investments are restricted, in accordance with the Company’s investment policy, to a concentration limit per issuer or sector.

Cash and Cash Equivalents

The Company considers all highly liquid investments with stated maturities of three months or less at the date of purchase to be cash equivalents.

Marketable Securities

The Company invests excess cash balances in marketable securities. The investments in marketable securities are classified as either held-to-maturity or available-for-sale based on facts and circumstances present at the time of purchase. Marketable securities with a remaining maturity date greater than one year are classified as non-current. The Company’s marketable securities consist of U.S. treasury securities, commercial paper, and corporate bonds. Marketable securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase of marketable debt securities is amortized and/or accreted to other income (expense), net over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense), net.

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

Restricted Cash

As of December 31, 2019, and 2018, the Company had $0.1 million of long-term restricted cash deposited with a financial institution. The entire amount is held in a separate bank account to support a letter of credit agreement related to the Company’s headquarter facility lease which expires in 2023.

Fair Value of Financial Instruments

Accounting Standards Codification (“ASC”) 820, Fair Value Measurement, establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company's own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company's assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following:

Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3—Unobservable inputs which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and inputs to the model.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts of the Company’s financial instruments consisting of cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities and other current liabilities, approximate fair value due to their relatively short maturities.

Leases

The Company adopted ASC 842 effective January 1, 2019 using the modified retrospective approach to recognize a cumulative-effect adjustment on the effective date and to not adjust financial information and disclosures required under the new lease standards for comparative prior periods. The Company did not elect for the package of practical expedients and assessed all contracts at the transition date. The Company did not utilize the practical expedient which allows the use of hindsight in determining lease term and assessing impairment in right-of-use assets. The Company elected to apply the practical expedient and accounted for each lease component and related non-lease component as one single component. The Company elected the practical expedient not to recognize leases with terms of one year or less on the balance sheet.

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

Property and Equipment, Net

Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, which is generally four years for laboratory equipment, three years for computer equipment and office equipment, five years for computer software and five to seven years for furniture and fixtures. Leasehold improvements are stated at cost and amortized over the shorter of the useful life of the assets or the length of the lease. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in operations. Maintenance and repairs are charged to operations as incurred.

Impairment of Long-Lived Assets

The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the assets. There have been no such impairments of long-lived assets in the years ended December 31, 2019 and 2018.

Research and Development Expenses

Costs related to research, design and development of products are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, including stock-based compensation, laboratory supplies, outside services and allocated overhead, including rent, equipment, depreciation and utilities.

Accrued Research and Development

The Company has entered into various agreements with contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”). The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the consolidated balance sheets. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.

Stock-Based Compensation

The Company accounts for stock-based compensation in accordance with the provisions of ASC 718, Compensation-Stock Compensation. The Company measures stock-based compensation expense for stock options and restricted stock units granted to its employees, directors and non-employees based on the estimated fair value of the awards on the grant date. The fair value of options is calculated using the Black-Scholes valuation model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. The expense is recorded on a straight-line basis over the requisite service period, which is generally the vesting period, for the entire award. The Company accounts for forfeitures as they occur.

Prior to the adoption of Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"), the measurement date for non-employee awards was generally the date the services are completed, resulting in financial reporting period adjustments to stock-based compensation during the vesting terms for changes in the fair value of the awards. After adoption of ASU 2018-07 as of January 1, 2019, the measurement date for non-employee awards is the date of grant without changes in the fair value of the award.

The Company has certain stock options and restricted stock units that vest in conjunction with certain performance conditions. At each reporting date, the Company is required to evaluate whether achievement of the performance conditions is probable. Compensation expense is recorded over the appropriate service period based upon the Company's assessment of accomplishing each performance provision.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses (“NOLs”) and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized.

The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

Comprehensive Loss

Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists primarily of unrealized gains and losses on debt securities.

Net Loss per Share Attributable to Common Stockholders

Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of this calculation, the redeemable convertible preferred stock, preferred stock warrants, convertible notes, common stock subject to repurchase, and stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security. The Company’s participating securities do not have a contractual obligation to share in the Company’s losses. As such, the net loss is attributed entirely to common stockholders. Since the Company has reported net loss for all periods presented, diluted net loss per share is the same as basic net loss per common share for those periods.

Recent accounting pronouncements

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

Recently Adopted Accounting Pronouncements

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

The adoption of the new lease standards on January 1, 2019 resulted in the initial recognition of right-of-use asset of $2.2 million and operating lease liability of $2.3 million and derecognition of noncurrent deferred liabilities of $0.2 million related to the operating lease for the Company’s office and laboratory space in Palo Alto, California on the consolidated balance sheets with no material impact to the consolidated statements of operations, stockholders’ equity or cash flows. Refer to Note 7.

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

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

New Accounting Pronouncements Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements and ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, which intends to improve financial reporting by requiring earlier recognition of credit losses on certain financial assets, such as available-for-sale debt securities. The standard is effective for interim and annual periods after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

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

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

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. The new standard will be effective beginning January 1, 2021. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

Property and Equipment

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, net

3. Property and Equipment, net

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

	December 31, 2019			December 31, 2018
Leasehold improvement	$	1,265		$	1,260
Laboratory equipment		1,125			1,133
Furniture and fixtures		204			225
Computer software		79			85
Office equipment		94			79
Total property and equipment		2,767			2,782
Less: Accumulated depreciation		(1,771	)		(1,685	)
Property and equipment, net	$	996		$	1,097

All property and equipment are maintained in the United States. Depreciation expense, including depreciation of assets under capital leases, was $0.5 million, $0.5 million and $0.5 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Accrued Liabilities and Other Current Liabilities

12 Months Ended

Dec. 31, 2019

Accounts Payable And Accrued Liabilities Current [Abstract]

Accrued Liabilities and Other Current Liabilities

4. Accrued Liabilities and Other Current Liabilities

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

	December 31, 2019		December 31, 2018
Accrued research and development	$	4,894	$	1,387
Accrued salaries and benefits		3,108		2,061
Accrued legal fees		302		82
Accrued professional fees		195		117
Accrued other liabilities		159		129
Total accrued and other current liabilities	$	8,658	$	3,776

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

5. Fair Value Measurements

The following tables present the Company’s fair value hierarchy for assets and liabilities measured at fair value on a recurring basis (in thousands):

	Fair Value Measurements at December 31, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	155,276	$	—	$	—	$	155,276
Repurchase agreement		50,000		—		—		50,000
Commercial paper		—		5,987		—		5,987
Marketable securities:
U.S. treasury securities		—		50,185		—		50,185
Commercial paper		—		34,533		—		34,533
Corporate notes		—		51,662		—		51,662
Total	$	205,276	$	142,367	$	—	$	347,643

	Fair Value Measurements at December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	87,957	$	—	$	—	$	87,957
Total	$	87,957	$	—	$	—	$	87,957

There were no liabilities measured at fair value on a recurring and non-recurring basis as of December 31, 2019 and 2018. There were no transfers of assets or liabilities between the fair value measurement levels during the years ended December 31, 2019 and 2018.

- 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

Marketable Securities

12 Months Ended

Dec. 31, 2019

Investments Debt And Equity Securities [Abstract]

Marketable Securities

6. Marketable Securities

The marketable securities are classified as available-for-sale and consist of U.S. treasury securities, corporate notes and commercial paper. The fair value measurement data for marketable securities is obtained from independent pricing services. The Company validates the prices provided by the third-party pricing services by understanding the valuation methods and data sources used and analyzing the pricing data in certain instances.

The following table summarizes the marketable securities held at December 31, 2019 (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
U.S. treasury securities	$	50,190	$	—	$	(5	)	$	50,185
Commercial paper		34,532		1		—			34,533
Corporate notes		39,956		13		(3	)		39,966
Total marketable securities, current	$	124,678	$	14	$	(8	)	$	124,684

Corporate notes	$	11,692	$	4	$	—		$	11,696
Total marketable securities, noncurrent	$	11,692	$	4	$	—		$	11,696

All marketable securities held at December 31, 2019 had effective maturities of less than two years. There were no realized gains or losses recognized on the sale or maturity of available-for-sale debt securities during the year ended December 31, 2019 and as a result, the Company did not reclassify any amounts out of accumulated comprehensive loss. All marketable securities with unrealized losses as of December 31, 2019 have been in a loss position for less than twelve months and the loss is not material. These marketable securities were not considered to be other-than-temporarily impaired as of December 31, 2019.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

7. Commitments and Contingencies

Leases

In January 2013, the Company executed a non-cancellable lease agreement for office and laboratory space in Palo Alto, California. The lease began in October 2013 and would expire in October 2018. In March 2016, the Company executed a lease amendment agreement which was effective March 2016 and extended the lease term until October 2023.

The Company recognizes rent expense on a straight-line basis over the lease period. Rent expense recognized under all leases was $0.6 million and $0.6 million and $0.6 million for the years ended December 31, 2019, 2018 and 2017, respectively. Note that the Company adopted ASC 842 using the required modified retrospective approach effective January 1, 2019. Therefore, the amounts disclosed for the years ended December 31, 2018 and 2017 are presented under previous accounting guidance and are therefore not comparable to the amounts recorded in the current period under ASC 842.

The Company recognized an operating lease right-of-use asset and corresponding liability on January 1, 2019 based on the present value of remaining lease payments discounted at the Company’s estimated incremental borrowing rate of 8.5% over the remaining lease term of 4.83 years for the facility in Palo Alto, California. For the year ended December 31, 2019, the operating lease cost was $0.6 million and the variable lease costs were $0.2 million. The cash paid for amounts included in the measurement of the operating lease liability was $0.6 million for the year ended December 31, 2019. The short-term lease costs were immaterial for the year ended December 31, 2019.

The maturities of the operating lease liabilities as of December 31, 2019 were as follows (in thousands):

Year ending December 31,	As of December 31, 2019
2020	$	533
2021		598
2022		616
2023		526
Total undiscounted lease payments		2,273
Less: imputed interest		(338	)
Total operating lease liabilities	$	1,935

The future minimum lease payments under all non-cancelable operating lease obligations as of December 31, 2018 were as follows (in thousands):

Year ending December 31,	As of December 31, 2018
2019	$	564
2020		581
2021		598
2022		616
2023		526
Total payments	$	2,885

Other Commitments and Contingencies

The Company has entered into service agreements with Lonza AG and its affiliates (“Lonza”), pursuant to which Lonza agreed to perform activities in connection with the manufacturing process of certain compounds. Such agreements, and related amendments, state that planned activities that are included in the signed work orders are, in some cases, binding and, hence, obligate the Company to pay the full price of the work order upon satisfactory delivery of products and services. Per the terms of the agreements, the Company has the option to cancel signed orders at any time upon written notice, which may or may not be subject to payment of a cancellation fee. The level of cancellation fees may be dependent on the timing of the written notice in relation to the commencement date of the work, with the maximum cancellation fee equal to the full price of the work order. As of December 31, 2019 and 2018, the total amount of unconditional purchase obligations, including accrued amounts, under these agreements were $4.7 million and $3.2 million, respectively. Purchases under this agreement for the years ended December 31, 2019, 2018 and 2017 were $7.9 million, $2.8 million and $13.9 million, respectively. As of December 31, 2019, the Company had not incurred any cancellation fees for the work performed by Lonza.

In the normal course of business, the Company enters into purchase commitments to support research and development activities that cannot be terminated without incurring cancellation fees. The level of cancellation fees may vary and are generally based on the passage of time within a 12-month period. As of December 31, 2019, the total amount of cancellation fees related to manufacturing equipment was $0.7 million and none as of December 31, 2019 and 2018, respectively.

The Company is also party to a cancellable assignment and license agreement that would require the Company to make milestone payments of up to $33.2 million and royalty payments on net sales of products utilizing KSI-201 and related technology. Such milestones and royalties are dependent on future activity or product sales and are not estimable.

Tenant Improvement Allowance Payable

In May 2013, the Company entered into a tenant improvement allowance agreement with its landlord. The agreement allowed the Company to draw down $0.3 million for tenant improvements related to the office lease over the period from the execution of the agreement to October 2018. The interest rate is 8% per year over the lease period. This tenant improvement allowance was repaid in October 2018.

In March 2016, the Company entered into a lease amendment, under which the Company is allowed to draw down an additional allowance of $0.4 million for tenant improvements related to the office lease over the period from the execution of the agreement to October 2023. The interest rate is 8% per year over 10 years. Principal and interest are payable on the first day of every month.

As of December 31, 2019 and 2018, the current portion of the tenant improvement allowance payable in accrued and other current liabilities was less than $0.1 million and less than $0.1 million, respectively. As of December 31, 2019 and 2018, the non-current portion of the tenant improvement allowance payable in other liabilities was $0.3 million and $0.3 million, respectively.

Legal Proceedings

From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. Management is currently not aware of any matters that could have a material adverse effect on the Company’s financial position, results of operations or cash flows. The Company records a legal liability when it believes that it is both probable that a liability may be imputed, and the amount of the liability can be reasonably estimated. Significant judgment by the Company is required to determine both probability and the estimated amount.

Indemnification

To the extent permitted under Delaware law, the Company has agreed to indemnify its directors and officers for certain events or occurrences while the director or officer is, or was serving, at the Company’s request in such capacity. The indemnification period covers all pertinent events and occurrences during the director’s or officer’s service. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is not specified in the agreements; however, the Company has director and officer insurance coverage that reduces its exposure and enables the Company to recover a portion of any future amounts paid. The Company believes the estimated fair value of these indemnification agreements in excess of applicable insurance coverage is minimal.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

8. Income Taxes

The Company has not recorded any income tax expense. The Company has a net operating loss and has provided a valuation allowance against net deferred tax assets due to uncertainties regarding the Company’s ability to realize these assets.

The components of loss before income taxes were as follows (in thousands):

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
United States	$	2,633		$	(17,273	)	$	(3,240	)
Foreign		(49,998	)		(24,170	)		(24,696	)
Total loss before income taxes	$	(47,365	)	$	(41,443	)	$	(27,936	)

The tax effects of temporary differences that give rise to significant components of the net deferred tax assets are as follows (in thousands):

	December 31, 2019			December 31, 2018			December 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$	18,523		$	11,044		$	11,270
Intangible assets		12,112			7,588			618
Research and development tax credits		4,037			1,559			788
Stock-based compensation		1,539			394			-
Accruals		885			700			393
Operating lease liability		577			-			-
Property and equipment		143			109			97
Other		—			—			26
Total deferred tax assets		37,816			21,394			13,192
Valuation allowance		(37,249	)		(21,394	)		(13,192	)
Net deferred tax assets		567			—			—
Deferred tax liabilities:
Operating lease right-of-use asset		(534	)		—			—
Capitalized legal fees		(33	)		—			—
Total deferred tax liabilities		(567	)		—			—
Total net deferred tax assets	$	—		$	—		$	—

The Company has recorded a full valuation allowance against its net deferred tax assets due to the uncertainty as to whether such assets will be realized. The net change in the total valuation allowance for the years ended December 31, 2019, 2018 and 2017 was an increase of approximately $15.9 million, $8.2 million and $4.8 million, respectively.

On December 22, 2017, H.R. 1 (the “Tax Act”) was enacted and included broad tax reforms. The Tax Act reduced the U.S. corporate tax rate from 35% to 21% effective January 1, 2018. The rate change resulted in a $2.5 million reduction in the Company’s deferred tax assets from 2016 to 2017. The Tax Act also imposed a deemed repatriation of foreign earnings of subsidiaries; Kodiak Sciences GmbH is considered an E&P deficit corporation for the purposes of this provision and thus no income inclusion was required. The Company has elected to treat taxes on Global Intangible Low Tax Income (“GILTI”) as period costs starting in 2018.

NOLs and tax credit carry-forwards are subject to review and possible adjustment by the Internal Revenue Service (“IRS”) and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50% as defined under Sections 382 and 383 in the Internal Revenue Code, which could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the Company’s value immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed a Section 382 study through December 31, 2019 which concluded no such ownership change had occurred through December 31, 2019.

As of December 31, 2019, the Company had $31.8 million of federal and $131.2 million of state net operating loss available to offset future taxable income. A portion of the federal net operating loss carryforwards begin to expire in 2035 and the state net operating loss carryforwards begin to expire in 2035, if not utilized. $13.7 million of the federal net operating loss are not subject to expiration.

As of December 31, 2019, the Company also had federal and state research and development credit carryforwards of $3.6 million and $2.3 million, respectively. The federal research and development credit carryforwards expire beginning 2035. The California tax credit can be carried forward indefinitely.

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	December 31, 2019			December 31, 2018			December 31, 2017
Federal statutory income tax rate		21.0	%		21.0	%		34.0	%
State taxes		11.0			5.6			7.1
Foreign tax rate differential		(12.4	)		(3.8	)		(17.1	)
Change in valuation allowance		(30.5	)		(18.3	)		(15.4	)
Stock-based compensation		7.7			(0.6	)		(0.3	)
Research tax credit		3.3			1.3			0.8
Other		(0.1	)		(0.1	)		(0.3	)
Remeasurement of deferred tax due to tax law change		—			—			(8.8	)
Fair value adjustments		—			(2.4	)		—
Extinguishment of convertible note		—			(2.7	)		—
Provision for income taxes		0.0	%		0.0	%		0.0	%

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. As of December 31, 2019, 2018 and 2017, none of the unrecognized tax benefits would affect income tax expense with consideration of the valuation allowance. The Company does not anticipate the uncertain tax positions will materially change in the next 12 months. It is the Company’s policy to include interest and penalties as a component of income tax expense or benefit as necessary.

The beginning and ending unrecognized tax benefits amounts are as follows (in thousands):

	December 31, 2019		December 31, 2018		December 31, 2017
Unrecognized tax benefits at beginning of period	$	398	$	357	$	235
Increases related to prior year tax positions		—		—		102
Increases related to current year tax positions		1,440		41		20
Unrecognized tax benefits at end of period	$	1,838	$	398	$	357

The Company files income tax returns in the United States and Switzerland. The Company is not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns remain open for examination by the federal and state authorities for three and four years, respectively, from the date of utilization of any net operating loss or credits.

Preferred Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Preferred Stock

9. Preferred Stock

As of December 31, 2019 and 2018, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue up to 10,000,000 shares of preferred stock at the par value of $0.0001 per share. As of December 31, 2019, there are no holders of the Company’s preferred stock.

Common Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Common Stock

10. Common Stock

As of December 31, 2019 and 2018, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 490,000,000 shares of common stock at the par value of $0.0001 per share. Each share of common stock is entitled to one vote. The board of directors may declare and pay dividends to holders of common stock. The Company has never declared or paid any dividends on common stock.

The Company had reserved common stock for future issuances as follows:

	December 31, 2019		December 31, 2018
Exercise of options outstanding		6,830,442		5,135,267
Exercise of common stock warrants outstanding		399,999		400,000
Issuance of common stock under the 2018 Equity Incentive Plan		2,118,877		3,024,404
Issuance of common stock under the 2018 Employee Share Purchase Plan		460,000		460,000
Total		9,809,318		9,019,671

Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

11. Stock-Based Compensation

2018 Equity Incentive Plan

In August 2018, the Company adopted the 2018 Equity Incentive Plan (“2018 Plan”), which became effective on the business day prior to the effectiveness of the registration statement relating to the IPO. The 2018 Plan initially reserved 4,300,000 shares of common stock for the issuance of incentive stock options ("ISOs"), nonstatutory stock options, restricted stock, restricted stock units (“RSUs”), stock appreciation rights, performance units and performance shares to employees, directors and consultants of the Company. The number of shares available for issuance will increase annually on the first day of each fiscal year beginning in 2019 equal to the least of (1) 4,300,000 shares, and (2) 4% of outstanding shares of common stock as of the last day of the immediately preceding year, and (3) such other amount as determined by the board of directors. The exercise price of options must be equal to at least the fair market value of the common stock on the grant date. For ISOs, the term may not exceed ten years, except in respect to any participant with more than 10% of voting power of all classes or stock, then the term may not exceed five years and the exercise price must be equal to at least 110% of the fair market value of the common stock on the grant date. Options granted generally vest over four years.

The 2015 Equity Incentive Plan was terminated in connection with the adoption of the 2018 Plan and the 63,359 shares that were then unissued and available for future award under the 2015 Equity Incentive Plan became available under the 2018 Plan. The awards outstanding under the 2015 Equity Incentive Plan continue to be governed by their existing terms.

The number of shares available for issuance increased by 1,473,194 shares in 2019 and there were 2,118,877 shares available for grant under the 2018 Plan as of December 31, 2019.

2015 Equity Incentive Plan

In September 2015, the Company adopted the 2015 Equity Incentive Plan (“2015 Plan”) under which 2,810,513 shares of common stock were reserved for issuance through grants of incentive stock options, nonqualified stock options and restricted stock awards (“RSAs”) to employees, directors and consultants of the Company. During 2018, the board of directors approved an increase of 2,125,000 shares to the common stock reserved under the 2015 Plan. The awards outstanding under the previously terminated 2009 Share Incentive Plan continue to be governed by their existing terms.

Stock Options

Stock option activity under the 2018 Plan and 2015 Plan is summarized as follows (in thousands, except share and per share data):

				Outstanding Awards
	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options			Weighted Average Exercise Price		Weighted Average Contractual Term (in years)		Aggregate Intrinsic Value
Balance, January 1, 2017		551,817			1,263,757		$	0.98		9.42	$	100
Options granted		(137,500	)		137,500		$	1.06
Options exercised		—			(5,603	)	$	0.50				3
Options forfeited or canceled		191,240			(191,240	)	$	1.04
Balance, December 31, 2017		605,557			1,204,414		$	0.98		8.49	$	41
Shares authorized		6,425,000			—
Options granted		(4,103,653	)		4,103,653		$	7.45
Options exercised		—			(47,800	)	$	1.03				290
RSAs granted		(27,500	)		—
RSUs granted		(60,000	)		60,000		$	10.00
Options forfeited or canceled		185,000			(185,000	)	$	5.19
Balance, December 31, 2018		3,024,404			5,135,267		$	5.99		9.07	$	10,681
Shares authorized		1,473,194			—
Options granted		(2,539,493	)		2,539,493		$	36.70
Options exercised		—			(662,079	)	$	3.45				45,355
RSUs granted		(128,900	)		128,900		$	73.51
RSUs vested		—			(21,467	)	$	9.90
Shares withheld related to net share settlement of RSUs		8,533			(8,533	)
Options forfeited or canceled		281,139			(281,139	)	$	7.86
Balance, December 31, 2019		2,118,877			6,830,442		$	17.49		8.32	$	373,514
Shares exercisable, December 31, 2018					1,766,385		$	3.11		8.28	$	7,182
Vested and expected to vest, December 31, 2018					5,135,267		$	5.99		9.07	$	10,681
Shares exercisable, December 31, 2019					2,773,881		$	11.82		8.17	$	126,346
Vested and expected to vest, December 31, 2019					6,830,442		$	17.49		8.32	$	373,514

Shares Subject to Repurchase

The Company has a right of repurchase with respect to unvested shares issued upon early exercise of options at an amount equal to the lower of (1) the exercise price of each restricted share being repurchased and (2) the fair market value of such restricted share at the time the Company’s right of repurchase is exercised. The Company’s right to repurchase these shares lapses as those shares vest over the requisite service period.

Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be issued until those shares vest according to their respective vesting schedules. Cash received for early exercised stock options is recorded as accrued liabilities and other current liabilities on the consolidated balance sheet and is reclassified to common stock and additional paid-in capital as such shares vest. At December 31, 2019 and 2018, there are no early exercised stock options that remained subject to the Company’s right of repurchase.

Employee Stock Options

Prior to the Company’s IPO, the fair value of the shares of common stock underlying the stock options was determined by the board of directors with assistance from management and external appraisers as there has been no historical public market for the Company’s common stock. Subsequent to the Company’s IPO, the fair value of the Company’s common stock is determined based on its closing market price.

During the years ended December 31, 2019, 2018 and 2017, the Company granted 2,354,343, 3,888,653 and 107,500 stock options, respectively, to employees with a weighted-average grant date fair value of $34.11, $4.36 and $0.61 per share, respectively.

The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Expected volatility		68	%		59	%		63	%
Risk-free interest rate		1.65	%		2.82	%		1.89	%
Dividend yield		0	%		0	%		0	%
Expected term		5.78			6.06			6.00

Expected Term. The expected term is calculated using the simplified method, which is available where there is insufficient historical data about exercise patterns and post-vesting employment termination behavior. The simplified method is based on the vesting period and the contractual term for each grant, or for each vesting-tranche for awards with graded vesting. The mid-point between the vesting date and the maximum contractual expiration date is used as the expected term under this method. For awards with multiple vesting-tranches, the times from grant until the mid-points for each of the tranches may be averaged to provide an overall expected term.

Expected Volatility. The Company used an average historical stock price volatility of a peer group of publicly traded companies to be representative of its expected future stock price volatility, as the Company does not have sufficient trading history for its common stock. For purposes of identifying these peer companies, the Company considered the industry, stage of development, size and financial leverage of potential comparable companies. For each grant, the Company measured historical volatility over a period equivalent to the expected term. 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 risk-free interest rate is based on the implied yield currently available on U.S. Treasury zero-coupon issues with a remaining term equivalent to the expected term of a stock award.

Expected Dividend Rate. The Company has not paid and does not anticipate paying any dividends in the near future. Accordingly, the Company has estimated the dividend yield to be zero.

The total fair value of employee options vested during the years ended December 31, 2019, 2018 and 2017 was $4.6 million, $1.2 million and $0.2 million, respectively. Stock-based compensation expense recognized during the years ended December 31, 2018, 2017 and 2016 for options granted to employees was $5.7 million, $2.0 million and $0.2 million, respectively.

Non-Employee Stock Options

The Company granted 15,000, 215,000 and 30,000 stock options to non-employees during the years ended December 31, 2019, 2018 and 2017, respectively.

Subsequent to the adoption of ASU 2018-07 effective January 1, 2019, existing stock options granted to non-employees will no longer be revalued, and the estimated fair value of new stock options granted to non-employees will be calculated on the date of grant and not remeasured, similar to stock options granted to employees. The fair value of non-employee stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Expected volatility		73	%		68	%		63	%
Risk-free interest rate		1.61	%		2.71	%		2.31	%
Dividend yield		0	%		0	%		0	%
Expected term		6.08			9.30			9.35

Stock-based compensation expense recognized during the years ended December 31, 2019, 2018 and 2017 for options granted to non-employees was $0.1 million, $0.4 million and less than $0.1 million, respectively.

Performance-Based Stock Options and Restricted Stock Units

The Company granted 170,150 performance-based stock options and 128,900 performance-based restricted stock units (“RSUs”) to employees in 2019. These performance-based equity awards will vest one-quarter upon the achievement of specific clinical development milestones. The remaining shares will then vest in three equal annual installments after that date. Performance-based stock options and performance-based restricted stock units are recorded as expense beginning when vesting events are determined to be probable.

The fair value of performance-based stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31, 2019
Expected volatility		72	%
Risk-free interest rate		1.67	%
Dividend yield		0	%
Expected term		6.31

The weighted-average grant date fair value was $47.89 per share for performance-based stock options and $73.51 per share for performance-based restricted stock units. None of these performance-based equity awards vested during 2019. The Company believes that the achievement of the requisite performance condition is probable and stock-based compensation expense recognized during the year ended December 31, 2019 was less than $0.1 million related to these awards.

Restricted Stock Awards

Restricted stock award (“RSAs”) activity is summarized as follows:

	Number of Shares Underlying Outstanding RSAs			Weighted Average Grant Date Fair Value
Unvested, December 31, 2016		549,351		$	0.41
Vested		(257,718	)	$	1.00
Unvested, December 31, 2017		291,633		$	0.45
Granted		27,500		$	5.38
Vested		(268,683	)	$	0.89
Unvested, December 31, 2018		50,450		$	0.79
Vested		(48,603	)	$	0.78
Unvested, December 31, 2019		1,847		$	1.04

Under the terms of the restricted stock agreements, 1/48th of the award vests monthly over four years, which is the requisite service period. Recipients of restricted stock awards generally have voting and dividend rights with respect to such shares upon grant without regard to vesting. Shares of restricted stock that do not vest are subject to forfeiture. The Company recognizes stock-based compensation expense for RSAs on a straight-line basis over the requisite service period for the entire award. The total fair value of RSAs vested during the years ended December 31, 2019, 2018 and 2017 was less than $0.1 million, $0.2 million and $0.1 million, respectively. Stock-based compensation expense recognized during the years ended December 31, 2019, 2018 and 2017 for RSAs was less than $0.1 million, $0.2 million and $0.1 million, respectively.

Restricted Stock Units

RSUs activity is summarized as follows:

	Number of Shares Underlying Outstanding RSUs			Weighted Average Grant Date Fair Value
Unvested, December 31, 2017		—		$	—
Granted		60,000		$	9.90
Unvested, December 31, 2018		60,000		$	9.90
Vested		(21,467	)	$	9.90
Shares withheld related to net share settlement of RSUs		(8,533	)	$	9.90
Unvested, December 31, 2019		30,000		$	9.90

The total fair value of RSUs vested during the year ended December 31, 2019 was $0.3 million. Stock-based compensation expense recognized during the years ended December 31, 2019 and 2018 for RSUs was $0.3 million and less than $0.1 million, respectively.

2018 Employee Share Purchase Plan

In August 2018, the Company adopted the 2018 Employee Share Purchase Plan (“ESPP”), which became effective on the business day prior to the effectiveness of the registration statement relating to the IPO. A total of 460,000 shares of common stock were initially reserved for issuance under the ESPP. The offering period and purchase period will be determined by the board of directors. As of December 31, 2019, no offerings have been authorized to date.

Stock-Based Compensation Expense

Stock-based compensation is classified in the consolidated statements of operations and comprehensive loss as follows (in thousands):

	Year Ended December 31, 2019		Year Ended December 31, 2018		Year Ended December 31, 2017
Research and development	$	3,496	$	1,535	$	171
General and administrative		2,614		1,073		104
Total stock-based compensation	$	6,110	$	2,608	$	275

As of December 31, 2019, the Company had $59.4 million and $0.2 million of unrecognized compensation expense related to unvested stock options and unvested restricted stock awards and units, respectively, that is expected to be recognized over a weighted-average period of 3.07 years and 0.77 years, respectively.

Net Loss per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

12. Net Loss per Share Attributable to Common Stockholders

The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders which excludes shares which are legally outstanding, but subject to repurchase by the Company (in thousands, except share and per share data):

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Numerator:
Net loss attributable to common stockholders	$	(47,365	)	$	(41,443	)	$	(27,936	)
Denominator:
Weighted-average shares outstanding		37,869,291			15,136,197			7,932,717
Less: weighted-average unvested restricted shares and shares subject to repurchase		(15,675	)		(159,682	)		(417,381	)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted		37,853,616			14,976,515			7,515,336
Net loss per share attributable to common stockholders, basic and diluted	$	(1.25	)	$	(2.77	)	$	(3.72	)

The following potentially dilutive securities, presented on an as-converted to common stock basis, were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive:

	Year Ended December 31, 2019		Year Ended December 31, 2018		Year Ended December 31, 2017
Exercise of options outstanding		6,671,542		5,135,267		1,204,414
Unvested restricted shares		160,747		50,450		291,633
Unvested early exercised common stock options		—		—		2,887
Conversion of redeemable convertible preferred stock		—		—		12,385,154
Conversion of convertible notes outstanding		—		—		2,153,781
Exercise of preferred stock warrants outstanding		—		—		500,000
Total		6,832,289		5,185,717		16,537,869

401(k) Plan

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

401(k) plan

13. 401(k) Plan

In 2011, the Company adopted a 401(k) retirement and savings plan covering all employees. The 401(k) plan allows employees to make pre- and post-tax contributions up to the maximum allowable amount set by the Internal Revenue Service. The 401(k) plan was amended to include an employer matching provision in 2019. The Company will make matching contributions of 100% of employee contributions up to a maximum of 50% of the individual maximum contribution limit allowed under the IRS rules. For the year ended December 31, 2019, the expense related to the matching contributions was $0.3 million.

Subsequent Events

12 Months Ended

Dec. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

14. Subsequent Events

Funding Agreement with Baker Bros. Advisors, LP

On December 1, 2019, the Company and its subsidiary Kodiak Sciences GmbH entered into a funding agreement with Baker Bros. Advisors, LP (“BBA”), pursuant to which BBA purchased the right to receive a capped 4.5% royalty on future net sales of KSI-301, the Company’s anti-VEGF antibody biopolymer conjugate therapy, in exchange for $225.0 million. The funding agreement with BBA, which holds more than 5% of the Company’s stock, was the result of a competitive process overseen by independent and disinterested members of the board of directors of the Company with the assistance of outside counsel.

The closing of the funding agreement was subject to certain conditions and occurred in February 2020. The Company received $100.0 million of the funding on February 4, 2020. The remaining $125.0 million shall be payable to the Company upon enrollment of 50% patients in the planned Phase 3 clinical trials of KSI-301 for branch retinal vein occlusion and central retinal vein occlusion.

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 table provides the selected quarterly financial information for the years 2019 and 2018 (in thousands, except per share data):

	Three Months Ended
	March 31, 2019			June 30, 2019			September 30, 2019			December 31, 2019
Loss from operations	$	(8,460	)	$	(11,814	)	$	(12,732	)	$	(16,184	)
Net loss	$	(7,984	)	$	(11,385	)	$	(12,380	)	$	(15,616	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.21	)	$	(0.31	)	$	(0.33	)	$	(0.40	)

	Three Months Ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
Loss from operations	$	(5,547	)	$	(5,090	)	$	(6,380	)	$	(9,357	)
Net loss and comprehensive loss	$	(8,920	)	$	(7,409	)	$	(10,452	)	$	(14,662	)
Net loss per share attributable to common stockholders, basic and diluted	$	(1.16	)	$	(0.96	)	$	(1.33	)	$	(0.40	)

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“US GAAP”).
Reclassification	Reclassification Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the Company’s accounts and the accounts of Kodiak Sciences Financing Corporation and Kodiak Sciences China, the Company’s direct wholly owned subsidiaries, incorporated in the United States and Cayman Islands, respectively, and Kodiak Sciences GmbH, the Company’s indirect wholly owned subsidiary, incorporated in Switzerland. All intercompany accounts and transactions have been eliminated. The functional and reporting currency of the Company and its subsidiaries is the U.S. dollar. The aggregate foreign currency transaction loss included in determining net loss was less than $0.1 million, $0.3 million and less than $0.1 million for the years ended December 31, 2019, 2018 and 2017, respectively.
Segments	Segments The Company operates and manages its business as one reportable and operating segment, which is the business of research and development of drugs for ophthalmic diseases. The chief operating decision maker reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and expenses during the reporting period. Such estimates include, but are not limited to, the accrual for research and development expenses, the valuation of deferred tax assets, useful lives of property and equipment, the measurement of right-of-use assets and lease liabilities, stock-based compensation, and the valuation of common shares, convertible debt, derivatives and redeemable convertible preferred stock warrants prior to the Company’s IPO. Actual results could differ from those estimates.
Risk and Uncertainties	Risk and Uncertainties The Company’s future results of operations involve a number of risks and uncertainties common to clinical-stage companies in the biotechnology industry. The Company’s product candidates are in development and the Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of any of the Company’s product candidates that receive regulatory approval, competition from new technological innovations, substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals, contract manufacturer and research organizations, and other suppliers. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that any of the Company’s product candidates will receive the necessary approvals. If the Company is denied approval, approval is delayed or the Company is unable to maintain approvals, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical trials and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be on terms acceptable by the Company.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and marketable securities. As of December 31, 2019 and 2018, cash, cash equivalents and marketable securities were invested primarily in money market funds, overnight repurchase agreements, U.S. treasury securities, commercial paper and corporate notes through highly rated financial institutions. Investments are restricted, in accordance with the Company’s investment policy, to a concentration limit per issuer or sector.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with stated maturities of three months or less at the date of purchase to be cash equivalents.
Marketable Securities	Marketable Securities The Company invests excess cash balances in marketable securities. The investments in marketable securities are classified as either held-to-maturity or available-for-sale based on facts and circumstances present at the time of purchase. Marketable securities with a remaining maturity date greater than one year are classified as non-current. The Company’s marketable securities consist of U.S. treasury securities, commercial paper, and corporate bonds. Marketable securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase of marketable debt securities is amortized and/or accreted to other income (expense), net over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense), net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss.
Restricted Cash	Restricted Cash As of December 31, 2019, and 2018, the Company had $0.1 million of long-term restricted cash deposited with a financial institution. The entire amount is held in a separate bank account to support a letter of credit agreement related to the Company’s headquarter facility lease which expires in 2023.
Fair Value Measurements	Fair Value of Financial Instruments Accounting Standards Codification (“ASC”) 820, Fair Value Measurement, establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company's own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company's assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3—Unobservable inputs which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and inputs to the model. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts of the Company’s financial instruments consisting of cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities and other current liabilities, approximate fair value due to their relatively short maturities.
Leases	Leases The Company adopted ASC 842 effective January 1, 2019 using the modified retrospective approach to recognize a cumulative-effect adjustment on the effective date and to not adjust financial information and disclosures required under the new lease standards for comparative prior periods. The Company did not elect for the package of practical expedients and assessed all contracts at the transition date. The Company did not utilize the practical expedient which allows the use of hindsight in determining lease term and assessing impairment in right-of-use assets. The Company elected to apply the practical expedient and accounted for each lease component and related non-lease component as one single component. The Company elected the practical expedient not to recognize leases with terms of one year or less on the balance sheet. The Company determines if an arrangement is, or contains, a lease at inception and then classifies the lease as operating or financing based on the underlying terms and conditions of the contract. Leases with terms greater than one year are initially recognized on the balance sheet as right-of-use assets and lease liabilities based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the incremental borrowing rate, which is the rate incurred to borrow, on a collateralized basis, an amount equal to the lease payments over a similar term and in a similar economic environment of the applicable country or region. Variable lease payments are excluded from the right of use assets and operating lease liabilities and are recognized in the period in which the obligation for those payments is incurred.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, which is generally four years for laboratory equipment, three years for computer equipment and office equipment, five years for computer software and five to seven years for furniture and fixtures. Leasehold improvements are stated at cost and amortized over the shorter of the useful life of the assets or the length of the lease. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in operations. Maintenance and repairs are charged to operations as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the assets. There have been no such impairments of long-lived assets in the years ended December 31, 2019 and 2018.
Research and Development Expenses	Research and Development Expenses Costs related to research, design and development of products are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, including stock-based compensation, laboratory supplies, outside services and allocated overhead, including rent, equipment, depreciation and utilities.
Accrued Research And Development	Accrued Research and Development The Company has entered into various agreements with contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”). The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the consolidated balance sheets. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation in accordance with the provisions of ASC 718, Compensation-Stock Compensation. The Company measures stock-based compensation expense for stock options and restricted stock units granted to its employees, directors and non-employees based on the estimated fair value of the awards on the grant date. The fair value of options is calculated using the Black-Scholes valuation model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. The expense is recorded on a straight-line basis over the requisite service period, which is generally the vesting period, for the entire award. The Company accounts for forfeitures as they occur. Prior to the adoption of Accounting Standards Update ("ASU") No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"), the measurement date for non-employee awards was generally the date the services are completed, resulting in financial reporting period adjustments to stock-based compensation during the vesting terms for changes in the fair value of the awards. After adoption of ASU 2018-07 as of January 1, 2019, the measurement date for non-employee awards is the date of grant without changes in the fair value of the award. The Company has certain stock options and restricted stock units that vest in conjunction with certain performance conditions. At each reporting date, the Company is required to evaluate whether achievement of the performance conditions is probable. Compensation expense is recorded over the appropriate service period based upon the Company's assessment of accomplishing each performance provision.
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses (“NOLs”) and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized. The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists primarily of unrealized gains and losses on debt securities.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of this calculation, the redeemable convertible preferred stock, preferred stock warrants, convertible notes, common stock subject to repurchase, and stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security. The Company’s participating securities do not have a contractual obligation to share in the Company’s losses. As such, the net loss is attributed entirely to common stockholders. Since the Company has reported net loss for all periods presented, diluted net loss per share is the same as basic net loss per common share for those periods.
Recent Accounting Pronouncements	Recent accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB, under its ASC or other standard setting bodies, and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which set out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). In July 2018, the FASB issued ASU 2018-10, Leases (Topic 842), Codification Improvements, and ASU 2018-11, Leases (Topic 842), Targeted Improvements. ASU 2018-10 clarified certain provisions and corrected unintended applications of the guidance such as the application of implicit rate, lessee reassessment of lease classification, and certain transition adjustments that should be recognized to earnings rather than to stockholders' equity. ASU 2018-11 provided an alternative transition method and practical expedient for separating contract components for the adoption of Topic 842. ASU 2016-02, ASU 2018-10, and ASU 2018-11 (collectively, "the new lease standards") superseded the previous leases standard, ASC 840 Leases. The Company adopted ASC 842 effective January 1, 2019 using the modified retrospective approach to recognize a cumulative-effect adjustment on the effective date and to not adjust financial information and disclosures required under the new lease standards for comparative prior periods. The Company did not elect for the package of practical expedients and assessed all contracts at the transition date. The Company did not utilize the practical expedient which allows the use of hindsight in determining lease term and assessing impairment in right-of-use assets. The Company elected to apply the practical expedient and accounted for each lease component and related non-lease component as one single component. The Company elected the practical expedient not to recognize leases with terms of one year or less on the balance sheet. The adoption of the new lease standards on January 1, 2019 resulted in the initial recognition of right-of-use asset of $2.2 million and operating lease liability of $2.3 million and derecognition of noncurrent deferred liabilities of $0.2 million related to the operating lease for the Company’s office and laboratory space in Palo Alto, California on the consolidated balance sheets with no material impact to the consolidated statements of operations, stockholders’ equity or cash flows. Refer to Note 7. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) Distinguishing Liabilities from Equity (Topic 480) Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. This update simplifies the accounting for certain financial instruments with down round features, a provision in an equity-linked financial instrument (or embedded feature) that provides a downward adjustment of the current exercise price based on the price of future equity offerings. Down round features are common in warrants, preferred shares, and convertible debt instruments issued by private companies and early-stage public companies. This update requires companies to disregard the down round feature when assessing whether the instrument is indexed to its own stock, for purposes of determining liability or equity classification. ASU 2017-11 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not impact its consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU 2018-07, Compensation — Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of Topic 718 to include all share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 specifies that Topic 718 applies to all share-based payment transactions in which the grantor acquires goods and services to be used or consumed in its own operations by issuing share-based payment awards. ASU 2018-07 also clarifies that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under ASC 606. ASU 2018-07 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this new guidance as of January 1, 2019, which did not result in a material impact on its consolidated financial statements and related disclosures. New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 362): Measurement of Credit Losses on Financial Statements and ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, which intends to improve financial reporting by requiring earlier recognition of credit losses on certain financial assets, such as available-for-sale debt securities. The standard is effective for interim and annual periods after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurements, which eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of the FASB’s disclosure framework project. The standard is effective for interim and annual periods beginning after December 15, 2019. The standard specifies certain amendments which should be applied prospectively while all other amendments should be applied retrospectively. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which clarifies the accounting for implementation, set-up, and other upfront costs incurred in cloud computing arrangements. The standard is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. The new standard will be effective beginning January 1, 2021. The Company is currently evaluating the impact of adopting this guidance on its consolidated financial statements and related disclosures.

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, net

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

	December 31, 2019			December 31, 2018
Leasehold improvement	$	1,265		$	1,260
Laboratory equipment		1,125			1,133
Furniture and fixtures		204			225
Computer software		79			85
Office equipment		94			79
Total property and equipment		2,767			2,782
Less: Accumulated depreciation		(1,771	)		(1,685	)
Property and equipment, net	$	996		$	1,097

Accrued Liabilities and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Accounts Payable And Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities and Other Current Liabilities

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

	December 31, 2019		December 31, 2018
Accrued research and development	$	4,894	$	1,387
Accrued salaries and benefits		3,108		2,061
Accrued legal fees		302		82
Accrued professional fees		195		117
Accrued other liabilities		159		129
Total accrued and other current liabilities	$	8,658	$	3,776

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Fair Value Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis

The following tables present the Company’s fair value hierarchy for assets and liabilities measured at fair value on a recurring basis (in thousands):

	Fair Value Measurements at December 31, 2019
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	155,276	$	—	$	—	$	155,276
Repurchase agreement		50,000		—		—		50,000
Commercial paper		—		5,987		—		5,987
Marketable securities:
U.S. treasury securities		—		50,185		—		50,185
Commercial paper		—		34,533		—		34,533
Corporate notes		—		51,662		—		51,662
Total	$	205,276	$	142,367	$	—	$	347,643

	Fair Value Measurements at December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	87,957	$	—	$	—	$	87,957
Total	$	87,957	$	—	$	—	$	87,957

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2019

Investments Debt And Equity Securities [Abstract]

Summary of Marketable Securities Held

The following table summarizes the marketable securities held at December 31, 2019 (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
U.S. treasury securities	$	50,190	$	—	$	(5	)	$	50,185
Commercial paper		34,532		1		—			34,533
Corporate notes		39,956		13		(3	)		39,966
Total marketable securities, current	$	124,678	$	14	$	(8	)	$	124,684

Corporate notes	$	11,692	$	4	$	—		$	11,696
Total marketable securities, noncurrent	$	11,692	$	4	$	—		$	11,696

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Summary of Maturities of Operating Lease Liabilities

The maturities of the operating lease liabilities as of December 31, 2019 were as follows (in thousands):

Year ending December 31,	As of December 31, 2019
2020	$	533
2021		598
2022		616
2023		526
Total undiscounted lease payments		2,273
Less: imputed interest		(338	)
Total operating lease liabilities	$	1,935

Summary of Future Minimum Lease Payments Under Non-Cancelable Operating Lease Obligations

The future minimum lease payments under all non-cancelable operating lease obligations as of December 31, 2018 were as follows (in thousands):

Year ending December 31,	As of December 31, 2018
2019	$	564
2020		581
2021		598
2022		616
2023		526
Total payments	$	2,885

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Components of Loss before Income Taxes

The components of loss before income taxes were as follows (in thousands):

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
United States	$	2,633		$	(17,273	)	$	(3,240	)
Foreign		(49,998	)		(24,170	)		(24,696	)
Total loss before income taxes	$	(47,365	)	$	(41,443	)	$	(27,936	)

Components of Deferred Tax Assets

The tax effects of temporary differences that give rise to significant components of the net deferred tax assets are as follows (in thousands):

	December 31, 2019			December 31, 2018			December 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$	18,523		$	11,044		$	11,270
Intangible assets		12,112			7,588			618
Research and development tax credits		4,037			1,559			788
Stock-based compensation		1,539			394			-
Accruals		885			700			393
Operating lease liability		577			-			-
Property and equipment		143			109			97
Other		—			—			26
Total deferred tax assets		37,816			21,394			13,192
Valuation allowance		(37,249	)		(21,394	)		(13,192	)
Net deferred tax assets		567			—			—
Deferred tax liabilities:
Operating lease right-of-use asset		(534	)		—			—
Capitalized legal fees		(33	)		—			—
Total deferred tax liabilities		(567	)		—			—
Total net deferred tax assets	$	—		$	—		$	—

Schedule of Reconciliation of Effective Tax Rate to Statutory U.S. Federal Rate

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	December 31, 2019			December 31, 2018			December 31, 2017
Federal statutory income tax rate		21.0	%		21.0	%		34.0	%
State taxes		11.0			5.6			7.1
Foreign tax rate differential		(12.4	)		(3.8	)		(17.1	)
Change in valuation allowance		(30.5	)		(18.3	)		(15.4	)
Stock-based compensation		7.7			(0.6	)		(0.3	)
Research tax credit		3.3			1.3			0.8
Other		(0.1	)		(0.1	)		(0.3	)
Remeasurement of deferred tax due to tax law change		—			—			(8.8	)
Fair value adjustments		—			(2.4	)		—
Extinguishment of convertible note		—			(2.7	)		—
Provision for income taxes		0.0	%		0.0	%		0.0	%

Summary of Unrecognized Tax Benefits Amounts

The beginning and ending unrecognized tax benefits amounts are as follows (in thousands):

	December 31, 2019		December 31, 2018		December 31, 2017
Unrecognized tax benefits at beginning of period	$	398	$	357	$	235
Increases related to prior year tax positions		—		—		102
Increases related to current year tax positions		1,440		41		20
Unrecognized tax benefits at end of period	$	1,838	$	398	$	357

Common Stock (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Schedule of Common Stock Reserved for Future Issuances

The Company had reserved common stock for future issuances as follows:

	December 31, 2019		December 31, 2018
Exercise of options outstanding		6,830,442		5,135,267
Exercise of common stock warrants outstanding		399,999		400,000
Issuance of common stock under the 2018 Equity Incentive Plan		2,118,877		3,024,404
Issuance of common stock under the 2018 Employee Share Purchase Plan		460,000		460,000
Total		9,809,318		9,019,671

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Stock Options Activity Under 2018 Plan and 2015 Plan

Stock option activity under the 2018 Plan and 2015 Plan is summarized as follows (in thousands, except share and per share data):

				Outstanding Awards
	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options			Weighted Average Exercise Price		Weighted Average Contractual Term (in years)		Aggregate Intrinsic Value
Balance, January 1, 2017		551,817			1,263,757		$	0.98		9.42	$	100
Options granted		(137,500	)		137,500		$	1.06
Options exercised		—			(5,603	)	$	0.50				3
Options forfeited or canceled		191,240			(191,240	)	$	1.04
Balance, December 31, 2017		605,557			1,204,414		$	0.98		8.49	$	41
Shares authorized		6,425,000			—
Options granted		(4,103,653	)		4,103,653		$	7.45
Options exercised		—			(47,800	)	$	1.03				290
RSAs granted		(27,500	)		—
RSUs granted		(60,000	)		60,000		$	10.00
Options forfeited or canceled		185,000			(185,000	)	$	5.19
Balance, December 31, 2018		3,024,404			5,135,267		$	5.99		9.07	$	10,681
Shares authorized		1,473,194			—
Options granted		(2,539,493	)		2,539,493		$	36.70
Options exercised		—			(662,079	)	$	3.45				45,355
RSUs granted		(128,900	)		128,900		$	73.51
RSUs vested		—			(21,467	)	$	9.90
Shares withheld related to net share settlement of RSUs		8,533			(8,533	)
Options forfeited or canceled		281,139			(281,139	)	$	7.86
Balance, December 31, 2019		2,118,877			6,830,442		$	17.49		8.32	$	373,514
Shares exercisable, December 31, 2018					1,766,385		$	3.11		8.28	$	7,182
Vested and expected to vest, December 31, 2018					5,135,267		$	5.99		9.07	$	10,681
Shares exercisable, December 31, 2019					2,773,881		$	11.82		8.17	$	126,346
Vested and expected to vest, December 31, 2019					6,830,442		$	17.49		8.32	$	373,514

Summary of Restricted Stock Award Activity

Restricted stock award (“RSAs”) activity is summarized as follows:

	Number of Shares Underlying Outstanding RSAs			Weighted Average Grant Date Fair Value
Unvested, December 31, 2016		549,351		$	0.41
Vested		(257,718	)	$	1.00
Unvested, December 31, 2017		291,633		$	0.45
Granted		27,500		$	5.38
Vested		(268,683	)	$	0.89
Unvested, December 31, 2018		50,450		$	0.79
Vested		(48,603	)	$	0.78
Unvested, December 31, 2019		1,847		$	1.04

Summary of Stock-based Compensation Classified in Condensed Consolidated Statements of Operations and Comprehensive Loss

Stock-based compensation is classified in the consolidated statements of operations and comprehensive loss as follows (in thousands):

	Year Ended December 31, 2019		Year Ended December 31, 2018		Year Ended December 31, 2017
Research and development	$	3,496	$	1,535	$	171
General and administrative		2,614		1,073		104
Total stock-based compensation	$	6,110	$	2,608	$	275

Employee Stock Options

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Fair Value of Stock Options Estimated Using Weighted-average Assumptions

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Expected volatility		68	%		59	%		63	%
Risk-free interest rate		1.65	%		2.82	%		1.89	%
Dividend yield		0	%		0	%		0	%
Expected term		5.78			6.06			6.00

Non-employee Stock Options

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Fair Value of Stock Options Estimated Using Weighted-average Assumptions

The fair value of non-employee stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Expected volatility		73	%		68	%		63	%
Risk-free interest rate		1.61	%		2.71	%		2.31	%
Dividend yield		0	%		0	%		0	%
Expected term		6.08			9.30			9.35

Performance-based Stock Options

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Fair Value of Stock Options Estimated Using Weighted-average Assumptions

The fair value of performance-based stock options was estimated using the following weighted-average assumptions:

	Year Ended December 31, 2019
Expected volatility		72	%
Risk-free interest rate		1.67	%
Dividend yield		0	%
Expected term		6.31

Restricted Stock Units (RSUs)

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Restricted Stock Award Activity

RSUs activity is summarized as follows:

	Number of Shares Underlying Outstanding RSUs			Weighted Average Grant Date Fair Value
Unvested, December 31, 2017		—		$	—
Granted		60,000		$	9.90
Unvested, December 31, 2018		60,000		$	9.90
Vested		(21,467	)	$	9.90
Shares withheld related to net share settlement of RSUs		(8,533	)	$	9.90
Unvested, December 31, 2019		30,000		$	9.90

Net Loss per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

	Year Ended December 31, 2019			Year Ended December 31, 2018			Year Ended December 31, 2017
Numerator:
Net loss attributable to common stockholders	$	(47,365	)	$	(41,443	)	$	(27,936	)
Denominator:
Weighted-average shares outstanding		37,869,291			15,136,197			7,932,717
Less: weighted-average unvested restricted shares and shares subject to repurchase		(15,675	)		(159,682	)		(417,381	)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted		37,853,616			14,976,515			7,515,336
Net loss per share attributable to common stockholders, basic and diluted	$	(1.25	)	$	(2.77	)	$	(3.72	)

Summary of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss per Share

	Year Ended December 31, 2019		Year Ended December 31, 2018		Year Ended December 31, 2017
Exercise of options outstanding		6,671,542		5,135,267		1,204,414
Unvested restricted shares		160,747		50,450		291,633
Unvested early exercised common stock options		—		—		2,887
Conversion of redeemable convertible preferred stock		—		—		12,385,154
Conversion of convertible notes outstanding		—		—		2,153,781
Exercise of preferred stock warrants outstanding		—		—		500,000
Total		6,832,289		5,185,717		16,537,869

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information

The following table provides the selected quarterly financial information for the years 2019 and 2018 (in thousands, except per share data):

	Three Months Ended
	March 31, 2019			June 30, 2019			September 30, 2019			December 31, 2019
Loss from operations	$	(8,460	)	$	(11,814	)	$	(12,732	)	$	(16,184	)
Net loss	$	(7,984	)	$	(11,385	)	$	(12,380	)	$	(15,616	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.21	)	$	(0.31	)	$	(0.33	)	$	(0.40	)

	Three Months Ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
Loss from operations	$	(5,547	)	$	(5,090	)	$	(6,380	)	$	(9,357	)
Net loss and comprehensive loss	$	(8,920	)	$	(7,409	)	$	(10,452	)	$	(14,662	)
Net loss per share attributable to common stockholders, basic and diluted	$	(1.16	)	$	(0.96	)	$	(1.33	)	$	(0.40	)

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Nov. 16, 2018	Oct. 09, 2018	Dec. 31, 2019	Oct. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from initial public offering				$ 83,500
Net proceeds from issuance of common stock					$ 297,616	$ 83,755
Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of redeemable convertible preferred stock into common stock, shares						(12,385,154)
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period					6,900,000	9,400,000
Conversion of 2017 and 2018 convertible notes into common stock, shares						6,932,969
Initial Public Offering \| Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period				9,000,000
Share issued price, per share				$ 10.00
Gross proceeds from initial public offering				$ 90,000
Conversion of redeemable convertible preferred stock into common stock, shares		12,385,154
Initial Public Offering \| Common Stock \| 2017 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of 2017 and 2018 convertible notes into common stock, shares		2,637,292
Initial Public Offering \| Common Stock \| 2018 Convertible Notes
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of 2017 and 2018 convertible notes into common stock, shares		4,295,677
Initial Public Offering \| Common Stock \| Redeemable Convertible Preferred Stock \| Warrants Exercised Into Convertible Preferred Shares Then Converted
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrants exercised into common stock shares		100,000
Initial Public Offering \| Common Stock \| Redeemable Convertible Preferred Stock \| Warrant
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Conversion of preferred stock warrants into common stock warrants, shares		500,000
Over-Allotment Option \| Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period	400,000
Share issued price, per share	$ 10.00
Gross proceeds from initial public offering	$ 4,000
Follow-On Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Gross proceeds from issuance of common stock			$ 317,400
Net proceeds from issuance of common stock			$ 297,600
Follow-On Offering \| Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during period			6,900,000
Share issued price, per share			$ 46.00		$ 46.00

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction loss		$ (300,000)
Number of reportable segments \| Segment	1
Number of operating segments \| Segment	1
Long-term restricted cash	$ 140,000	$ 140,000	$ 140,000
Lease expiration year		2023
Impairment of long-lived assets	0	$ 0
Unrecognized tax benefits, interest and penalties charged	0
Operating lease right-of-use asset	1,790,000
Operating lease liability	$ 1,935,000
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use asset				$ 2,200,000
Operating lease liability				2,300,000
Derecognition of noncurrent deferred liabilities				$ 200,000
Laboratory Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	4 years
Computer Equipment and Office Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	3 years
Computer Software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	5 years
Deposited with Financial Institution
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 100,000	$ 100,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction loss	$ (100,000)		$ (100,000)
Maximum \| Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	7 years
Minimum \| Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives of assets	5 years

Property and Equipment, Net - Summary 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	$ 2,767	$ 2,782
Less: Accumulated depreciation	(1,771)	(1,685)
Property and equipment, net	996	1,097
Leasehold Improvement
Property Plant and Equipment [Line Items]
Total property and equipment	1,265	1,260
Laboratory Equipment
Property Plant and Equipment [Line Items]
Total property and equipment	1,125	1,133
Computer Software
Property Plant and Equipment [Line Items]
Total property and equipment	79	85
Furniture and Fixtures
Property Plant and Equipment [Line Items]
Total property and equipment	204	225
Office Equipment
Property Plant and Equipment [Line Items]
Total property and equipment	$ 94	$ 79

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 538	$ 490	$ 549

Accrued Liabilities and Other Current Liabilities - Schedule of Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts Payable And Accrued Liabilities Current [Abstract]
Accrued research and development	$ 4,894	$ 1,387
Accrued salaries and benefits	3,108	2,061
Accrued legal fees	302	82
Accrued professional fees	195	117
Accrued other liabilities	159	129
Total accrued and other current liabilities	$ 8,658	$ 3,776

Fair Value Measurements - Summary of Fair Value Hierarchy of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	$ 347,643	$ 87,957
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	155,276	87,957
Repurchase Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	50,000
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	50,185
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	5,987
Marketable securities	34,533
Corporate Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	51,662
Quoted Price in Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	205,276	87,957
Quoted Price in Active Markets (Level 1) \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	155,276	87,957
Quoted Price in Active Markets (Level 1) \| Repurchase Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	50,000
Quoted Price in Active Markets (Level 1) \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0
Quoted Price in Active Markets (Level 1) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable securities	0
Quoted Price in Active Markets (Level 1) \| Corporate Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0
Significant Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	142,367	0
Significant Observable Inputs (Level 2) \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Significant Observable Inputs (Level 2) \| Repurchase Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Significant Observable Inputs (Level 2) \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	50,185
Significant Observable Inputs (Level 2) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	5,987
Marketable securities	34,533
Significant Observable Inputs (Level 2) \| Corporate Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	51,662
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	0	0
Significant Unobservable Inputs (Level 3) \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	$ 0
Significant Unobservable Inputs (Level 3) \| Repurchase Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Significant Unobservable Inputs (Level 3) \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0
Significant Unobservable Inputs (Level 3) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable securities	0
Significant Unobservable Inputs (Level 3) \| Corporate Notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 0

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers of assets or liabilities between the fair value measurement levels	$ 0	$ 0
Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability measured at fair value	0	0
Nonrecurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability measured at fair value	$ 0	$ 0

Marketable Securities - Summary of Marketable Securities Held (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Marketable Securities [Line Items]
Amortized Cost, current	$ 124,678
Unrealized Gains, current	14
Unrealized Losses, current	(8)
Fair Value, current	124,684
Amortized Cost, noncurrent	11,692
Unrealized Gains, noncurrent	4
Fair Value, noncurrent	11,696
U.S. Treasury Securities
Marketable Securities [Line Items]
Amortized Cost, current	50,190
Unrealized Losses, current	(5)
Fair Value, current	50,185
Commercial Paper
Marketable Securities [Line Items]
Amortized Cost, current	34,532
Unrealized Gains, current	1
Fair Value, current	34,533
Corporate Notes
Marketable Securities [Line Items]
Amortized Cost, current	39,956
Unrealized Gains, current	13
Unrealized Losses, current	(3)
Fair Value, current	39,966
Amortized Cost, noncurrent	11,692
Unrealized Gains, noncurrent	4
Fair Value, noncurrent	$ 11,696

Marketable Securities - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Marketable Securities [Line Items]
Realized gains or losses recognized on sale or maturity of available-for-sale debt securities	$ 0
Maximum
Marketable Securities [Line Items]
Marketable securities, contractual maturities	2 years

Commitments and Contingencies - Additional Information (Details) - USD ($)			1 Months Ended		12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2016	May 31, 2013	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Line Items]
Rent expense					$ 600,000	$ 600,000	$ 600,000
Estimated incremental borrowing rate used to determine discounted present value of remaining lease payments	8.50%				8.50%
Remaining operating lease term	4 years 9 months 29 days				4 years 9 months 29 days
Operating lease cost					$ 600,000
Variable lease costs					200,000
Operating lease liability					600,000
Current portion of tenant improvement allowance payable	$ 8,658,000	$ 3,776,000			8,658,000	3,776,000
Other liabilities, non-current portion of tenant Improvement allowance payable	295,000	530,000			295,000	530,000
Tenant Improvement Allowance Payable
Commitments And Contingencies Disclosure [Line Items]
Other liabilities, non-current portion of tenant Improvement allowance payable	300,000	300,000			300,000	300,000
Leasehold Improvements \| Palo Alto, California
Commitments And Contingencies Disclosure [Line Items]
Improvements allowance payable to be drawn			$ 400,000
Interest rate			8.00%
Lease term			Oct. 31, 2023
Debt instrument term			10 years
Tenant Improvement Allowance Agreement \| Leasehold Improvements \| Palo Alto, California
Commitments And Contingencies Disclosure [Line Items]
Improvements allowance payable to be drawn				$ 300,000
Interest rate				8.00%
Lease term				Oct. 31, 2018
Maximum \| Tenant Improvement Allowance Payable
Commitments And Contingencies Disclosure [Line Items]
Current portion of tenant improvement allowance payable	100,000	100,000			100,000	100,000
Purchase Commitments
Commitments And Contingencies Disclosure [Line Items]
Cancellation fees related to manufacturing equipment	700,000	0
Cancellable Assignment and License Agreement \| Maximum
Commitments And Contingencies Disclosure [Line Items]
Milestone payments					33,200,000
Lonza \| Service Agreements
Commitments And Contingencies Disclosure [Line Items]
Unconditional purchase obligations, including accrued amounts	$ 4,700,000	$ 3,200,000			4,700,000	3,200,000
Unrecorded unconditional purchase obligation, purchases					7,900,000	$ 2,800,000	$ 13,900,000
Unrecorded unconditional purchase obligation, cancellation fees					$ 0

Commitments and Contingencies - Summary of Maturities of Operating Lease Liabilities (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Operating Lease
2020	$ 533
2021	598
2022	616
2023	526
Total undiscounted lease payments	2,273
Less: imputed interest	(338)
Total operating lease liabilities	$ 1,935

Commitments and Contingencies - Summary of Future Minimum Lease Payments Under Non-Cancelable Operating Lease Obligations (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Lease
2019	$ 564
2020	581
2021	598
2022	616
2023	526
Total payments	$ 2,885

Income Taxes - Components of Loss before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ 2,633	$ (17,273)	$ (3,240)
Foreign	(49,998)	(24,170)	(24,696)
Total loss before income taxes	$ (47,365)	$ (41,443)	$ (27,936)

Income Taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 18,523	$ 11,044	$ 11,270
Intangible assets	12,112	7,588	618
Research and development tax credits	4,037	1,559	788
Stock-based compensation	1,539	394	0
Accruals	885	700	393
Operating lease liability	577	0	0
Property and equipment	143	109	97
Other	0	0	26
Total deferred tax assets	37,816	21,394	13,192
Valuation allowance	(37,249)	(21,394)	(13,192)
Net deferred tax assets	567	0	0
Deferred tax liabilities:
Operating lease right-of-use asset	(534)	0	0
Capitalized legal fees	(33)	0	0
Total deferred tax liabilities	567	0	0
Total net deferred tax assets	$ 0	$ 0	$ 0

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Increase in valuation allowance	$ 15,900	$ 8,200	$ 4,800
U.S. corporate tax rate	21.00%	21.00%	34.00%
Tax Acts, reduction in deferred tax assets			$ 2,500
Research and development credit carryforwards	$ 4,037	$ 1,559	$ 788
Federal
Income Taxes [Line Items]
Net operating loss	$ 31,800
Net operating loss carryforwards expiration start year	2035
Research and development credit carryforwards	$ 3,600
Tax credit carryforwards expiration start year	2035
Number of periods open for examination	3 years
Federal \| Net Operating Loss Not Subject to Expiration
Income Taxes [Line Items]
Net operating loss	$ 13,700
State
Income Taxes [Line Items]
Net operating loss	$ 131,200
Net operating loss carryforwards expiration start year	2035
Research and development credit carryforwards	$ 2,300
Number of periods open for examination	4 years
California
Income Taxes [Line Items]
Tax credit carryforward, expiration year	0

Income Taxes - Schedule of Reconciliation of Effective Tax Rate to Statutory U.S. Federal Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State taxes	11.00%	5.60%	7.10%
Foreign tax rate differential	(12.40%)	(3.80%)	(17.10%)
Change in valuation allowance	(30.50%)	(18.30%)	(15.40%)
Stock-based compensation	7.70%	(0.60%)	(0.30%)
Research tax credit	3.30%	1.30%	0.80%
Other	(0.10%)	(0.10%)	(0.30%)
Remeasurement of deferred tax due to tax law change			(8.80%)
Fair value adjustments		(2.40%)
Extinguishment of convertible note		(2.70%)
Provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Summary of Unrecognized Tax Benefits Amounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of period	$ 398	$ 357	$ 235
Increases related to prior year tax positions	0	0	102
Increases related to current year tax positions	1,440	41	20
Unrecognized tax benefits at end of period	$ 1,838	$ 398	$ 357

Preferred Stock - Additional Information (Details) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.0001	$ 0.0001
Maximum
Class Of Stock [Line Items]
Preferred stock, shares authorized	10,000,000	10,000,000

Common Stock - Additional Information (Details)	12 Months Ended
Common Stock - Additional Information (Details)	Dec. 31, 2019 Vote $ / shares shares	Dec. 31, 2018 Vote $ / shares shares
Equity [Abstract]
Common stock, shares authorized \| shares	490,000,000	490,000,000
Common stock, par value \| $ / shares	$ 0.0001	$ 0.0001
Common stock, number of vote per share \| Vote	1	1

Common Stock - Schedule of Common Stock Reserved for Future Issuances (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Common stock reserved for future issuances	9,809,318	9,019,671
Employee Stock Options
Class Of Stock [Line Items]
Common stock reserved for future issuances	6,830,442	5,135,267
2018 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved for future issuances	2,118,877	3,024,404
2018 Employee Share Purchase Plan
Class Of Stock [Line Items]
Common stock reserved for future issuances	460,000	460,000
Warrant
Class Of Stock [Line Items]
Common stock reserved for future issuances	399,999	400,000

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		9,809,318	9,019,671
Right of shares repurchase as early exercised stock options.		0	0
Stock-based compensation expense recognized		$ 6,110,000	$ 2,608,000	$ 275,000
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 100,000
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		6,830,442	5,135,267
Stock options, granted		2,354,343	3,888,653	107,500
Stock options, weighted-average grant date fair value		$ 34.11	$ 4.36	$ 0.61
Dividends paid		$ 0
Expected dividend payment		$ 0
Dividend yield		0.00%	0.00%	0.00%
Total fair value of employee options vested		$ 4,600,000	$ 1,200,000	$ 200,000
Stock-based compensation expense recognized		5,700,000	$ 2,000,000	$ 200,000
Unrecognized stock-based compensation expense		$ 59,400,000
Unrecognized stock-based compensation weighted-average period expected for recognition		3 years 25 days
Non-employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted		15,000	215,000	30,000
Dividend yield		0.00%	0.00%	0.00%
Options Granted to Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 100,000	$ 400,000	$ 100,000
Performance-based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, granted		170,150
Dividend yield		0.00%
Awards vested		0
Weighted-average grant date fair value		$ 47.89
Performance-based Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock units, granted		128,900
Weighted-average grant date fair value		$ 73.51
Performance Based Equity Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		3 months
Restricted Stock Awards (RSAs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards vesting period		4 years
Stock-based compensation expense recognized			$ 200,000	$ 100,000
Stock units, granted			27,500
Awards vested		48,603	268,683	257,718
Weighted-average grant date fair value		$ 1.04	$ 0.79	$ 0.45	$ 0.41
Stock-based compensation of total fair value vested			$ 200,000	$ 100,000
Restricted Stock Awards (RSAs) \| Share Based Compensation Award Monthly Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award monthly vesting percentage		2.083%
Restricted Stock Awards (RSAs) \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 100,000
Stock-based compensation of total fair value vested		100,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized		$ 300,000
Stock units, granted		128,900	60,000
Awards vested		21,467
Weighted-average grant date fair value		$ 9.90	$ 9.90
Stock-based compensation of total fair value vested		$ 300,000
Restricted Stock Units (RSUs) \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense recognized			$ 100,000
Restricted Stock Awards and Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 200,000
Unrecognized stock-based compensation weighted-average period expected for recognition		9 months 7 days
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		63,359
Number of shares increases annually		1,473,194
Percentage of outstanding stock		4.00%
Awards vesting period		4 years
Number of shares available for grant		2,118,877
2018 Equity Incentive Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares increases annually		4,300,000
2018 Equity Incentive Plan \| Incentive Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		4,300,000
2018 Equity Incentive Plan \| Incentive Stock Options \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options, periods granted		10 years
2018 Equity Incentive Plan \| ISO Granted to a Greater than 10% Stockholder \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options, periods granted		5 years
Options exercise price, percentage of estimated fair value of shares on grant date		110.00%
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance	2,125,000
Common stock, shares reserved for issuance	2,810,513
2018 Employee Share Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		460,000
Common stock, shares authorized		0

Stock-Based Compensation - Summary of Stock Options Activity Under 2018 Plan and 2015 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
2018 Plan And 2015 Equity Incentive Plan
Number of Shares Available for Grant
Beginning balance	3,024,404	605,557	551,817
Shares authorized	1,473,194	6,425,000
Options granted	(2,539,493)	(4,103,653)	(137,500)
Options forfeited or canceled	281,139	185,000	191,240
Ending balance	2,118,877	3,024,404	605,557	551,817
Number of Shares Underlying Outstanding Options
Beginning balance	5,135,267	1,204,414	1,263,757
Options granted	2,539,493	4,103,653	137,500
Options exercised	(662,079)	(47,800)	(5,603)
Options forfeited or canceled	(281,139)	(185,000)	(191,240)
Ending balance	6,830,442	5,135,267	1,204,414	1,263,757
Shares exercisable	2,773,881	1,766,385
Vested and expected to vest	6,830,442	5,135,267
Weighted Average Exercise Price
Beginning balance	$ 5.99	$ 0.98	$ 0.98
Options granted	36.70	7.45	1.06
Options exercised	3.45	1.03	0.50
Options forfeited or canceled	7.86	5.19	1.04
Ending balance	17.49	5.99	$ 0.98	$ 0.98
Shares exercisable	11.82	3.11
Vested and expected to vest	$ 17.49	$ 5.99
Weighted Average Contractual Term (in years)
Weighted Average Contractual Term (in years)	8 years 3 months 25 days	9 years 25 days	8 years 5 months 26 days	9 years 5 months 1 day
Shares exercisable	8 years 2 months 1 day	8 years 3 months 10 days
Vested and expected to vest	8 years 3 months 25 days	9 years 25 days
Aggregate Intrinsic Value
Beginning balance	$ 10,681	$ 41	$ 100
Options exercised	45,355	290	3
Ending balance	373,514	10,681	$ 41	$ 100
Shares exercisable	126,346	7,182
Vested and expected to vest	$ 373,514	$ 10,681
Restricted Stock Awards (RSAs)
Number of Shares Available for Grant
Shares granted		(27,500)
Number of Shares Underlying Outstanding Options
RSUs vested	(48,603)	(268,683)	(257,718)
Restricted Stock Units (RSUs)
Number of Shares Available for Grant
Shares granted	(128,900)	(60,000)
Shares withheld related to net share settlement of RSUs	8,533
Number of Shares Underlying Outstanding Options
RSUs granted	128,900	60,000
RSUs vested	(21,467)
Shares withheld related to net share settlement of RSUs	(8,533)
Weighted Average Exercise Price
RSUs granted	$ 73.51	$ 10.00
RSUs vested	$ 9.90

Stock-Based Compensation - Fair Value of Employee Stock Options Estimated Using Weighted-average Assumptions (Details) - Employee Stock Options	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	68.00%	59.00%	63.00%
Risk-free interest rate	1.65%	2.82%	1.89%
Dividend yield	0.00%	0.00%	0.00%
Expected term	5 years 9 months 10 days	6 years 21 days	6 years

Stock-Based Compensation - Fair Value of Non-employee Stock Options Estimated Using Weighted-average Assumptions (Details) - Non-employee Stock Options	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	73.00%	68.00%	63.00%
Risk-free interest rate	1.61%	2.71%	2.31%
Dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 29 days	9 years 3 months 18 days	9 years 4 months 6 days

Stock-Based Compensation - Fair Value of Performance-based Stock Options Estimated Using Weighted-average Assumptions (Details) - Performance-based Stock Options	12 Months Ended
	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	72.00%
Risk-free interest rate	1.67%
Dividend yield	0.00%
Expected term	6 years 3 months 21 days

Stock-Based Compensation - Summary of Restricted Stock Award Activity (Details) - Restricted Stock Awards (RSAs) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares Underlying Outstanding RSAs
Unvested, beginning balance	50,450	291,633	549,351
Granted		27,500
Vested	(48,603)	(268,683)	(257,718)
Unvested, ending balance	1,847	50,450	291,633
Weighted Average Grant Date Fair Value
Unvested, beginning balance	$ 0.79	$ 0.45	$ 0.41
Granted		5.38
Vested	0.78	0.89	1.00
Unvested, ending balance	$ 1.04	$ 0.79	$ 0.45

Stock-Based Compensation - Summary of RSUs Activity (Details) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares Underlying Outstanding RSAs
Unvested, beginning balance	60,000
Granted	128,900	60,000
Vested	(21,467)
Shares withheld related to net share settlement of RSUs	(8,533)
Unvested, ending balance	30,000	60,000
Weighted Average Grant Date Fair Value
Unvested, beginning balance	$ 9.90
Granted		$ 9.90
Vested	9.90
Shares withheld related to net share settlement of RSUs	9.90
Unvested, ending balance	$ 9.90	$ 9.90

Stock-Based Compensation - Summary of Stock-based Compensation Classified in Condensed Consolidated Statements of Operations and Comprehensive Loss (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]
Total stock-based compensation	$ 6,110	$ 2,608	$ 275
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	3,496	1,535	171
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 2,614	$ 1,073	$ 104

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (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	$ (15,616)	$ (12,380)	$ (11,385)	$ (7,984)					$ (47,365)	$ (41,443)	$ (27,936)
Denominator:
Weighted-average shares outstanding									37,869,291	15,136,197	7,932,717
Less: weighted-average unvested restricted shares and shares subject to repurchase									(15,675)	(159,682)	(417,381)
Weighted-average common shares outstanding used in computing net loss per common share, basic and diluted									37,853,616	14,976,515	7,515,336
Net loss per share attributable to common stockholders, basic and diluted	$ (0.40)	$ (0.33)	$ (0.31)	$ (0.21)	$ (0.40)	$ (1.33)	$ (0.96)	$ (1.16)	$ (1.25)	$ (2.77)	$ (3.72)

- 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 Share Attributable to Common Stockholders - Summary of Anti-dilutive Securities Excluded from Computation of Diluted Net Loss 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 the computation of diluted net loss per share	6,832,289	5,185,717	16,537,869
Employee Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	6,671,542	5,135,267	1,204,414
Unvested Restricted Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	160,747	50,450	291,633
Unvested Early Exercised Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	0	0	2,887
Convertible Notes
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	0	0	2,153,781
Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	0	0	500,000
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	0	0	12,385,154

401(k) Plan - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Defined Contribution Plan Disclosure [Line Items]
Plan name	401(k)
Employer matching contribution, percent of match	100.00%
Expense related to matching contributions	$ 0.3
IRS
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution cap on employee contribution limit	50.00%

Subsequent Events - Additional Information (Details) - Baker Bros. Advisors LP - Funding Agreement - USD ($) $ in Millions	Dec. 01, 2019	Feb. 29, 2020	Feb. 04, 2020
Subsequent Event [Line Items]
Capped percentage of royalty on future net sales	4.50%
Amount under funding agreement	$ 225.0
Subsequent Event
Subsequent Event [Line Items]
Amount under funding agreement			$ 100.0
Funds to be payable upon enrollment of patients		$ 125.0
Percentage of patients to be enrolled		50.00%
Minimum
Subsequent Event [Line Items]
Ownership percentage threshold	5.00%

Selected Quarterly Financial Data - Schedule of Quarterly Financial Information (Unaudited) (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	$ (16,184)	$ (12,732)	$ (11,814)	$ (8,460)	$ (9,357)	$ (6,380)	$ (5,090)	$ (5,547)	$ (49,190)	$ (26,374)	$ (25,521)
Net loss	$ (15,616)	$ (12,380)	$ (11,385)	$ (7,984)					(47,365)	(41,443)	(27,936)
Net loss and comprehensive loss					$ (14,662)	$ (10,452)	$ (7,409)	$ (8,920)	$ (47,355)	$ (41,443)	$ (27,936)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.40)	$ (0.33)	$ (0.31)	$ (0.21)	$ (0.40)	$ (1.33)	$ (0.96)	$ (1.16)	$ (1.25)	$ (2.77)	$ (3.72)

- Definition

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

+ References + Details