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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2018	Nov. 05, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	KZR
Entity Registrant Name	Kezar Life Sciences, Inc.
Entity Central Index Key	0001645666
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		19,108,221

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 39,594	$ 51,033
Marketable securities	73,161
Prepaid expenses	1,676	785
Other current assets	894	508
Total current assets	115,325	52,326
Restricted cash		13
Property and equipment, net	4,660	1,540
Other assets	282	343
Total assets	120,267	54,222
Current liabilities:
Accounts payable	662	547
Accrued liabilities	2,345	911
Deferred rent, current	343
Other liabilities, current	152	26
Total current liabilities	3,502	1,484
Deferred rent, noncurrent	2,638	494
Total liabilities	6,140	1,978
Redeemable convertible preferred stock, $0.001 par value, zero and 75,533,240 shares authorized as of September 30, 2018 and December 31, 2017, respectively; zero and 12,263,126 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively		77,931
Stockholders' equity (deficit):
Common stock, $0.001 par value, 125,000,000 and 96,000,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 19,108,221 and 948,578 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively	19	5
Preferred stock, $0.001 par value, 10,000,000 and zero shares authorized as of September 30, 2018 and December 31, 2017, respectively; zero shares issued and outstanding as of September 30, 2018 and December 31, 2017
Additional paid-in capital	157,681	447
Accumulated other comprehensive loss	(160)	(111)
Accumulated deficit	(43,413)	(26,028)
Total stockholders' equity (deficit)	114,127	(25,687)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 120,267	$ 54,222

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	0	75,533,240
Redeemable convertible preferred stock, shares issued	0	12,263,126
Redeemable convertible preferred stock, shares outstanding	0	12,263,126
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	125,000,000	96,000,000
Common stock, shares issued	19,108,221	948,578
Common stock, shares outstanding	19,108,221	948,578
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 4,664	$ 1,557	$ 13,463	$ 4,833
General and administrative	1,600	635	4,837	1,497
Total operating expenses	6,264	2,192	18,300	6,330
Loss from operations	(6,264)	(2,192)	(18,300)	(6,330)
Interest income	601	112	915	113
Net loss	$ (5,663)	$ (2,080)	$ (17,385)	$ (6,217)
Net loss per common share, basic and diluted	$ (0.30)	$ (3.31)	$ (2.38)	$ (10.92)
Weighted-average shares used to compute net loss per common share, basic and diluted	18,955,384	628,435	7,319,012	569,417

- Definition

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

+ References + Details

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (5,663)	$ (2,080)	$ (17,385)	$ (6,217)
Other comprehensive (loss) income, net of tax:
Foreign currency translation adjustments	(14)	(3)	(23)	42
Unrealized loss on marketable securities	(26)		(26)
Total other comprehensive (loss) income, net of tax	(40)	(3)	(49)	42
Comprehensive loss	$ (5,703)	$ (2,083)	$ (17,434)	$ (6,175)

- Definition

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

+ References + Details

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (17,385)	$ (6,217)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	486	130
Stock-based compensation	1,544	112
Amortization of premiums and discounts on marketable securities	(192)
Loss on disposal of property and equipment	97
Changes in operating assets and liabilities
Prepaid expenses & other current assets	(1,333)	(965)
Other assets	61	1
Accounts payable & accrued liabilities	1,628	443
Other liabilities, current	(9)	(2)
Deferred rent	(216)	5
Net cash used in operating activities	(15,319)	(6,493)
Cash flows from investing activities:
Purchases of property and equipment	(1,003)	(326)
Purchases of marketable securities	(72,995)
Proceeds from sale of property and equipment	10
Net cash used in investing activities	(73,988)	(326)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	77,695
Proceeds from issuance of preferred stock, net of issuance costs		49,755
Proceeds from the exercise of stock options	213
Net cash provided by financing activities	77,908	49,755
Effect of exchange rate changes on cash and cash equivalents and restricted cash	(53)	42
Net (decrease) increase in cash, cash equivalents and restricted cash	(11,452)	42,978
Cash, cash equivalents and restricted cash at the beginning of period	51,046	9,760
Cash, cash equivalents and restricted cash at the end of period	39,594	52,738
Supplemental disclosures of noncash financing information:
Reclassification of employee stock liability to equity upon vesting	29	$ 14
Addition of tenant improvement paid by landlord	2,703
Purchase of property and equipment in accounts payable	7
Conversion of redeemable convertible preferred stock into common stock	$ 77,931

Organization and Description of the Business	9 Months Ended
Organization and Description of the Business	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Description of the Business	1. Organization and Description of the Business Description of Business Kezar Life Sciences, Inc. (the “Company”) was incorporated in Delaware on February 19, 2015, and commenced operations in June 2015. The Company is a clinical-stage biotechnology company, discovering and developing novel small molecule therapeutics to treat unmet needs in autoimmunity and cancer. The Company’s lead product candidate, KZR-616, a first-in-class selective immunoproteasome inhibitor, has completed testing in healthy volunteers and is now enrolling a Phase 1b/2 clinical trial in lupus and lupus nephritis. The Company is also leveraging its protein secretion pathway platform to discover and develop small molecule therapies targeting cancer and immuno-oncology. To date, the Company’s primary activities have been related to the establishment of its facilities, recruitment of personnel and conducting development of its product candidates, including clinical trials. The Company’s principal operations are in South San Francisco, California, and it operates in one segment. Reverse Stock Split On June 8, 2018, the Company filed an Amended and Restated Certificate of Incorporation effecting a 1-for-5.62 reverse stock split of its issued and outstanding shares of common stock and redeemable convertible preferred stock. The par value of the authorized stock was not adjusted as a result of the reverse stock split. In connection with the reverse stock split, the filed Amended and Restated Certificate of Incorporation also adjusted the minimum price per share required in a firm-commitment underwritten public offering of the Company’s common stock in order for the preferred stock to automatically convert to common stock. The minimum price post-split was $15.884 and was adjusted to $7.942. The Company did not adjust the number of authorized shares of common stock or redeemable convertible preferred stock. Other than the par value and the number of authorized shares of common stock, all share and per share data shown in the accompanying condensed consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. Initial Public Offering On June 25, 2018, the Company completed its initial public offering (“IPO”), whereby the Company issued 5,750,000 shares of its common stock (inclusive of 750,000 shares of common stock pursuant to the full exercise of an overallotment option granted to the underwriters in connection with the offering) at a price of $15.00 per share. The shares began trading on The Nasdaq Global Select Market on June 21, 2018. The net proceeds received by the Company from the offering were approximately $77.7 million, after deducting underwriting discounts and commissions and other offering expenses payable by the Company of $8.6 million. Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock converted into 12,263,126 shares of common stock. Additionally, the Company is now authorized to issue 125,000,000 shares of common stock and 10,000,000 shares of preferred stock. Liquidity Since commencing operations in mid-2015, substantially all of the Company’s efforts have been focused on research, development, and the advancement of the Company’s lead product candidate, KZR-616. The Company’s ultimate success depends on the outcome of the ongoing research and development activities. The Company has not yet generated product sales and as a result has experienced operating losses since inception and had an accumulated deficit of $43.4 million as of September 30, 2018. The Company expects to incur additional losses in the future to conduct research and development and will need to raise additional capital to fully implement management’s business plan. The Company intends to raise such capital through the issuance of additional equity, and potentially through borrowings, strategic alliances with partner companies and other licensing transactions. However, if such financing is not available at adequate levels, the Company may need to reevaluate its operating plans. Management believes that its existing cash, cash equivalents and marketable securities will be sufficient to fund the Company’s cash requirements for at least 12 months following the issuance of these financial statements.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Significant Accounting Policies

The Company’s significant accounting policies are disclosed in the audited consolidated financial statements for the year ended December 31, 2017 and the notes thereto, which are included in the Company’s prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-225194), which was filed with the Securities and Exchange Commission (“SEC”) pursuant to Rule 424 on June 21, 2018 (the “Prospectus”), and have had no material changes during the three and nine months ended September 30, 2018.

Basis of Presentation and Consolidation

The condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“U.S. GAAP”) and include the Company’s accounts and those of its wholly owned Australian subsidiary, Kezar Life Sciences Australia Pty Ltd, which is a proprietary company limited by shares. All intercompany balances and transactions have been eliminated upon consolidation.

Unaudited Interim Condensed Consolidated Financial Statements

The interim condensed consolidated balance sheet as of September 30, 2018, the condensed consolidated statements of operations and the condensed consolidated statements of comprehensive loss for the three and nine months ended September 30, 2017 and 2018, and the condensed consolidated statement of cash flows for the nine months ended September 30, 2017 and 2018 are unaudited. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair statement of the Company’s financial position as of September 30, 2018 and its results of operations for the three and nine months ended September 30, 2017 and 2018 and cash flows for the nine months ended September 30, 2017 and 2018. The financial data and the other financial information disclosed in these notes to the condensed consolidated financial statements related to the three and nine-month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018, or for any other future annual or interim period. The balance sheet as of December 31, 2017, included herein was derived from the audited consolidated financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements contained in the Company’s Prospectus.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant items subject to such estimates and assumptions include the useful lives of fixed assets, stock-based compensation, and accrued research and development costs. Management bases its estimates on historical experience and on various other market-specific relevant assumptions that management believes to be reasonable under the circumstances. Actual results may differ from those estimates.

Cash and Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents consist of corporate debt securities and highly liquid money market funds.

Restricted cash consisted of deposits at the bank held as collateral for the Company’s credit card program. The collateral requirement was removed and released in May 2018.

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

	September 30, 2018		December 31, 2017
Cash and cash equivalents	$	39,594	$	51,033
Restricted cash		—		13
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	39,594	$	51,046

Marketable Securities

All marketable securities have been classified as “available-for-sale” in accordance with the Company’s investment policy and cash management strategy. Short-term marketable securities mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses reported within accumulated other comprehensive income as a separate component of stockholders’ deficit until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income on the Company’s condensed consolidated statements of operations.

Accounting Pronouncements Adopted in 2018

In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting. This ASU provides guidance about which changes to the terms or conditions of a share-based payment award requires the Company to apply modification accounting. This ASU will be effective for the Company for annual reporting periods, including interim reporting periods, beginning after December 15, 2017. The Company adopted this standard on January 1, 2018 noting it did not have a material impact on the Company’s financial statements.

New Accounting Pronouncements Not Yet Adopted

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842). ASU 2016-02 increases transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and requires disclosing key information about leasing arrangements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, which provides entities the option to initially apply the transition provisions of ASU 2016-02 at its adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company plans to adopt ASU 2016-02 on January 1, 2019 using the effective date approach as allowed under ASU 2018-11. In addition, the Company plans to apply the practical expedients provided by the FASB. The Company expects that its recognition of expense on its statement of operations under ASU 2016-02 will be similar to its recognition of expense under the current accounting standard. Further, the Company expects to recognize lease liabilities and right-of use assets on its balance sheet.

In June 2018, the FASB issued ASU 2018-07, Compensation – Stock Compensation (Topic 718). ASU 2018-07 simplifies the accounting for nonemployee share-based payment transactions. This ASU is effective for public entities for interim and annual reporting periods beginning after December 15, 2018. The Company has evaluated the potential impact of this guidance and does not believe that it will have a material impact on the Company’s financial statements.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement, which modifies the disclosure requirements on fair value measurements. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is currently evaluating the effect the new guidance will have on its consolidated financial statements.

Fair Value Measurements

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

Financial assets and liabilities are recorded at fair value. The carrying amount of certain financial instruments, including cash and cash equivalents, restricted cash, accounts payable and accrued liabilities, approximate fair value due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the condensed consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

Level 1: Quoted prices in active markets for identical assets or liabilities.

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 that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability.

We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, which are obtained from various third-party data providers, represent quoted prices for similar assets in active markets and were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data. There were no transfers between Level 1 and Level 2 securities in the periods presented.

In certain cases, where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. The Company does not have any assets or liabilities measured using Level 3 inputs as of September 30, 2018 or December 31, 2017.

As of September 30, 2018 and December 31, 2017, financial assets and liabilities measured and recognized at fair value on a recurring basis and classified under the appropriate level of the fair value hierarchy as described above were as follows (in thousands):

	September 30, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Assets
Level 1:
Cash		691		—		—			691
Money market funds		13,072		—		—			13,072
U.S. Treasury securities		53,597		—		(18	)		53,579
Subtotal		67,360		—		(18	)		67,342
Level 2:
Commercial paper		13,712		—		(1	)		13,711
Corporate debt securities		6,571		—		(1	)		6,570
Government agency bonds		25,137		—		(5	)		25,132
Subtotal		45,420		—		(7	)		45,413
Total	$	112,780	$	—	$	(25	)	$	112,755
Reported as:
Cash and cash equivalents								$	39,594
Marketable securities								$	73,161

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Assets
Level 1:
Cash		804		—		—		804
Money market funds		50,229		—		—		50,229
Restricted cash		13		—		—		13
Subtotal		51,046		—		—		51,046
Total	$	51,046	$	—	$	—	$	51,046
Reported as:
Cash and cash equivalents							$	51,033
Restricted cash							$	13

As of September 30, 2018, the amortized cost and estimated fair value of the Company’s available-for-sale securities by contractual maturity are shown below (in thousands):

	Amortized		Estimated
	Cost		Fair Value
Marketable securities maturing:
In one year or less	$	73,184	$	73,161
Total marketable securities	$	73,184	$	73,161

The Company determined that the gross unrealized losses on its marketable securities as of September 30, 2018 were temporary in nature. The Company currently does not intend to sell these securities prior to maturity and does not consider these investments to be other-than-temporarily impaired at September 30, 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

Balance Sheet Components

9 Months Ended

Sep. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

4. Balance Sheet Components

Property and Equipment, Net

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

	September 30, 2018			December 31, 2017
Leasehold improvements	$	3,268		$	155
Furniture, laboratory and office equipment		1,984			1,242
Computer equipment		161			34
Construction in progress		—			464
Total property and equipment		5,413			1,895
Less accumulated depreciation and amortization		(753	)		(355	)
Property and equipment, net	$	4,660		$	1,540

Under the terms of its lease for office and laboratory space at 4000 Shoreline Court, South San Francisco, the Company received an incentive from the landlord for $3.2 million to construct leasehold improvements, which have been recorded in fixed assets and as deferred rent in other liabilities that will be amortized over the remaining lease term. During the three and nine months ended September 30, 2018, the Company disposed of leasehold improvements, laboratory equipment and office equipment resulting in a loss of $0 and $97,000, respectively. There was no such loss during the three and nine months ended September 30, 2017.

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	September 30, 2018		December 31, 2017
Accrued preclinical and research costs	$	1,075	$	108
Accrued clinical costs		403		340
Accrued employee-related costs		722		422
Accrued professional services		74		—
Other		71		41
Accrued liabilities	$	2,345	$	911

Stock-Based Compensation

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

6. Stock-Based Compensation

Stock Incentive Plans

2018 Equity Incentive Plan

In June 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Equity Incentive Plan (the “2018 Plan”), which became effective as of June 20, 2018, at which point no further grants will be made under the 2015 Equity Incentive Plan (the “2015 Plan”) described below. Under the 2018 Plan, the Company may grant incentive stock options (“ISOs”), nonstatutory stock options (“NSOs”), stock appreciation rights, restricted stock awards, restricted stock units (“RSUs”) and other stock-based awards for the purchase of that number of shares of common stock. As of September 30, 2018, 75,621 shares of options and 10,189 RSUs have been granted and 1,812,145 shares were available for future issuance under the 2018 Plan.

Initially, subject to adjustment as provided in the 2018 Plan, the aggregate number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2018 Plan will not exceed 4,000,000 shares, which is the sum of (i) 1,600,692 shares plus (ii) the number of shares reserved, and remaining available for issuance, under the 2015 Plan at the time the 2018 Plan became effective and (iii) the number of shares subject to stock options or other stock awards granted under the 2015 Plan that expire, terminate are forfeited or otherwise not issued, or are withheld to satisfy a tax withholding obligation in connection with an award or to satisfy a purchase or exercise price of an award (such as upon the expiration or termination of a stock award prior to vesting). The number of shares of the Company’s common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 5% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2018 Plan is 12,500,000 shares.

2015 Equity Incentive Plan

The Company’s 2015 Equity Incentive Plan provided for the granting of ISOs and NSOs to employees, directors and consultants at the discretion of the board of directors. The 2015 Plan was terminated as to future awards in June 2018, although it continues to govern the terms of options that remain outstanding under the 2015 Plan.

No additional stock awards will be granted under the 2015 Plan, and all outstanding stock awards granted under the 2015 Plan that are repurchased, forfeited, expire or are cancelled will become available for grant under the 2018 Plan in accordance with its terms.

Options granted under the 2015 Plan expire no later than 10 years from the date of grant. Options granted under the 2015 Plan vest over periods determined by the board of directors, generally over four years. The 2015 Plan allows for early exercise of certain options prior to vesting. Upon termination of employment, the unvested shares are subject to repurchase at the original exercise price. As of September 30, 2018, options to purchase 2,102,045 shares of common stock were outstanding under the 2015 Plan.

2018 Employee Stock Purchase Plan

In June 2018, the Company’s board of directors and its stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective as of June 20, 2018. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the U.S. Internal Revenue Code of 1986, as amended. The number of shares of common stock initially reserved for issuance under the ESPP was 200,000 shares. The ESPP provides for an annual increase on the first day of each year beginning in 2019 and ending in 2028, in each case subject to the approval of the board of directors, equal to the lesser of (i) 1% of the shares of common stock outstanding on the last day of the prior fiscal year or (ii) 375,000 shares; provided, that prior to the date of any such increase, the board of directors may determine that such increase will be less than the amount set forth in clauses (i) and (ii). As of September 30, 2018, no shares of common stock had been issued under the ESPP and 200,000 shares remained available for future issuance under the ESPP. The option price per share of common stock to be paid by a participant upon exercise of the participant’s option on the applicable exercise date for an offering period shall be equal to 85% of the lesser of the fair market value of a share of common stock on (a) the applicable grant date or (b) the applicable exercise date. The Board authorized an initial offering beginning on November 16, 2018 and ending on May 15, 2019. Following the end of the initial offering, a new offering will automatically begin on the day that immediately follows the conclusion of the preceding offering.

Stock Option Activity

The following table summarizes activity under the Company’s stock option plan and related information (in thousands, except share and per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at December 31, 2017		1,213,010		$	1.54		8.7	$	999
Options granted		1,111,901		$	5.77
Options exercised		(136,328	)	$	1.56			$	237
Options cancelled		(10,917	)	$	0.90
Outstanding at September 30, 2018		2,177,666		$	3.70		8.7	$	38,559
Vested and exercisable at September 30, 2018		747,477		$	2.02		7.9	$	14,493

The weighted average grant date fair value of options granted during the nine months ended September 30, 2018 was $4.12 per share. The 2015 Plan allows for early exercisable option grants, which permit the grantee to exercise a stock option in exchange for stock before the requisite service is provided (e.g., before the award is vested under its original terms); however, such arrangements permit the Company to subsequently repurchase such shares at the exercise price if the vesting conditions are not satisfied. To date, the Company has made such grants only to non-employee board members. The total intrinsic value of exercised stock options during the three and nine months ended September 30, 2018 was $0 and $237,000, respectively. The aggregate intrinsic value is calculated as the difference between the exercise price and the estimated fair value of the Company’s common stock at the date of exercise.

Early Exercise Stock Purchase Agreements

As of September 30, 2018 and December 31, 2017, there were 92,466 and 45,224, respectively, of nonvested common shares outstanding that were exercised early and subject to repurchase by the Company at the original issuance price upon termination of the stockholder’s services. The right to repurchase these shares generally lapses with respect to 25% of the shares underlying the option after the applicable vesting commencement date and 1/48 of the shares underlying the original grant per month for 36 months thereafter. The shares purchased pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be issued until those shares vest. The cash received in exchange for exercised and nonvested shares related to stock options granted is recorded as a liability for the early exercise of stock options on the balance sheets with the corresponding par value in common stock and an offset in additional paid-in capital. As of September 30, 2018 and December 31, 2017, the Company recorded in other current liabilities $152,000 and $18,000, respectively, associated with shares issued upon the early exercise of stock options that are subject to repurchase.

Restricted Stock Units Granted to Employees

There were no RSUs granted during the three months ended September 30, 2018. During the nine months ended September 30, 2018, the Company granted RSUs to certain employees to receive 10,189 shares of common stock pursuant to the 2018 Plan with a weighted-average estimated grant-date fair value of $17.75 per share. These RSUs were fully vested on the grant date. The valuation for these RSUs totaled $181,000 and was recognized as stock-based compensation expense in June 2018. There were no RSUs granted during the three and nine months ended September 30, 2017.

Stock Options Granted to Employees That Contain Performance Condition

There were no options granted that contained performance condition during the three months ended September 30, 2018. During nine months ended September 30, 2018, the Company granted options to two of its executive officers to purchase an aggregate 115,657 shares of common stock that vested fully upon the closing of the Company’s IPO on June 25, 2018. The aggregate fair value of these options was estimated at $474,000 and was recognized as stock-based compensation expense in June 2018.

Restricted Stock

In addition to the nonvested common shares outstanding described above at “Early Exercise Stock Purchase Agreements,” the Company issued restricted stock to its founders. The fair value of restricted stock on the issuance date is deemed equal to the cash consideration paid by the founders. Restricted stock vests over a four-year period from the applicable vesting commencement date. The following summarizes the activity of nonvested restricted stock:

	Number of Shares
Nonvested—December 31, 2017		193,394
Vested		(156,806	)
Nonvested—September 30, 2018		36,588

Stock-Based Compensation Expense

Total stock-based compensation recognized by function was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
General and administrative	$	247	$	19	$	905	$	60
Research and development		183		17		639		52
Total stock-based compensation expense	$	430	$	36	$	1,544	$	112

As of September 30, 2018, the unrecognized stock-based compensation cost and the estimated weighted average amortization period, using the straight-line attribution method, was as follows (dollars in thousands):

	Unrecognized Compensation Cost		Weighted Average Remaining Amortization Period (Years)
Employee options	$	4,236		2.9
Nonemployee options		6		0.7
Total unrecognized stock-based compensation expense	$	4,242

The fair value of the employee stock options granted is calculated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Expected term (years)		—		—	5.5 - 6.1			6.1
Expected volatility		—		—	82.0 - 82.8%			82.5	%
Risk-free interest rate		—		—	2.6 - 2.8%			2.1	%
Expected dividend yield		—		—		—		—

There were no stock options granted during the three months ended September 30, 2018 and 2017.

Net Loss per Share

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

8. Net Loss Per Share

Net Loss Per Share

The following table sets forth the calculation of basic and diluted net loss per share during the periods presented (in thousands, except share and per share data):

	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(5,663	)	$	(2,080	)	$	(17,385	)	$	(6,217	)
Denominator:
Weighted-average shares of common stock outstanding		18,955,384			628,435			7,319,012			569,417
Net loss per share, basic and diluted	$	(0.30	)	$	(3.31	)	$	(2.38	)	$	(10.92	)

The following outstanding shares of common stock equivalents were excluded from the computation of the diluted net loss per share for the periods presented because their effect would have been anti-dilutive:

	Three and Nine Months Ended September 30,
	2018		2017
Redeemable convertible preferred stock on an if converted basis		—		12,263,126
Stock options to purchase common stock		2,177,666		734,663
Common stock subject to future vesting		129,054		297,902
Total		2,306,720		13,295,691

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2018

Accounting Policies [Abstract]

Basis of presentation and consolidation

Basis of Presentation and Consolidation

Unaudited Interim Condensed Consolidated Financial Statements

Use of Estimates

Cash and Cash Equivalents and Restricted Cash

Restricted cash consisted of deposits at the bank held as collateral for the Company’s credit card program. The collateral requirement was removed and released in May 2018.

	September 30, 2018		December 31, 2017
Cash and cash equivalents	$	39,594	$	51,033
Restricted cash		—		13
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	$	39,594	$	51,046

Marketable Securities

Accounting Pronouncements Adopted in 2018 and New Accounting Pronouncements Not Yet Adopted

Accounting Pronouncements Adopted in 2018

New Accounting Pronouncements Not Yet Adopted

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

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

	September 30, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Assets
Level 1:
Cash		691		—		—			691
Money market funds		13,072		—		—			13,072
U.S. Treasury securities		53,597		—		(18	)		53,579
Subtotal		67,360		—		(18	)		67,342
Level 2:
Commercial paper		13,712		—		(1	)		13,711
Corporate debt securities		6,571		—		(1	)		6,570
Government agency bonds		25,137		—		(5	)		25,132
Subtotal		45,420		—		(7	)		45,413
Total	$	112,780	$	—	$	(25	)	$	112,755
Reported as:
Cash and cash equivalents								$	39,594
Marketable securities								$	73,161

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Assets
Level 1:
Cash		804		—		—		804
Money market funds		50,229		—		—		50,229
Restricted cash		13		—		—		13
Subtotal		51,046		—		—		51,046
Total	$	51,046	$	—	$	—	$	51,046
Reported as:
Cash and cash equivalents							$	51,033
Restricted cash							$	13

Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities by Contractual Maturity

As of September 30, 2018, the amortized cost and estimated fair value of the Company’s available-for-sale securities by contractual maturity are shown below (in thousands):

	Amortized		Estimated
	Cost		Fair Value
Marketable securities maturing:
In one year or less	$	73,184	$	73,161
Total marketable securities	$	73,184	$	73,161

Balance Sheet Components (Tables)

9 Months Ended

Sep. 30, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Property and Equipment

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

	September 30, 2018			December 31, 2017
Leasehold improvements	$	3,268		$	155
Furniture, laboratory and office equipment		1,984			1,242
Computer equipment		161			34
Construction in progress		—			464
Total property and equipment		5,413			1,895
Less accumulated depreciation and amortization		(753	)		(355	)
Property and equipment, net	$	4,660		$	1,540

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	September 30, 2018		December 31, 2017
Accrued preclinical and research costs	$	1,075	$	108
Accrued clinical costs		403		340
Accrued employee-related costs		722		422
Accrued professional services		74		—
Other		71		41
Accrued liabilities	$	2,345	$	911

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity under Stock Option Plan and Related Information

The following table summarizes activity under the Company’s stock option plan and related information (in thousands, except share and per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at December 31, 2017		1,213,010		$	1.54		8.7	$	999
Options granted		1,111,901		$	5.77
Options exercised		(136,328	)	$	1.56			$	237
Options cancelled		(10,917	)	$	0.90
Outstanding at September 30, 2018		2,177,666		$	3.70		8.7	$	38,559
Vested and exercisable at September 30, 2018		747,477		$	2.02		7.9	$	14,493

Summarizes of Activity of Nonvested Restricted Stock

The following summarizes the activity of nonvested restricted stock:

	Number of Shares
Nonvested—December 31, 2017		193,394
Vested		(156,806	)
Nonvested—September 30, 2018		36,588

Stock-Based Compensation Recognized

Total stock-based compensation recognized by function was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
General and administrative	$	247	$	19	$	905	$	60
Research and development		183		17		639		52
Total stock-based compensation expense	$	430	$	36	$	1,544	$	112

Unrecognized Stock-based Compensation Cost and Estimated Weighted Average Amortization Period

	Unrecognized Compensation Cost		Weighted Average Remaining Amortization Period (Years)
Employee options	$	4,236		2.9
Nonemployee options		6		0.7
Total unrecognized stock-based compensation expense	$	4,242

Fair Value of Employee Stock Options Granted is Calculated Using Black Scholes Option Pricing Model with Weighted Average Assumptions

The fair value of the employee stock options granted is calculated using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Expected term (years)		—		—	5.5 - 6.1			6.1
Expected volatility		—		—	82.0 - 82.8%			82.5	%
Risk-free interest rate		—		—	2.6 - 2.8%			2.1	%
Expected dividend yield		—		—		—		—

Net Loss per Share (Tables)

9 Months Ended

Sep. 30, 2018

Earnings Per Share [Abstract]

Calculation of Basic and Diluted Net Loss per Share

The following table sets forth the calculation of basic and diluted net loss per share during the periods presented (in thousands, except share and per share data):

	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(5,663	)	$	(2,080	)	$	(17,385	)	$	(6,217	)
Denominator:
Weighted-average shares of common stock outstanding		18,955,384			628,435			7,319,012			569,417
Net loss per share, basic and diluted	$	(0.30	)	$	(3.31	)	$	(2.38	)	$	(10.92	)

Anti-dilutive Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share

	Three and Nine Months Ended September 30,
	2018		2017
Redeemable convertible preferred stock on an if converted basis		—		12,263,126
Stock options to purchase common stock		2,177,666		734,663
Common stock subject to future vesting		129,054		297,902
Total		2,306,720		13,295,691

Organization and Description of the Business - Additional Information (Details) $ / shares in Units, $ in Thousands			9 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Jun. 08, 2018 $ / shares	Sep. 30, 2018 USD ($) Segment shares	Dec. 31, 2017 USD ($) shares
Organization And Description Of Business [Line Items]
Number of operating segments \| Segment			1
Stockholders equity, reverse stock split			1-for-5.62
Stockholders equity, stock split, conversion ratio			0.1779
Minimum price post-split \| $ / shares		$ 15.884
Adjusted price post split \| $ / shares		$ 7.942
Net proceeds from issuance of common stock \| $			$ 77,695
Common stock, shares authorized			125,000,000	96,000,000
Preferred stock, shares authorized			10,000,000	0
Accumulated deficit \| $			$ 43,413	$ 26,028
Minimum
Organization And Description Of Business [Line Items]
Sufficient cash and cash equivalents available period term			12 months
IPO
Organization And Description Of Business [Line Items]
Common stock, shares, issued	5,750,000
Common stock, shares issued, price per share \| $ / shares	$ 15.00
Net proceeds from issuance of common stock \| $	$ 77,700
Payments of stock issuance costs \| $	$ 8,600
Convertible preferred stock converted into common stock	12,263,126
Common stock, shares authorized	125,000,000
Preferred stock, shares authorized	10,000,000
Overallotment Option
Organization And Description Of Business [Line Items]
Common stock, shares, issued	750,000

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured and Recognized at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 73,184
Fair Value	73,161
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	112,780	$ 51,046
Unrealized Losses	(25)
Fair Value	112,755	51,046
Cash and cash equivalents	39,594	51,033
Marketable securities	73,161
Restricted cash		13
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	67,360	51,046
Unrealized Losses	(18)
Fair Value	67,342	51,046
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	691	804
Fair Value	691	804
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	13,072	50,229
Fair Value	13,072	50,229
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	53,597
Unrealized Losses	(18)
Fair Value	53,579
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1 \| Restricted Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		13
Fair Value		$ 13
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	45,420
Unrealized Losses	(7)
Fair Value	45,413
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	13,712
Unrealized Losses	(1)
Fair Value	13,711
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	6,571
Unrealized Losses	(1)
Fair Value	6,570
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Government agency bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	25,137
Unrealized Losses	(5)
Fair Value	$ 25,132

Stock-Based Compensation - Additional Information (Details)		3 Months Ended		9 Months Ended
	Jun. 25, 2018 shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) ExecutiveOfficer $ / shares shares	Sep. 30, 2017 USD ($) shares	Dec. 31, 2017 USD ($) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options granted				1,111,901
Maximum number of shares issued upon exercise of incentive stock options		747,477		747,477
Common stock, shares issued		19,108,221		19,108,221		948,578
Weighted average grant date fair value of options granted \| $ / shares				$ 5.77
Total intrinsic value of exercised stock options \| $				$ 237,000
Stock-based compensation expense recognized \| $		$ 430,000	$ 36,000	$ 1,544,000	$ 112,000
Stock options granted \| $		$ 0	$ 0
Early Exercise Stock Purchase Agreements
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period				36 months
Nonvested common shares outstanding		92,466		92,466		45,224
Percentage of underlying option shares whose right to purchase lapses after applicable vesting commencement date				25.00%
Right to repurchase shares underlying shares				0.0208
Other Current Liabilities \| Early Exercise Stock Purchase Agreements
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued upon early exercise of stock options are subject to repurchase \| $		$ 152,000		$ 152,000		$ 18,000
Stock Options Granted to Employees that Contain Performance Condition
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options granted		0		115,657
Stock-based compensation expense recognized \| $				$ 474,000
Number of executive officers granted with purchase option \| ExecutiveOfficer				2
Number of option vested	115,657
Options vesting date				Jun. 25, 2018
Stock-based compensation expense recognition period				2018-06
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period				4 years
Nonvested common shares outstanding		36,588		36,588		193,394
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future issuance		1,812,145		1,812,145
Number of shares will increase automatically through every year on specified date				--01-01
Number of shares increase automatically continuing through maximum period				beginning on January 1, 2019 and continuing through and including January 1, 2028
2018 Equity Incentive Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares available for issuance		1,600,692		1,600,692
Increase in number of shares reserved for issuance as percentage of total number of shares of capital stock outstanding on last date of preceding calendar year				5.00%
2018 Equity Incentive Plan \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares available for issuance		4,000,000		4,000,000
2018 Equity Incentive Plan \| Employee options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options granted				75,621
2018 Equity Incentive Plan \| Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSU granted		0	0	10,189	0
Weighted-average estimated grant-date fair value \| $ / shares				$ 17.75
Stock-based compensation expense recognized \| $				$ 181,000
2018 Equity Incentive Plan \| Incentive Stock Options \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum number of shares issued upon exercise of incentive stock options		12,500,000		12,500,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period				4 years
Options to purchase of common stock outstanding		2,102,045		2,102,045
Weighted average grant date fair value of options granted \| $ / shares				$ 4.12
Total intrinsic value of exercised stock options \| $		$ 0		$ 237,000
2015 Equity Incentive Plan \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted expiry period				10 years
2018 Employee Stock Purchase Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future issuance		200,000		200,000
Common stock, shares issued		0		0
Authorized an offering beginning period				Nov. 16, 2018
Authorized an offering ending period				May 15, 2019
2018 Employee Stock Purchase Plan \| Maximum [Member] \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual increase in number of common stock shares				1.00%
Annual increase in number of common stock shares				375,000
Option price per share of common stock at fair market value				85.00%

Stock-Based Compensation - Summary of Activity under Stock Option Plan and Related Information (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of options outstanding, Outstanding, Beginning balance \| shares	1,213,010
Number of options outstanding, Options granted \| shares	1,111,901
Number of options outstanding, Options exercised \| shares	(136,328)
Number of options outstanding, Options cancelled \| shares	(10,917)
Number of options outstanding, Outstanding, Ending balance \| shares	2,177,666	1,213,010
Number of options outstanding, options vested and exercisable \| shares	747,477
Weighted average exercise price, Outstanding, Beginning balance \| $ / shares	$ 1.54
Weighted average exercise price, Options granted \| $ / shares	5.77
Weighted average exercise price, Options exercised \| $ / shares	1.56
Weighted average exercise price, Options cancelled \| $ / shares	0.90
Weighted average exercise price, Outstanding, Ending balance \| $ / shares	3.70	$ 1.54
Weighted average exercise price, options vested and exercisable \| $ / shares	$ 2.02
Weighted average remaining contractual term, Outstanding, Beginning balance	8 years 8 months 12 days	8 years 8 months 12 days
Weighted average remaining contractual term,options vested and exercisable	7 years 10 months 24 days
Aggregate intrinsic value, Outstanding, Beginning balance \| $	$ 999
Aggregate intrinsic value, Options exercised \| $	237
Aggregate intrinsic value, Outstanding, Ending balance \| $	38,559	$ 999
Aggregate intrinsic value, options vested and exercisable \| $	$ 14,493

Stock-Based Compensation - Fair Value of Employee Stock Options Granted is Calculated Using Black Scholes Option Pricing Model with Weighted Average Assumptions (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)		6 years 1 month 6 days		6 years 1 month 6 days
Expected volatility				82.50%
Expected volatility, Minimum			82.00%
Expected volatility, Maximum			82.80%
Risk-free interest rate				2.10%
Risk-free interest rate, Minimum			2.60%
Risk-free interest rate, Maximum			2.80%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 6 months		5 years 6 months
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 1 month 6 days		6 years 1 month 6 days

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	7. Income Taxes For the three and nine months ended September 30, 2018 and 2017, respectively, the Company did not record an income tax provision. The U.S. federal deferred tax assets generated from the Company’s net operating losses have been fully reserved, as the Company believes it is not more likely than not that the benefit will be realized. The Tax Cuts and Jobs Act of 2017 (the “Tax Act”) contains tax law changes that are effective in tax years 2018 and onward. The Company believes that these changes may alter the amount of tax loss generated, but does not believe that the changes will create a tax liability in 2018. In December 2015, the Protecting Americans from Tax Hikes (PATH) Act of 2015 was signed into law, which created several new research and development (“R&D”) tax credit provisions, including allowing qualified small businesses to utilize the R&D credit against the employer’s portion of payroll tax up to a maximum of $250,000 per year. The Company qualified as a small business under PATH for both 2016 and 2017. During the three and nine months ended September 30, 2018, the Company has utilized $152,000 of R&D tax credits as a reduction of research and development costs to offset its payroll tax liabilities. The remaining R&D tax credits available for future payroll tax liabilities have been recorded as deferred tax assets and a full valuation allowance.

Fair Value Measurements - Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Marketable securities maturing, Amortized Cost:
In one year or less, Amortized Cost	$ 73,184
Total marketable securities, Amortized Cost	73,184
Marketable securities maturing, Estimated Fair Value:
In one year or less, Estimated Fair Value	73,161
Total marketable securities, Estimated Fair Value	$ 73,161

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	0	75,533,240
Shares Issued and Outstanding		12,263,126
Net Proceeds After Issuance Costs		$ 77,931
Liquidation Preference		$ 78,369
Series A
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		33,533,240
Shares Issued and Outstanding		5,966,753
Net Proceeds After Issuance Costs		$ 28,176
Liquidation Preference		$ 28,369
Series B
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		42,000,000
Shares Issued and Outstanding		6,296,373
Net Proceeds After Issuance Costs		$ 49,755
Liquidation Preference		$ 50,000

Redeemable Convertible Preferred Stock - Additional Information (Details)	1 Months Ended
	Jun. 30, 2018 shares
IPO
Temporary Equity [Line Items]
Number of outstanding shares of convertible preferred stock converted into common stock	12,263,126

Stock-Based Compensation - Summarizes of Activity of Nonvested Restricted Stock (Details) - Restricted Stock	9 Months Ended
	Sep. 30, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Nonvested beginning balance	193,394
Number of Shares Outstanding, Vested	(156,806)
Nonvested ending balance	36,588

Fair Value Measurements - Additional Information (Detail)	Sep. 30, 2018 USD ($)
Fair Value Disclosures [Abstract]
Transfers from Level 1 to Level 2	$ 0
Transfers from Level 2 to Level 1	$ 0

Balance Sheet Components - Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 5,413	$ 1,895
Less accumulated depreciation and amortization	(753)	(355)
Property and equipment, net	4,660	1,540
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	3,268	155
Furniture, Laboratory and Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	1,984	1,242
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 161	34
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment		$ 464

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accrued preclinical and research costs	$ 1,075	$ 108
Accrued clinical costs	403	340
Accrued employee-related costs	722	422
Accrued professional services	74
Other	71	41
Accrued liabilities	$ 2,345	$ 911

Stock-Based Compensation - Unrecognized Stock-based Compensation Cost and Estimated Weighted Average Amortization Period (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation cost	$ 4,242
Employee options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation cost	$ 4,236
Weighted average remaining amortization period (years)	2 years 10 months 24 days
Nonemployee options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized costs of unvested options awarded	$ 6
Weighted average remaining amortization period (years)	8 months 12 days