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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. 01, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	82-2933343
Entity Public Float			$ 548.8
Entity Address, Address Line One	131 Oyster Point Blvd
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	415
Local Phone Number	231-5660
Document Transition Report	false
Document Annual Report	true
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		78,982,116
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Incorporation, State or Country Code	DE
Documents Incorporated by Reference	Portions of the registrant’s Definitive Proxy Statement relating to the registrant’s Annual Meeting of Shareholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 89,641	$ 65,470
Marketable securities	263,432	224,938
Prepaid expenses and other current assets	4,364	2,768
Total current assets	357,437	293,176
Property and equipment, net	33,852	10,937
Operating lease right-of-use assets	28,476
Restricted cash	1,472	1,472
Other assets	676	2,774
Total assets	421,913	308,359
Current liabilities:
Accounts payable	278	126
Accrued clinical supply costs	6,706	4,463
Accrued liabilities	18,256	8,439
Deferred revenue, current portion	30,165	34,905
Deferred rent, current portion		15
Operating lease liabilities, current portion	6,565
Total current liabilities	61,970	47,948
Deferred revenue, long-term portion	123,236	139,715
Deferred rent, long-term portion		7,478
Operating lease liabilities, long-term portion	41,471
Other long-term liabilities	493	96
Total liabilities	227,170	195,237
Commitments and contingencies (Note 5)
Convertible preferred stock; $0.0001 par value; zero and 45,849,677 shares authorized as of December 31, 2019 and 2018, respectively; zero and 45,374,836 shares issued and outstanding as of December 31, 2019 and 2018, respectively		210,520
Stockholders' equity (deficit):
Common stock, $0.0001 par value; 200,000,000 and 65,000,000 shares authorized as of December 31, 2019 and 2018, respectively; 69,052,873 and 13,764,829 shares issued and outstanding as of December 31, 2019 and 2018, respectively	7	1
Additional paid-in capital	414,414	17,078
Accumulated other comprehensive income (loss)	142	(42)
Accumulated deficit	(219,820)	(114,435)
Total stockholders' equity (deficit)	194,743	(97,398)
Total liabilities, convertible preferred stock, and stockholders' equity (deficit)	$ 421,913	$ 308,359

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	45,849,677
Convertible preferred stock, shares issued	0	45,374,836
Convertible preferred stock, shares outstanding	0	45,374,836
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	65,000,000
Common stock, shares issued	69,052,873	13,764,829
Common stock, shares outstanding	69,052,873	13,764,829

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Total revenue	$ 21,219	$ 27,508	$ 2,872
Type of Revenue [Extensible List]	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Grant revenue	$ 0	$ 169	$ 863
Total revenue	21,219	27,677	3,735
Operating expenses:
Research and development	100,528	73,031	29,911
General and administrative	35,095	11,934	6,503
Total operating expenses	135,623	84,965	36,414
Loss from operations	(114,404)	(57,288)	(32,679)
Other income, net	9,019	5,040	199
Net loss	(105,385)	(52,248)	(32,480)
Unrealized gain (loss) on marketable securities	184	(42)
Comprehensive loss	$ (105,201)	$ (52,290)	$ (32,480)
Net loss per share, basic and diluted	$ (1.71)	$ (4.62)	$ (3.55)
Shares used in computing net loss per share, basic and diluted	61,734,492	11,302,788	9,142,688

Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Preferred Units	Convertible Preferred Stock	Common Units	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ (24,907)					$ 4,800		$ (29,707)
Preferred units, beginning balance (in shares) at Dec. 31, 2016		36,001,203
Preferred units, beginning balance at Dec. 31, 2016		$ 77,485
Common units, beginning balance (in shares) at Dec. 31, 2016				11,517,585
Issuance of common units				2,483,500
Cancellation of common units				(8,333)
Conversion from LLC to corporation		$ (77,485)	$ 77,485		$ 1	(1)
Conversion from llc to corporation (units)		(36,001,203)		(13,992,752)
Conversion from llc to corporation (in shares)			36,001,203		13,825,387
Forfeiture/Cancellation of restricted common stock					(49,234)
Stock-based compensation	5,354					5,354
Net loss and comprehensive loss	(32,480)							(32,480)
Ending Balance at Dec. 31, 2017	(52,033)				$ 1	10,153		(62,187)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2017			36,001,203
Convertible preferred stock, ending balance at Dec. 31, 2017			$ 77,485
Ending balance (in shares) at Dec. 31, 2017					13,776,153
Issuance of Series E convertible preferred stock, net of issuance costs			$ 133,035
Issuance of Series E convertible preferred stock, net of costs (in shares)			9,373,633
Forfeiture/Cancellation of restricted common stock					(11,324)
Stock-based compensation	6,925					6,925
Unrealized gain (loss) on marketable securities	(42)						$ (42)
Net loss and comprehensive loss	(52,248)							(52,248)
Ending Balance at Dec. 31, 2018	$ (97,398)				$ 1	17,078	(42)	(114,435)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2018	45,374,836		45,374,836
Convertible preferred stock, ending balance at Dec. 31, 2018	$ 210,520		$ 210,520
Ending balance (in shares) at Dec. 31, 2018	13,764,829				13,764,829
Conversion of convertible preferred stock into common stock	$ 210,521		$ (210,520)		$ 5	210,516
Conversion of convertible preferred stock into common stock (in shares)			(45,374,836)		45,374,836
Issuance of common stock upon initial public offering, net of issuance costs	168,223				$ 1	168,222
Issuance of common stock upon initial public offering, net of issuance costs (in shares)					9,739,541
Exercise of stock options	1,519					1,519
Exercise of stock options (in shares)					180,287
Purchase of common stock under employee stock purchase plan	798					798
Purchase of common stock under employee stock purchase plan (in shares)					50,353
Forfeiture/Cancellation of restricted common stock					(56,973)
Stock-based compensation	16,281					16,281
Unrealized gain (loss) on marketable securities	184						184
Net loss and comprehensive loss	(105,385)							(105,385)
Ending Balance at Dec. 31, 2019	$ 194,743				$ 7	$ 414,414	$ 142	$ (219,820)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2019	0
Ending balance (in shares) at Dec. 31, 2019	69,052,873				69,052,873

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (105,385)	$ (52,248)	$ (32,480)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,775	1,019	680
Stock-based compensation	16,281	6,925	5,354
Accretion of discount on marketable securities	(4,701)	(2,745)
Amortization of right-of-use assets	1,799
Impairment loss on right-of-use asset	1,158
Loss from disposal of property and equipment, net	39	151
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,596)	(2,245)	(170)
Other assets	(42)	106
Accounts payable	152	(809)	(41)
Accrued liabilities and accrued clinical supply costs	9,210	4,743	6,716
Deferred revenue	(21,219)	172,492	2,128
Deferred rent	(44)	(7)	28
Lease liabilities	868
Other long-term liabilities	397	82	14
Net cash provided by (used in) operating activities	(99,308)	127,464	(17,771)
Cash flows from investing activities:
Purchase of property and equipment	(15,265)	(1,884)	(801)
Purchase of marketable securities	(529,609)	(472,235)
Maturities of marketable securities	496,000	250,000
Net cash used in investing activities	(48,874)	(224,119)	(801)
Cash flows from financing activities:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	170,128	(1,904)
Payment of deferred offering costs	(92)
Proceeds from issuance of convertible preferred stock, net of issuance costs		133,050	(15)
Proceeds from the exercise of options to purchase common stock	1,519
Proceeds from issuance of stock from employee stock purchase plan	798
Net cash provided by (used in) financing activities	172,353	131,146	(15)
Net increase (decrease) in cash, cash equivalents, and restricted cash	24,171	34,491	(18,587)
Cash, cash equivalents, and restricted cash at beginning of period	66,942	32,451	51,038
Cash, cash equivalents, and restricted cash at end of period	91,113	66,942	32,451
Non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	3,471	293	353
Issuance costs for convertible preferred stock included in accounts payable			$ 53
Deferred offering costs in accrued liabilities	465	792
Tenant improvements paid by landlord	$ 8,286	$ 7,449

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 consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States (“GAAP”) as defined by the Financial Accounting Standards Board (“FASB”). The consolidated financial statements include the accounts of Alector, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of consolidated financial statements in conformity with 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 consolidated financial statements and the reported amounts of revenue and expense during the reporting period. The Company evaluates its estimates, including those related to revenue recognition, manufacturing accruals, clinical accruals, fair value of assets and liabilities, income taxes uncertainties, stock-based compensation, and related assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, and short-term marketable securities. Cash and cash equivalents are deposited in checking and sweep accounts at a financial institution. Such deposits may, at times, exceed federally insured limits.

Cash, Cash Equivalents, and Restricted Cash

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash and cash equivalents. Cash equivalents, which consist of amounts invested in money market funds, are stated at fair value. There are no unrealized gains or losses on the money market funds for the periods presented.

Restricted cash as of December 31, 2019 relates to a letter of credit established for a lease entered into in June 2018.

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

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Cash and cash equivalents	$	89,641	$	65,470	$	32,451
Restricted cash		1,472		1,472		—
Total cash, cash equivalents, and restricted cash	$	91,113	$	66,942	$	32,451

Marketable Securities

All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. The Company considers its available-for-sale portfolio as available for use in current operations. Accordingly, the Company may classify certain investments as short-term marketable securities,

even though the stated maturity date may be one year or more beyond the current balance sheet date. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive income and reported as a separate component of stockholders’ equity (deficit) until realized. Interest income, realized gains and losses, and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income, net. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. In accordance with our investment policy, management invests in money market funds, U.S. treasury securities, corporate bonds, commercial paper, and overnight repurchase agreements collateralized by U.S. treasury securities. The Company has not experienced any losses on its deposits of cash, cash equivalents, and marketable securities.

The Company periodically evaluates whether declines in the fair value of our investments below their cost are other than temporary. This evaluation consists of qualitative and quantitative factors regarding the severity and duration of the unrealized loss, as well as our ability and intent to hold the marketable securities until a forecasted recovery occurs. If it is determined that the decline in fair value of an investment is below its accounting basis and this decline is other than temporary, the Company would reduce the carrying value of the security and record a loss for the amount of such decline. The Company has not recorded any realized losses or declines in value judged to be other than temporary on any investments.

Fair Value of Financial Instruments

The Company’s believes the carrying value of financial instruments, including cash and cash equivalents, receivables, accounts payable, and accrued liabilities, approximate fair value due to their short-term nature.

The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels:

Level 1 – Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities 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 related assets or liabilities; and

Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

Property and Equipment

Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the lesser of their useful lives or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statements of operations in the period realized. Maintenance and repairs are charged to the consolidated statements of operations as incurred.

Leases

The Company adopted Accounting Standards Update No. 2016-02, Leases on January 1, 2019. Upon adoption, the Company recorded a right-of-use asset and a lease liability for all leases with a term of greater than 12 months. The Company adopted the guidance on a modified retrospective basis and did not restate comparative periods. The Company elected to adopt the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows the historical lease classification of existing leases be carried forward. As a result of the adoption, the Company recorded operating right-of-use assets of $31.4 million and lease liabilities of $38.9 million on January 1, 2019. The prior deferred rent balances of $7.5 million under legacy guidance, primarily related to a tenant improvement allowance, was derecognized. The lease liabilities balance increased through December 31, 2019 due to additional tenant improvement allowance paid by a landlord. There was no effect on accumulated deficit as a result of the adoption.

The Company determines whether the arrangement is or contains a lease at the inception of the lease. Leases are recognized on the balance sheet as right-of-use assets and lease liabilities. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and any prepaid or accrued rent. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses.

The Company elected to exclude balance sheet recognition of operating leases having a term of 12 months or less (“short-term leases”) and elected to not separate lease components and non-lease components for its long-term leases.

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 comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If the total of the undiscounted future cash flows is less than the carrying amount of the assets, an impairment loss is recognized for the amount by which the carrying amount of the assets exceeds is fair value. In June 2019, the Company recognized a $1.2 million impairment loss on its right-of-use asset related to the portion of its headquarters being subleased.

Revenue Recognition

The Company recognizes revenue when control of promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under arrangements, the Company performs the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies the performance obligation.

Collaboration Revenue

The Company signed an agreement in October 2017 with AbbVie Biotechnology, Ltd. (“AbbVie”) to co-develop antibodies to two program targets in preclinical development (“AbbVie Agreement”). Under the terms of the AbbVie Agreement, AbbVie made $205.0 million in upfront payments, of which $5.0 million and $200.0 million was received by the Company in October 2017 and January 2018, respectively. The Company will perform research and development services for the antibodies to the two programs through the end of Phase 2 clinical trials which the Company expects to conduct through 2023. AbbVie will then have the exclusive right to exercise an option to enter into a license and collaboration agreement with the Company for one or both of the programs for $250.0 million each. If AbbVie exercises its option for the programs, AbbVie will take over the development of the product candidates for such program and costs will be split between the parties. The Company will also share in profits and losses upon commercialization of any products from such program. However, following AbbVie’s exercise of its option for a program, the Company may opt out of sharing in development costs and profits or losses for that program and instead receive tiered royalties. Additionally, under the terms of the AbbVie Agreement, the Company will be eligible to earn up to an additional $242.8 million in milestone payments per program related to the initiation of certain clinical studies and regulatory approval for up to three indications per program. The Company assessed its collaboration agreement with AbbVie in the context of the delivery of the research and development services.

The Company has determined that there are two research and development performance obligations as part of the agreement with AbbVie, one research and development performance obligation for each of the two research and development programs. The non-refundable upfront cash payments of $5.0 million and $200.0 million received in October 2017 and January 2018, respectively, were included in the transaction price. None of the remaining development and regulatory milestone and program opt-in payment amounts have been included in the transaction price, as all these amounts were fully constrained as of December 31, 2019. As part of the Company’s evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestone amounts is outside the control of the Company and contingent upon success in future clinical trials. Any consideration related to royalties on net product sales will be recognized when the related sales occur and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur.

The Company recognizes collaboration revenue by measuring the progress toward complete satisfaction of the performance obligation using an input measure. In order to recognize revenue over the research and development period, the Company measures actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligation. Revenues are recognized as the program costs are incurred. The Company will re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Clinical trials are expensive and can take many years to complete, and the outcome is inherently uncertain. Changes in the Company’s forecasted costs are likely to occur over time based upon changes in clinical trial procedures set forth in protocols, changes in estimates of manufacturing costs, or feedback from regulators on the design or operation of clinical trials. The Company has had changes to the overall expected costs to satisfy the performance obligations from period to period. For the year ended December 31, 2019, the Company had a 12% increase in the forecast of total expected costs due to changes to the clinical trial plans. For the year ended December 31, 2019, the increase in the overall expected costs to satisfy the performance obligation resulted in an approximately $6 million reduction in revenue compared to if the expected costs had remained the same, as a result of the cumulative catch up for the change in estimate. Collaboration revenue under the Company’s collaboration agreement with AbbVie during 2019, 2018, and 2017 was $21.2 million, $27.5 million, and $2.9 million, respectively. Collaboration revenue recognized during the year ended December 31, 2019, was included in deferred revenue at the beginning of the period. The Company recorded deferred revenue of $153.4 million as of December 31, 2019. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of Phase 2 clinical trials.

Grant Revenue

The Company has grant revenue from the U.S. government. In March 2016, the National Institute on Aging, a division of the National Institute of Health of the U.S. government, awarded the Company a Small Business Innovation Research grant. The Company recognized zero, $0.2 million and $0.9 million for the years ended December 31, 2019, 2018, and 2017, respectively, related to research performed under the grant. The Company recognizes grant revenue as the related research services are performed.

Research and Development Costs

Research and development costs are expensed as incurred and consist primarily of new product development. Research and development costs include salaries and benefits, consultants’ fees, process development costs, stock-based compensation, and laboratory supplies, as well as fees paid to third parties that conduct certain research and development activities on the Company’s behalf. In addition, research and development costs include the reimbursable costs incurred for the grant agreements, which includes payroll costs for time incurred on projects, laboratory supplies, and third-party research and development activities.

A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers. The Company records accrued expenses for estimated preclinical study and clinical trial expenses. Estimates are based on the services performed pursuant to contracts with research institutions, contract research organizations in connection with clinical studies, investigative sites in connection with clinical studies, vendors in connection with preclinical development activities, and contract manufacturing organizations in connection with the production of materials for clinical trials. Further, the Company accrues expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. The Company monitors patient enrollment levels and related activity to the extent reasonably possible and make judgments and estimates in determining the accrued balance in each reporting period. If the Company underestimates or overestimates the level of services performed or the costs of these services, actual expenses could differ from estimates. To date, the Company has not experienced significant changes in its estimates of preclinical studies and clinical trial accruals.

Stock-based Compensation

Stock-based compensation for employee awards is measured on the grant date based on the fair value of the award and recognized on a straight-line basis over the requisite service period. The fair value of options to purchase common stock are measured using the Black-Scholes option-pricing model. The Company accounts for forfeitures as they occur.

The Company adopted Accounting Standards Update (“ASU”) No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”) in June 2018. The new standard simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees. The Company early adopted ASU 2018-07 effective July 1, 2018. The early adoption of this new standard did not have a material impact on the Company’s consolidated financial statements.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ equity that are the result of transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive loss was net unrealized gain (loss) on marketable securities.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statement and tax bases of assets and liabilities by using enacted tax rates in effect for the year in which the differences are expected to recover or settle. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date.

The deferred tax assets are recognized to the extent the Company believes that these assets are more likely than not to be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net deferred tax assets have been fully offset by a valuation allowance.

The Company records uncertain tax positions using a two-step process. First, the Company determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position. Second, for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority.

The Company recognizes interest and penalties related to unrecognized tax benefits on the interest expense line and other expense line, respectively, in the accompanying statements of operations. Accrued interest and penalties are included on the related liability lines in the balance sheet.

Employee 401(k) Plan

The Company has a qualified contributory savings plan under Section 401(k) of the Internal Revenue Code (the “Code”) covering substantially all U.S. employees of Alector. The 401(k) plan is designed to provide tax-deferred retirement benefits in accordance with the provisions of Section 401(k) of the Code. Eligible employees may defer up to 100% of their eligible compensation up to the annual maximum as determined by the Internal Revenue Service. The Company’s contributions to the plan are discretionary. For the years ended December 31, 2019 and 2018, the Company did not make any contributions to the plan.

Segments

The Company operates in one segment. The Company’s chief operating decision maker, its Chief Executive Officer, manages the Company’s operations on a consolidated basis for purposes of allocating resources.

Deferred Offering Costs

Offering costs, consisting of legal, accounting, printer, and filing fees related to the IPO and secondary offering were deferred and offset against proceeds upon the effectiveness of the offerings. As of December 31, 2019 and 2018, $0.6 million and $2.7 million, respectively, of deferred offering costs were recorded as other assets in the accompanying consolidated balance sheets.

Recent Accounting Pronouncements

In September 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) (“ASU 2016-13”). This ASU implements an impairment model, known as the current expected credit loss model that is based on expected losses rather than incurred losses. For available-for-sale debt securities, entities will be required to recognize an allowance for credit losses rather than an other-than-temporary impairment that reduces the cost basis of the investment. Further, an entity will recognize any changes in estimated credit losses immediately in earnings. The Company will adopt this ASU on January 1, 2020. The Company does not expect the adoption of ASU 2016-13 to have a material impact on its consolidated financial statements.

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The following tables summarize the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy:

	December 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	61,104	$	—	$	—		$	61,104
U.S. government treasury securities	Level 1		227,446		149		(8	)		227,587
Commercial paper	Level 2		43,735		—		—			43,735
Corporate bonds	Level 2		10,103		3		(2	)		10,104
Total cash equivalents and marketable securities		$	342,388	$	152	$	(10	)	$	342,530

	December 31, 2018
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	65,222	$	—	$	—		$	65,222
U.S. government treasury securities	Level 1		224,980		3		(45	)		224,938
Total cash equivalents and marketable securities		$	290,202	$	3	$	(45	)	$	290,160

The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models for which all significant inputs are observable. There were no transfers between levels of the fair value hierarchy during the year ended December 31, 2019. The Company classifies marketable securities available to fund current operations as current assets. As of December 31, 2019, the remaining contractual maturities of $311.8 million investments were less than one year and $30.7 million investments were after one year through two years. The Company does not intend to sell the investments that are currently in an unrealized loss position, and it is highly unlikely that the Company will be required to sell the investments before recovery of their amortized cost basis, which may be maturity.

- 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

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Balance Sheet Components

4.	Balance Sheet Components

Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Leasehold improvements	$	25,222		$	210
Lab equipment		10,458			4,599
Furniture and fixtures		2,031			131
Computer equipment		1,554			449
Construction-in-progress		—			7,449
Property and equipment, gross		39,265			12,838
Less accumulated depreciation and amortization		(5,413	)		(1,901	)
Total property and equipment, net	$	33,852		$	10,937

Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2019		2018
	(In thousands)
Accrued research and development costs	$	6,400	$	3,821
Accrued employee compensation		5,730		2,766
Accrued property and equipment		3,471		588
Accrued professional services		2,040		293
Other		615		971
Total accrued liabilities	$	18,256	$	8,439

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Convertible Preferred Stock

7.	Convertible Preferred Stock

The Company issued 9,373,633 shares of Series E convertible preferred stock in April 2018, July 2018, and October 2018 at an issuance price of $14.2154 per share. The 24,621 shares issued in October 2018 were to a board member, a related party. Upon the effectiveness of the IPO, all shares of convertible preferred stock automatically converted on a 1-to-1 basis into shares of common stock

The following tables summarizes the authorized, issued, and outstanding convertible preferred stock of the Company as of December 31, 2018:

	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529
Series E		9,848,474		9,373,633		14.22		133,035		133,250
Total convertible preferred stock		45,849,677		45,374,836			$	210,520	$	211,054

The Company recorded its convertible preferred stock at the issuance price on the dates of issuance, net of issuance costs.

Stock-based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8.	Stock-based Compensation

The Company recognized stock-based compensation as follows:

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Research and development	$	8,031	$	4,195	$	4,392
General and administrative		8,250		2,730		962
Total stock-based compensation	$	16,281	$	6,925	$	5,354

Determination of Fair Value

The estimated grant-date fair value of all the Company’s profit interest units and options to purchase common stock was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2019	2018	2017
Expected term (in years)	5.3 – 6.1	5.1 – 6.1	6.0 – 10.0
Expected volatility	76% – 82%	73% – 81%	75%
Risk-free interest rate	1.4% – 2.6%	2.5% – 3.1%	1.9 – 2.4%
Dividend yield	0%	0%	0%

The fair value of each profit interest unit and stock option was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management.

Expected Term—The expected term represents the period that stock-based awards are expected to be outstanding. The Company’s profit interest units did not have a contractual term. However, there is a constructive maturity of the profit interest units based on the expected exit or liquidity scenarios for the Company. The Company’s historical share option exercise is limited due to a lack of sufficient data points and did not provide a reasonable basis upon which to estimate an expected term. The expected term was derived by using the simplified method which uses the midpoint between the average vesting term and the contractual expiration period of the stock-based award.

Expected Volatility—The Company has limited information on the volatility of profit interest units and stock options as the shares were not actively traded on any public markets prior to February 7, 2019. The expected volatility was derived from the historical stock volatilities of comparable peer public companies within its industry. These companies are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards.

Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the profit interest units’ and stock options’ expected term.

Expected Dividend Rate—The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its common stock underlying its stock options in the foreseeable future.

Restricted Common Stock

The restricted common stock generally vest over a four-year period with straight-line vesting with a 25% one-year cliff.

Activity for the restricted common stock is shown below:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted common stock as of December 31, 2017		3,148,813		$	6.95
Vested		(1,219,641	)		6.95
Forfeited		(11,324	)		6.95
Unvested restricted common stock as of December 31, 2018		1,917,848			6.95
Vested		(866,037	)		6.95
Forfeited		(56,973	)		6.95
Unvested restricted common stock as of December 31, 2019		994,838			6.95

As of December 31, 2019, total unrecognized stock-based compensation related to unvested restricted common stock issued to employees was $4.5 million, which the Company expects to recognize over a remaining weighted-average period of 1.7 years.

2019 Equity Incentive Plan

On February 6, 2019, the Company adopted the 2019 Equity Incentive Plan (“2019 Plan”) under which the Board may issue incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units, and performance shares to the Company’s employees, directors, and consultants. The Company initially reserved for issuance 7,688,156 shares of common stock pursuant to the 2019 Plan. The Company’s 2017 Stock Option and Grant Plan (“2017 Plan”) was terminated; however, shares subject to awards granted under it will continue to be governed by the 2017 Plan. Shares reserved for issuance but not issued pursuant to, or not subject to, awards granted under the 2017 Plan were added to the available shares in the 2019 Plan. Shares subject to awards granted under the 2017 Plan that are repurchased by, or forfeited to, the Company will also be reserved for issuance under the 2019 Plan. The board of directors, or a committee appointed by the board of directors, has the authority to determine to whom options or shares will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the exercise price of each share shall be at least 110% of the fair market value and the term of the award shall not exceed than five years. All other options granted under the 2019 Plan must have an exercise price at least equal to the fair market value on the date of grant and have a term not to exceed ten years. The shares generally vest over a four-year period with straight-line vesting with a 25% one-year cliff. As of December 31, 2019, the Company had reserved 12,749,592 shares of common stock for issuance under the 2019 Plan, of which 4,306,768 shares were available for issuance.

Activity for the options to purchase common stock shown below (in thousands, except share and per share amounts):

	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of December 31, 2017		22,500		$	6.95
Granted		5,063,584			8.94
Forfeited		(22,396	)		8.01
Outstanding as of December 31, 2018		5,063,688			8.94
Granted		3,738,071			17.74
Exercised		(180,287	)		8.43
Forfeited		(178,648	)		11.53
Outstanding as of December 31, 2019		8,442,824		$	12.79		9.1	$	39,840
Exercisable as of December 31, 2019		1,603,073		$	9.60		8.7	$	12,416
Vested and expected to vest as of December 31, 2019		8,442,824		$	12.79		9.1	$	39,840

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. The aggregate intrinsic value of options exercised was $1.8 million for the year ended December 31, 2019. There were no options exercised in 2018 and 2017. The weighted-average grant-date fair value per share of options granted was $12.06, $6.27, and $4.62, for the years ended December 31, 2019, 2018, and 2017, respectively. As of December 31, 2019, total unrecognized stock-based compensation related to unvested stock options was $60.2 million, which the Company expects to recognize over a remaining weighted-average period of 3.1 years.

2019 Employee Stock Purchase Plan

On February 6, 2019, the Company adopted the 2019 Employee Stock Option Plan (“2019 ESPP”). The 2019 ESPP will enable eligible employees of the Company to purchase shares of common stock at a discount. The Company has initially reserved for issuance 1,478,492 shares of common stock pursuant to the 2019 ESPP. The initial offering period began upon completion of the IPO on February 7, 2019 and ended on the last trading day on or before December 1, 2019. Each subsequent offering period will be approximately six months long beginning on the first trading day after December 1, 2019. ESPP participants purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the first trading day of the offering period or (2) the fair market value of the common stock on the purchase date. As of December 31, 2019, there was $0.2 million in unrecognized compensation expense related to the 2019 ESPP to be recognized over five months.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Tax

9.	Income Taxes

The Company was classified as a partnership for U.S. income tax purposes through the Conversion on October 12, 2017. The Company incurred net losses for the years ended December 31, 2019, 2018, and 2017. The Company has reflected a benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its net deferred tax assets due to the uncertainty surrounding realization of such assets.

A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2019, 2018, and 2017 (in thousands):

	Year Ended December 31,
	2019			2018			2017
Tax benefit at federal statutory rate	$	(22,081	)	$	(10,972	)	$	(11,038	)
State income taxes		(4,694	)	—			—
Pre-conversion passthrough loss	—			—				7,331
Tax credits		(6,530	)		(2,387	)		(351	)
Federal uncertain tax positions		2,044			708			102
Stock-based compensation		1,493			1,251			470
Impact of federal rate change	—			—				2,329
Nondeductible expense		124			58			126
Tax benefit recognized on utilization of attributes previous reserved		(13,582	)	—			—
Deferred adjustment from amended tax return		(1,073	)	—			—
Change of valuation allowance		44,748			11,346			782
Others		(449	)		(4	)		249
Income tax provision	$	—		$	—		$	—

The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets consist of (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Net operating loss	$	2,223		$	13,867
Accrued bonus		370			487
Tax credits		7,319			3,098
Stock-based compensation		2,015			213
Deferred rent		—			1,647
Deferred revenue		37,621			—
Lease liability		11,781			—
Others		209			98
Gross deferred tax assets		61,538			19,410
Less valuation allowance		(47,053	)		(17,286	)
Total deferred tax assets	$	14,485		$	2,124
Deferred tax liabilities:
Depreciation and amortization	$	(7,495	)	$	(2,124	)
Right-of-use assets		(6,990	)		—
Gross deferred tax liabilities		(14,485	)		(2,124	)
Deferred tax assets, net	$	—		$	—

In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred assets will be realized. The ultimate realization of the deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Evaluating the need for a valuation allowance for deferred tax assets often requires judgment and analysis of all the positive and negative evidence available, including cumulative losses in recent years and projected future taxable income, to determine whether all or some portion of the deferred tax assets will not be realized. As of December 31, 2019, the Company has utilized a full valuation allowance to offset the net deferred tax assets as the Company believes it is not more likely than not that the net deferred tax assets will be fully realizable. The valuation allowance increased on a net basis by $29.8 million during the year ended December 31, 2019.

As of December 31, 2019, the Company had federal and state net operating loss (“NOL”) carryforwards of approximately $10.4 million and $0.7 million, respectively. Federal NOL carryforwards have an indefinite life and deductions cannot exceed 80% of taxable income. State NOL carryforwards will begin to expire as early as 2028, if not utilized, or have an indefinite life. As of December 31, 2019, the Company also had federal and California tax credit carryforward of approximately $8.1 million and $2.0 million, respectively. The federal tax credits will begin to expire in 2036 while the California tax credits have no expiration date.

Generally, utilization of the NOL carryforwards and credits may be subject to an annual limitation due to the ownership change limitations provided by Section 382, which provides for limitations on NOL carryforwards and certain built-in losses following ownership changes, and Section 383, which provides for special limitations on certain excess credits, etc., of the Code, and similar state provisions. Accordingly, the Company’s ability to utilize NOL carryforwards may be limited as the result of such an “ownership change.” The carryforwards could be subject to an annual limitation, resulting in a reduction in the gross deferred tax assets before considering the valuation allowance. Further, a portion of the carryforwards may expire before being applied to reduce future earnings. The Company is not aware of any changes in ownership that would result in material limitations under Section 382 at this time.

The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2019 and 2018 (in thousands):

Balance as of December 31, 2016	$	—
Increases related to tax positions taken during the prior year		227
Increases related to tax positions taken during the current year		403
Balance as of December 31, 2017		630
Increases related to tax positions taken during the current year		877
Balance as of December 31, 2018		1,507
Decreases related to tax positions taken during the prior year		(80	)
Increases related to tax positions taken during the current year		1,608
Balance as of December 31, 2019	$	3,035

If the unrecognized tax benefits for uncertain tax positions as of December 31, 2019, is recognized, there will be no impact to the effective tax rate as the tax benefit would increase the net deferred tax assets, which is currently offset with a full valuation allowance. The Company’s policy is to include interest and penalties related to unrecognized tax benefits, if any, within the provision for taxes in the consolidated statements of operations. The Company did not accrue any interest or penalties and does not have any tax positions for interest or penalties for the year ended December 31, 2019. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized tax benefits will significantly change within 12 months of December 31, 2019.

The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. The Company uses a two-step approach to recognize and measure uncertain tax positions. During the year ended December 31, 2019, the Company recorded a tax reserve of $1.6 million as a reduction of the tax credit carryover.

The Company’s income tax returns generally remain subject to examination by federal and most state tax authorities. The Company is currently not subject to any income tax audits by federal or state taxing authorities. The statute of limitations for tax liabilities for all years remains open.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

12.	Selected Quarterly Financial Data (Unaudited)

The following table provides the selected quarterly financial data for 2019 and 2018. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented.

	Three months ended
	December 31, 2019			September 30, 2019			June 30, 2019			March 31, 2019
	(in thousands, except per share data)
Revenue	$	6,001		$	2,696		$	6,917		$	5,605
Loss from operations	$	(32,342	)	$	(34,149	)	$	(27,152	)	$	(20,761	)
Net loss	$	(30,527	)	$	(31,738	)	$	(24,560	)	$	(18,560	)
Net loss per share, basic and diluted	$	(0.45	)	$	(0.47	)	$	(0.36	)	$	(0.42	)

	Three months ended
	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
	(in thousands, except per share data)
Revenue	$	9,145		$	6,503		$	7,109		$	4,920
Loss from operations	$	(19,017	)	$	(16,815	)	$	(12,231	)	$	(9,225	)
Net loss	$	(17,373	)	$	(15,317	)	$	(11,121	)	$	(8,437	)
Net loss per share, basic and diluted	$	(1.48	)	$	(1.34	)	$	(1.00	)	$	(0.78	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Concentration of Credit Risk

Cash, Cash Equivalents, and Restricted Cash

Restricted cash as of December 31, 2019 relates to a letter of credit established for a lease entered into in June 2018.

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Cash and cash equivalents	$	89,641	$	65,470	$	32,451
Restricted cash		1,472		1,472		—
Total cash, cash equivalents, and restricted cash	$	91,113	$	66,942	$	32,451

Marketable Securities

Fair Value of Financial Instruments

Level 1 – Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

Property, Plant and Equipment

Property and Equipment

Leases

Impairment or Disposal of Long-Lived Assets

Impairment of Long-Lived Assets

Revenue Recognition

Collaboration Revenue

Grant Revenue

Research and Development Expense

Research and Development Costs

Stock-based Compensation

Comprehensive Loss

Income Tax

Income Taxes

Employee 401 (K) Plan

Employee 401(k) Plan

Segment

Segments

Deferred Offering Costs

New Accounting Pronouncements

Recent Accounting Pronouncements

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on a Recurring Basis

The following tables summarize the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy:

	December 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	61,104	$	—	$	—		$	61,104
U.S. government treasury securities	Level 1		227,446		149		(8	)		227,587
Commercial paper	Level 2		43,735		—		—			43,735
Corporate bonds	Level 2		10,103		3		(2	)		10,104
Total cash equivalents and marketable securities		$	342,388	$	152	$	(10	)	$	342,530

	December 31, 2018
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	65,222	$	—	$	—		$	65,222
U.S. government treasury securities	Level 1		224,980		3		(45	)		224,938
Total cash equivalents and marketable securities		$	290,202	$	3	$	(45	)	$	290,160

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Leasehold improvements	$	25,222		$	210
Lab equipment		10,458			4,599
Furniture and fixtures		2,031			131
Computer equipment		1,554			449
Construction-in-progress		—			7,449
Property and equipment, gross		39,265			12,838
Less accumulated depreciation and amortization		(5,413	)		(1,901	)
Total property and equipment, net	$	33,852		$	10,937

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	December 31,
	2019		2018
	(In thousands)
Accrued research and development costs	$	6,400	$	3,821
Accrued employee compensation		5,730		2,766
Accrued property and equipment		3,471		588
Accrued professional services		2,040		293
Other		615		971
Total accrued liabilities	$	18,256	$	8,439

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Summary of Authorized, Issued, and Outstanding Convertible Preferred Stock

The following tables summarizes the authorized, issued, and outstanding convertible preferred stock of the Company as of December 31, 2018:

	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529
Series E		9,848,474		9,373,633		14.22		133,035		133,250
Total convertible preferred stock		45,849,677		45,374,836			$	210,520	$	211,054

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Recognized Stock-Based Compensation

The Company recognized stock-based compensation as follows:

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Research and development	$	8,031	$	4,195	$	4,392
General and administrative		8,250		2,730		962
Total stock-based compensation	$	16,281	$	6,925	$	5,354

Summary of Share-based Payment Award, Stock Options, Valuation Assumptions

	Year Ended December 31,
	2019	2018	2017
Expected term (in years)	5.3 – 6.1	5.1 – 6.1	6.0 – 10.0
Expected volatility	76% – 82%	73% – 81%	75%
Risk-free interest rate	1.4% – 2.6%	2.5% – 3.1%	1.9 – 2.4%
Dividend yield	0%	0%	0%

Summary of Restricted Common Stock

Activity for the restricted common stock is shown below:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted common stock as of December 31, 2017		3,148,813		$	6.95
Vested		(1,219,641	)		6.95
Forfeited		(11,324	)		6.95
Unvested restricted common stock as of December 31, 2018		1,917,848			6.95
Vested		(866,037	)		6.95
Forfeited		(56,973	)		6.95
Unvested restricted common stock as of December 31, 2019		994,838			6.95

Summary of Options to Purchase Common Stock

Activity for the options to purchase common stock shown below (in thousands, except share and per share amounts):

	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of December 31, 2017		22,500		$	6.95
Granted		5,063,584			8.94
Forfeited		(22,396	)		8.01
Outstanding as of December 31, 2018		5,063,688			8.94
Granted		3,738,071			17.74
Exercised		(180,287	)		8.43
Forfeited		(178,648	)		11.53
Outstanding as of December 31, 2019		8,442,824		$	12.79		9.1	$	39,840
Exercisable as of December 31, 2019		1,603,073		$	9.60		8.7	$	12,416
Vested and expected to vest as of December 31, 2019		8,442,824		$	12.79		9.1	$	39,840

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2019, 2018, and 2017 (in thousands):

	Year Ended December 31,
	2019			2018			2017
Tax benefit at federal statutory rate	$	(22,081	)	$	(10,972	)	$	(11,038	)
State income taxes		(4,694	)	—			—
Pre-conversion passthrough loss	—			—				7,331
Tax credits		(6,530	)		(2,387	)		(351	)
Federal uncertain tax positions		2,044			708			102
Stock-based compensation		1,493			1,251			470
Impact of federal rate change	—			—				2,329
Nondeductible expense		124			58			126
Tax benefit recognized on utilization of attributes previous reserved		(13,582	)	—			—
Deferred adjustment from amended tax return		(1,073	)	—			—
Change of valuation allowance		44,748			11,346			782
Others		(449	)		(4	)		249
Income tax provision	$	—		$	—		$	—

Schedule of Deferred Tax Assets and Liabilities

The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets consist of (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Net operating loss	$	2,223		$	13,867
Accrued bonus		370			487
Tax credits		7,319			3,098
Stock-based compensation		2,015			213
Deferred rent		—			1,647
Deferred revenue		37,621			—
Lease liability		11,781			—
Others		209			98
Gross deferred tax assets		61,538			19,410
Less valuation allowance		(47,053	)		(17,286	)
Total deferred tax assets	$	14,485		$	2,124
Deferred tax liabilities:
Depreciation and amortization	$	(7,495	)	$	(2,124	)
Right-of-use assets		(6,990	)		—
Gross deferred tax liabilities		(14,485	)		(2,124	)
Deferred tax assets, net	$	—		$	—

Schedule of Unrecognized Tax Benefits

The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2019 and 2018 (in thousands):

Balance as of December 31, 2016	$	—
Increases related to tax positions taken during the prior year		227
Increases related to tax positions taken during the current year		403
Balance as of December 31, 2017		630
Increases related to tax positions taken during the current year		877
Balance as of December 31, 2018		1,507
Decreases related to tax positions taken during the prior year		(80	)
Increases related to tax positions taken during the current year		1,608
Balance as of December 31, 2019	$	3,035

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Quarterly Financial Data

	Three months ended
	December 31, 2019			September 30, 2019			June 30, 2019			March 31, 2019
	(in thousands, except per share data)
Revenue	$	6,001		$	2,696		$	6,917		$	5,605
Loss from operations	$	(32,342	)	$	(34,149	)	$	(27,152	)	$	(20,761	)
Net loss	$	(30,527	)	$	(31,738	)	$	(24,560	)	$	(18,560	)
Net loss per share, basic and diluted	$	(0.45	)	$	(0.47	)	$	(0.36	)	$	(0.42	)

	Three months ended
	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
	(in thousands, except per share data)
Revenue	$	9,145		$	6,503		$	7,109		$	4,920
Loss from operations	$	(19,017	)	$	(16,815	)	$	(12,231	)	$	(9,225	)
Net loss	$	(17,373	)	$	(15,317	)	$	(11,121	)	$	(8,437	)
Net loss per share, basic and diluted	$	(1.48	)	$	(1.34	)	$	(1.00	)	$	(0.78	)

The Company and Liquidity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	12 Months Ended
	Jan. 30, 2020	Feb. 07, 2019	Jan. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock, par value				$ 0.0001	$ 0.0001		$ 0.0001
Common stock, shares issued				69,052,873	13,764,829
Convertible preferred stock, shares outstanding				0	45,374,836
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings				9,739,541
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings		9,739,541
Share price		$ 19.00
Proceeds from issuance of common stock upon initial public offering, net of issuance costs		$ 168.2
Convertible preferred stock, shares outstanding		0
Initial Public Offering \| Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Conversion of convertible preferred stock to common stock		45,374,836
Over-Allotment Option
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings		489,541
Over-Allotment Option \| Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	1,252,500		1,252,500
Follow-on-offering \| Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	9,602,500		9,602,500
Share price	$ 25.00		$ 25.00
Proceeds from issuance of common stock upon Follow on public offering, net of issuance costs	$ 224.5		$ 224.5
Restricted Stock Units (RSUs)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock, shares issued							202,924
Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Preferred stock, par value							$ 0.0001
Convertible preferred stock, shares outstanding					45,374,836	36,001,203

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended			12 Months Ended
	Jun. 30, 2019	Jan. 31, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets				$ 28,476,000			$ 31,400,000
Lease liabilities				48,036,000			38,900,000
Deferred rent, long-term portion					$ 7,478,000		$ 7,500,000
Impairment of long-lived assets	$ 1,200,000
Deferred revenue				$ 153,400,000
Increase in the forecast of total expected costs				12.00%
Reduction in revenue due to increase in budget during the year				$ 6,000,000
Grant revenue				0	169,000	$ 863,000
Deferred costs recorded as other assets				600,000	2,700,000
Abb Vie Biotechnology Limited
Summary Of Significant Accounting Policies [Line Items]
Upfront payments			$ 205,000,000
Upfront payment received		$ 200,000,000	5,000,000
Exclusive option rights exercised for each program			250,000,000
Cash payment received, non-refundable		$ 200,000,000	5,000,000
Abb Vie Biotechnology Limited \| Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue				$ 21,200,000	$ 27,500,000	$ 2,900,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life				3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life				5 years
Maximum \| Abb Vie Biotechnology Limited
Summary Of Significant Accounting Policies [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval			$ 242,800,000
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Unrealized gains or loss on money market funds				$ 0

Leases - Additional Information (Detail)	1 Months Ended		12 Months Ended
Leases - Additional Information (Detail)	May 31, 2019 ft²	Jun. 30, 2018 ft²	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee Lease Description [Line Items]
Tenant improvement allowance			$ 15,700,000
Amount form letter of credit			$ 1,472,000	$ 1,472,000
Operating term			10 years
Operating lease, renewal period			10 years
Operating lease term			The lease is over a ten-year term with an option to renew for a period of ten years.
Operating sublease, impairment charges			$ 1,158,000
Rent expense				$ 1,100,000	$ 1,100,000
Weighted average remaining lease term for operating leases			9 years 3 months 18 days
Weighted average discount rate for operating leases			8.80%
Cash paid for amounts included In measurement of lease liabilities			$ 3,200,000
Offices and Laboratory Facilities \| South San Francisco
Lessee Lease Description [Line Items]
Area under lease \| ft²		105,000
Area under lease \| ft²	25,000		16,000
Lease expiration date			2019-04
Sublease commencement date			2019-06
Sublease term	30 months
Operating sublease, option to extend			The sublessor has the option to extend the Sublease for one additional year subject to the consent of the Company.
Sublease renewal period	1 year
Operating sublease, existence of option to extend			true
Ongoing loss over sublease period			$ 0
Offices and Laboratory Facilities \| South San Francisco \| General and Administrative Expense
Lessee Lease Description [Line Items]
Operating sublease, impairment charges			$ 1,200,000
Laboratory Facilities \| Milpitas
Lessee Lease Description [Line Items]
Area under lease \| ft²			9,000
Operating lease, renewal period			4 years
Operating lease term			The Company also leases approximately 9,000 square feet of laboratory facilities in Milpitas under an agreement that ends in January 2022 with an option to extend for four years.
Lease expiration date			2022-01

Convertible Preferred Stock - Summary of Authorized, Issued, and Outstanding Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2018
Temporary Equity [Line Items]
(In thousands, except share and per share data)	0	45,849,677
(In thousands, except share and per share data)	0	45,374,836
(In thousands, except share and per share data)		$ 210,520
(In thousands, except share and per share data)		$ 211,054
Series A-1 Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		1,000,000
(In thousands, except share and per share data)		1,000,000
(In thousands, except share and per share data)		$ 0.40
(In thousands, except share and per share data)		$ 354
(In thousands, except share and per share data)		$ 400
Series A-2 Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		10,549,450
(In thousands, except share and per share data)		10,549,450
(In thousands, except share and per share data)		$ 0.91
(In thousands, except share and per share data)		$ 9,554
(In thousands, except share and per share data)		$ 9,600
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		5,000,000
(In thousands, except share and per share data)		5,000,000
(In thousands, except share and per share data)		$ 1.20
(In thousands, except share and per share data)		$ 5,968
(In thousands, except share and per share data)		$ 6,000
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		12,088,016
(In thousands, except share and per share data)		12,088,016
(In thousands, except share and per share data)		$ 2.67
(In thousands, except share and per share data)		$ 32,158
(In thousands, except share and per share data)		$ 32,275
Series D Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		7,363,737
(In thousands, except share and per share data)		7,363,737
(In thousands, except share and per share data)		$ 4.01
(In thousands, except share and per share data)		$ 29,451
(In thousands, except share and per share data)		$ 29,529
Series E Convertible Preferred Stock
Temporary Equity [Line Items]
(In thousands, except share and per share data)		9,848,474
(In thousands, except share and per share data)		9,373,633
(In thousands, except share and per share data)		$ 14.22	$ 14.2154
(In thousands, except share and per share data)		$ 133,035
(In thousands, except share and per share data)		$ 133,250

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Feb. 06, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Expected weighted average period		3 years 1 month 6 days
Common stock reserved for issuance	7,688,156	12,749,592
Percentage of stock owned by Individual	10.00%
Percentage of exercise price per share from fair market value	110.00%
Stock options, term	5 years
Cliff vesting percentage	25.00%
Cliff vesting period	1 year
Common stock, shares available for issuance	4,306,768
Aggregate intrinsic value of option exercised		$ 1.8	$ 0.0	$ 0.0
Unrecognized stock-based compensation		$ 60.2
Weighted-average grant-date fair value per share of options granted		$ 12.06	$ 6.27	$ 4.62
2019 Equity Incentive Plan \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, term	10 years
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	1,478,492
Purchase price of common stock, percentage	85.00%
Subsequent offering period	6 months
Un-recognized compensation expense		$ 0.2
Restricted Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage		25.00%
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount		$ 4.5
Expected weighted average period		1 year 8 months 12 days

Stock-based Compensation - Summary of Options to Purchase Common Stock (Details) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance	5,063,688	22,500
Number of Options, Granted	3,738,071	5,063,584
Number of Options, Forfeited	(178,648)	(22,396)
Number of Options, Outstanding, ending balance	8,442,824	5,063,688
Number of Options, Exercised	(180,287)
Number of Options, Exercisable	1,603,073
Number of Options, Vested and expected to vest	8,442,824
Weighted average exercise price, Outstanding, beginning balance	$ 8.94	$ 6.95
Weighted average exercise price, Granted	17.74	8.94
Weighted average exercise price, Forfeited	11.53	8.01
Weighted average exercise price, Outstanding, ending balance	12.79	$ 8.94
Weighted average exercise price, Exercised	8.43
Weighted average exercise price, Exercisable	9.60
Weighted average exercise price, Vested and expected to vest	$ 12.79
Weighted Average Remaining Contractual Term, Outstanding	9 years 1 month 6 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	8 years 8 months 12 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	9 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 39,840	$ 0	$ 0
Aggregate Intrinsic Value, Exercisable	12,416
Aggregate Intrinsic Value, Vested or expected to vest	$ 39,840

Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ (22,081)	$ (10,972)	$ (11,038)
State income taxes	(4,694)
Pre-conversion passthrough loss			7,331
Tax credits	(6,530)	(2,387)	(351)
Federal uncertain tax positions	2,044	708	102
Stock-based compensation	1,493	1,251	470
Impact of federal rate change			2,329
Nondeductible expense	124	58	126
Tax benefit recognized on utilization of attributes previous reserved	(13,582)
Deferred adjustment from amended tax return	(1,073)
Change of valuation allowance	44,748	11,346	782
Others	$ (449)	$ (4)	$ 249

Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss	$ 2,223	$ 13,867
Accrued bonus	370	487
Tax credits	7,319	3,098
Stock-based compensation	2,015	213
Deferred rent		1,647
Deferred revenue	37,621
Lease liability	11,781
Others	209	98
Gross deferred tax assets	61,538	19,410
Less valuation allowance	(47,053)	(17,286)
Total deferred tax assets	14,485	2,124
Deferred tax liabilities:
Depreciation and amortization	(7,495)	(2,124)
Right-of-use assets	(6,990)
Gross deferred tax liabilities	$ (14,485)	$ (2,124)

Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Issuance cost of common stock	$ 3,874
Series E Preferred Stock [Member]
Issuance cost of convertible preferred Stock		$ 214

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		$ 61,104	$ 65,222
Cash equivalents and marketable securities, Fair Market Value		61,104	65,222
U.S Government Treasury Securities \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		227,446	224,980
Cash equivalents and marketable securities, Unrealized Gains		149	3
Cash equivalents and marketable securities, Unrealized Losses		(8)	(45)
Cash equivalents and marketable securities, Fair Market Value		227,587	224,938
Commercial Paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		43,735
Cash equivalents and marketable securities, Fair Market Value		43,735
Corporate Bonds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		10,103
Cash equivalents and marketable securities, Unrealized Gains		3
Cash equivalents and marketable securities, Unrealized Losses		(2)
Cash equivalents and marketable securities, Fair Market Value		$ 10,104
Cash Equivalents and Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	$ 342,388		290,202
Cash equivalents and marketable securities, Unrealized Gains	152		3
Cash equivalents and marketable securities, Unrealized Losses	(10)		(45)
Cash equivalents and marketable securities, Fair Market Value	$ 342,530		$ 290,160

Fair Value Measurements - Additional Information (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Fair value of assets transferred from level 1 to level 2	$ 0
Fair value of assets transferred from level 2 to level 1	0
Remaining contractual maturities of investments were less than one year	311,800
Remaining contractual maturities of investments were less than one year through two years	$ 30,700

Leases - Components of Lease Expense (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 5,875
Variable lease cost	1,328
Short-term lease cost	68
Sublease income and reimbursement of variable lease cost	1,263
Total	$ 6,008

Leases - Schedule of Undiscounted Cash Flows Owned Under Company's Operating Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2020	$ 6,882
2021	7,122
2022	7,146
2023	7,376
2024	7,634
Thereafter	35,512
Total undiscounted lease payments	71,672
Less: Present value adjustment	(23,636)
Total lease liability	$ 48,036	$ 38,900

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 89,641	$ 65,470	$ 32,451
Restricted cash	1,472	1,472
Total cash, cash equivalents, and restricted cash	$ 91,113	$ 66,942	$ 32,451	$ 51,038

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 39,265	$ 12,838
Less accumulated depreciation and amortization	(5,413)	(1,901)
Total property and equipment, net	33,852	10,937
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	10,458	4,599
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,554	449
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	25,222	210
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,031	131
Construction-in-Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross		$ 7,449

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development costs	$ 6,400	$ 3,821
Accrued employee compensation	5,730	2,766
Accrued property and equipment	3,471	588
Accrued professional services	2,040	293
Other	615	971
Total accrued liabilities	$ 18,256	$ 8,439

Stock-based Compensation - Summary of Restricted Common Stock (Details) - Restricted Common Stock - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Shares
Number of Shares, Unvested, beginning balance \| shares	1,917,848	3,148,813
Number of Shares, Vested \| shares	(866,037)	(1,219,641)
Number of Shares, Forfeited \| shares	(56,973)	(11,324)
Number of Shares, Unvested, ending balance \| shares	994,838	1,917,848
Weighted-average grant-date fair value
Weighted Average Grant Date Fair Value per Share, Unvested, beginning balance \| $ / shares	$ 6.95	$ 6.95
Weighted Average Grant Date Fair Value per Share, Vested \| $ / shares	6.95	6.95
Weighted Average Grant Date Fair Value per Share, Forfeited \| $ / shares	6.95	6.95
Weighted Average Grant Date Fair Value per Share, Unvested, ending balance \| $ / shares	$ 6.95	$ 6.95

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Change of valuation allowance	$ 29.8
Tax Credit Carryforward, Amount	1.6
California Tax Authority
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount		$ 2.0
Federal and State Tax Authorities
Income Tax Contingency [Line Items]
Operating loss carry forward, net	10.4
Federal Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward, net	$ 0.7
Tax Credit Carryforward, Amount		$ 8.1
Tax Credit Beginning Expiration Year		2036
Federal Tax Authority \| Maximum
Income Tax Contingency [Line Items]
Operating loss carry forward, deductions	80.00%
State Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward beginning expiration year	2028

Selected Quarterly Financial Data (Unaudited) - Schedule of Selected Quarterly Financial Data (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]
Revenue	$ 6,001	$ 2,696	$ 6,917	$ 5,605	$ 9,145	$ 6,503	$ 7,109	$ 4,920	$ 21,219	$ 27,677	$ 3,735
Loss from operations	(32,342)	(34,149)	(27,152)	(20,761)	(19,017)	(16,815)	(12,231)	(9,225)	(114,404)	(57,288)	(32,679)
Net loss	$ (30,527)	$ (31,738)	$ (24,560)	$ (18,560)	$ (17,373)	$ (15,317)	$ (11,121)	$ (8,437)	$ (105,385)	$ (52,248)	$ (32,480)
Net loss per share, basic and diluted	$ (0.45)	$ (0.47)	$ (0.36)	$ (0.42)	$ (1.48)	$ (1.34)	$ (1.00)	$ (0.78)	$ (1.71)	$ (4.62)	$ (3.55)