Alector (ALEC) 10-Q 4 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 31, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	82-2933343
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 Quarterly Report	true
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		81,276,097
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	DE

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 516,532	$ 49,969
Marketable securities	261,318	363,339
Receivable from Collaboration partner	1,584
Prepaid expenses and other current assets	8,585	8,203
Total current assets	788,019	421,511
Property and equipment, net	27,898	30,181
Operating lease right-of-use assets	31,121	32,470
Restricted cash	1,472	1,472
Other assets	5,716	2,617
Total assets	854,226	488,251
Current liabilities:
Accounts payable	2,341	3,004
Accrued clinical supply costs	7,418	11,148
Accrued liabilities	20,586	22,538
Deferred revenue, current portion	94,683	23,886
Operating lease liabilities, current portion	7,787	7,512
Total current liabilities	132,815	68,088
Deferred revenue, long-term portion	344,529	108,417
Operating lease liabilities, long-term portion	40,834	43,744
Other long-term liabilities	158	472
Total liabilities	518,336	220,721
Stockholders' equity:
Common stock, $0.0001 par value; 200,000,000 shares authorized; 81,248,646 and 79,316,261 shares issued and outstanding as of September 30, 2021 and December 31, 2020	8	8
Additional paid-in capital	726,692	676,956
Accumulated other comprehensive income (loss)	(41)	614
Accumulated deficit	(390,769)	(410,048)
Total stockholders' equity	335,890	267,530
Total liabilities and stockholders’ equity	$ 854,226	$ 488,251

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	81,248,646	81,248,646
Common stock, shares outstanding	79,316,261	79,316,261

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total revenue	$ 182,413	$ 5,904				$ 193,091	$ 16,245
Type of Revenue [Extensible List]	Collaboration Revenue [Member]	Collaboration Revenue [Member]	Collaboration Revenue [Member]	Collaboration Revenue [Member]	Collaboration Revenue [Member]	Collaboration Revenue [Member]	Collaboration Revenue [Member]
Operating expenses:
Research and development	$ 43,066	$ 43,819				$ 136,617	$ 112,486
General and administrative	13,018	15,834				38,105	46,175
Total operating expenses	56,084	59,653				174,722	158,661
Income(loss) from operations	126,329	(53,749)				18,369	(142,416)
Other income, net	268	1,045				910	4,367
Net income (loss)	126,597	(52,704)	$ (45,326)			19,279	(138,049)
Unrealized gain (loss) on marketable securities	(244)	(752)				(655)	952
Comprehensive income (loss)	$ 126,353	$ (53,456)				$ 18,624	$ (137,097)
Net income (loss) per share:
Basic net income (loss) per share	$ 1.56	$ (0.67)				$ 0.24	$ (1.78)
Diluted net income (loss) per share	$ 1.49	$ (0.67)				$ 0.23	$ (1.78)
Weighted-average shares used in calculating:
Basic net income (loss) per share	80,964,701	78,771,930				80,048,758	77,340,896
Diluted net income (loss) per share	85,232,690	78,771,930				82,871,254	77,340,896

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2019	$ 194,743	$ 7	$ 414,414	$ 142	$ (219,820)
Beginning balance (in shares) at Dec. 31, 2019		69,052,873
Issuance of common stock upon follow-on public offering, net of issuance costs	224,511	$ 1	224,510
Issuance of common stock upon follow-on public offering, net of issuance costs (in shares)		9,602,500
Exercise of stock options	3,217		3,217
Exercise of stock options (in shares)		361,096
Stock-based compensation	6,642		6,642
Unrealized loss on marketable securities	2,640			2,640
Net income (loss)	(40,019)				(40,019)
Ending balance at Mar. 31, 2020	391,734	$ 8	648,783	2,782	(259,839)
Ending balance (in shares) at Mar. 31, 2020		79,016,469
Beginning balance at Dec. 31, 2019	194,743	$ 7	414,414	142	(219,820)
Beginning balance (in shares) at Dec. 31, 2019		69,052,873
Net income (loss)	(138,049)
Ending balance at Sep. 30, 2020	310,191	$ 8	666,958	1,094	(357,869)
Ending balance (in shares) at Sep. 30, 2020		79,284,181
Beginning balance at Mar. 31, 2020	391,734	$ 8	648,783	2,782	(259,839)
Beginning balance (in shares) at Mar. 31, 2020		79,016,469
Exercise of stock options	2,196		2,196
Exercise of stock options (in shares)		190,709
Purchase of common stock under employee stock purchase plan	715		715
Purchase of common stock under employee stock purchase plan (in shares)		45,217
Stock-based compensation	6,948		6,948
Unrealized loss on marketable securities	(936)			(936)
Net income (loss)	(45,326)				(45,326)
Ending balance at Jun. 30, 2020	$ 355,331	$ 8	$ 658,642	1,846	(305,165)
Ending balance (in shares) at Jun. 30, 2020		79,252,395
Exercise of stock options (in shares)	286	31,786	286
Stock-based compensation	$ 8,030		$ 8,030
Unrealized loss on marketable securities	(752)			(752)
Net income (loss)	(52,704)				(52,704)
Ending balance at Sep. 30, 2020	310,191	$ 8	666,958	1,094	(357,869)
Ending balance (in shares) at Sep. 30, 2020		79,284,181
Beginning balance at Dec. 31, 2020	$ 267,530	$ 8	676,956	614	(410,048)
Beginning balance (in shares) at Dec. 31, 2020	79,316,261	79,316,261
Exercise of stock options	$ 3,874		3,874
Exercise of stock options (in shares)		415,386
Stock-based compensation	8,800		8,800
Unrealized loss on marketable securities	(204)			(204)
Net income (loss)	(52,171)				(52,171)
Ending balance at Mar. 31, 2021	227,829	$ 8	689,630	410	(462,219)
Ending balance (in shares) at Mar. 31, 2021		79,731,647
Beginning balance at Dec. 31, 2020	$ 267,530	$ 8	676,956	614	(410,048)
Beginning balance (in shares) at Dec. 31, 2020	79,316,261	79,316,261
Net income (loss)	$ 19,279
Ending balance at Sep. 30, 2021	$ 335,890	$ 8	726,692	(41)	(390,769)
Ending balance (in shares) at Sep. 30, 2021	79,316,261	81,248,646
Beginning balance at Mar. 31, 2021	$ 227,829	$ 8	689,630	410	(462,219)
Beginning balance (in shares) at Mar. 31, 2021		79,731,647
Exercise of stock options	1,993		1,993
Exercise of stock options (in shares)		207,453
Purchase of common stock under employee stock purchase plan	969		969
Purchase of common stock under employee stock purchase plan (in shares)		84,105
Forfeiture of restricted common stock		(18,043)
Stock-based compensation	9,078		9,078
Unrealized loss on marketable securities	(207)			(207)
Net income (loss)	(55,147)				(55,147)
Ending balance at Jun. 30, 2021	184,515	$ 8	701,670	203	(517,366)
Ending balance (in shares) at Jun. 30, 2021		80,005,162
Exercise of stock options	14,821	$ 0	14,821
Exercise of stock options (in shares)		1,178,085
Vesting of restricted stock units		65,399
Stock-based compensation	10,201		10,201
Unrealized loss on marketable securities	(244)			(244)
Net income (loss)	126,597				126,597
Ending balance at Sep. 30, 2021	$ 335,890	$ 8	$ 726,692	$ (41)	$ (390,769)
Ending balance (in shares) at Sep. 30, 2021	79,316,261	81,248,646

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance cost of common stock	$ 1,148

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net income (loss)	$ 19,279	$ (138,049)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization	4,735	4,367
Stock-based compensation	28,079	21,620
Accretion of discounts on marketable securities	1,586	8
Amortization of right-of-use assets	1,434	934
Impairment loss on right-of-use-assets		238
Loss from disposal of property and equipment, net		(12)
Changes in operating assets and liabilities:
Receivable from collaboration partner	(1,584)
Prepaid expenses and other current assets	(382)	(3,934)
Other assets	(3,099)	(2,098)
Accounts payable	(347)	1,087
Accrued liabilities and accrued clinical supply costs	(5,488)	15,027
Deferred revenue	306,909	(16,245)
Lease liabilities	(2,992)	(2,043)
Other long-term liabilities	(314)
Net cash provided by (used in) operating activities	347,816	(119,076)
Cash flows from investing activities:
Purchase of property and equipment	(2,690)	(4,244)
Purchase of marketable securities	(139,716)	(414,949)
Sale of marketable secuirities	10,696	0
Maturities of marketable securities	228,800	302,590
Net cash provided by (used in) investing activities	97,090	(116,603)
Cash flows from financing activities:
Proceeds from issuance of common stock upon follow-on public offering, net of issuance costs		224,603
Proceeds from the exercise of options to purchase common stock	20,688	5,699
Purchase of common stock under employee stock option plan	969	715
Net cash provided by financing activities	21,657	231,017
Net increase (decrease) in cash, cash equivalents, and restricted cash	466,563	(4,662)
Cash, cash equivalents, and restricted cash at beginning of period	51,441	91,113
Cash, cash equivalents, and restricted cash at end of period	518,004	86,451
Non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	$ 236	$ 337

The Company and Liquidity

The Company and Liquidity	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Liquidity	1. The Company and Liquidity Alector, Inc. (Alector or the Company) is a Delaware corporation headquartered in South San Francisco, California. Alector is a clinical stage biopharmaceutical company pioneering immuno-neurology, a novel therapeutic approach for the treatment of neurodegeneration. Follow-on Offering On January 30, 2020, the Company completed a follow-on offering through issuing and selling 9,602,500 shares of common stock at a public offering price of $ 25.00 per share, including 1,252,500 shares sold pursuant to the underwriters’ full exercise of their option to purchase additional shares, resulting in aggregate net proceeds of $ 224.5 million, after deducting underwriting discounts and commissions and offering costs.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The condensed 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). In the opinion of management, these unaudited condensed consolidated financial statements include all normal, recurring adjustments that are necessary to present fairly the results of the interim periods presented. The condensed consolidated financial statements include the accounts of Alector, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on February 25, 2021. Use of Estimates The preparation of condensed 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 condensed 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 significant unrealized gains or losses on the money market funds for the periods presented. Restricted cash as of September 30, 2021 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 condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows: Nine Months Ended September 30, 2021 2020 (In thousands) Cash and cash equivalents $ 516,532 $ 84,979 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 518,004 $ 86,451 Fair Value of Financial Instruments The Company’s financial instruments include cash and cash equivalents, marketable securities, accounts payable, and accrued liabilities. The Company’s financial instruments approximate fair value due to their relatively short maturities. 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. Revenue Recognition The Company recognizes revenue when control of promised goods or services is 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. If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling price (SSP). The relative SSP for each deliverable is estimated using external sourced evidence if it is available. If external sourced evidence is not available, we use our best estimate of the SSP for the deliverable. The Company recognizes collaboration revenue at a point in time if control of the promised good or service has been transferred to the customer. The Company recognizes collaboration revenue over time 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 re-evaluates the estimate of expected costs to satisfy the performance obligation each reporting period. Stock-based Compensation Stock-based compensation is measured on the grant date based on the fair value of the awards. The fair value of options to purchase common stock are measured using the Black-Scholes option-pricing model. Stock-based compensation associated with restricted stock units (RSUs) is based on the fair value of the Company's common stock on the grant date, which equals the closing price of the Company's common stock on the grant date. The Company recognizes expense over the vesting period of the awards. Expense for options and RSUs are recognized on a straight-line basis. The Company also granted RSUs with market conditions to certain executives. The fair value of the RSUs with market conditions are estimated using a Monte Carlo simulation model. Assumptions and estimates utilized in the model include the stock price on grant date, risk-free interest rate, dividend yield, expected stock volatility, and the estimated period to achieve the market condition. The expense is recognized based on continued employment of the participants, regardless of achievement of the market condition. Expense related to the RSUs with market conditions is recognized using the accelerated attribution method. The Company accounts for forfeitures as they occur for all awards. Comprehensive Income (Loss) Comprehensive Income (Loss) includes net income (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.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
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: September 30, 2021 Fair Value Amortized Unrealized Unrealized Fair Market (In thousands) Money market funds Level 1 $ 219,553 $ — $ — $ 219,553 U.S. government treasury securities Level 1 261,358 61 ( 103 ) 261,316 Total cash equivalents and $ 480,911 $ 61 $ ( 103 ) $ 480,869 December 31, 2020 Fair Value Amortized Unrealized Unrealized Fair Market (In thousands) Money market funds Level 1 $ 37,951 $ — $ — $ 37,951 U.S. government treasury securities Level 1 357,725 620 ( 6 ) 358,339 Corporate bonds Level 2 5,000 $ — $ — 5,000 Total cash equivalents and $ 400,676 $ 620 $ ( 6 ) $ 401,290 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. The Company classifies marketable securities available to fund current operations as current assets. As of September 30, 2021 , the remaining contractual maturities of $ 332.4 million of investments were less than one year and $ 148.5 million of 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. As of September 30, 2021 , the Company considered any unrealized losses on our marketable securities to be driven by factors other than credit risk. The Company sold marketable securities for total proceeds of $ 10.7 million for the three and nine months ended September 30, 2021, for an immaterial realized gain based on the specific identification method. The Company did no t sell any marketable securities for the three and nine months ended September 30, 2020.

Collaboration Agreements

Collaboration Agreements	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreements	4. Collaboration Agreements GSK On July 1, 2021, the Company entered into a Collaboration and License Agreement with Glaxo Wellcome UK Limited, a subsidiary of GlaxoSmithKline plc (GSK), pursuant to which the Company and GSK will collaborate on the global development and commercialization of progranulin-elevating monoclonal antibodies, including AL001 and AL101 (GSK Agreement). The GSK Agreement was made effective on August 17, 2021 . Under the terms of the GSK Agreement, the Company receives $ 700 million in upfront payments, of which $ 500 million was received in the third quarter of 2021 and $ 200 million is expected to be invoiceable in the first quarter of 2022. In addition, based on the development and commercialization plan for AL001 and AL101, the Company may be eligible to receive up to an additional $ 1.5 billion in clinical development, regulatory, and commercial launch-related milestone payments. In the United States, the Company and GSK will equally share profits and losses from commercialization of AL001 and AL101. Outside of the United States, the Company will be eligible for double-digit tiered royalties. The Company and GSK will jointly develop AL001 and AL101. The Company will lead the global clinical development of AL001 and AL101, other than with respect to Phase 3 clinical trials for Alzheimer’s disease and Parkinson’s disease and other non-orphan indications, which will be led by GSK. The Company and GSK will share development costs 60 % by GSK and 40 % by the Company, except that the Company will solely bear the development costs of the initial Phase 2 clinical trials under the development plan, and the parties will share manufacturing development costs equally. In the United States, the Company and GSK will be jointly responsible for commercialization of AL001 and AL101, with the Company leading the commercialization for orphan indications and GSK leading the commercialization for Alzheimer’s disease and Parkinson’s disease and other non-orphan indications. Outside of the United States, GSK will be solely responsible for commercialization of AL001 and AL101 for all indications. The Company may opt out of the sharing of development costs and of profit and losses from commercialization in the United States on a product-by-product basis. In such case, the Company will no longer conduct development or commercialization of that product and the Company will receive tiered royalties on net sales in the United States instead of a share of profits or losses. GSK may terminate the agreement with 180 days' notice at any time, but the Company does not need to repay any portion of the payments received. The Company concluded that the GSK Agreement is within the scope of ASC 808, Collaborative Arrangements, as both parties are active participants in the activities and are exposed to significant risks and rewards dependent on the success of the commercialization of indications for AL001 and AL101. Certain elements are required to be accounted for under ASC 606, Revenue From Contracts With Customers, where the counterparty is a customer for a good or service that is a distinct unit of account. The Company concluded that the 2021 GSK Agreement contained the following units of account: (i) license and know-how for FTD- GRN in Phase 3 clinical development, (ii) the research and development activities, including license rights and know-how, relating to products in Phase 2 or earlier stages of development, and (iii) research and development services under the co-development plan to be accounted for outside of ASC 606, including all products that move into Phase 3 clinical development. The Company determined that the distinct performance obligations under ASC 606 consisted of: (i) license and know-how to AL001 FTD- GRN , which is currently in Phase 3 clinical development and (ii) the research and development activities, including license rights and know-how, relating to products in Phase 2 or earlier stages of development. The transaction price at inception included fixed consideration consisting of the upfront payments of $700 million. All potential future milestones and other payments were considered constrained at the inception of the GSK Agreement since the Company could not conclude it was probable that a significant reversal in the amount of revenue recognized would not occur. The respective standalone value for each of the performance obligations was allocated to the transaction price. The estimated SSP of each performance obligation was determined using discounted cash flows from the expected commercialization of AL001 and AL101 and estimated research and development costs to be incurred by the Company in the initial Phase 2 clinical trials. For the license for FTD- GRN , the Company determined that GSK can benefit from the license at the time the license was granted and therefore, the related performance obligation was satisfied at a point in time. The Company determined that GSK cannot benefit from the licenses without the corresponding development services that the Company has committed to perform due the earlier stage of development for these licenses. The Company will perform research and development activities through the end of the initial Phase 2 clinical trials. Revenue will be recognized over time as the research and development activities are performed. The Company will measure progress based on actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligations. The research and development activities for products in Phase 3 clinical development were determined to be within the scope of ASC 808. Both parties will be active participants in the development, manufacturing, and commercialization of the product and are exposed to significant risks and rewards that are dependent on the commercial success of the products. The Company and GSK participate in profit and loss sharing for each program commensurate with each party's cost-sharing responsibilities during research and development. ASC 808 does not provide recognition and measurement guidance. As such, the Company determined that ASC 730, Research and Development, was appropriate to analogize to based on the nature of the cost-sharing provision of the agreement. The Company has concluded that payments to or reimbursements from GSK related to these services will be accounted for as an increase to or reduction of research and development expenses, respectively. The Company also concluded that any payments from GSK related to the profit and loss sharing arrangement (including royalties) contingent upon the commercialization of the related products will be analogized to ASC 606 and therefore, will be recognized when the related sales occur. Collaboration revenue under the GSK Agreement during the three and nine months ended September 30, 2021 was $ 179.8 million, no ne of which was included in deferred revenue at the beginning of the period. $ 173.4 million was revenue from the Phase 3 license for FTD- GRN recognized when the license and know-how was delivered following the effective date of the agreement. The deferred revenue was $ 320.2 million as of September 30, 2021. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of initial Phase 2 clinical trials. Costs associated with co-development activities performed under the agreement are included in research and development expenses in the condensed consolidated statements of operations, with any reimbursement of costs by GSK reflected as a reduction of such expenses. During the three and nine months ended September 30, 2021, the Company recognized a reduction of research and development expense of $ 1.6 million under the GSK Agreement. AbbVie The Company entered into 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 a program, 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. Collaboration revenue under the Company’s collaboration agreement with AbbVie during the three and nine months ended September 30, 2021 was $ 2.6 million and $ 13.3 million, respectively, the entire amount of which was included in deferred revenue at the beginning of the period. The deferred revenue was $ 119.0 million as of September 30, 2021. 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. The Company has had changes to the overall expected costs to satisfy the performance obligations from period to period for the AbbVie Agreement. For the three months ended September 30, 2021 , the Company had a 1 % increase in the forecast of the total expected costs for the AbbVie Agreement. For the three months ended September 30, 2021 , the increase in the overall expected costs to satisfy the performance obligation resulted in an approximately $ 1.0 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. Innovent The Company entered into an agreement in March 2020 with Innovent Biologics (Innovent) to license, develop, and commercialize AL008 in China (Innovent Agreement). AL008 is the Company’s novel antibody targeting the CD47-SIRP-alpha pathway, a potent survival pathway co-opted by tumors to evade the innate immune system. Under the terms of the Innovent Agreement, Innovent may pay the Company up to $ 11.5 million in development milestones, $ 112.5 million in sales milestones, and future royalties for any sales. The Company retains the rights to develop and commercialize AL008 outside of China. The Company has determined there is one performance obligation for the delivery of the license and will recognize revenue when it is probable that there will not be significant reversal of cumulative revenue. Development and sales milestones under the Innovent Agreement have not been included in the transaction price, as all these amounts were fully constrained as of September 30, 2021. As of September 30, 2021 , no revenue has been recognized or payments received under the Innovent Agreement.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	5. Stock-based Compensation The Company recognized stock-based compensation as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 (In thousands) (In thousands) Research and development $ 5,414 $ 4,058 $ 14,902 $ 10,631 General and administrative 4,787 3,972 13,177 10,989 Total stock-based compensation $ 10,201 $ 8,030 $ 28,079 $ 21,620 2019 Equity Incentive Plan On January 1, 2021, the Company added 3,965,813 shares to the shares reserved for issuance under the 2019 Equity Incentive Plan (2019 Plan). As of September 30, 2021 , the Company had reserved 17,811,149 shares of common stock for issuance under the 2019 Plan, of which 5,414,472 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 Weighted Weighted Aggregate Outstanding as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633 Granted 1,420,878 19.56 Exercised ( 1,800,924 ) 11.48 Forfeited ( 1,109,290 ) 14.87 Outstanding as of September 30, 2021 11,165,662 $ 14.99 8.3 $ 93,091 Exercisable as of September 30, 2021 4,294,184 $ 14.51 7.8 $ 38,208 Vested and expected to vest as of September 30, 2021 11,165,662 $ 14.99 8.3 $ 93,091 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. As of September 30, 2021 , total unrecognized stock-based compensation related to unvested stock options was $ 66.7 million, which the Company expects to recognize over a remaining weighted-average period of 2.0 years. Restricted Stock Activity Activity for the restricted stock awards and RSUs is shown below. In May 2021, the Company issued RSUs with market conditions to certain executives, which are included in the table below. The RSUs with market conditions are earned based on stock price performance and continued service by the employee. Number of Awards Weighted Unvested restricted stock awards and restricted stock units as of December 31, 2020 186,425 $ 6.95 Granted 1,305,345 14.80 Vested ( 233,781 ) 7.04 Forfeited ( 92,373 ) 14.51 Unvested restricted stock awards and restricted stock units as of September 30, 2021 1,165,616 $ 15.13 As of September 30, 2021 , total unrecognized stock-based compensation related to unvested restricted stock awards and restricted stock units was $ 15.6 million, which the Company expects to recognize over a remaining weighted-average period of 2.4 years. 2019 Employee Stock Purchase Plan The 2019 Employee Stock Purchase Plan (2019 ESPP) enables eligible employees of the Company to purchase shares of common stock at a discount. As of September 30, 2021 , the Company has reserved for issuance 2,439,958 shares of common stock pursuant to the 2019 ESPP. Each offering period is approximately six months long. 2019 ESPP participants will 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.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	6. Related Party Transactions The Company has a collaboration agreement with Adimab, LLC (Adimab) under which the Company is developing antibodies discovered by Adimab in its AL001 and AL101 programs, and the Company is developing antibodies optimized by Adimab in its AL002 and AL003 programs (2014 Adimab Agreement). In August 2019, the Company signed a new collaboration agreement with Adimab for research and development of additional antibodies (2019 Adimab Agreement). The Chief Executive Officer of Adimab is a Co-founder and Chairperson of the board of directors of the Company. For the three and nine months ended September 30, 2021 , the Company incurred expenses of zero and $ 1.0 million for a milestone payment for first patient dosed in the AL002 Phase 2 trial, respectively. For the three and nine months ended September 30, 2020 , the Company incurred no expenses to Adimab. The Company had no accrued liabilities due to Adimab as of September 30, 2021 and December 31, 2020. Under the 2014 Adimab Agreement, the Company will owe up to $ 3.5 million in milestone payments per program to Adimab for its product candidates. The Company will also owe low to mid single-digit royalty payments for commercial sales of such product candidates. Under the 2019 Adimab Agreement, the Company will owe certain milestone payments per program for its product candidates and low single-digit royalty payments for commercial sales of such product candidates.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	7. Net Income (Loss) Per Share The following tables set forth the computation of the basic and diluted net income (loss) per share (in thousands, except share and per share data. Three Months Ended Nine Months Ended 2021 2020 2021 2020 Numerator: Net income (loss) $ 126,597 $ ( 52,704 ) $ 19,279 $ ( 138,049 ) Denominator: Weighted-average number of shares outstanding basic 80,964,701 78,771,930 80,048,758 77,340,896 Dilutive effect of outstanding common stock options, ESPP shares issuable, and restricted stock units 4,267,989 — 2,822,496 — Weighted-average number of shares outstanding, diluted 85,232,690 78,771,930 82,871,254 77,340,896 Net income (loss) per share, basic $ 1.56 $ ( 0.67 ) $ 0.24 $ ( 1.78 ) Net income (loss) per share, diluted $ 1.49 $ ( 0.67 ) $ 0.23 $ ( 1.78 ) Potentially dilutive securities have been excluded from the calculation of diluted net income (loss) per share for the period presented due to their anti-dilutive effect: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Restricted stock subject to future vesting — 415,521 — 415,521 Restricted stock with market conditions subject to future vesting 466,000 — 466,000 — Options to purchase common stock 1,690,083 10,033,270 4,485,816 10,033,270 Shares committed under 2019 ESPP — 25,822 — 25,822 Total 2,156,083 10,474,613 4,951,816 10,474,613

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed 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). In the opinion of management, these unaudited condensed consolidated financial statements include all normal, recurring adjustments that are necessary to present fairly the results of the interim periods presented. The condensed consolidated financial statements include the accounts of Alector, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on February 25, 2021.
Use of Estimates	Use of Estimates The preparation of condensed 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 condensed 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	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	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 significant unrealized gains or losses on the money market funds for the periods presented. Restricted cash as of September 30, 2021 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 condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows: Nine Months Ended September 30, 2021 2020 (In thousands) Cash and cash equivalents $ 516,532 $ 84,979 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 518,004 $ 86,451
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments include cash and cash equivalents, marketable securities, accounts payable, and accrued liabilities. The Company’s financial instruments approximate fair value due to their relatively short maturities. 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.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when control of promised goods or services is 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. If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling price (SSP). The relative SSP for each deliverable is estimated using external sourced evidence if it is available. If external sourced evidence is not available, we use our best estimate of the SSP for the deliverable. The Company recognizes collaboration revenue at a point in time if control of the promised good or service has been transferred to the customer. The Company recognizes collaboration revenue over time 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 re-evaluates the estimate of expected costs to satisfy the performance obligation each reporting period.
Stock-based Compensation	Stock-based Compensation Stock-based compensation is measured on the grant date based on the fair value of the awards. The fair value of options to purchase common stock are measured using the Black-Scholes option-pricing model. Stock-based compensation associated with restricted stock units (RSUs) is based on the fair value of the Company's common stock on the grant date, which equals the closing price of the Company's common stock on the grant date. The Company recognizes expense over the vesting period of the awards. Expense for options and RSUs are recognized on a straight-line basis. The Company also granted RSUs with market conditions to certain executives. The fair value of the RSUs with market conditions are estimated using a Monte Carlo simulation model. Assumptions and estimates utilized in the model include the stock price on grant date, risk-free interest rate, dividend yield, expected stock volatility, and the estimated period to achieve the market condition. The expense is recognized based on continued employment of the participants, regardless of achievement of the market condition. Expense related to the RSUs with market conditions is recognized using the accelerated attribution method. The Company accounts for forfeitures as they occur for all awards.
Comprehensive Loss	Comprehensive Income (Loss) Comprehensive Income (Loss) includes net income (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.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	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 condensed consolidated statements of cash flows: Nine Months Ended September 30, 2021 2020 (In thousands) Cash and cash equivalents $ 516,532 $ 84,979 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 518,004 $ 86,451

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
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: September 30, 2021 Fair Value Amortized Unrealized Unrealized Fair Market (In thousands) Money market funds Level 1 $ 219,553 $ — $ — $ 219,553 U.S. government treasury securities Level 1 261,358 61 ( 103 ) 261,316 Total cash equivalents and $ 480,911 $ 61 $ ( 103 ) $ 480,869 December 31, 2020 Fair Value Amortized Unrealized Unrealized Fair Market (In thousands) Money market funds Level 1 $ 37,951 $ — $ — $ 37,951 U.S. government treasury securities Level 1 357,725 620 ( 6 ) 358,339 Corporate bonds Level 2 5,000 $ — $ — 5,000 Total cash equivalents and $ 400,676 $ 620 $ ( 6 ) $ 401,290

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Summary of Recognized Stock-Based Compensation	The Company recognized stock-based compensation as follows: Three Months Ended Nine Months Ended 2021 2020 2021 2020 (In thousands) (In thousands) Research and development $ 5,414 $ 4,058 $ 14,902 $ 10,631 General and administrative 4,787 3,972 13,177 10,989 Total stock-based compensation $ 10,201 $ 8,030 $ 28,079 $ 21,620
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 Weighted Weighted Aggregate Outstanding as of December 31, 2020 12,654,998 $ 13.96 8.7 $ 39,633 Granted 1,420,878 19.56 Exercised ( 1,800,924 ) 11.48 Forfeited ( 1,109,290 ) 14.87 Outstanding as of September 30, 2021 11,165,662 $ 14.99 8.3 $ 93,091 Exercisable as of September 30, 2021 4,294,184 $ 14.51 7.8 $ 38,208 Vested and expected to vest as of September 30, 2021 11,165,662 $ 14.99 8.3 $ 93,091
Summary of Restricted Stock Awards and Restricted Stock Units	Activity for the restricted stock awards and RSUs is shown below. In May 2021, the Company issued RSUs with market conditions to certain executives, which are included in the table below. The RSUs with market conditions are earned based on stock price performance and continued service by the employee. Number of Awards Weighted Unvested restricted stock awards and restricted stock units as of December 31, 2020 186,425 $ 6.95 Granted 1,305,345 14.80 Vested ( 233,781 ) 7.04 Forfeited ( 92,373 ) 14.51 Unvested restricted stock awards and restricted stock units as of September 30, 2021 1,165,616 $ 15.13

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Income (Loss) per Share	The following tables set forth the computation of the basic and diluted net income (loss) per share (in thousands, except share and per share data. Three Months Ended Nine Months Ended 2021 2020 2021 2020 Numerator: Net income (loss) $ 126,597 $ ( 52,704 ) $ 19,279 $ ( 138,049 ) Denominator: Weighted-average number of shares outstanding basic 80,964,701 78,771,930 80,048,758 77,340,896 Dilutive effect of outstanding common stock options, ESPP shares issuable, and restricted stock units 4,267,989 — 2,822,496 — Weighted-average number of shares outstanding, diluted 85,232,690 78,771,930 82,871,254 77,340,896 Net income (loss) per share, basic $ 1.56 $ ( 0.67 ) $ 0.24 $ ( 1.78 ) Net income (loss) per share, diluted $ 1.49 $ ( 0.67 ) $ 0.23 $ ( 1.78 )
Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	Potentially dilutive securities have been excluded from the calculation of diluted net income (loss) per share for the period presented due to their anti-dilutive effect: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Restricted stock subject to future vesting — 415,521 — 415,521 Restricted stock with market conditions subject to future vesting 466,000 — 466,000 — Options to purchase common stock 1,690,083 10,033,270 4,485,816 10,033,270 Shares committed under 2019 ESPP — 25,822 — 25,822 Total 2,156,083 10,474,613 4,951,816 10,474,613

The Company and Liquidity - Add

The Company and Liquidity - Additional Information (Details) $ / shares in Units, $ in Millions	Jan. 30, 2020USD ($)$ / sharesshares
Follow-on-offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	9,602,500
Share price | $ / shares	$ 25
Proceeds from issuance of common stock, net of underwriting discounts and commissions and offering expenses | $	$ 224.5
Over-Allotment Option
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offerings	1,252,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Sep. 30, 2021	Sep. 30, 2021	Jul. 01, 2021
Summary Of Significant Accounting Policies [Line Items]
Increase in expected cost percentage			1.00%
Reduced revenue			$ 1,000
Deferred revenue			320,200	$ 320,200	$ 0
AbbVie Biotechnology Limited
Summary Of Significant Accounting Policies [Line Items]
Upfront payments		$ 205,000
Upfront payment received	$ 200,000	5,000
Exclusive option rights exercised for each program		250,000
Collaboration revenue			2,600	13,300
Deferred revenue			$ 119,000	119,000
AbbVie Biotechnology Limited | Maximum
Summary Of Significant Accounting Policies [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval		$ 242,800
Innovent
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue				0
Development milestone				11,500
Sales milestone				112,500
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Unrealized gains or loss on money market funds				$ 0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 516,532	$ 49,969	$ 84,979
Restricted cash	1,472	1,472	1,472
Total cash, cash equivalents, and restricted cash	$ 518,004	$ 51,441	$ 86,451	$ 91,113

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
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	$ 219,553	$ 37,951
Cash equivalents and marketable securities, Unrealized Gains	0
Cash equivalents and marketable securities, Unrealized Losses	0
Cash equivalents and marketable securities, Fair Market Value	219,553	37,951
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	261,358	357,725
Cash equivalents and marketable securities, Unrealized Gains	61	620
Cash equivalents and marketable securities, Unrealized Losses	(103)	(6)
Cash equivalents and marketable securities, Fair Market Value	261,316	358,339
Corporate Bonds | Fair Value, Inputs, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost		5,000
Cash equivalents and marketable securities, Fair Market Value		5,000
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	480,911	400,676
Cash equivalents and marketable securities, Unrealized Gains	61	620
Cash equivalents and marketable securities, Unrealized Losses	(103)	(6)
Cash equivalents and marketable securities, Fair Market Value	$ 480,869	$ 401,290

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Remaining contractual maturities of investments were less than one year	$ 332,400		$ 332,400
Remaining contractual maturities of investments were less than one year through two years	148,500		148,500
Sale of marketable secuirities	$ 10,700	$ 0	$ 10,696	$ 0

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Details) $ in Thousands	Aug. 17, 2021	Jul. 01, 2021USD ($)	Jan. 31, 2018USD ($)	Oct. 31, 2017USD ($)	Mar. 31, 2022USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		$ 0				$ 320,200		$ 320,200
Research and development expense						$ 43,066	$ 43,819	$ 136,617	$ 112,486
Increase in expected cost percentage						1.00%
Reduced revenue						$ 1,000
GlaxoSmithKline plc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Effective date of agreement	Aug. 17, 2021
Upfront payments		$ 700
Upfront payment received					$ 200,000	500,000
Percentage of development cost								0.60
Collaboration revenue						179,800		$ 179,800
Research and development expense						1,600		1,600
GlaxoSmithKline plc | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval						1,500,000		$ 1,500,000
GlaxoSmithKline plc | Phase 3 License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue						173,400
Alector
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of development cost								0.40
AbbVie Biotechnology Limited
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payments				$ 205,000
Upfront payment received			$ 200,000	5,000
Exclusive option rights exercised for each program				250,000
Collaboration revenue						2,600		$ 13,300
Deferred revenue						$ 119,000		119,000
AbbVie Biotechnology Limited | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval				$ 242,800
Innovent
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue								0
Development milestone								11,500
Sales milestone								$ 112,500

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 10,201	$ 8,030	$ 28,079	$ 21,620
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	5,414	4,058	14,902	10,631
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 4,787	$ 3,972	$ 13,177	$ 10,989

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ in Millions	Feb. 06, 2019	Sep. 30, 2021	Jan. 01, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected weighted average period		2 years
Unrecognized stock-based compensation		$ 66.7
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance		17,811,149	3,965,813
Common stock, shares available for issuance		5,414,472
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance		2,439,958
Purchase price of common stock, percentage	85.00%
Subsequent offering period	6 months
Restricted Stock Awards and Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation		$ 15.6
Expected weighted average period		2 years 4 months 24 days

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Options to Purchase Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Exercised	(286)
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance		12,654,998
Number of Options, Granted		1,420,878
Number of Options, Exercised		(1,800,924)
Number of Options, Forfeited		(1,109,290)
Number of Options, Outstanding, ending balance		11,165,662	12,654,998
Number of Options, Exercisable		4,294,184
Number of Options, Vested and expected to vest		11,165,662
Weighted average exercise price, Outstanding, beginning balance		$ 13.96
Weighted average exercise price, Granted		19.56
Weighted average exercise price, Exercised		11.48
Weighted average exercise price, Forfeited		14.87
Weighted average exercise price, Outstanding, ending balance		14.99	$ 13.96
Weighted average exercise price, Exercisable		14.51
Weighted average exercise price, Vested and expected to vest		$ 14.99
Weighted Average Remaining Contractual Term, Outstanding (in years)		8 years 3 months 18 days	8 years 8 months 12 days
Weighted Average Remaining Contractual Term, Exercisable (In years)		7 years 9 months 18 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)		8 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding		$ 93,091	$ 39,633
Aggregate Intrinsic Value, Exercisable		38,208
Aggregate Intrinsic Value, Vested or expected to vest		$ 93,091

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Restricted Stock Awards and Restricted Stock Units (Details) - Restricted Stock Awards and Restricted Stock Units	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Number of Shares
Number of Shares, Unvested, beginning balance | shares	186,425
Number of Shares, Granted | shares	1,305,345
Number of Shares, Vested | shares	(233,781)
Number of Shares, Forfeited | shares	(92,373)
Number of Shares, Unvested, ending balance | shares	1,165,616
Weighted-average grant-date fair value
Weighted Average Grant Date Fair Value per Share, Unvested, beginning balance | $ / shares	$ 6.95
Weighted Average Grant Date Fair Value per Share, Granted | $ / shares	14.80
Weighted Average Grant Date Fair Value per Share, Vested | $ / shares	7.04
Weighted Average Grant Date Fair Value per Share, Forfeited | $ / shares	14.51
Weighted Average Grant Date Fair Value per Share, Unvested, ending balance | $ / shares	$ 15.13

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($)	Jan. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Future milestone payments owed		$ 3,500,000		$ 3,500,000
Follow-on-offering
Related Party Transaction [Line Items]
Public offerings	9,602,500
Share price	$ 25
Two Thousand Fourteen Adimab Agreement
Related Party Transaction [Line Items]
Related party transaction expenses		0	$ 0	1,000	$ 0
Adimab
Related Party Transaction [Line Items]
Accrued liabilities due to related parties		$ 0		$ 0		$ 0

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computation of Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net income (loss)	$ 126,597	$ (55,147)	$ (52,171)	$ (52,704)	$ (45,326)	$ (40,019)	$ 19,279	$ (138,049)
Denominator:
Weighted average number of shares outstanding basic	80,964,701			78,771,930			80,048,758	77,340,896
Dilutive effect of outstanding common stock options, ESPP shares issuable, and restricted stock units	4,267,989						2,822,496
Weighted Average Number of Shares Outstanding, Diluted, Total	85,232,690			78,771,930			82,871,254	77,340,896
Net income (loss) per share, basic	$ 1.56			$ (0.67)			$ 0.24	$ (1.78)
Net income (loss) per share, diluted	$ 1.49			$ (0.67)			$ 0.23	$ (1.78)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	2,156,083	10,474,613	4,951,816	10,474,613
Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	0	415,521	0	415,521
Restricted Stock With Market Conditions Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	466,000		466,000
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	1,690,083	10,033,270	4,485,816	10,033,270
Shares Committed Under 2019 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	0	25,822	0	25,822