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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	ALEC
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 Common Stock, Shares Outstanding		68,871,792

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
CURRENT ASSETS:
Cash and cash equivalents	$ 60,954	$ 65,470
Marketable securities	375,641	224,938
Prepaid expenses and other current assets	5,519	2,768
Total current assets	442,114	293,176
Property and equipment, net	21,733	10,937
Operating lease right-of-use assets	30,665
Restricted cash	1,472	1,472
Other assets	60	2,774
TOTAL ASSETS	496,044	308,359
CURRENT LIABILITIES:
Accounts payable	1,200	126
Accrued clinical supply costs	3,723	4,463
Accrued liabilities	8,000	8,439
Deferred revenue, current portion	38,526	34,905
Deferred rent, current portion		15
Operating lease liabilities, current portion	4,385
Total current liabilities	55,834	47,948
Deferred revenue, long-term portion	130,489	139,715
Deferred rent, long-term portion		7,478
Operating lease liabilities, long-term portion	43,439
Other long-term liabilities	96	96
TOTAL LIABILITIES	229,858	195,237
Convertible preferred stock; $0.0001 par value; zero and 45,849,677 shares authorized as of March 31, 2019 and December 31, 2018, respectively; zero and 45,374,836 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively		210,520
STOCKHOLDERS’ EQUITY (DEFICIT):
Common stock, $0.0001 par value; 200,000,000 and 65,000,000 shares authorized as of March 31, 2019 and December 31, 2018, respectively; 68,871,792 and 13,764,829 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively	7	1
Additional paid-in capital	399,066	17,078
Accumulated other comprehensive income (loss)	108	(42)
Accumulated deficit	(132,995)	(114,435)
TOTAL STOCKHOLDERS’ EQUITY (DEFICIT)	266,186	(97,398)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK, AND STOCKHOLDERS’ EQUITY (DEFICIT)	$ 496,044	$ 308,359

Condensed Consolidated Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Mar. 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	68,871,792	13,764,829
Common stock, shares outstanding	68,871,792	13,764,829

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue:
Total revenue	$ 5,605	$ 4,751
Type of Revenue [Extensible List]	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Grant revenue		$ 169
Total revenue	$ 5,605	4,920
Operating expenses:
Research and development	20,607	11,724
General and administrative	5,759	2,421
Total operating expenses	26,366	14,145
Loss from operations	(20,761)	(9,225)
Other income, net	2,201	788
Net loss	(18,560)	(8,437)
Unrealized income (loss) on marketable securities	150	(154)
Comprehensive loss	$ (18,410)	$ (8,591)
Net loss per share, basic and diluted	$ (0.42)	$ (0.78)
Shares used in computing net loss per share, basic and diluted	43,828,106	10,858,473

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (52,033)		$ 1	$ 10,153		$ (62,187)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2017		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017			13,776,153
Cancellation of restricted common stock (in shares)			(11,324)
Stock-based compensation	1,124			1,124
Accumulated other comprehensive income	(154)				$ (154)
Net loss	(8,437)					(8,437)
Ending balance at Mar. 31, 2018	(59,500)		$ 1	11,277	(154)	(70,624)
Convertible preferred stock, ending balance (in shares) at Mar. 31, 2018		36,001,203
Ending balance (in shares) at Mar. 31, 2018			13,764,829
Convertible preferred stock, ending balance at Mar. 31, 2018		$ 77,485
Beginning balance at Dec. 31, 2018	$ (97,398)		$ 1	17,078	(42)	(114,435)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018	45,374,836	45,374,836
Convertible preferred stock, beginning balance at Dec. 31, 2018	$ 210,520	$ 210,520
Beginning 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	5			5
Exercise of stock options (in shares)			625
Cancellation of restricted common stock (in shares)			(8,039)
Stock-based compensation	3,245			3,245
Accumulated other comprehensive income	150				150
Net loss	(18,560)					(18,560)
Ending balance at Mar. 31, 2019	$ 266,186		$ 7	$ 399,066	$ 108	$ (132,995)
Convertible preferred stock, ending balance (in shares) at Mar. 31, 2019	0
Ending balance (in shares) at Mar. 31, 2019	68,871,792		68,871,792

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (18,560)	$ (8,437)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	298	212
Stock-based compensation	3,245	1,124
Accretion of discount on marketable securities	(1,321)	(352)
Loss from disposal of property and equipment, net		76
Changes in operating assets and liabilities:
Accounts receivable		69
Prepaid expenses and other current assets	(2,751)	(1,051)
Other assets	17	0
Accounts payable	683	(783)
Accrued liabilities and accrued clinical supply costs	(952)	(1,616)
Deferred revenue	(5,605)	195,249
Deferred rent	(44)	(1)
Lease liabilities and other long-term liabilities	1,424	35
Net cash provided by (used in) operating activities	(23,566)	184,525
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(2,030)	(613)
Purchase of marketable securities	(267,232)	(178,902)
Maturities of marketable securities	118,000	0
Net cash used in investing activities	(151,262)	(179,515)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	170,307	0
Payment of convertible preferred stock issuance costs		(62)
Proceeds from the exercise of options to purchase common stock	5
Net cash provided by (used in) financing activities	170,312	(62)
Net increase (decrease) in cash, cash equivalents, and restricted cash	(4,516)	4,948
Cash, cash equivalents, and restricted cash at beginning of period	66,942	32,451
Cash, cash equivalents, and restricted cash at end of period	62,426	37,399
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued liabilities	1,071	109
Issuance costs for convertible preferred stock included in accrued liabilities		33
Deferred offering costs for initial public offering included in accounts payable and accrued liabilities	179
Tenant improvements paid by landlord	$ 8,286	$ 0

The Company and Liquidity

3 Months Ended

Mar. 31, 2019

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 biotechnology company focused on harnessing the immune system to cure neurodegenerative diseases.

Initial Public Offering

On February 7, 2019, the Company completed an initial public offering (“IPO”) through issuing and selling 9,739,541 shares of common stock at a public offering price of $19.00 per share, including 489,541 shares sold pursuant to the underwriters’ partial exercise of their option to purchase additional shares. The aggregate net proceeds received by the Company from the offering, net of underwriting discounts and commissions and offering expenses, were $168.2 million. Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 45,374,836 shares of common stock. Subsequent to the closing of the IPO, there were no shares of convertible preferred stock outstanding.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

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, 2018, 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 March 26, 2019.

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 unrealized gains or losses on the money market funds for the periods presented.

Restricted cash as of March 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 condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows:

	Three Months Ended March 31,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	60,954	$	37,399
Restricted cash		1,472		—
Total cash, cash equivalents, and restricted cash	$	62,426	$	37,399

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 increase through March 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 from its balance sheets recognition of 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.

Fair Value of Financial Instruments

The Company’s financial instruments include cash and cash equivalents, receivables, 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 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 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. Collaboration revenue under the Company’s collaboration agreement with AbbVie during the three months ended March 31, 2019 and 2018 was $5.6 million and $4.8 million, respectively, the entire amount of which was included in deferred revenue at the beginning of the respective periods. The Company recorded deferred revenue of $169.0 million as of March 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.

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 losses on marketable securities.

Fair Value Measurements

3 Months Ended

Mar. 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:

	March 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	60,703	$	—	$	—		$	60,703
U.S. government treasury securities	Level 1		375,533		111		(3	)		375,641
Total cash equivalents and marketable securities		$	436,236	$	111	$	(3	)	$	436,344

	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

- 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

3 Months Ended

Mar. 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:

	March 31, 2019			December 31, 2018
	(In thousands)
Lab equipment	$	4,905		$	4,599
Computer equipment		538			449
Leasehold improvements		210			210
Office equipment		131			131
Construction-in-progress		18,147			7,449
Property and equipment, gross		23,931			12,838
Less accumulated depreciation and amortization		(2,198	)		(1,901	)
Total property and equipment, net	$	21,733		$	10,937

Accrued Liabilities

Accrued liabilities consist of the following:

	March 31, 2019		December 31, 2018
	(In thousands)
Accrued research and development costs	$	4,566	$	3,821
Accrued employee compensation		2,009		2,766
Accrued property and equipment		810		293
Accrued professional services		401		588
Accrued offering costs		49		792
Other		165		179
Total accrued liabilities	$	8,000	$	8,439

Leases

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Leases

In June 2018, the Company signed a lease agreement to lease approximately 105,000 square feet in new office and laboratory space in South San Francisco which serves as the new headquarters. The lease is over a ten-year term with an option to renew for a period of ten years. The Company occupied the premises and began to make rent payments on May 1, 2019. The lease commencement date was in January 2019, as that is when the Company was given control of the space. The landlord paid for $15.7 million of tenant improvements. In connection with the lease, the Company entered into a letter of credit arrangement in the amount of $1.5 million as collateral for the lease, which is classified as restricted cash on the consolidated balance sheets. The Company leased approximately 16,000 square feet as its headquarters with its main offices and laboratory facilities in South San Francisco under a sublease agreement that ended in April 2019. 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. None of the leases included the options to extend in the calculation of the lease liabilities as such extensions are not reasonably certain to occur. Variable lease costs for all of the Company’s leases consist of operating expenses for the spaces.

The components of lease expense were as follows (in thousands):

	Three Months Ended March 31, 2019
Operating lease cost	$	1,685
Variable lease cost		184
Short-term lease cost		34
Total	$	1,903

Rent expense under legacy leasing guidance for the three months ended March 31, 2018 was $0.3 million.

As of March 31, 2019, the weighted-average remaining lease term for operating leases was 10 years and the weighted-average discount rate was 8.8%.

The following are the undiscounted cash flows owed under the Company’s operating leases as of March 31, 2019:

	(In thousands)
2019 (remaining nine months)	$	2,945
2020		6,882
2021		7,122
2022		7,146
2023		7,376
Thereafter		43,146
Total undiscounted lease payments	$	74,617
Less: Present value adjustment		(26,793	)
Total	$	47,824

Stock-based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

6.	Stock-based Compensation

The Company recognized stock-based compensation as follows:

	Three Months Ended March 31,
	2019		2018
	(In thousands)
Research and development	$	1,704	$	609
General and administrative		1,541		515
Total stock-based compensation	$	3,245	$	1,124

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, 2018		1,917,848		$	6.95
Vested		(240,427	)		6.95
Cancelled/forfeited		(8,039	)		6.95
Unvested restricted common stock as of March 31, 2019		1,669,382		$	6.95

As of March 31, 2019, total unrecognized stock-based compensation related to unvested restricted common stock was $7.5 million, which the Company expects to recognize over a remaining weighted-average period of 2.4 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 and a 25% one-year cliff. As of March 31, 2019, the Company had reserved 16,587,389 shares of common stock for issuance under the 2019 Plan, of which 7,404,136 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 (In years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2018		5,063,688		$	8.94	9.6	$	6,067
Granted		422,600			19.50
Exercised		(625	)		8.16
Cancelled/forfeited		(9,479	)		9.20
Outstanding as of March 31, 2019		5,476,184		$	9.75	9.4	$	49,417
Exercisable as of March 31, 2019		273,561		$	8.10	9.2	$	4,662
Vested and expected to vest as of March 31, 2019		5,476,184		$	9.75	9.4	$	49,417

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 March 31, 2019, total unrecognized stock-based compensation related to unvested stock options was $32.4 million, which the Company expects to recognize over a remaining weighted-average period of 3.3 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 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 will end on the last trading day on or before December 1, 2019. Each subsequent offering period will be approximately six months long. 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

3 Months Ended

Mar. 31, 2019

Related Party Transactions

7.	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 product candidates, and the Company is developing antibodies optimized by Adimab in its AL002 and AL003 product candidates. The Chief Executive Officer of Adimab is a Co-Founder and Chairperson of the board of directors of Alector. For the three months ended March 31, 2019 and 2018, Alector incurred expenses of $0.8 million and less than $0.1 million, respectively, for services provided by Adimab. The total amount for the three months ended March 31, 2019 was related to milestone payments for the product candidate AL003. As of March 31, 2019, the Company had an accrued liabilities balance of $0.5 million related to one of the milestones achieved near the end of the first quarter. There were no payables as of December 31, 2018. 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.

Net Loss Per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

8.	Net Loss Per Share

The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect:

	Three Months Ended March 31,
	2019		2018
Convertible preferred stock		—		36,001,203
Restricted stock subject to future vesting		1,669,382		2,770,409
Options to purchase common stock		5,476,184		100,000
Shares committed under ESPP		58,127		—
Total		7,203,693		38,871,612

Subsequent Events

3 Months Ended

Mar. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

9.	Subsequent Event

In May 2019, the Company entered in a sublease agreement with a third party to sublease approximately 25,000 square feet of the Company’s office and lab space. The sublease will commence on June 1, 2019 with a term of 30 months. The third party is obligated to pay the Company base rent of $4.5 million and a prorated portion of operating expenses over the term of the sublease.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 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 March 31, 2019 relates to a letter of credit established for a lease entered into in June 2018.

	Three Months Ended March 31,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	60,954	$	37,399
Restricted cash		1,472		—
Total cash, cash equivalents, and restricted cash	$	62,426	$	37,399

Leases

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.

Revenue Recognition

Comprehensive Loss

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash

	Three Months Ended March 31,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	60,954	$	37,399
Restricted cash		1,472		—
Total cash, cash equivalents, and restricted cash	$	62,426	$	37,399

Fair Value Measurements (Tables)

3 Months Ended

Mar. 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:

	March 31, 2019
	Fair Value Hierarchy	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds	Level 1	$	60,703	$	—	$	—		$	60,703
U.S. government treasury securities	Level 1		375,533		111		(3	)		375,641
Total cash equivalents and marketable securities		$	436,236	$	111	$	(3	)	$	436,344

	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)

3 Months Ended

Mar. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	March 31, 2019			December 31, 2018
	(In thousands)
Lab equipment	$	4,905		$	4,599
Computer equipment		538			449
Leasehold improvements		210			210
Office equipment		131			131
Construction-in-progress		18,147			7,449
Property and equipment, gross		23,931			12,838
Less accumulated depreciation and amortization		(2,198	)		(1,901	)
Total property and equipment, net	$	21,733		$	10,937

Summary of Accrued Liabilities

Accrued liabilities consist of the following:

	March 31, 2019		December 31, 2018
	(In thousands)
Accrued research and development costs	$	4,566	$	3,821
Accrued employee compensation		2,009		2,766
Accrued property and equipment		810		293
Accrued professional services		401		588
Accrued offering costs		49		792
Other		165		179
Total accrued liabilities	$	8,000	$	8,439

Leases (Tables)

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Components of Lease Expense

The components of lease expense were as follows (in thousands):

	Three Months Ended March 31, 2019
Operating lease cost	$	1,685
Variable lease cost		184
Short-term lease cost		34
Total	$	1,903

Schedule of Undiscounted Cash Flows Owned Under Company's

The following are the undiscounted cash flows owed under the Company’s operating leases as of March 31, 2019:

	(In thousands)
2019 (remaining nine months)	$	2,945
2020		6,882
2021		7,122
2022		7,146
2023		7,376
Thereafter		43,146
Total undiscounted lease payments	$	74,617
Less: Present value adjustment		(26,793	)
Total	$	47,824

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Recognized Stock-Based Compensation

The Company recognized stock-based compensation as follows:

	Three Months Ended March 31,
	2019		2018
	(In thousands)
Research and development	$	1,704	$	609
General and administrative		1,541		515
Total stock-based compensation	$	3,245	$	1,124

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, 2018		1,917,848		$	6.95
Vested		(240,427	)		6.95
Cancelled/forfeited		(8,039	)		6.95
Unvested restricted common stock as of March 31, 2019		1,669,382		$	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 (In years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2018		5,063,688		$	8.94	9.6	$	6,067
Granted		422,600			19.50
Exercised		(625	)		8.16
Cancelled/forfeited		(9,479	)		9.20
Outstanding as of March 31, 2019		5,476,184		$	9.75	9.4	$	49,417
Exercisable as of March 31, 2019		273,561		$	8.10	9.2	$	4,662
Vested and expected to vest as of March 31, 2019		5,476,184		$	9.75	9.4	$	49,417

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share

The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect:

	Three Months Ended March 31,
	2019		2018
Convertible preferred stock		—		36,001,203
Restricted stock subject to future vesting		1,669,382		2,770,409
Options to purchase common stock		5,476,184		100,000
Shares committed under ESPP		58,127		—
Total		7,203,693		38,871,612

The Company and Liquidity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Feb. 07, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from issuance of common stock upon initial public offering, net of issuance costs		$ 170,307	$ 0
Convertible preferred stock, shares outstanding		0		45,374,836
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issuing and selling of common stock		9,739,541
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issuing and selling of common stock	9,739,541
Share price	$ 19.00
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	$ 168,200
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]
Shares issuing and selling of common stock	489,541

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets			$ 30,665,000
Lease liabilities			47,824,000
Deferred Rent						$ 7,478,000
AbbVie
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
Collaboration revenue			5,600,000	$ 4,800,000
Deferred revenue			169,000,000
AbbVie \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval		$ 242,800,000
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets					$ 31,400,000
Lease liabilities					$ 38,900,000
Deferred Rent						$ 7,500,000
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Unrealized gains or loss on money market funds			$ 0

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 60,954	$ 65,470	$ 37,399
Restricted cash	1,472	1,472
Total cash, cash equivalents, and restricted cash	$ 62,426	$ 66,942	$ 37,399	$ 32,451

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Money Market Funds \| Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	$ 60,703	$ 65,222
Cash equivalents and marketable securities, Fair Market Value	60,703	65,222
U.S Government Treasury Securities [Member] \| Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	375,533	224,980
Cash equivalents and marketable securities, Unrealized Gains	111	3
Cash equivalents and marketable securities, Unrealized Losses	(3)	(45)
Cash equivalents and marketable securities, Fair Market Value	375,641	224,938
Cash Equivalents and Marketable Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	436,236	290,202
Cash equivalents and marketable securities, Unrealized Gains	111	3
Cash equivalents and marketable securities, Unrealized Losses	(3)	(45)
Cash equivalents and marketable securities, Fair Market Value	$ 436,344	$ 290,160

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 23,931	$ 12,838
Less accumulated depreciation and amortization	(2,198)	(1,901)
Total property and equipment, net	21,733	10,937
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,905	4,599
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	538	449
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	210	210
Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	131	131
Construction-in-Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 18,147	$ 7,449

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development costs	$ 4,566	$ 3,821
Accrued employee compensation	2,009	2,766
Accrued property and equipment	810	293
Accrued professional services	401	588
Accrued offering costs	49	792
Other	165	179
Total accrued liabilities	$ 8,000	$ 8,439

Leases - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended
Leases - Additional Information (Detail) $ in Thousands	Jun. 30, 2018 ft²	Mar. 31, 2019 USD ($) ft²	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Lessee Lease Description [Line Items]
Tenant improvement allowance \| $		$ 15,700
Amount form letter of credit \| $		$ 1,472		$ 1,472
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.
Rent expense \| $			$ 300
Weighted average remaining lease term for operating leases		10 years
Weighted average discount rate for operating leases		8.80%
Offices and Laboratory Facilities \| South San Francisco
Lessee Lease Description [Line Items]
Area under lease \| ft²	105,000
Area under lease \| ft²		16,000
Lease expiration date		2019-04
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

Leases - Components of Lease Expense (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 1,685
Variable lease cost	184
Short-term lease cost	34
Total	$ 1,903

Leases - Schedule of Undiscounted Cash Flows Owned Under Company's Operating Leases (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
2019 (remaining nine months)	$ 2,945
2020	6,882
2021	7,122
2022	7,146
2023	7,376
Thereafter	43,146
Total undiscounted lease payments	74,617
Less: Present value adjustment	(26,793)
Total	$ 47,824

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 3,245	$ 1,124
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	1,704	609
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,541	$ 515

Stock-based Compensation - Summary of Restricted Common Stock (Details) - Restricted Common Stock	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, beginning balance \| shares	1,917,848
Number of Shares, Vested \| shares	(240,427)
Number of Shares, Cancelled/forfeited \| shares	(8,039)
Number of Shares, Unvested, ending balance \| shares	1,669,382
Weighted Average Grant Date Fair Value per Share, Unvested, beginning balance \| $ / shares	$ 6.95
Weighted Average Grant Date Fair Value per Share, Vested \| $ / shares	6.95
Weighted Average Grant Date Fair Value per Share, Cancelled/forfeited \| $ / shares	6.95
Weighted Average Grant Date Fair Value per Share, Unvested, ending balance \| $ / shares	$ 6.95

Stock-based Compensation - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended
	Feb. 06, 2019	Mar. 31, 2019
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period		3 years 3 months 18 days
Common stock reserved for issuance	7,688,156	16,587,389
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
Vesting period	4 years
Cliff vesting percentage	25.00%
Cliff vesting period	1 year
Common stock, shares available for issuance	7,404,136
Unrecognized stock-based compensation		$ 32.4
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
Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation		$ 7.5
Expected weighted average period		2 years 4 months 24 days

Stock-based Compensation - Summary of Options to Purchase Common Stock (Details) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance	5,063,688
Number of Options, Granted	422,600
Number of Options, Exercised	(625)
Number of Options, Cancelled/forfeited	(9,479)
Number of Options, Outstanding, ending balance	5,476,184	5,063,688
Number of Options, Exercisable	273,561
Number of Options, Vested and expected to vest	5,476,184
Weighted average exercise price, Outstanding, beginning balance	$ 8.94
Weighted average exercise price, Granted	19.50
Weighted average exercise price, Exercised	8.16
Weighted average exercise price, Cancelled/forfeited	9.20
Weighted average exercise price, Outstanding, ending balance	9.75	$ 8.94
Weighted average exercise price, Exercisable	8.10
Weighted average exercise price, Vested and expected to vest	$ 9.75
Weighted Average Remaining Contractual Term, Outstanding	9 years 4 months 24 days	9 years 7 months 6 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	9 years 2 months 12 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	9 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding	$ 49,417	$ 6,067
Aggregate Intrinsic Value, Exercisable	4,662
Aggregate Intrinsic Value, Vested or expected to vest	$ 49,417

Related Party Transactions - Additional Information (Details) - Adimab - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Related party transaction expenses	$ 800,000
Related parties accounts payable related to milestones			$ 0
Related parties accrued liabilities related to milestones	500,000
Maximum
Related Party Transaction [Line Items]
Related party transaction expenses		$ 100,000
Future milestone payments owed	$ 3,500,000

Net Loss Per Share - Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	7,203,693	38,871,612
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares		36,001,203
Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	1,669,382	2,770,409
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	5,476,184	100,000
Shares Committed Under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	58,127

Subsequent Events - Additional Information (Details) - Offices and Laboratory Facilities - Subsequent Event $ in Millions	1 Months Ended
	May 31, 2019 USD ($) ft²
Subsequent Event [Line Items]
Area under sublease \| ft²	25,000
Sublease commencement date	Jun. 01, 2019
Sublease term	30 months
Base rent for sublease \| $	$ 4.5