Alector (ALEC) 10-Q 12 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 01, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	822933343
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	California
Entity Address, Postal Zip Code	94080
City Area Code	415
Local Phone Number	231-5660
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		68,831,857

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
CURRENT ASSETS:
Cash and cash equivalents	$ 69,208	$ 65,470
Marketable securities	341,725	224,938
Prepaid expenses and other current assets	3,664	2,768
Total current assets	414,597	293,176
Property and equipment, net	32,014	10,937
Operating lease right-of-use assets	29,058
Restricted cash	1,472	1,472
Other assets	110	2,774
TOTAL ASSETS	477,251	308,359
CURRENT LIABILITIES:
Accounts payable	540	126
Accrued clinical supply costs	5,981	4,463
Accrued liabilities	14,577	8,439
Deferred revenue, current portion	40,871	34,905
Deferred rent, current portion		15
Operating lease liabilities, current portion	5,133
Total current liabilities	67,102	47,948
Deferred revenue, long-term portion	121,227	139,715
Deferred rent, long-term portion		7,478
Operating lease liabilities, long-term portion	42,797
Other long-term liabilities	409	96
TOTAL LIABILITIES	231,535	195,237
Convertible preferred stock; $0.0001 par value; zero and 45,849,677 shares authorized as of June 30, 2019 and December 31, 2018, respectively; zero and 45,374,836 shares issued and outstanding as of June 30, 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 June 30, 2019 and December 31, 2018, respectively; 68,831,857 and 13,764,829 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	7	1
Additional paid-in capital	402,823	17,078
Accumulated other comprehensive income (loss)	441	(42)
Accumulated deficit	(157,555)	(114,435)
TOTAL STOCKHOLDERS’ EQUITY (DEFICIT)	245,716	(97,398)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK, AND STOCKHOLDERS’ EQUITY (DEFICIT)	$ 477,251	$ 308,359

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Jun. 30, 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	68,831,857	13,764,829
Common stock, shares outstanding	68,831,857	13,764,829

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue:
Total revenue	$ 6,917	$ 7,109	$ 12,522	$ 11,860
Type of Revenue [Extensible List]	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Grant revenue				$ 169
Total revenue	$ 6,917	$ 7,109	$ 12,522	12,029
Operating expenses:
Research and development	25,640	16,818	46,247	28,542
General and administrative	8,429	2,522	14,188	4,943
Total operating expenses	34,069	19,340	60,435	33,485
Loss from operations	(27,152)	(12,231)	(47,913)	(21,456)
Other income, net	2,592	1,110	4,793	1,898
Net loss	(24,560)	(11,121)	(43,120)	(19,558)
Unrealized income (loss) on marketable securities	333	62	483	(92)
Comprehensive loss	$ (24,227)	$ (11,059)	$ (42,637)	$ (19,650)
Net loss per share, basic and diluted	$ (0.36)	$ (1)	$ (0.77)	$ (1.78)
Shares used in computing net loss per share, basic and diluted	67,327,975	11,157,024	55,643,532	11,008,568

Condensed Consolidated Statem_2

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
Forfeiture 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, 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
Net loss	(19,558)
Ending balance at Jun. 30, 2018	(69,398)		$ 1	12,438	(92)	(81,745)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2018		40,943,028
Ending balance (in shares) at Jun. 30, 2018			13,764,829
Convertible preferred stock, ending balance at Jun. 30, 2018		$ 147,569
Beginning balance at Mar. 31, 2018	(59,500)		$ 1	11,277	(154)	(70,624)
Convertible preferred stock, beginning balance (in shares) at Mar. 31, 2018		36,001,203
Convertible preferred stock, beginning balance at Mar. 31, 2018		$ 77,485
Beginning balance (in shares) at Mar. 31, 2018			13,764,829
Issuance of Series E convertible preferred stock, net of issuance costs		$ 70,084
Issuance of Series E convertible preferred stock, net of costs (in shares)		4,941,825
Stock-based compensation	1,161			1,161
Accumulated other comprehensive income	62				62
Net loss	(11,121)					(11,121)
Ending balance at Jun. 30, 2018	(69,398)		$ 1	12,438	(92)	(81,745)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2018		40,943,028
Ending balance (in shares) at Jun. 30, 2018			13,764,829
Convertible preferred stock, ending balance at Jun. 30, 2018		$ 147,569
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
Forfeiture 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)
Ending balance (in shares) at Mar. 31, 2019			68,871,792
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
Net loss	$ (43,120)
Ending balance at Jun. 30, 2019	$ 245,716		$ 7	402,823	441	(157,555)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2019	0
Ending balance (in shares) at Jun. 30, 2019	68,831,857		68,831,857
Beginning balance at Mar. 31, 2019	$ 266,186		$ 7	399,066	108	(132,995)
Beginning balance (in shares) at Mar. 31, 2019			68,871,792
Exercise of stock options	63		$ 8,999	63
Forfeiture of restricted common stock (in shares)			(48,934)
Stock-based compensation	3,694			3,694
Accumulated other comprehensive income	333				333
Net loss	(24,560)					(24,560)
Ending balance at Jun. 30, 2019	$ 245,716		$ 7	$ 402,823	$ 441	$ (157,555)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2019	0
Ending balance (in shares) at Jun. 30, 2019	68,831,857		68,831,857

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (43,120)	$ (19,558)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,119	464
Stock-based compensation	6,939	2,285
Accretion of discount on marketable securities	(2,857)	(953)
Impairment loss on right-of-use asset	1,158
Loss from disposal of property and equipment, net	39	89
Changes in operating assets and liabilities:
Accounts receivable		238
Prepaid expenses and other current assets	(896)	(1,636)
Other assets	17	153
Accounts payable	414	(830)
Accrued liabilities and accrued clinical supply costs	5,843	(949)
Deferred revenue	(12,522)	188,140
Deferred rent	(44)	(1)
Lease liabilities and other long-term liabilities	2,292	35
Net cash provided by (used in) operating activities	(41,618)	167,477
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(11,393)	(1,310)
Purchase of marketable securities	(339,447)	(268,328)
Maturities of marketable securities	226,000	50,000
Net cash used in investing activities	(124,840)	(219,638)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	170,128	0
Proceeds from issuance of convertible preferred stock, net of issuance costs		70,092
Proceeds from the exercise of options to purchase common stock	68
Net cash provided by financing activities	170,196	70,092
Net increase in cash, cash equivalents, and restricted cash	3,738	17,931
Cash, cash equivalents, and restricted cash at beginning of period	66,942	32,451
Cash, cash equivalents, and restricted cash at end of period	70,680	50,382
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accrued liabilities	2,849	8
Issuance costs for convertible preferred stock included in accrued liabilities		3
Deferred offering costs for initial public offering included in accounts payable and accrued liabilities		104
Tenant improvements paid by landlord	$ 8,286	$ 0

The Company and Liquidity

The Company and Liquidity	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company and Liquidity	1. 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 Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. 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 June 30, 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: Six Months Ended June 30, 2019 2018 (In thousands) Cash and cash equivalents $ 69,208 $ 48,910 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 70,680 $ 50,382 Leases The Company adopted Accounting Standards Update No. 2016-02, Leases 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 and six months ended June 30, 2019 was $6.9 million and $12.5 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 $162.1 million as of June 30, 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

Fair Value Measurements	6 Months Ended
	Jun. 30, 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: June 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 68,958 $ — $ — $ 68,958 U.S. government treasury securities Level 1 341,284 441 — 341,725 Total cash equivalents and marketable securities $ 410,242 $ 441 $ — $ 410,683 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 As of June 30, 2019, the remaining balance of all investments were less than one year.

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Property and Equipment, Net Property and equipment, net consists of the following: June 30, 2019 December 31, 2018 (In thousands) Leasehold improvements $ 23,735 $ 210 Lab equipment 7,733 4,599 Furniture and fixtures 1,870 131 Computer equipment 1,377 449 Construction-in-progress 56 7,449 Property and equipment, gross 34,771 12,838 Less accumulated depreciation and amortization (2,757 ) (1,901 ) Total property and equipment, net $ 32,014 $ 10,937 Accrued Liabilities Accrued liabilities consist of the following: June 30, 2019 December 31, 2018 (In thousands) Accrued research and development costs $ 7,814 $ 3,821 Accrued employee compensation 2,922 2,766 Accrued property and equipment 2,849 293 Accrued professional services 836 588 Accrued offering costs — 792 Other 156 179 Total accrued liabilities $ 14,577 $ 8,439

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	5. 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 “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 previously 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. In May 2019, the Company entered into an agreement to sublease approximately 25,000 square feet of the Headquarters (the “Sublease”). The term of the Sublease commenced in June 2019 and lasts for 30 months. The sublessor has the option to extend the Sublease for one additional year subject to the consent of the Company. The sublessee is required to pay its proportionate share of operating expenses for the space. In June 2019, in connection with the Sublease, the Company evaluated the related right-of-use asset for impairment. The lease costs plus amortization expense of the leasehold improvements in the subleased space exceeded the sublease income over the remaining sublease term. As such, the Company recorded an impairment charge of $1.2 million in general and administrative expenses to write-down the right-of-use asset such that no ongoing loss is expected to be recognized over the sublease period. The components of lease expense were as follows (in thousands): Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Operating lease cost $ 1,498 $ 3,183 Variable lease cost 294 478 Short-term lease cost 34 68 Sublease income and reimbursement of variable lease cost (183 ) (183 ) Total $ 1,643 $ 3,546 Rent expense under legacy leasing guidance for the three and six months ended June 30, 2018 was $0.3 million and $0.6 million, respectively. As of June 30, 2019, the weighted-average remaining lease term for operating leases was 9.8 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 June 30, 2019: (In thousands) 2019 (remaining six months) $ 2,002 2020 6,882 2021 7,122 2022 7,146 2023 7,376 Thereafter 43,146 Total undiscounted lease payments $ 73,674 Less: Present value adjustment (25,744 ) Total $ 47,930

Stock-based Compensation

Stock-based Compensation	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	6. The Company recognized stock-based compensation as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 1,876 $ 635 $ 3,580 $ 1,244 General and administrative 1,818 526 3,359 1,041 Total stock-based compensation $ 3,694 $ 1,161 $ 6,939 $ 2,285 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 (464,434 ) 6.95 Forfeited (56,973 ) 6.95 Unvested restricted common stock as of June 30, 2019 1,396,441 $ 6.95 As of June 30, 2019, total unrecognized stock-based compensation related to unvested restricted common stock was $6.2 million, which the Company expects to recognize over a remaining weighted-average period of 1.9 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 June 30, 2019, the Company had reserved 16,578,390 shares of common stock for issuance under the 2019 Plan, of which 7,254,739 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 765,100 19.42 Exercised (9,624 ) 7.10 Forfeited (153,648 ) 10.27 Outstanding as of June 30, 2019 5,665,516 $ 10.32 9.2 $ 49,593 Exercisable as of June 30, 2019 633,851 $ 8.29 9.0 $ 6,798 Vested and expected to vest as of June 30, 2019 5,665,516 $ 10.32 9.2 $ 49,593 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 June 30, 2019, total unrecognized stock-based compensation related to unvested stock options was $33.2 million, which the Company expects to recognize over a remaining weighted-average period of 3.2 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

Related Party Transactions	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	7. 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 the Company. For the three and six months ended June 30, 2019, the Company incurred expenses of zero and $0.8 million, respectively. The total amount for the six months ended June 30, 2019 was related to milestone payments for the product candidate AL003. For the three and six months ended June 30, 2018, the Company incurred expenses of zero and less than $0.1 million, respectively, for services provided by Adimab. The Company had no payables due to Adimab as of June 30, 2019 and 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

Net Loss Per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	8. 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 and Six Months Ended June 30, 2019 2018 Convertible preferred stock — 40,943,028 Restricted stock subject to future vesting 1,396,441 2,498,339 Options to purchase common stock 5,665,516 286,750 Shares committed under ESPP 53,384 — Total 7,115,341 43,728,117

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
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, 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	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 unrealized gains or losses on the money market funds for the periods presented. Restricted cash as of June 30, 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: Six Months Ended June 30, 2019 2018 (In thousands) Cash and cash equivalents $ 69,208 $ 48,910 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 70,680 $ 50,382
Leases	Leases The Company adopted Accounting Standards Update No. 2016-02, Leases 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	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	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 and six months ended June 30, 2019 was $6.9 million and $12.5 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 $162.1 million as of June 30, 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 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.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
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: Six Months Ended June 30, 2019 2018 (In thousands) Cash and cash equivalents $ 69,208 $ 48,910 Restricted cash 1,472 1,472 Total cash, cash equivalents, and restricted cash $ 70,680 $ 50,382

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 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: June 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Fair Market Value (In thousands) Money market funds Level 1 $ 68,958 $ — $ — $ 68,958 U.S. government treasury securities Level 1 341,284 441 — 341,725 Total cash equivalents and marketable securities $ 410,242 $ 441 $ — $ 410,683 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 (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: June 30, 2019 December 31, 2018 (In thousands) Leasehold improvements $ 23,735 $ 210 Lab equipment 7,733 4,599 Furniture and fixtures 1,870 131 Computer equipment 1,377 449 Construction-in-progress 56 7,449 Property and equipment, gross 34,771 12,838 Less accumulated depreciation and amortization (2,757 ) (1,901 ) Total property and equipment, net $ 32,014 $ 10,937
Summary of Accrued Liabilities	Accrued liabilities consist of the following: June 30, 2019 December 31, 2018 (In thousands) Accrued research and development costs $ 7,814 $ 3,821 Accrued employee compensation 2,922 2,766 Accrued property and equipment 2,849 293 Accrued professional services 836 588 Accrued offering costs — 792 Other 156 179 Total accrued liabilities $ 14,577 $ 8,439

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Components of Lease Expense	The components of lease expense were as follows (in thousands): Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Operating lease cost $ 1,498 $ 3,183 Variable lease cost 294 478 Short-term lease cost 34 68 Sublease income and reimbursement of variable lease cost (183 ) (183 ) Total $ 1,643 $ 3,546
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 June 30, 2019: (In thousands) 2019 (remaining six months) $ 2,002 2020 6,882 2021 7,122 2022 7,146 2023 7,376 Thereafter 43,146 Total undiscounted lease payments $ 73,674 Less: Present value adjustment (25,744 ) Total $ 47,930

Stock-based Compensation (Table

Stock-based Compensation (Tables)	6 Months Ended
	Jun. 30, 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 June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 1,876 $ 635 $ 3,580 $ 1,244 General and administrative 1,818 526 3,359 1,041 Total stock-based compensation $ 3,694 $ 1,161 $ 6,939 $ 2,285
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 (464,434 ) 6.95 Forfeited (56,973 ) 6.95 Unvested restricted common stock as of June 30, 2019 1,396,441 $ 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 765,100 19.42 Exercised (9,624 ) 7.10 Forfeited (153,648 ) 10.27 Outstanding as of June 30, 2019 5,665,516 $ 10.32 9.2 $ 49,593 Exercisable as of June 30, 2019 633,851 $ 8.29 9.0 $ 6,798 Vested and expected to vest as of June 30, 2019 5,665,516 $ 10.32 9.2 $ 49,593

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 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 and Six Months Ended June 30, 2019 2018 Convertible preferred stock — 40,943,028 Restricted stock subject to future vesting 1,396,441 2,498,339 Options to purchase common stock 5,665,516 286,750 Shares committed under ESPP 53,384 — Total 7,115,341 43,728,117

The Company and Liquidity - Add

The Company and Liquidity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 07, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 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			$ 170,128	$ 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
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 Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended	6 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Jun. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets			$ 29,058,000	$ 29,058,000
Lease liabilities			47,930,000	47,930,000
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			6,900,000	12,500,000
Deferred revenue			$ 162,100,000	162,100,000
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 Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 69,208	$ 65,470	$ 48,910
Restricted cash	1,472	1,472	1,472
Total cash, cash equivalents, and restricted cash	$ 70,680	$ 66,942	$ 50,382	$ 32,451

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 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	$ 68,958	$ 65,222
Cash equivalents and marketable securities, Fair Market Value	68,958	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	341,284	224,980
Cash equivalents and marketable securities, Unrealized Gains	441	3
Cash equivalents and marketable securities, Unrealized Losses		(45)
Cash equivalents and marketable securities, Fair Market Value	341,725	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	410,242	290,202
Cash equivalents and marketable securities, Unrealized Gains	441	3
Cash equivalents and marketable securities, Unrealized Losses		(45)
Cash equivalents and marketable securities, Fair Market Value	$ 410,683	$ 290,160

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Maximum remaining maturity period of investments	1 year

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 34,771	$ 12,838
Less accumulated depreciation and amortization	(2,757)	(1,901)
Total property and equipment, net	32,014	10,937
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	23,735	210
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	7,733	4,599
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,870	131
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,377	449
Construction-in-Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 56	$ 7,449

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development costs	$ 7,814	$ 3,821
Accrued employee compensation	2,922	2,766
Accrued property and equipment	2,849	293
Accrued professional services	836	588
Accrued offering costs		792
Other	156	179
Total accrued liabilities	$ 14,577	$ 8,439

Leases - Additional Information

Leases - Additional Information (Detail)	1 Months Ended		3 Months Ended	6 Months Ended
	May 31, 2019ft²	Jun. 30, 2018USD ($)ft²	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)ft²	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Tenant improvement allowance				$ 15,700,000
Amount form letter of credit		$ 1,472,000	$ 1,472,000	$ 1,472,000	$ 1,472,000	$ 1,472,000
Operating term				10 years
Operating lease, renewal period				10 years
Operating lease term				The lease is over a ten-year term with an option to renew for a period of ten years.
Operating sublease, impairment charges				$ 1,158,000
Rent expense			$ 300,000		$ 600,000
Weighted average remaining lease term for operating leases				9 years 9 months 18 days
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	25,000			16,000
Lease expiration date				2019-04
Sublease commencement date				2019-06
Sublease term	30 months
Operating sublease, option to extend				The sublessor has the option to extend the Sublease for one additional year subject to the consent of the Company.
Sublease renewal period	1 year
Operating sublease, existence of option to extend				true
Ongoing loss over sublease period				$ 0
Offices and Laboratory Facilities | South San Francisco | General and Administrative Expense
Lessee Lease Description [Line Items]
Operating sublease, impairment charges				$ 1,200,000
Laboratory Facilities | Milpitas
Lessee Lease Description [Line Items]
Area under lease | ft²				9,000
Operating lease, renewal period				4 years
Operating lease term				The Company also leases approximately 9,000 square feet of laboratory facilities in Milpitas under an agreement that ends in January 2022 with an option to extend for four years.
Lease expiration date				2022-01

Leases - Components of Lease Ex

Leases - Components of Lease Expense (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating lease cost	$ 1,498	$ 3,183
Variable lease cost	294	478
Short-term lease cost	34	68
Sublease income and reimbursement of variable lease cost	(183)	(183)
Total	$ 1,643	$ 3,546

Leases - Schedule of Undiscount

Leases - Schedule of Undiscounted Cash Flows Owned Under Company's Operating Leases (Detail) $ in Thousands	Jun. 30, 2019USD ($)
Leases [Abstract]
2019 (remaining six months)	$ 2,002
2020	6,882
2021	7,122
2022	7,146
2023	7,376
Thereafter	43,146
Total undiscounted lease payments	73,674
Less: Present value adjustment	(25,744)
Total	$ 47,930

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 3,694	$ 1,161	$ 6,939	$ 2,285
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	1,876	635	3,580	1,244
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,818	$ 526	$ 3,359	$ 1,041

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Restricted Common Stock (Details) - Restricted Common Stock	6 Months Ended
	Jun. 30, 2019$ / sharesshares
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	(464,434)
Number of Shares, Forfeited | shares	(56,973)
Number of Shares, Unvested, ending balance | shares	1,396,441
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, Forfeited | $ / shares	6.95
Weighted Average Grant Date Fair Value per Share, Unvested, ending balance | $ / shares	$ 6.95

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ in Millions	Feb. 06, 2019	Jun. 30, 2019
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period		3 years 2 months 12 days
Common stock reserved for issuance	7,688,156	16,578,390
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	7,254,739
Unrecognized stock-based compensation		$ 33.2
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		$ 6.2
Expected weighted average period		1 year 10 months 24 days

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Options to Purchase Common Stock (Details) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 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	765,100
Number of Options, Exercised	(9,624)
Number of Options, Forfeited	(153,648)
Number of Options, Outstanding, ending balance	5,665,516	5,063,688
Number of Options, Exercisable	633,851
Number of Options, Vested and expected to vest	5,665,516
Weighted average exercise price, Outstanding, beginning balance	$ 8.94
Weighted average exercise price, Granted	19.42
Weighted average exercise price, Exercised	7.10
Weighted average exercise price, Forfeited	10.27
Weighted average exercise price, Outstanding, ending balance	10.32	$ 8.94
Weighted average exercise price, Exercisable	8.29
Weighted average exercise price, Vested and expected to vest	$ 10.32
Weighted Average Remaining Contractual Term, Outstanding	9 years 2 months 12 days	9 years 7 months 6 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	9 years
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	9 years 2 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 49,593	$ 6,067
Aggregate Intrinsic Value, Exercisable	6,798
Aggregate Intrinsic Value, Vested or expected to vest	$ 49,593

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Adimab - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Related party transaction expenses	$ 0	$ 0	$ 800,000
Related parties accounts payable related to milestones	0		0		$ 0
Maximum
Related Party Transaction [Line Items]
Related party transaction expenses				$ 100,000
Future milestone payments owed	$ 3,500,000		$ 3,500,000

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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	7,115,341	43,728,117	7,115,341	43,728,117
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares		40,943,028		40,943,028
Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	1,396,441	2,498,339	1,396,441	2,498,339
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	5,665,516	286,750	5,665,516	286,750
Shares Committed Under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	53,384		53,384