View Filing Data

Cover	9 Months Ended
Cover	Sep. 30, 2019
Cover page.
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
CURRENT ASSETS:
Cash and cash equivalents	$ 86,974	$ 65,470	$ 32,451
Marketable securities	294,472	224,938
Receivable from collaboration partner			200,000
Accounts receivable			238
Prepaid expenses and other current assets	4,037	2,768	285
Total current assets	385,483	293,176	232,974
Property and equipment, net	33,861	10,937	2,834
Operating lease right-of-use assets	28,765
Restricted cash	1,472	1,472
Other assets	149	2,774	252
TOTAL ASSETS	449,730	308,359	236,060
CURRENT LIABILITIES:
Accounts payable	258	126	1,140
Accrued clinical supply costs	6,412	4,463	3,820
Accrued liabilities	16,546	8,439	3,455
Deferred revenue, current portion	29,527	34,905	18,978
Deferred rent, current portion		15	10
Operating lease liabilities, current portion	5,975
Total current liabilities	58,718	47,948	27,403
Deferred revenue, long-term portion	129,875	139,715	183,150
Deferred rent, long-term portion		7,478	41
Operating lease liabilities, long-term portion	42,141
Other long-term liabilities	409	96	14
TOTAL LIABILITIES	231,143	195,237	210,608
Commitments and Contingencies
Convertible preferred stock		210,520	77,485
STOCKHOLDERS' DEFICIT:
Common stock, par value	7	1	1
Additional paid-in capital	407,647	17,078	10,153
Accumulated other comprehensive income (loss)	226	(42)
Accumulated deficit	(189,293)	(114,435)	(62,187)
TOTAL STOCKHOLDERS' DEFICIT	218,587	(97,398)	(52,033)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK, AND STOCKHOLDERS' DEFICIT	$ 449,730	$ 308,359	$ 236,060

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	45,849,677	36,001,203
Convertible preferred stock, shares issued	0	45,374,836	36,001,203
Convertible preferred stock, shares outstanding	0	45,374,836	36,001,203
Liquidation preference		$ 211,054
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	65,000,000	52,000,000
Common stock, shares issued	68,922,249	13,764,829	13,776,153
Common stock, shares outstanding	68,922,249	13,764,829	13,776,153

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Collaboration revenue	$ 2,696	$ 6,503	$ 15,218	$ 18,363	$ 27,508	$ 2,872
Grant revenue				169	169	863	$ 416
Total revenue	2,696	6,503	15,218	18,532	27,677	3,735	416
Operating expenses:
Research and development	28,519	20,392	74,766	48,934	73,031	29,911	13,674
General and administrative	8,326	2,926	22,514	7,869	11,934	6,503	1,874
Total operating expenses	36,845	23,318	97,280	56,803	84,965	36,414	15,548
Loss from operations	(34,149)	(16,815)	(82,062)	(38,271)	(57,288)	(32,679)	(15,132)
Other income, net	2,411	1,498	7,204	3,396	5,040	199	22
Net loss	(31,738)	(15,317)	(74,858)	(34,875)	(52,248)	(32,480)	(15,110)
Unrealized gain (loss) on marketable securities	(215)	(48)	268	(140)	(42)
Comprehensive loss	$ (31,953)	$ (15,365)	$ (74,590)	$ (35,015)	$ (52,290)	$ (32,480)	$ (15,110)
Net loss per share, basic and diluted	$ (0.47)	$ (1.34)	$ (1.25)	$ (3.13)	$ (4.62)	$ (3.55)	$ (2.11)
Shares used in computing net loss per share, basic and diluted	67,572,452	11,441,285	59,663,773	11,154,391	11,302,788	9,142,688	7,173,441

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Preferred Units	Convertible Preferred Stock	Common Units	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ (11,760)					$ 2,837		$ (14,597)
Preferred units, beginning balance (in shares) at Dec. 31, 2015		36,001,203
Preferred units, beginning balance at Dec. 31, 2015		$ 77,485
Common units, beginning balance (in shares) at Dec. 31, 2015				10,707,167
Issuance of common units				1,136,500
Cancellation of common units				(326,082)
Stock-based compensation	1,963					1,963
Net loss	(15,110)							(15,110)
Ending balance at Dec. 31, 2016	(24,907)					4,800		(29,707)
Preferred units, ending balance (in shares) at Dec. 31, 2016		36,001,203
Preferred units, ending balance at Dec. 31, 2016		$ 77,485
Common units, ending balance (in shares) at Dec. 31, 2016				11,517,585
Issuance of common units				2,483,500
Cancellation of common units				(8,333)
Conversion from LLC to corporation		$ (77,485)	$ 77,485		$ 1	(1)
Conversion from llc to corporation (units)		(36,001,203)		(13,992,752)
Conversion from llc to corporation (in shares)			36,001,203		13,825,387
Forfeiture of restricted common stock (in shares)					(49,234)
Stock-based compensation	5,354					5,354
Net loss	(32,480)							(32,480)
Ending balance at Dec. 31, 2017	$ (52,033)				$ 1	10,153		(62,187)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2017	36,001,203		36,001,203
Convertible preferred stock, ending balance at Dec. 31, 2017	$ 77,485		$ 77,485
Ending balance (in shares) at Dec. 31, 2017	13,776,153				13,776,153
Forfeiture of restricted common stock (in shares)					(11,324)
Stock-based compensation	$ 1,124					1,124
Unrealized gain on marketable securities	(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
Convertible preferred stock, ending balance at Mar. 31, 2018			$ 77,485
Ending balance (in shares) at Mar. 31, 2018					13,764,829
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		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017	$ 77,485		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017	13,776,153				13,776,153
Net loss	$ (34,875)
Ending balance at Sep. 30, 2018	(82,783)				$ 1	14,418	(140)	(97,062)
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2018			45,350,215
Convertible preferred stock, ending balance at Sep. 30, 2018			$ 210,170
Ending balance (in shares) at Sep. 30, 2018					13,764,829
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		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017	$ 77,485		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017	13,776,153				13,776,153
Issuance of Series E convertible preferred stock, net of issuance costs			$ 133,035
Issuance of Series E convertible preferred stock, net of costs (in shares)			9,373,633
Forfeiture of restricted common stock (in shares)					(11,324)
Stock-based compensation	$ 6,925					6,925
Unrealized gain on marketable securities	(42)						(42)
Net loss	(52,248)							(52,248)
Ending balance at Dec. 31, 2018	$ (97,398)				$ 1	17,078	(42)	(114,435)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2018	45,374,836		45,374,836
Convertible preferred stock, ending balance at Dec. 31, 2018	$ 210,520		$ 210,520
Ending balance (in shares) at Dec. 31, 2018	13,764,829				13,764,829
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
Unrealized gain on marketable securities	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
Convertible preferred stock, ending balance at Jun. 30, 2018			$ 147,569
Ending balance (in shares) at Jun. 30, 2018					13,764,829
Issuance of Series E convertible preferred stock, net of issuance costs			$ 62,601
Issuance of Series E convertible preferred stock, net of costs (in shares)			4,407,187
Stock-based compensation	1,980					1,980
Unrealized gain on marketable securities	(48)						(48)
Net loss	(15,317)							(15,317)
Ending balance at Sep. 30, 2018	(82,783)				$ 1	14,418	(140)	(97,062)
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2018			45,350,215
Convertible preferred stock, ending balance at Sep. 30, 2018			$ 210,170
Ending balance (in shares) at Sep. 30, 2018					13,764,829
Net loss	(17,373)
Ending balance at Dec. 31, 2018	$ (97,398)				$ 1	17,078	(42)	(114,435)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2018	45,374,836		45,374,836
Convertible preferred stock, ending balance at Dec. 31, 2018	$ 210,520		$ 210,520
Ending balance (in shares) at Dec. 31, 2018	13,764,829				13,764,829
Conversion of convertible preferred stock into common stock	$ 210,521		$ (210,520)		$ 5	210,516
Conversion of convertible preferred stock into common stock (in shares)			(45,374,836)		45,374,836
Issuance of common stock upon initial public offering, net of issuance costs	168,223				$ 1	168,222
Issuance of common stock upon initial public offering, net of issuance costs (in shares)					9,739,541
Exercise of stock options	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
Unrealized gain on marketable securities	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	$ (74,858)
Ending balance at Sep. 30, 2019	$ 218,587				$ 7	407,647	226	(189,293)
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2019	0
Ending balance (in shares) at Sep. 30, 2019	68,922,249				68,922,249
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					63
Exercise of stock options (in shares)					8,999
Forfeiture of restricted common stock (in shares)					(48,934)
Stock-based compensation	3,694					3,694
Unrealized gain on marketable securities	333						333
Net loss	(24,560)							(24,560)
Ending balance at Jun. 30, 2019	245,716				$ 7	402,823	441	(157,555)
Ending balance (in shares) at Jun. 30, 2019					68,831,857
Exercise of stock options	720					720
Exercise of stock options (in shares)					90,392
Stock-based compensation	4,104					4,104
Unrealized gain on marketable securities	(215)						(215)
Net loss	(31,738)							(31,738)
Ending balance at Sep. 30, 2019	$ 218,587				$ 7	$ 407,647	$ 226	$ (189,293)
Convertible preferred stock, ending balance (in shares) at Sep. 30, 2019	0
Ending balance (in shares) at Sep. 30, 2019	68,922,249				68,922,249

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

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (74,858)	$ (34,875)	$ (52,248)	$ (32,480)	$ (15,110)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,397	743	1,019	680	201
Stock-based compensation	11,043	4,265	6,925	5,354	1,963
Accretion of discount on marketable securities	(4,027)	(1,778)	(2,745)
Impairment loss on right-of-use asset	1,158
Loss from disposal of property and equipment, net	39	89	151
Changes in operating assets and liabilities:
Accounts receivable		238	238	(20)	(218)
Prepaid expenses and other current assets	(1,269)	(3,638)	(2,483)	(150)	(22)
Other assets	(72)	132	106		(184)
Accounts payable	132	(604)	(809)	(41)	708
Accrued liabilities and accrued clinical supply costs	7,467	(1,125)	4,743	6,716	(354)
Deferred revenue	(15,218)	181,637	172,492	2,128
Deferred rent	(44)	(1)	(7)	28	23
Lease liabilities and other long-term liabilities	2,771	35
Other long-term liabilities			82	14
Net cash provided by (used in) operating activities	(70,481)	145,118	127,464	(17,771)	(12,993)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(13,692)	(1,467)	(1,884)	(801)	(2,250)
Purchase of marketable securities	(411,239)	(395,117)	(472,235)
Maturities of marketable securities	346,000	130,000	250,000
Net cash used in investing activities	(78,931)	(266,584)	(224,119)	(801)	(2,250)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	170,128	(278)
Proceeds from issuance of convertible preferred stock, net of issuance costs		132,704	133,050	(15)	(77)
Proceeds from the exercise of options to purchase common stock	788
Payment of deferred offering costs for initial public offering			(1,904)
Net cash provided by (used in) financing activities	170,916	132,426	131,146	(15)	(77)
Net increase (decrease) in cash, cash equivalents, and restricted cash	21,504	10,960	34,491	(18,587)	(15,320)
Cash, cash equivalents, and restricted cash at beginning of period	66,942	32,451	32,451	51,038	66,358
Cash, cash equivalents, and restricted cash at end of period	88,446	43,411	66,942	32,451	51,038
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued liabilities	3,675	30	293	353	$ 176
Deferred offering costs for initial public offering included in accounts payable and accrued liabilities		729
Issuance costs for convertible preferred stock included in accounts payable				$ 53
Deferred offering costs for initial public offering included in accrued liabilities			792
Tenant improvements paid by landlord	$ 8,286	$ 401	$ 7,449

The Company and Liquidity

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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.

1.	The Company and Liquidity

Conversion

Alector was originally formed in May 2013 as a Delaware limited liability company under the name Alector LLC. In October 2017, the Company completed a reorganization whereby the Company converted from a Delaware limited liability company named Alector LLC to a Delaware corporation named under the name Alector, Inc. (the “Conversion”). In conjunction with the Conversion, (i) all of the Company’s outstanding common units converted on a 1-for-1 basis into shares of common stock, par value $0.0001; (ii) all of the Company’s outstanding preferred units converted on a 1-for-1 basis into shares of convertible preferred stock, par value $0.0001; and (iii) the Company’s 202,924 unvested restricted units converted on a 1-for-1 basis into shares of unvested restricted common stock. Prior to the Conversion, the Company had issued profit interest units to employees. The Company’s vested profit interest units converted on a net issuance basis into shares of common stock and the Company’s unvested profit interest units converted on a net issuance basis into restricted common stock. Fractional shares related to the conversion of profit interest grants were settled in cash. All vesting provisions remained the same following the Conversion.

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 estimated offering expenses, were $168.2 million. Upon the closing of the IPO, all of the outstanding shares of convertible preferred stock automatically converted into 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

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 elsewhere in this prospectus.

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 September 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:


	Nine Months Ended September 30,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	86,974	$	41,939
Restricted cash		1,472		1,472

Total cash, cash equivalents, and restricted cash	$	88,446	$	43,411

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 September 30, 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, 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. We re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Clinical trials are expensive and can take many years to complete, and the outcome is inherently uncertain. Changes in our forecasted costs are likely to occur over time based upon changes in clinical trial procedures set forth in protocols, changes in estimates of manufacturing costs, or feedback from regulators on the design or operation of our clinical trials. Collaboration revenue under the Company’s collaboration agreement with AbbVie during the three and nine months ended September 30, 2019 was $2.7 million and $15.2 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 $159.4 million as of September 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 gain (loss) on marketable securities.

2.	Summary of Significant Accounting Policies

Basis of Presentation

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

Use of Estimates

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

Concentration of Credit Risk

Cash, Cash Equivalents, and Restricted Cash

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

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


	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Cash and cash equivalents	$	65,470	$	32,451	$	50,838
Restricted cash		1,472		—		200

Total cash, cash equivalents, and restricted cash	$	66,942	$	32,451	$	51,038

Marketable Securities

All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. The Company considers its available-for-sale portfolio as available for use in current operations. Accordingly, the Company may classify certain investments as short-term marketable securities, even though the stated maturity date may be one year or more beyond the current balance sheet date. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive income and reported as a separate component of stockholders’ deficit until realized. Interest income, realized gains and losses, and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income, net. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. In accordance with our investment policy, management only invests in money market funds and U.S. treasury securities. The Company has not experienced any losses on its deposits of cash, cash equivalents, and marketable securities.

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

Fair Value of Financial Instruments

The Company’s 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.

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

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

Property and Equipment

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

Impairment of Long-Lived Assets

The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If the total of the undiscounted future cash flows is less than the carrying amount of the assets, an impairment loss is recognized for the amount by which the carrying amount of the assets exceeds is fair value. The Company has not identified any such impairment losses to date.

Revenue Recognition

The Company early adopted the new revenue accounting standard (“ASC 606”) using the full retrospective method effective January 1, 2017, which retrospectively applied the standard to all prior periods presented. Under ASC 606, an entity recognizes revenue when control of promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under arrangements, the Company performs the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies the performance obligation.

Collaboration Revenue

The Company signed an agreement in October 2017 with AbbVie to co-develop antibodies to two program targets in preclinical development. Under the terms of the 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 agreement, the Company will be eligible to earn up to an additional $242.8 million in milestone payments per program related to the initiation of certain clinical studies and regulatory approval for up to three indications per program. The Company assessed its collaboration agreement with AbbVie in the context of the delivery of the research and development services.

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

The Company recognizes collaboration revenue by measuring the progress toward complete satisfaction of the performance obligation using an input measure. In order to recognize revenue over the research and development period, the Company measures actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligation. Revenues are recognized as the program costs are incurred. The Company will re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Collaboration revenue under the Company’s collaboration agreement with AbbVie during 2018 and 2017 was $27.5 million and $2.9 million, respectively. All revenue recognized during the year ended December 31, 2018, was included in deferred revenue at the beginning of the period. The Company recorded deferred revenue of $174.6 million as of December 31, 2018. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of Phase 2 clinical trials.

Grant Revenue

The Company has grant revenue from the U.S. government. In March 2016, the National Institute on Aging, a division of the National Institute of Health of the U.S. government, awarded the Company a Small Business Innovation Research grant. The Company recognized $0.2 million, $0.9 million, and $0.4 million for the years ended December 31, 2018, 2017, and 2016, respectively, related to research performed under the grant. The Company recognizes grant revenue as the related research services are performed.

Research and Development Costs

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

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

Stock-based Compensation

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

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

Comprehensive Loss

Income Taxes

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

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

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

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

Employee 401(k) Plan

The Company has a qualified contributory savings plan under Section 401(k) of the Internal Revenue Code (the “Code”) covering substantially all U.S. employees of Alector. The 401(k) plan is designed to provide tax-deferred retirement benefits in accordance with the provisions of Section 401(k) of the Code. Eligible employees may defer up to 100% of their eligible compensation up to the annual maximum as determined by the Internal Revenue Service. The Company’s contributions to the plan are discretionary. For the years ended December 31, 2018 and 2017, the Company did not make any contributions to the plan.

Segments

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

Deferred Offering Costs

Offering costs, consisting of legal, accounting, printer, and filing fees related to the IPO were deferred and offset against proceeds from the IPO upon the effectiveness of the offering. As of December 31, 2018, $2.7 million of deferred offering costs were recorded as other assets in the accompanying consolidated balance sheets. There were no deferred offering costs for the IPO as of December 31, 2017.

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. The Company will make an accounting policy election to account for leases with a term of 12 months or less similar to existing guidance for operating leases. The Company will adopt the new standard effective January 1, 2019 on a modified retrospective basis and will not restate comparative periods. Management estimates that the adoption of this standard will result in recognition of approximately $27 – $32 million as a right-of-use asset for leased facilities and recognition of approximately $44 – $50 million as a corresponding liability for the lease payments remaining on the lease less tenant improvement incentives on the consolidated balance sheets as of March 31, 2019. Management does not expect a material change to the consolidated statement of operations and comprehensive loss or cash flows.

Fair Value Measurements

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

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


	September 30, 2019
	Fair Value Hierarchy		Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds		Level 1	$	76,549	$	—	$	—		$	76,549
U.S. government treasury securities		Level 1		299,336		242		(16	)		299,562
Corporate bonds		Level 2		5,025		—		—			5,025

Total cash equivalents and marketable securities			$	380,910	$	242	$	(16	)	$	381,136


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

Total cash equivalents and marketable securities			$	290,202	$	3	$	(45	)	$	290,160

The Company’s Level 2 securities are valued using third-party pricing sources. The pricing services utilize industry standard valuation models for which all significant inputs are observable. There were no transfers between levels of the fair value hierarchy during the three and nine months ended September 30, 2019. The Company classifies marketable securities available to fund current operations as current assets. As of September 30, 2019, the remaining contractual maturities of $349.3 million investments were less than one year and $31.8 million investments were after one year through two years.

3.	Fair Value Measurements

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


	December 31, 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


	December 31, 2017
	Fair Value Hierarchy		Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Market Value
	(In thousands)
Money market funds		Level 1	$	27,201	$	—	$	—	$	27,201

Total cash equivalents and marketable securities			$	27,201	$	—	$	—	$	27,201

As of December 31, 2018, the remaining maturities of all investments were less than one year.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Balance Sheet Related Disclosures Abstract

Balance Sheet Components

4.	Balance Sheet Components

Property and Equipment, Net

Property and equipment, net consists of the following:


	September 30, 2019			December 31, 2018
	(In thousands)
Leasehold improvements	$	25,224		$	210
Lab equipment		9,110			4,599
Furniture and fixtures		2,066			131
Computer equipment		1,457			449
Construction-in-progress		39			7,449

Property and equipment, gross		37,896			12,838
Less accumulated depreciation and amortization		(4,035	)		(1,901	)

Total property and equipment, net	$	33,861		$	10,937

Accrued Liabilities

Accrued liabilities consist of the following:


	September 30, 2019		December 31, 2018
	(In thousands)
Accrued research and development costs	$	7,431	$	3,821
Accrued employee compensation		4,272		2,766
Accrued property and equipment		3,675		293
Accrued professional services		1,046		588
Accrued offering costs		—		792
Other		122		179

Total accrued liabilities	$	16,546	$	8,439

4.	Balance Sheet Components

Property and Equipment, Net

Property and equipment, net consists of the following:


	December 31,
	2018			2017
	(In thousands)
Lab equipment	$	4,599		$	3,527
Computer equipment		449			157
Leasehold improvements		210			113
Office equipment		131			17
Construction-in-progress		7,449			—

Property and equipment, gross		12,838			3,814
Less accumulated depreciation and amortization		(1,901	)		(980	)

Total property and equipment, net	$	10,937		$	2,834

Accrued Liabilities

Accrued liabilities consist of the following:


	December 31,
	2018		2017
	(In thousands)
Accrued research and development costs	$	3,821	$	1,471
Accrued employee compensation		2,766		1,294
Deferred offering costs		792		—
Accrued professional services		588		387
Accrued property and equipment		293		201
Other		179		102

Total accrued liabilities	$	8,439	$	3,455

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Operating Lease

The Company leases its headquarters with its main offices and laboratory facilities in South San Francisco under a sublease agreement that ends in April 2019. The Company also has laboratory facilities in Milpitas under an agreement that ends in January 2022. 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 that will be the new headquarters when the current lease ends in April 2019. 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 lease is over a ten-year term with an option to renew for a period of ten years. Rent increases, including the impact of a rent holiday, were recognized as deferred rent, which is included in other liabilities in the accompanying consolidated balance sheets. Rent expense is recognized on a straight-line basis over the term of the original lease.

The following are minimum future rental payments owed under the Company’s operating leases as of December 31, 2018:


	(In thousands)
2019	$	3,231
2020		6,882
2021		7,122
2022		7,146
2023 and thereafter		50,522

Total	$	74,903

Rent expense for the years ended December 31, 2018, 2017, and 2016, was $1.1 million, $1.1 million, and $0.6 million, respectively.

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Convertible Preferred Stock

6.	Convertible Preferred Stock

The Company issued 9,373,633 shares of Series E convertible preferred stock in April 2018, July 2018, and October 2018 at an issuance price of $14.2154 per share. The 24,621 shares issued in October 2018 were to a board member, a related party.

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


	December 31, 2018
	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529
Series E		9,848,474		9,373,633		14.22		133,035		133,250

Total convertible preferred stock		45,849,677		45,374,836			$	210,520	$	211,054


	December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529

Total convertible preferred stock		36,001,203		36,001,203			$	77,485	$	77,804

The Company recorded its convertible preferred stock at the issuance price on the dates of issuance, net of issuance costs. As of December 31, 2018 and 2017, the Company classifies the convertible preferred stock as temporary equity because the shares are contingently redeemable outside the control of the Company. Upon the effectiveness of the IPO, all shares of convertible preferred stock automatically converted on a 1-to-1 basis into shares of common stock.

Stock-based Compensation

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
	(In thousands)				(In thousands)
Research and development	$	1,935	$	1,276	$	5,515	$	2,520
General and administrative		2,169		704		5,528		1,745

Total stock-based compensation	$	4,104	$	1,980	$	11,043	$	4,265

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		(669,125	)		6.95
Forfeited		(56,973	)		6.95

Unvested restricted common stock as of September 30, 2019		1,191,750		$	6.95

As of September 30, 2019, total unrecognized stock-based compensation related to unvested restricted common stock was $5.4 million, which the Company expects to recognize over a remaining weighted-average period of 1.7 years.

2019 Equity Incentive Plan

On February 6, 2019, the Company adopted the 2019 Equity Incentive Plan (“2019 Plan”) under which the Board may issue incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units, and performance shares to the Company’s employees, directors, and consultants. The Company initially reserved for issuance 7,688,156 shares of common stock pursuant to the 2019 Plan. The Company’s 2017 Stock Option and Grant Plan (“2017 Plan”) was terminated; however, shares subject to awards granted under it will continue to be governed by the 2017 Plan. Shares reserved for issuance but not issued pursuant to, or not subject to, awards granted under the 2017 Plan were added to the available shares in the 2019 Plan. Shares subject to awards granted under the 2017 Plan that are repurchased by, or forfeited to, the Company will also be reserved for issuance under the 2019 Plan. The board of directors, or a committee appointed by the board of directors, has the authority to determine to whom options or shares will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the exercise price of each share shall be at least 110% of the fair market value and the term of the award shall not exceed than five years. All other options granted under the 2019 Plan must have an exercise price at least equal to the fair market value on the date of grant and have a term not to exceed ten years. The shares generally vest over a four-year period with straight-line vesting and a 25% one-year cliff. As of September 30, 2019, the Company had reserved 12,829,863 shares of common stock for issuance under the 2019 Plan, of which 6,911,739 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		1,133,100			19.17
Exercised		(100,016	)		7.89
Forfeited		(178,648	)		11.52

Outstanding as of September 30, 2019		5,918,124		$	10.84		9.0	$	26,336

Exercisable as of September 30, 2019		891,664		$	8.69		8.8	$	5,296

Vested and expected to vest as of September 30, 2019		5,918,124		$	10.84		9.0	$	26,336

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. As of September 30, 2019, total unrecognized stock-based compensation related to unvested stock options was $34.6 million, which the Company expects to recognize over a remaining weighted-average period of 2.8 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.

7.	Stock-based Compensation

The Company recognized stock-based compensation as follows:


	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Research and development	$	4,195	$	4,392	$	1,727
General and administrative		2,730		962		236

Total stock-based compensation	$	6,925	$	5,354	$	1,963

Determination of Fair Value

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


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	5.1 – 6.1	6.0 – 10.0	6.1 – 10.0
Expected volatility	73 – 81%	75%	77%
Risk-free interest rate	2.5 – 3.1%	1.9 – 2.4%	1.2 – 2.5%
Dividend yield	0%	0%	0%

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

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

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

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

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

Profit Interest Units

The profit interest units have a “strike price” and are economically similar to a stock option with an exercise price. In the event of a distribution by the Company, the proceeds distributed to the holder would be reduced by the strike price. Profit interest units generally vest 25% after one-year with the remainder vesting quarterly over the following three-year period.

Activity for the profit interest units is shown below:


	Number of Units			Weighted Average Grant Date Fair Value per Unit
Unvested as of December 31, 2016		1,205,563			1.72
Granted		2,483,500			4.76
Vested		(363,387	)		1.89
Cancelled/forfeited		(8,338	)		1.17
Exchange of unvested profit interest units for restricted common stock upon the Conversion		(3,317,338	)		3.98

Unvested as of December 31, 2017		—			—

As discussed in Note 1, on October 12, 2017, the profit interest units were all converted to common stock upon the Conversion.

Restricted Units and Restricted Common Stock

The restricted units have no strike price and generally vest 25% upfront with the remainder vesting quarterly over a four-year period. The restricted units converted into restricted common stock upon the Conversion in October 2017.

Activity for the restricted units is shown below:


	Number of Units			Weighted Average Grant Date Fair Value per Unit
Unvested restricted units as of December 31, 2016		1,480,199			0.26
Vested prior to conversion from LLC to corporation		(1,277,275	)		0.09
Exchange of unvested restricted units for restricted common stock upon the Conversion		(202,924	)		1.33

Unvested restricted units as of December 31, 2017		—			—

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, 2016		—		$	—
Exchange of unvested profit interest units for restricted common stock upon the Conversion		3,165,350			6.95
Exchange of unvested restricted units for restricted common stock upon the Conversion		202,924			6.95
Vested		(170,227	)		6.95
Cancelled/forfeited		(49,234	)		6.95

Unvested restricted common stock as of December 31, 2017		3,148,813			6.95
Vested		(1,219,641	)		6.95
Cancelled/forfeited		(11,324	)		6.95

Unvested restricted common stock as of December 31, 2018		1,917,848			6.95

In the Conversion, the profit interest units and restricted units were cancelled and replaced with replacement awards. The profit interest units were replaced on a net issuance basis and the restricted units were replaced on a 1-for-1 basis. Unvested units were replaced with restricted common stock and vested units were replaced with common stock. The grant date fair value of the replacement awards was determined to be the fair value of the common stock on the date of the Conversion. The Company’s outstanding common stock, as presented, includes shares which are subject to repurchase.

The fair value of shares vested during the year ended December 31, 2018, 2017, and 2016, was $10.5 million, $9.1 million, and $4.1 million, respectively. As of December 31, 2018, total unrecognized stock-based compensation related to unvested restricted common stock issued to employees was $8.5 million, which the Company expects to recognize over a remaining weighted-average period of 2.4 years.

2017 Stock Option and Grant Plan

On October 13, 2017, the Company adopted the 2017 Stock Option and Grant Plan (“2017 Plan”) under which the Board may issue stock options, restricted stock awards, unrestricted stock awards, and restricted stock units to employees, directors, and consultants. The board of directors has the authority to determine to whom options or stock 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 price of each share shall be at least 110% of the fair market value. Options granted under the 2017 Plan have a term of up to ten years and generally vest over a four-year period with straight-line vesting and a 25% one-year cliff. As of December 31, 2018, the Company had reserved 8,899,858 shares of common stock for issuance under the 2017 Plan, of which 121,062 shares were available for issuance.

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


	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of December 31, 2016		—		$	—
Granted		22,500			6.95

Outstanding as of December 31, 2017		22,500			6.95
Granted		5,063,584			8.94
Cancelled/forfeited		(22,396	)		8.01

Outstanding as of December 31, 2018		5,063,688			8.94		9.6	$	6,067

Exercisable as of December 31, 2018		273,561			8.09		9.4	$	561

Vested and expected to vest as of December 31, 2018		5,063,688			8.94		9.6	$	6,067

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 December 31, 2018, Alector was still not a public company and the latest board-approved stock price of $10.14 was used to estimate the intrinsic value. The fair value of shares vested during the year ended December 31, 2018, was $2.5 million. As of December 31, 2018, total unrecognized stock-based compensation related to unvested stock options was $28.9 million, which the Company expects to recognize over a remaining weighted-average period of 3.6 years.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

8.	Income Taxes

The Company was classified as a partnership for U.S. income tax purposes through the Conversion on October 12, 2017. The Company incurred net losses for the years ended December 31, 2018 and 2017. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements and has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding realization of such assets.

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


	Year Ended December 31,
	2018			2017
Tax benefit at federal statutory rate	$	(10,972	)	$	(11,038	)
Pre-conversion passthrough loss		—			7,331
Tax credits		(2,387	)		(351	)
Federal uncertain tax positions		708			102
Stock-based compensation		1,251			470
Impact of federal rate change		—			2,329
Nondeductible expense		58			126
Change of valuation allowance		11,346			782
Others		(4	)		249

Income tax provision	$	—		$	—

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


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss	$	13,867		$	4,363
Accrued bonus		487			230
Tax credits		3,098			574
Stock-based compensation		213			—
Deferred rent		1,647			—
Others		98			73

Gross deferred tax assets		19,410			5,240
Less valuation allowance		(17,286	)		(5,026	)

Total deferred tax assets	$	2,124		$	214

Deferred tax liabilities:
Depreciation and amortization	$	(2,124	)	$	(214	)

Gross deferred tax liabilities		(2,124	)		(214	)

Deferred tax assets, net	$	—		$	—

In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred assets will be realized. The ultimate realization of the deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Evaluating the need for a valuation allowance for deferred tax assets often requires judgment and analysis of all the positive and negative evidence available, including cumulative losses in recent years and projected future taxable income, to determine whether all or some portion of the deferred tax assets will not be realized. As of December 31, 2018, the Company has utilized a full valuation allowance to offset the net deferred tax assets as the Company believes it is not more likely than not that the net deferred tax assets will be fully realizable. The valuation allowance increased on a net basis by $12.3 million during the year ended December 31, 2018.

As of December 31, 2018, the Company had federal and state net operating loss (“NOL”) carryforwards of approximately $17.5 million and $17.1 million, respectively. Federal and state NOL carryforwards will begin to expire in 2037, if not utilized. Due to tax reform, the Company also has federal NOL carryforwards of $42.9 million generated after December 31, 2017, which do not expire. As of December 31, 2018, the Company also had federal and California tax credit carryforward of approximately $3.3 million and $1.1 million, respectively. The federal tax credits will begin to expire in 2036 while the California tax credits have no expiration date.

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

On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act reduces the federal corporate income tax rate from 35% to 21% effective January 1, 2018, which the Company expects will positively impact the Company’s future effective tax rate and after-tax earnings in the United States. The Company has recorded a decrease related to deferred tax assets and deferred tax liabilities, with an offset by the valuation allowance. As a result, the tax provision impact related to the federal tax rate change is zero. On December 22, 2017, Staff Account Bulletin No. 118 was issued to allow companies to record provisional amounts for those impacts, with the requirement that the accounting be completed in a period not to exceed one year from the date of enactment. As a result, as of December 31, 2017, the Company performed a provisional estimate of the effect of the Tax Act in the financial statements. The primary impact of the Tax Act resulted from the re-measurement of deferred tax assets and liabilities due to the change in the corporate tax rate, reducing our deferred tax assets by $2.3 million with a corresponding reduction in the valuation allowance. This had no effect on the Company’s effective tax rate. In the fourth quarter of 2018, we completed our analysis as of December 31, 2018, with no material changes from our provisional estimate.

The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2018 and 2017 (in thousands):


Balance as of December 31, 2016	$	—
Increases related to tax positions taken during the prior year		227
Increases related to tax positions taken during the current year		403

Balance as of December 31, 2017		630
Increases related to tax positions taken during the current year		877

Balance as of December 31, 2018	$	1,507

If the unrecognized tax benefits for uncertain tax positions as of December 31, 2018, is recognized, there will be no impact to the effective tax rate as the tax benefit would increase the deferred tax assets, which is currently offset with a full valuation allowance. The Company’s policy is to include interest and penalties related to unrecognized tax benefits, if any, within the provision for taxes in the consolidated statements of operations. The Company did not accrue any interest or penalties and does not have any tax positions for interest or penalties for the year ended December 31, 2018. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized tax benefits will significantly change within 12 months of December 31, 2018.

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

Related Party Transactions

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 (“2013 Adimab Agreement”). In August 2019, the Company signed a new collaboration agreement with Adimab for research and development of additional antibodies (“2019 Adimab Agreement”). The Chief Executive Officer of Adimab is a Co-Founder and Chairperson of the board of directors of Alector. For the three and nine months ended September 30, 2019, the Company incurred expenses under the 2013 Adimab Agreement of $2.0 million and $2.8 million, respectively, the majority of which is related to milestone payments. For the three and nine months ended September 30, 2018, the Company incurred expenses under the 2013 Adimab Agreement of $1.8 million, the majority of which is related to milestone payments. The Company had $2.0 million and zero in accrued liabilities due to Adimab as of September 30, 2019 and December 31, 2018. Under the 2013 Adimab Agreement, the Company will owe up to $3.5 million in milestone payments per program to Adimab for its product candidates. The Company will also owe low- to mid- single-digit royalty payments for commercial sales of such product candidates. Under the 2019 Adimab Agreement, the Company will owe certain milestone payments per program for its product candidates and low single-digit royalty payments for commercial sales of such product candidates.

9.	Related Party Transactions

In 2014, the Company entered into a collaboration agreement with Adimab, LLC (“Adimab”), which (as amended, and together with certain applicable option exercise letters the Company sent to Adimab, the “Adimab Collaboration Agreement”). The Company works with Adimab to discover and optimize antibodies directed against certain targets selected by the Company. Under the Adimab Collaboration Agreement, the Company is required to fund, and the Company and Adimab are required to use commercially reasonable efforts to conduct certain research to discover and optimize antibodies directed against targets selected by the Company. 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 years ended December 31, 2018, 2017, and 2016, Alector incurred expenses of $2.3 million, $0.4 million, and $0.6 million for services provided by Adimab, respectively. The year ended December 31, 2018, included expenses of $2.3 million related to milestones. 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

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 and Nine Months Ended September 30,
	2019		2018
Convertible preferred stock		—		45,350,215
Restricted stock subject to future vesting		1,191,750		2,161,047
Options to purchase common stock		5,918,124		3,048,500
Shares committed under ESPP		51,667		—

Total		7,161,541		50,559,762

10.

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:


	Year Ended December 31,
	2018		2017		2016
Convertible preferred stock		45,374,836		36,001,203		36,001,203
Restricted stock subject to future vesting		1,917,848		3,148,813		1,480,199
Options to purchase common stock		5,063,688		22,500		—
Profit interest units		—		—		1,893,201

Total		52,356,372		39,172,516		39,374,603

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

11.

Selected Quarterly Financial Data (Unaudited)

The following table provides the selected quarterly financial data for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented.


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


	Three months ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
	(in thousands, except per share data)
Revenue	$	3,059		$	153		$	280		$	243
Loss from operations	$	(9,807	)	$	(10,091	)	$	(7,286	)	$	(5,495	)
Net loss	$	(9,778	)	$	(10,033	)	$	(7,226	)	$	(5,443	)
Net loss per share, basic and diluted	$	(0.94	)	$	(1.10	)	$	(0.83	)	$	(0.66	)

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

12.

Subsequent Events

On February 7, 2019, the Company completed an 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 estimated offering expenses, were $168.2 million. Upon the closing of our IPO, all of the outstanding shares of convertible preferred stock automatically converted into shares of common stock. Subsequent to the closing of our IPO, there were no shares of convertible preferred stock outstanding.

The Company’s board of directors adopted, and the Company’s stockholders approved, the 2019 Equity Incentive Plan (the “2019 Plan”), effective February 6, 2019. The 2019 Plan provides for the grant of 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 has reserved for issuance 7,688,156 shares of common stock pursuant to the 2019 Plan.

The Company’s board of directors adopted, and the Company’s stockholders approved, the 2019 Employee Stock Option Plan (“2019 ESPP”), effective on February 6, 2019. 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.

Leases

9 Months Ended

Sep. 30, 2019

Leases [Abstract]

Leases

5.	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 “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 September 30, 2019			Nine Months Ended September 30, 2018
Operating lease cost	$	1,346		$	4,529
Variable lease cost		425			903
Short-term lease cost		—			68
Sublease income and reimbursement of variable lease cost		(540	)		(723	)

Total	$	1,231		$	4,777

Rent expense under legacy leasing guidance for the three and nine months ended September 30, 2018 was $0.2 million and $0.8 million, respectively.

As of September 30, 2019, the weighted-average remaining lease term for operating leases was 9.5 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 September 30, 2019:


	(In thousands)
2019 (remaining three months)	$	1,136
2020		6,882
2021		7,122
2022		7,146
2023		7,376
Thereafter		43,146

Total undiscounted lease payments	$	72,808
Less: Present value adjustment		(24,692	)

Total	$	48,116

Summary of Significant Accounting Policies (Policies)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Basis of Presentation

Use of Estimates

Use of Estimates

Concentration of Credit Risk

Concentration of Credit Risk

Cash, Cash Equivalents, and Restricted Cash

Cash, Cash Equivalents, and Restricted Cash

Restricted cash as of September 30, 2019 relates to a letter of credit established for a lease entered into in June 2018.


	Nine Months Ended September 30,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	86,974	$	41,939
Restricted cash		1,472		1,472

Total cash, cash equivalents, and restricted cash	$	88,446	$	43,411

Cash, Cash Equivalents, and Restricted Cash

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


	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Cash and cash equivalents	$	65,470	$	32,451	$	50,838
Restricted cash		1,472		—		200

Total cash, cash equivalents, and restricted cash	$	66,942	$	32,451	$	51,038

Marketable Securities

Marketable Securities

Fair Value of Financial Instruments

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.

Fair Value of Financial Instruments

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

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

Property and Equipment

Property and Equipment

Impairment of Long-Lived Assets

Impairment of Long-Lived Assets

Revenue Recognition

Revenue Recognition

Collaboration Revenue

The Company recognizes collaboration revenue by measuring the progress toward complete satisfaction of the performance obligation using an input measure. In order to recognize revenue over the research and development period, the Company measures actual costs incurred to date compared to the overall total expected costs to satisfy the performance obligation. Revenues are recognized as the program costs are incurred. The Company will re-evaluate the estimate of expected costs to satisfy the performance obligation each reporting period and make adjustments for any significant changes. Collaboration revenue under the Company’s collaboration agreement with AbbVie during 2018 and 2017 was $27.5 million and $2.9 million, respectively. All revenue recognized during the year ended December 31, 2018, was included in deferred revenue at the beginning of the period. The Company recorded deferred revenue of $174.6 million as of December 31, 2018. The deferred revenue is expected to be recognized over the research and development period of the programs through the completion of Phase 2 clinical trials.

Grant Revenue

Research and Development Costs

Research and Development Costs

Stock-based Compensation

Stock-based Compensation

Comprehensive Loss

Comprehensive Loss

Income Taxes

Income Taxes

Employee 401(k) Plan

Employee 401(k) Plan

Segments

Segments

Deferred Offering Costs

Deferred Offering Costs

Recent Accounting Pronouncements

Recent Accounting Pronouncements

Leases

Leases

Summary of Significant Accounting Policies (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Accounting Policies [Abstract]

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


	Nine Months Ended September 30,
	2019		2018
	(In thousands)
Cash and cash equivalents	$	86,974	$	41,939
Restricted cash		1,472		1,472

Total cash, cash equivalents, and restricted cash	$	88,446	$	43,411


	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Cash and cash equivalents	$	65,470	$	32,451	$	50,838
Restricted cash		1,472		—		200

Total cash, cash equivalents, and restricted cash	$	66,942	$	32,451	$	51,038

Fair Value Measurements (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Fair Value Disclosures [Abstract]

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

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


	September 30, 2019
	Fair Value Hierarchy		Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Market Value
	(In thousands)
Money market funds		Level 1	$	76,549	$	—	$	—		$	76,549
U.S. government treasury securities		Level 1		299,336		242		(16	)		299,562
Corporate bonds		Level 2		5,025		—		—			5,025

Total cash equivalents and marketable securities			$	380,910	$	242	$	(16	)	$	381,136


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

Total cash equivalents and marketable securities			$	290,202	$	3	$	(45	)	$	290,160

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


	December 31, 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


	December 31, 2017
	Fair Value Hierarchy		Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Market Value
	(In thousands)
Money market funds		Level 1	$	27,201	$	—	$	—	$	27,201

Total cash equivalents and marketable securities			$	27,201	$	—	$	—	$	27,201

Balance Sheet Components (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Balance Sheet Related Disclosures Abstract

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:


	September 30, 2019			December 31, 2018
	(In thousands)
Leasehold improvements	$	25,224		$	210
Lab equipment		9,110			4,599
Furniture and fixtures		2,066			131
Computer equipment		1,457			449
Construction-in-progress		39			7,449

Property and equipment, gross		37,896			12,838
Less accumulated depreciation and amortization		(4,035	)		(1,901	)

Total property and equipment, net	$	33,861		$	10,937

Property and equipment, net consists of the following:


	December 31,
	2018			2017
	(In thousands)
Lab equipment	$	4,599		$	3,527
Computer equipment		449			157
Leasehold improvements		210			113
Office equipment		131			17
Construction-in-progress		7,449			—

Property and equipment, gross		12,838			3,814
Less accumulated depreciation and amortization		(1,901	)		(980	)

Total property and equipment, net	$	10,937		$	2,834

Summary of Accrued Liabilities

Accrued liabilities consist of the following:


	September 30, 2019		December 31, 2018
	(In thousands)
Accrued research and development costs	$	7,431	$	3,821
Accrued employee compensation		4,272		2,766
Accrued property and equipment		3,675		293
Accrued professional services		1,046		588
Accrued offering costs		—		792
Other		122		179

Total accrued liabilities	$	16,546	$	8,439

Accrued liabilities consist of the following:


	December 31,
	2018		2017
	(In thousands)
Accrued research and development costs	$	3,821	$	1,471
Accrued employee compensation		2,766		1,294
Deferred offering costs		792		—
Accrued professional services		588		387
Accrued property and equipment		293		201
Other		179		102

Total accrued liabilities	$	8,439	$	3,455

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Summary of Operating Leases Minimum Future Rental Payments

The following are minimum future rental payments owed under the Company’s operating leases as of December 31, 2018:


	(In thousands)
2019	$	3,231
2020		6,882
2021		7,122
2022		7,146
2023 and thereafter		50,522

Total	$	74,903

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Summary of Authorized, Issued, and Outstanding Convertible Preferred Stock

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


	December 31, 2018
	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529
Series E		9,848,474		9,373,633		14.22		133,035		133,250

Total convertible preferred stock		45,849,677		45,374,836			$	210,520	$	211,054


	December 31, 2017
	Shares Authorized		Shares Issued and Outstanding		Issuance Price per Share		Net Proceeds		Aggregate Liquidation Preference
	(In thousands, except share and per share data)
Convertible Preferred Stock:
Series A-1		1,000,000		1,000,000	$	0.40	$	354	$	400
Series A-2		10,549,450		10,549,450		0.91		9,554		9,600
Series B		5,000,000		5,000,000		1.20		5,968		6,000
Series C		12,088,016		12,088,016		2.67		32,158		32,275
Series D		7,363,737		7,363,737		4.01		29,451		29,529

Total convertible preferred stock		36,001,203		36,001,203			$	77,485	$	77,804

Stock-based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Summary of Recognized Stock-Based Compensation

The Company recognized stock-based compensation as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
	(In thousands)				(In thousands)
Research and development	$	1,935	$	1,276	$	5,515	$	2,520
General and administrative		2,169		704		5,528		1,745

Total stock-based compensation	$	4,104	$	1,980	$	11,043	$	4,265

The Company recognized stock-based compensation as follows:


	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Research and development	$	4,195	$	4,392	$	1,727
General and administrative		2,730		962		236

Total stock-based compensation	$	6,925	$	5,354	$	1,963

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


	Year Ended December 31,
	2018	2017	2016
Expected term (in years)	5.1 – 6.1	6.0 – 10.0	6.1 – 10.0
Expected volatility	73 – 81%	75%	77%
Risk-free interest rate	2.5 – 3.1%	1.9 – 2.4%	1.2 – 2.5%
Dividend yield	0%	0%	0%

Summary of Restricted Common Stock

Activity for the restricted common stock is shown below:


	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted common stock as of December 31, 2018		1,917,848		$	6.95
Vested		(669,125	)		6.95
Forfeited		(56,973	)		6.95

Unvested restricted common stock as of September 30, 2019		1,191,750		$	6.95

Activity for the restricted units is shown below:


	Number of Units			Weighted Average Grant Date Fair Value per Unit
Unvested restricted units as of December 31, 2016		1,480,199			0.26
Vested prior to conversion from LLC to corporation		(1,277,275	)		0.09
Exchange of unvested restricted units for restricted common stock upon the Conversion		(202,924	)		1.33

Unvested restricted units as of December 31, 2017		—			—

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, 2016		—		$	—
Exchange of unvested profit interest units for restricted common stock upon the Conversion		3,165,350			6.95
Exchange of unvested restricted units for restricted common stock upon the Conversion		202,924			6.95
Vested		(170,227	)		6.95
Cancelled/forfeited		(49,234	)		6.95

Unvested restricted common stock as of December 31, 2017		3,148,813			6.95
Vested		(1,219,641	)		6.95
Cancelled/forfeited		(11,324	)		6.95

Unvested restricted common stock as of December 31, 2018		1,917,848			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		1,133,100			19.17
Exercised		(100,016	)		7.89
Forfeited		(178,648	)		11.52

Outstanding as of September 30, 2019		5,918,124		$	10.84		9.0	$	26,336

Exercisable as of September 30, 2019		891,664		$	8.69		8.8	$	5,296

Vested and expected to vest as of September 30, 2019		5,918,124		$	10.84		9.0	$	26,336

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


	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of December 31, 2016		—		$	—
Granted		22,500			6.95

Outstanding as of December 31, 2017		22,500			6.95
Granted		5,063,584			8.94
Cancelled/forfeited		(22,396	)		8.01

Outstanding as of December 31, 2018		5,063,688			8.94		9.6	$	6,067

Exercisable as of December 31, 2018		273,561			8.09		9.4	$	561

Vested and expected to vest as of December 31, 2018		5,063,688			8.94		9.6	$	6,067

Profit interest units

Summary of Restricted Common Stock

Activity for the profit interest units is shown below:


	Number of Units			Weighted Average Grant Date Fair Value per Unit
Unvested as of December 31, 2016		1,205,563			1.72
Granted		2,483,500			4.76
Vested		(363,387	)		1.89
Cancelled/forfeited		(8,338	)		1.17
Exchange of unvested profit interest units for restricted common stock upon the Conversion		(3,317,338	)		3.98

Unvested as of December 31, 2017		—			—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

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


	Year Ended December 31,
	2018			2017
Tax benefit at federal statutory rate	$	(10,972	)	$	(11,038	)
Pre-conversion passthrough loss		—			7,331
Tax credits		(2,387	)		(351	)
Federal uncertain tax positions		708			102
Stock-based compensation		1,251			470
Impact of federal rate change		—			2,329
Nondeductible expense		58			126
Change of valuation allowance		11,346			782
Others		(4	)		249

Income tax provision	$	—		$	—

Schedule of Deferred Tax Assets and Liabilities

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


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss	$	13,867		$	4,363
Accrued bonus		487			230
Tax credits		3,098			574
Stock-based compensation		213			—
Deferred rent		1,647			—
Others		98			73

Gross deferred tax assets		19,410			5,240
Less valuation allowance		(17,286	)		(5,026	)

Total deferred tax assets	$	2,124		$	214

Deferred tax liabilities:
Depreciation and amortization	$	(2,124	)	$	(214	)

Gross deferred tax liabilities		(2,124	)		(214	)

Deferred tax assets, net	$	—		$	—

Schedule of Unrecognized Tax Benefits

The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2018 and 2017 (in thousands):


Balance as of December 31, 2016	$	—
Increases related to tax positions taken during the prior year		227
Increases related to tax positions taken during the current year		403

Balance as of December 31, 2017		630
Increases related to tax positions taken during the current year		877

Balance as of December 31, 2018	$	1,507

Net Loss Per Share (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 Nine Months Ended September 30,
	2019		2018
Convertible preferred stock		—		45,350,215
Restricted stock subject to future vesting		1,191,750		2,161,047
Options to purchase common stock		5,918,124		3,048,500
Shares committed under ESPP		51,667		—

Total		7,161,541		50,559,762

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:


	Year Ended December 31,
	2018		2017		2016
Convertible preferred stock		45,374,836		36,001,203		36,001,203
Restricted stock subject to future vesting		1,917,848		3,148,813		1,480,199
Options to purchase common stock		5,063,688		22,500		—
Profit interest units		—		—		1,893,201

Total		52,356,372		39,172,516		39,374,603

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Quarterly Financial Data


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


	Three months ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
	(in thousands, except per share data)
Revenue	$	3,059		$	153		$	280		$	243
Loss from operations	$	(9,807	)	$	(10,091	)	$	(7,286	)	$	(5,495	)
Net loss	$	(9,778	)	$	(10,033	)	$	(7,226	)	$	(5,443	)
Net loss per share, basic and diluted	$	(0.94	)	$	(1.10	)	$	(0.83	)	$	(0.66	)

Leases (Tables)

9 Months Ended

Sep. 30, 2019

Leases [Abstract]

Components of Lease Expense

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


	Three Months Ended September 30, 2019			Nine Months Ended September 30, 2018
Operating lease cost	$	1,346		$	4,529
Variable lease cost		425			903
Short-term lease cost		—			68
Sublease income and reimbursement of variable lease cost		(540	)		(723	)

Total	$	1,231		$	4,777

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 September 30, 2019:


	(In thousands)
2019 (remaining three months)	$	1,136
2020		6,882
2021		7,122
2022		7,146
2023		7,376
Thereafter		43,146

Total undiscounted lease payments	$	72,808
Less: Present value adjustment		(24,692	)

Total	$	48,116

The Company and Liquidity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended
	Feb. 07, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Oct. 31, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock, par value			$ 0.0001		$ 0.0001			$ 0.0001	$ 0.0001
Common Stock Shares Issued			68,922,249		13,764,829			13,776,153
Proceeds from issuance of common stock upon initial public offering, net of issuance costs			$ 170,128	$ (278)
Convertible preferred stock, shares outstanding			0		45,374,836			36,001,203
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
Restricted Stock Units (RSUs)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common Stock Shares Issued									202,924
Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Preferred stock, par value									$ 0.0001
Convertible preferred stock, shares outstanding				45,350,215	45,374,836	40,943,028	36,001,203	36,001,203

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2018 USD ($)	Oct. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Grant revenue					$ 169,000	$ 169,000	$ 863,000	$ 416,000
Number of operating segment \| Segment						1
Deferred offering costs for initial public offering included in accrued liabilities						$ 792,000
Deferred costs recorded as other assets						2,700,000
Operating lease right-of-use assets			$ 28,765,000	$ 28,765,000
Lease liabilities			48,116,000	48,116,000
Deferred Rent						$ 7,478,000	41,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life						5 years
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant, and equipment useful life						3 years
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Unrealized gains or loss on money market funds				0		$ 0
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			2,700,000	15,200,000		27,500,000	$ 2,900,000
Deferred revenue			$ 159,400,000	$ 159,400,000		174,600,000
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
Accounting Standards Update 2016-02 \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets									$ 32,000,000
Lease liabilities									50,000,000
Accounting Standards Update 2016-02 \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets									27,000,000
Lease liabilities									$ 44,000,000

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 86,974	$ 65,470	$ 41,939	$ 32,451	$ 50,838
Restricted cash	1,472	1,472	1,472		200
Total cash, cash equivalents, and restricted cash	$ 88,446	$ 66,942	$ 43,411	$ 32,451	$ 51,038	$ 66,358

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	$ 76,549	$ 65,222	$ 27,201
Cash equivalents and marketable securities, Fair Market Value	76,549	65,222	27,201
U.S Government Treasury Securities \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	299,336	224,980
Cash equivalents and marketable securities, Unrealized Gains	242	3
Cash equivalents and marketable securities, Unrealized Losses	(16)	(45)
Cash equivalents and marketable securities, Fair Market Value	299,562	224,938
Corporate Bonds \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	5,025
Cash equivalents and marketable securities, Fair Market Value	5,025
Cash Equivalents And Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents and marketable securities, Amortized Cost	380,910	290,202	27,201
Cash equivalents and marketable securities, Unrealized Gains	242	3
Cash equivalents and marketable securities, Unrealized Losses	(16)	(45)
Cash equivalents and marketable securities, Fair Market Value	$ 381,136	$ 290,160	$ 27,201

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 37,896	$ 12,838	$ 3,814
Less accumulated depreciation and amortization	(4,035)	(1,901)	(980)
Total property and equipment, net	33,861	10,937	2,834
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	25,224	210	113
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	9,110	4,599	3,527
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,066	131
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,457	449	157
Construction-in-Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 39	7,449
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross		$ 131	$ 17

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities, Current [Abstract]
Accrued research and development costs	$ 7,431	$ 3,821	$ 1,471
Accrued employee compensation	4,272	2,766	1,294
Accrued offering costs		792
Accrued professional services	1,046	588	387
Accrued property and equipment	3,675	293	201
Other	122	179	102
Total accrued liabilities	$ 16,546	$ 8,439	$ 3,455

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2018 ft²	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($)
Amount form letter of credit		$ 1,472	$ 1,472		$ 200	$ 1,472
Operating term		10 years	10 years
Operating lease, renewal period		10 years	10 years
Operating lease term		The lease is over a ten-year term with an option to renew for a period of ten years.	The lease is over a ten-year term with an option to renew for a period of ten years.
Rent expense			$ 1,100	$ 1,100	$ 600
Offices and Laboratory Facilities \| South San Francisco
Area under lease \| ft²	105,000

Commitments and Contingencies - Summary of Operating leases Minimum Future Rental Payments (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 3,231
2020	6,882
2021	7,122
2022	7,146
2023 and thereafter	50,522
Total	$ 74,903

Convertible Preferred Stock - Additional Information (Detail) - $ / shares		1 Months Ended	7 Months Ended
	Feb. 07, 2019	Oct. 31, 2018	Oct. 31, 2018	Dec. 31, 2018
Temporary Equity [Line Items]
Convertible preferred stock, conversion rate	100.00%
Series E Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares issued			9,373,633
Convertible preferred stock, issuance price per share		$ 14.2154	$ 14.2154	$ 14.22
Series E Convertible Preferred Stock \| Board Member
Temporary Equity [Line Items]
Convertible preferred stock, shares issued		24,621

Convertible Preferred Stock - Summary of Authorized, Issued, and Outstanding Convertible Preferred Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Oct. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized	0	45,849,677		36,001,203
Shares Issued and Outstanding	0	45,374,836		36,001,203
Net Proceeds		$ 210,520		$ 77,485
Aggregate Liquidation Preference		$ 211,054		$ 77,804
Series A-1 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		1,000,000		1,000,000
Shares Issued and Outstanding		1,000,000		1,000,000
Issuance Price per Share		$ 0.40		$ 0.40
Net Proceeds		$ 354		$ 354
Aggregate Liquidation Preference		$ 400		$ 400
Series A-2 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		10,549,450		10,549,450
Shares Issued and Outstanding		10,549,450		10,549,450
Issuance Price per Share		$ 0.91		$ 0.91
Net Proceeds		$ 9,554		$ 9,554
Aggregate Liquidation Preference		$ 9,600		$ 9,600
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		5,000,000		5,000,000
Shares Issued and Outstanding		5,000,000		5,000,000
Issuance Price per Share		$ 1.20		$ 1.20
Net Proceeds		$ 5,968		$ 5,968
Aggregate Liquidation Preference		$ 6,000		$ 6,000
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		12,088,016		12,088,016
Shares Issued and Outstanding		12,088,016		12,088,016
Issuance Price per Share		$ 2.67		$ 2.67
Net Proceeds		$ 32,158		$ 32,158
Aggregate Liquidation Preference		$ 32,275		$ 32,275
Series D Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		7,363,737		7,363,737
Shares Issued and Outstanding		7,363,737		7,363,737
Issuance Price per Share		$ 4.01		$ 4.01
Net Proceeds		$ 29,451		$ 29,451
Aggregate Liquidation Preference		$ 29,529		$ 29,529
Series E Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		9,848,474
Shares Issued and Outstanding		9,373,633
Issuance Price per Share		$ 14.22	$ 14.2154
Net Proceeds		$ 133,035
Aggregate Liquidation Preference		$ 133,250

Stock-based Compensation - Summary of Recognized Stock-Based Compensation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 4,104	$ 1,980	$ 11,043	$ 4,265	$ 6,925	$ 5,354	$ 1,963
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	1,935	1,276	5,515	2,520	4,195	4,392	1,727
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 2,169	$ 704	$ 5,528	$ 1,745	$ 2,730	$ 962	$ 236

Stock-based Compensation - Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions (Detail) - Options to Purchase Common Stock	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility		75.00%	77.00%
Dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 1 month 6 days	6 years	6 years 1 month 6 days
Expected volatility	73.00%
Risk-free interest rate	2.50%	1.90%	1.20%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	10 years	10 years
Expected volatility	81.00%
Risk-free interest rate	3.10%	2.40%	2.50%

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			9 Months Ended	12 Months Ended
	Feb. 06, 2019	Oct. 13, 2017	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	4 years
Expected weighted average period			2 years 9 months 18 days
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
Common stock reserved for issuance	7,688,156		12,829,863
Common stock, shares available for issuance			6,911,739
Cliff vesting percentage	25.00%
Cliff vesting period	1 year
Unrecognized stock-based compensation			$ 34.6
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
2017 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of stock owned by individual		10.00%
Percentage of exercise price per share from fair market value		110.00%
stock options, term		10 years
Common stock reserved for issuance				8,899,858
Common stock, shares available for issuance				121,062
Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				4 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage				25.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value				$ 10.5	$ 9.1	$ 4.1
Unrecognized stock-based compensation			$ 5.4	$ 8.5
Expected weighted average period			1 year 8 months 12 days	2 years 4 months 24 days
Profit interest units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage				25.00%
Options to Purchase Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value				$ 2.5
Unrecognized stock-based compensation				$ 28.9
Expected weighted average period				3 years 7 months 6 days
Stock price				$ 10.14

Stock-based Compensation - Activity for Profit Interest Units (Detail) - Profit interest units	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, beginning balance \| shares	1,205,563
Granted \| shares	2,483,500
Vested \| shares	(363,387)
Cancelled/forfeited \| shares	(8,338)
Exchange of unvested profit interest units for restricted common stock upon the Conversion \| shares	(3,317,338)
Unvested restricted units,beginning of period \| $ / shares	$ 1.72
Granted \| $ / shares	4.76
Vested \| $ / shares	1.89
Cancelled/forfeited \| $ / shares	1.17
Exchange of unvested profit interest units for restricted common stock upon the Conversion \| $ / shares	$ 3.98

Stock-based Compensation - Summary of Restricted Units (Detail) - Restricted units [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, beginning balance \| shares	1,480,199
Vested prior to conversion from LLC to corporation \| shares	(1,277,275)
Exchange of unvested restricted units for restricted common stock upon the Conversion \| shares	(202,924)
Unvested restricted units,beginning of period \| $ / shares	$ 0.26
Vested prior to conversion from LLC to corporation \| $ / shares	0.09
Exchange of unvested restricted units for restricted common stock upon the Conversion \| $ / shares	$ 1.33

Stock-based Compensation - Summary of Restricted Common Stock (Detail) - Restricted Common Stock - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, beginning balance	1,917,848	3,148,813
Exchange of unvested profit interest units for restricted common stock upon the Conversion			3,165,350
Exchange of unvested restricted units for restricted common stock upon the Conversion			202,924
Number of Shares, Vested	(669,125)	(1,219,641)	(170,227)
Number of shares, Cancelled/forfeited	(56,973)	(11,324)	(49,234)
Number of Shares, Unvested, ending balance	1,191,750	1,917,848	3,148,813
Unvested restricted units,beginning of period	$ 6.95	$ 6.95
Weighted Average Grant Date Fair Value per Share, Exchange of unvested profit interest units for restricted common stock upon the Conversion			$ 6.95
Weighted Average Grant Date Fair Value per Share, Exchange of unvested restricted units for restricted common stock upon the Conversion			6.95
Weighted Average Grant Date Fair Value per Share, Vested	6.95	6.95	6.95
Weighted Average Grant Date Fair Value per Share, Cancelled/forfeited	6.95	6.95	6.95
Unvested restricted units, end of period	$ 6.95	$ 6.95	$ 6.95

Stock-based Compensation - Summary of Options to Purchase Common Stock (Detail) - 2019 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding, beginning balance	5,063,688	22,500
Number of Options, Granted	1,133,100	5,063,584	22,500
Number of Options, Exercised	(100,016)
Number of Options, Cancelled/forfeited	(178,648)	(22,396)
Number of Options, Outstanding, ending balance	5,918,124	5,063,688	22,500
Number of Options, Exercisable	891,664	273,561
Number of Options, Vested and expected to vest	5,918,124	5,063,688
Weighted average exercise price, Outstanding, beginning balance	$ 8.94	$ 6.95
Weighted average exercise price, Granted	19.17	8.94	$ 6.95
Weighted average exercise price, Exercised	7.89
Weighted average exercise price, Forfeited	11.52	8.01
Weighted average exercise price, Outstanding, ending balance	10.84	8.94	$ 6.95
Weighted average exercise price, Exercisable	8.69	8.09
Weighted average exercise price, Vested and expected to vest	$ 10.84	$ 8.94
Weighted Average Remaining Contractual Term, Outstanding	9 years	9 years 7 months 6 days
Weighted Average Remaining Contractual Term, Exercisable (In years)	8 years 9 months 18 days	9 years 4 months 24 days
Weighted Average Remaining Contractual Term, Vested and expected to vest (In years)	9 years	9 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding	$ 26,336	$ 6,067
Aggregate Intrinsic Value, Exercisable	5,296	561
Aggregate Intrinsic Value, Vested or expected to vest	$ 26,336	$ 6,067

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax benefit at federal statutory rate	$ (10,972)	$ (11,038)
Pre-conversion passthrough loss		7,331
Tax credits	(2,387)	(351)
Federal uncertain tax positions	708	102
Stock-based compensation	1,251	470
Impact of federal rate change		2,329
Nondeductible expense	58	126
Change of valuation allowance	11,346	782
Others	(4)	249
Income tax provision	$ 0	$ 0

Income Taxes -Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss	$ 13,867	$ 4,363
Accrued bonus	487	230
Tax credits	3,098	574
Stock-based compensation	213
Deferred rent	1,647
Others	98	73
Gross deferred tax assets	19,410	5,240
Less valuation allowance	(17,286)	(5,026)
Total deferred tax assets	2,124	214
Deferred tax liabilities:
Depreciation and amortization	(2,124)	(214)
Gross deferred tax liabilities	(2,124)	(214)
Deferred tax assets, net	$ 0	$ 0

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Change of valuation allowance	$ 12.3
Operating loss carry forward, net	$ 42.9
Reduction to deferred tax assets		$ 2.3
Effective Income Tax Rate Reconciliation, Percent	21.00%	35.00%
Change In Corporate Tax Rate
Income Tax Contingency [Line Items]
Change of valuation allowance		$ (2.3)
Federal Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forward, net	$ 17.5
Tax Credit Carryforward, Amount	$ 3.3
Tax Credit Beginning Expiration Year	2036
Federal and State Tax Authorities
Income Tax Contingency [Line Items]
Operating loss carry forward beginning expiration year	2037
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carry forward, net	$ 17.1
California Tax Authority
Income Tax Contingency [Line Items]
Tax Credit Carryforward, Amount	$ 1.1

Income Taxes - Schedule of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 630
Increases related to tax positions taken during the prior year		$ 227
Increases related to tax positions taken during the current year	877	403
Unrecognized tax benefits, ending balance	$ 1,507	$ 630

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Milestones
Related Party Transaction [Line Items]
Related party transaction expenses					$ 2.3
Adimab
Related Party Transaction [Line Items]
Related party transaction expenses	$ 2.0	$ 1.8	$ 2.8	$ 1.8	2.3	$ 0.4	$ 0.6
Accrued liabilities due to related parties	2.0		2.0		0.0
Adimab \| Maximum
Related Party Transaction [Line Items]
Future milestone payments owed	$ 3.5		$ 3.5		$ 3.5

Net Loss Per Share - Schedule of Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares	7,161,541	50,559,762	7,161,541	50,559,762	52,356,372	39,172,516	39,374,603
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares		45,350,215		45,350,215	45,374,836	36,001,203	36,001,203
Restricted Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares	1,191,750	2,161,047	1,191,750	2,161,047	1,917,848	3,148,813	1,480,199
Options to Purchase Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares	5,918,124	3,048,500	5,918,124	3,048,500	5,063,688	22,500
Shares committed under ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares	51,667		51,667
Profit interest units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares							1,893,201

Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended											9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,696			$ 9,145	$ 6,503	$ 7,109	$ 4,920	$ 3,059	$ 153	$ 280	$ 243	$ 15,218	$ 18,532	$ 27,677	$ 3,735	$ 416
Loss from operations	(34,149)			(19,017)	(16,815)	(12,231)	(9,225)	(9,807)	(10,091)	(7,286)	(5,495)	(82,062)	(38,271)	(57,288)	(32,679)	(15,132)
Net loss	$ (31,738)	$ (24,560)	$ (18,560)	$ (17,373)	$ (15,317)	$ (11,121)	$ (8,437)	$ (9,778)	$ (10,033)	$ (7,226)	$ (5,443)	$ (74,858)	$ (34,875)	$ (52,248)	$ (32,480)	$ (15,110)
Net loss per share, basic and diluted	$ (0.47)			$ (1.48)	$ (1.34)	$ (1.00)	$ (0.78)	$ (0.94)	$ (1.10)	$ (0.83)	$ (0.66)	$ (1.25)	$ (3.13)	$ (4.62)	$ (3.55)	$ (2.11)

- Definition

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

+ References + Details

Subsequent Event - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Feb. 07, 2019	Sep. 30, 2019	Sep. 30, 2018	Feb. 06, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Proceeds from issuance of common stock upon initial public offering, net of issuance costs		$ 170,128	$ (278)
Convertible preferred stock, shares outstanding		0			45,374,836	36,001,203
2019 Equity Incentive Plan
Subsequent Event [Line Items]
Common stock reserved for issuance		12,829,863		7,688,156
2019 Employee Stock Purchase Plan
Subsequent Event [Line Items]
Common stock reserved for issuance				1,478,492
Initial Public Offering
Subsequent Event [Line Items]
Issuance of common stock upon initial public offering, net of issuance costs (in shares)	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
Over-Allotment Option
Subsequent Event [Line Items]
Issuance of common stock upon initial public offering, net of issuance costs (in shares)	489,541

Fair Value Measurements - Additional Information (Detail) $ in Thousands	Sep. 30, 2019 USD ($)
Fair Value Disclosures [Abstract]
Fair value of assets transferred from level 1 to level 2	$ 0
Fair value of assets transferred from level 2 to level 1	0
Remaining contractual maturities of investments were less than one year	349,300
Remaining contractual maturities of investments were less than one year through two years	$ 31,800

Leases - Additional Information (Detail)	1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
Leases - Additional Information (Detail)	May 31, 2019 ft²	Jun. 30, 2018 ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)
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	$ 200,000
Operating term				10 years		10 years
Operating lease, renewal period				10 years		10 years
Operating lease term				The lease is over a ten-year term with an option to renew for a period of ten years.		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			$ 200,000		$ 800,000
Weighted average remaining lease term for operating leases				9 years 6 months
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
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 Expense (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 1,346	$ 4,529
Variable lease cost	425	903
Short-term lease cost		68
Sublease income and reimbursement of variable lease cost	(540)	(723)
Total	$ 1,231	$ 4,777

Leases - Schedule of Undiscounted Cash Flows Owned Under Company's Operating Leases (Detail) $ in Thousands	Sep. 30, 2019 USD ($)
Leases [Abstract]
2019 (remaining three months)	$ 1,136
2020	6,882
2021	7,122
2022	7,146
2023	7,376
Thereafter	43,146
Total undiscounted lease payments	72,808
Less: Present value adjustment	(24,692)
Total	$ 48,116