CytomX Therapeutics (CTMX) 10-K 27 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Jan. 31, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-37587
Entity Tax Identification Number	27-3521219
Entity Address, Address Line One	151 Oyster Point Boulevard,
Entity Address, Address Line Two	Suite 400
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	515-3185
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.00001 par value
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Entity Public Float			$ 501.8
Entity Common Stock, Shares Outstanding		45,566,551
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement to be filed for its 2020 Annual Meeting of Stockholders are incorporated by reference into Part III hereof. Such proxy statement will be filed with the Securities and Exchange Commission within 120 days of the end of the fiscal year covered by this Annual Report on Form 10-K.

BALANCE SHEETS

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 188,425	$ 247,577
Short-term investments	107,720	188,550
Accounts receivable	13	97
Prepaid expenses and other current assets	7,177	9,251
Total current assets	303,335	445,475
Property and equipment, net	7,372	6,934
Intangible assets, net	1,312	1,458
Goodwill	949	949
Restricted cash	917	917
Operating lease right-of-use asset	25,382
Other assets	2,015	1,375
Total assets	341,282	457,108
Current liabilities:
Accounts payable	4,158	5,132
Accrued liabilities	30,051	26,724
Income tax payable		13,339
Deferred revenues, current portion	51,381	52,713
Total current liabilities	85,590	97,908
Deferred revenue, net of current portion	178,858	225,267
Operating lease liabilities - long term	24,871
Other long-term liabilities	850	3,050
Total liabilities	290,169	326,225
Commitments and contingencies (Note 11)
Stockholders' equity
Common stock, $0.00001 par value; 75,000,000 shares authorized at December 31, 2019 and 2018; 45,523,088 and 45,083,209 shares issued and outstanding at December 31, 2019 and 2018, respectively	1	1
Additional paid-in capital	468,285	445,956
Accumulated other comprehensive income (loss)	57	(93)
Accumulated deficit	(417,230)	(314,981)
Total stockholders' equity	51,113	130,883
Total liabilities and stockholders' equity	341,282	457,108
Convertible Preferred Stock
Stockholders' equity
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized at December 31, 2019 and 2018; no shares issued and outstanding at December 31, 2019 and 2018, respectively

BALANCE SHEETS (Parenthetical)

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	45,523,088	45,083,209
Common stock, shares outstanding	45,523,088	45,083,209
Convertible Preferred Stock
Convertible Preferred stock, par value	$ 0.00001	$ 0.00001
Convertible Preferred stock, shares authorized	10,000,000	10,000,000
Convertible Preferred stock, shares issued	0	0
Convertible Preferred stock, shares outstanding	0	0

STATEMENTS OF OPERATIONS AND CO

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 57,489	$ 59,502	$ 71,623
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 131,619	$ 103,866	$ 92,277
General and administrative	36,765	33,510	25,605
Total operating expenses	168,384	137,376	117,882
Loss from operations	(110,895)	(77,874)	(46,259)
Interest income	8,365	7,641	2,674
Other expense, net	(135)	(68)	(27)
Loss before income taxes	(102,665)	(70,301)	(43,612)
Provision for (benefit from) income taxes	(427)	14,303	(513)
Net loss	$ (102,238)	$ (84,604)	$ (43,099)
Net loss per share, basic and diluted	$ (2.26)	$ (2.03)	$ (1.16)
Shares used to compute net loss per share, basic and diluted	45,335,927	41,664,382	37,166,830
Other comprehensive income (loss):
Changes in unrealized gain (loss) on short-term investments, net of tax	$ 139	$ 1	$ (67)
Impact of adoption of new accounting pronouncement	11
Total comprehensive loss	$ (102,088)	$ (84,603)	$ (43,166)

STATEMENTS OF STOCKHOLDERS' EQU

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 78,479,000	$ 1,000	$ 254,871,000	$ (27,000)	$ (176,366,000)
Beginning balance, shares at Dec. 31, 2016		36,490,169
Exercise of stock options	$ 3,165,000		3,165,000
Exercise of stock options, shares	764,576	764,576
Issuance of common stock under the Employee Stock Purchase Plan	$ 674,000		674,000
Issuance of common stock under the Employee Stock Purchase Plan, shares		67,746
Issuance of common stock pursuant to the Amgen Stock Purchase Agreement	19,457,000		19,457,000
Issuance of common stock pursuant to the Amgen Stock Purchase Agreement, shares		1,156,069
Stock-based compensation	11,287,000		11,287,000
Other comprehensive income (loss)	(67,000)			(67,000)
Net loss	(43,099,000)				(43,099,000)
Ending balance at Dec. 31, 2017	69,896,000	$ 1,000	289,454,000	(94,000)	(219,465,000)
Ending balance, shares at Dec. 31, 2017		38,478,560
Impact of adoption of new accounting pronouncements | ASU 2014-09	(10,912,000)				(10,912,000)
Issuance of common stock in follow-on offering, net of issuance costs	134,596,000		134,596,000
Issuance of common stock in follow-on offering, net of issuance costs, shares		5,867,347
Exercise of stock options	$ 4,156,000		4,156,000
Exercise of stock options, shares	673,382	673,382
Issuance of common stock under the Employee Stock Purchase Plan	$ 872,000		872,000
Issuance of common stock under the Employee Stock Purchase Plan, shares	63,920	63,920
Stock-based compensation	$ 16,878,000		16,878,000
Other comprehensive income (loss)	1,000			1,000
Net loss	(84,604,000)				(84,604,000)
Ending balance at Dec. 31, 2018	130,883,000	$ 1,000	445,956,000	(93,000)	(314,981,000)
Ending balance, shares at Dec. 31, 2018		45,083,209
Impact of adoption of new accounting pronouncements | ASU 2018-02	11,000			11,000	(11,000)
Issuance of common stock in follow-on offering, net of issuance costs	1,602,000		1,602,000
Issuance of common stock in follow-on offering, net of issuance costs, shares		150,000
Exercise of stock options	$ 633,000		633,000
Exercise of stock options, shares	146,930	146,930
Issuance of common stock under the Employee Stock Purchase Plan	$ 994,000		994,000
Issuance of common stock under the Employee Stock Purchase Plan, shares	142,949	142,949
Stock-based compensation	$ 19,100,000		19,100,000
Other comprehensive income (loss)	139,000			139,000
Net loss	(102,238,000)				(102,238,000)
Ending balance at Dec. 31, 2019	$ 51,113,000	$ 1,000	$ 468,285,000	$ 57,000	$ (417,230,000)
Ending balance, shares at Dec. 31, 2019		45,523,088

STATEMENTS OF CASH FLOWS

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (102,238)	$ (84,604)	$ (43,099)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Amortization of intangible assets	146	146	146
Depreciation and amortization	2,459	1,738	1,516
Amortization of premium (accretion of discount) on investments	(2,228)	(1,701)	371
Stock-based compensation expense	19,100	16,878	11,287
Issuance of common stock in connection with UCSB sublicense fee	1,602
Non-cash acquisition of in process research and development asset charged to expense			10,700
Deferred income taxes			(513)
Changes in operating assets and liabilities
Accounts receivable	84	10,042	(7,980)
Related party accounts receivable			154
Prepaid expenses and other current assets	2,074	(4,899)	(456)
Other assets	(640)	(20)	1,618
Accounts payable	(374)	261	(2,441)
Accrued liabilities, income tax payable and other long-term liabilities	(12,724)	24,833	9,157
Deferred revenue	(47,741)	(38,195)	189,913
Net cash (used in) provided by operating activities	(140,480)	(75,521)	170,373
Cash flows from investing activities:
Purchases of property and equipment	(3,497)	(3,788)	(1,559)
Purchases of short-term investments	(174,992)	(204,601)	(218,707)
Maturities of short-term investments	258,190	214,315	99,000
Net cash provided by (used in) investing activities	79,701	5,926	(121,266)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs		134,596	19,957
Proceeds from employee stock purchase plan and exercise of stock options	1,627	5,028	3,839
Net cash provided by financing activities	1,627	139,624	23,796
Net (decrease) increase in cash, cash equivalents and restricted cash	(59,152)	70,029	72,903
Cash, cash equivalents and restricted cash, beginning of year	248,494	178,465	105,562
Cash, cash equivalents and restricted cash, end of year	189,342	248,494	178,465
Supplemental disclosures of cash flow information:
Cash paid for income taxes	13,061
Supplemental disclosures of noncash investing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 428	$ 1,027	$ 361

Description of the Business

Description of the Business	12 Months Ended
	Dec. 31, 2019
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “ ” ®

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ( “ Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Concentration of Credit Risk and Other Risks and Uncertainties The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability. Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments in highly rated money market funds and its short-term investments in U.S. Government Bonds. Customers and collaboration partners who represent 10% or more of the Company’s total revenue during each period presented or accounts receivable balance at each respective balance sheet date are as follows (in thousands): Revenue Accounts Receivable, net Year Ended December 31, December 31, 2019 2018 2017 2019 2018 AbbVie Ireland Unlimited Company $ 5,878 $ 18,997 $ 19,434 $ — $ — Bristol-Myers Squibb Company 47,740 32,780 36,492 13 97 ImmunoGen, Inc. — * 12,503 — — Total revenue from customers who represent 10% or more of the Company's total revenue $ 53,618 $ 51,777 $ 68,429 $ 13 $ 97 * Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented. All of the Company’s customers are located in the United States of America. Segments Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer and principal financial officer, who are the Company’s chief operating decision makers. All long-lived assets are maintained in the United States of America. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the amounts shown in the statement of cash flows (in thousands): December 31 2019 2018 Cash and cash equivalents $ 188,425 $ 247,577 Restricted cash 917 917 $ 189,342 $ 248,494 Short-term Investments All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities greater than 12 months are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax. A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. Property and Equipment, net Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred. Goodwill and Intangible Assets Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development. The Company assesses impairment indicators annually as of December 31 or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2019, 2018 and 2017. Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the years ended December 31, 201 9 , 201 8 and 201 7 . Revenue Recognition On January 1, 2018, the Company adopted Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue Recognition (ASC 605) The Company ’ ’ The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company ’ The Company ’ The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, t he Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’ Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments and impact of adoption of new accounting pronouncements during the period represents the components of other comprehensive income (loss) that is excluded from the reported net loss. Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company ’ Stock-Based Compensation The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the ratable method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. Forfeitures are recognized as they occur. On January 2019, the Company adopted ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. related Prior to the adoption of ASU 2018-07, such stock-based compensation expense was measured on the date of performance at the fair value of the consideration received or the fair value Income Taxes The Company accounts for income taxes using an asset and liability approach. Deferred tax assets and liabilities reflect the net tax effects of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The Company records a valuation allowance to reduce its deferred tax assets to reflect the net amount that it believes as more likely than not to be realized. Realization of the deferred tax assets is dependent on the generation of future taxable income, the amount and timing of which are uncertain. The valuation allowance requires an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable. Based upon the weight of available evidence at December 31, 2019, the Company continues to maintain a full valuation allowance against all of its deferred tax assets after management considered all available evidence, both positive and negative, including but not limited to its historical operating results, income or loss in recent periods, cumulative income in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts. The Compa n y recognize s the tax effects of an uncertain tax position only if it is more likely than not that it will be sustained based solely on its technical merits as of the reporting date and only in an amount more likely than not that it will be sustained upon review by the tax authorities. The Company evaluate s uncertain tax positions on a quarterly basis and adjust the liability for changes in facts and circumstances, such as new regulations or interpretations by the taxing authorities, new information obtained during a tax examination, significant amendment to an existing tax law, or resolution of an examination. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences will impact the income tax provision in the period in which such determination is made. The resolution of its uncertain income tax positions is dependent on uncontrollable factors such as law changes, new case law, and the willingness of the income tax authorities to settle, including the timing thereof and other factors. Although the Company does not anticipate significant changes to its uncertain income tax positions in the next twelve months, items outside of its control could cause its uncertain income tax positions to change in the future, which would be recorded in its statements of operations. Interest and/or penalties related to income tax matters are recognized as a component of income tax expense. Research and Development Expenses The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred. In January 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology from a third party for $5.0 million. The Company plans to use this technology in certain of the Company’s discovery stage projects, and has concluded that the technology acquired does not have an alternative future use. Accordingly, the $5.0 million has been recorded as research and development expense for 2019. Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company elected the short-term lease recognition exemption .

Adopted and Recent Accounting P

Adopted and Recent Accounting Pronouncements	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Adopted and Recent Accounting Pronouncements	3. Adopted and Recent Accounting Pronouncements Adopted Accounting Pronouncements Leases The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) lessees recognize on the balance sheet the assets and liabilities that arise from leases, including leases classified as operating leases under current GAAP, and disclose qualitative and quantitative information about leasing arrangements. I n July 2018, the Financial Accounting Standard Board (“FASB”) further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company has elected such option and accordingly the comparative periods are not recast. The Company has also elected the package of practical expedients permitted under the new standard (“ASC 842”). Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. There was no impact on the Company’s accumulated deficit as of the adoption date. This standard does not have material impact on the Company’s results of operations or cash flows. Reclassification of Certain Tax Effects In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Nonemployee Share-Based Payment In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) periods presented upon their effective date. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this ASU and delay adoption of the additional disclosures until their effective date. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606 In May 2019, the FASB issued ASU 2019-05, Financial Instruments - Credit Losses (Topic 326): Targeted Transition Relief , to provide entities with an option to irrevocably elect the fair value option in Subtopic 825-10, applied on an instrument-by-instrument basis for eligible instruments, that are within the scope of Subtopic 326-20, upon adoption of Topic 326. The fair value option election does not apply to held-to-maturity debt securities. This targeted transition relief is intended to increase comparability of financial statement information for some entities that otherwise would have measured similar financial instruments using different measuring methodologies. The effective date of this ASU is the same as ASU No. 2016-13, for the Company on January 1, 2020. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In November 2019, the FASB issued ASU 2019-08, Compensation - Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606): Codification Improvements – Share-Based Consideration Payable to a Customer. The amendments in this ASU require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. Under ASC 606, these awards are considered a reduction of the transaction price, unless the awards are payment for a distinct good or service received from the customer and should be recorded as a reduction of the transaction price. However, the ASU requires these awards to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718 and should be recognized at the later of when the award is promised and when the entity recognizes revenue for the transfer of the related goods or services in accordance with ASC 606. The grant date is the date at which a grantor (supplier) and a grantee (customer) reach a mutual understanding of the key terms and conditions of a share-based payment award. The classification and subsequent measurement of the award are subject to the guidance in Topic 718 unless the share-based payment award is subsequently modified and the grantee is no longer a customer. The ASU is effective for the Company on January 1, 2020, and interim periods within those fiscal years. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	4. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is c alculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share is the same as basic net loss per share since the effect of the potentially dilutive securities is anti-dilutive. The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented, because including them would have been anti-dilutive: Year Ended December 31, 2019 2018 2017 Options to purchase common stock 9,687,844 7,478,755 6,891,123

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Investments	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements and Short-Term Investments	5. Fair Value Measurements and Short-Term Investments In accordance with Accounting Standards Codification (“ASC”) 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows: • Level I: Inputs which include quoted prices in active markets for identical assets and liabilities. • Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices 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 assets or liabilities. • Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. The Company’s financial instruments consist of Level I assets which consist primarily of highly liquid money market funds, some of which is included in restricted cash and U.S. government bonds that are included in short-term investments. The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): December 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets Money market funds Level I $ 170,757 $ — $ — $ 170,757 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 107,610 110 — 107,720 Total Securities $ 279,284 $ 110 $ — $ 279,394 December 31, 2018 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets Money market funds Level I $ 226,979 $ — $ — $ 226,979 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 188,616 — (66 ) 188,550 Total Securities $ 416,512 $ — $ (66 ) $ 416,446 No securities have contractual maturities of longer than one year

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Property and Equipment	6. Property and Equipment Property and equipment, net consisted of the following (in thousands): December 31 2019 2018 Machinery and equipment $ 12,124 $ 10,498 Computer equipment and software 1,555 955 Furniture and fixtures 1,024 749 Leasehold improvements 1,483 893 Construction in progress 236 785 16,422 13,880 Less: accumulated depreciation and amortization (9,050 ) (6,946 ) $ 7,372 $ 6,934 Depreciation and amortization expense was $2.5 million, $1.7 million and $1.5 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	7. Goodwill and Intangible Assets Goodwill and in-process research and development assets resulted from a series of integrated financing transactions in 2010 that was accounted for as a business combination. The in-process research and development relates to the Company’s proprietary Probody Platform and was accounted for as an indefinite-lived intangible asset until the underlying project was completed or abandoned. In connection with the collaboration agreements, the Company began amortizing the intangible asset in 2017. The intangible asset is being amortized over the estimated lives of the patents which average 12 years. The amortization expense for the years ended December 31, 2019, 2018 and 2017 was $0.1 million, $0.1 million and $0.1 million, respectively. Goodwill and intangible assets consisted of the following (in thousands): Goodwill December 31, 2019 2018 Goodwill $ 949 $ 949 Intangible assets December 31, 2019 2018 In-process research and development $ 1,750 $ 1,750 Less accumulated amortization (438 ) (292 ) $ 1,312 $ 1,458

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued Liabilities	8. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Research and clinical expenses $ 19,006 $ 18,520 Payroll and related expenses 6,721 6,585 Legal and professional expenses 1,062 830 Operating lease liabilities - short term 2,810 — Other accrued expenses 452 789 Total $ 30,051 $ 26,724

Research and Collaboration Agre

Research and Collaboration Agreements	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	9. Research and Collaboration Agreements The following table summarizes the revenue by collaboration partner (in thousands): Year Ended December 31, 2019 2018 2017 AbbVie $ 5,878 $ 18,997 $ 19,434 Amgen 3,871 4,899 1,311 Bristol-Myers Squibb 47,740 32,780 36,492 ImmunoGen - 1,471 12,503 Pfizer - 1,355 1,883 Total Revenue $ 57,489 $ 59,502 $ 71,623 AbbVie Ireland Unlimited Company In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “ ” “ ” “ ” “ ” Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and is eligible to receive up to $470.0 million in development, regulatory and commercial milestone payments, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 PDC. The Company ’ ’ “ ” Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics and create PDCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such PDCs ( “ ” Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and subsequently earned an additional $10.0 million milestone payment triggered by selection of the second target by AbbVie in June 2019. The Company is also eligible to receive up to $275.0 million in development, regulatory and commercial milestone payments and royalties in the high single to low teens from commercial sales of any resulting PDCs. The second target was selected under the Amended and Restated Discovery Collaboration and License Agreement entered into in June 2019 that allows AbbVie to select a target for developing a PDC or a Probody. The Company has determined that the CD71 Agreement and Discovery Agreement with AbbVie should be combined and evaluated as a single arrangement in determining revenue recognition, because both agreements were concurrently negotiated and executed. The Company identified the following performance obligations at the inception of the AbbVie Agreements: (1) the research, development and commercialization license for CD71 Probody therapeutic, (2) the research services related to CD71 Probody therapeutic, (3) the obligation to participate in the CD71 Agreement joint research committee, (4) the research services related to the first discovery target (5) the research, development and commercialization license for the first discovery target, and (6) the obligation to participate in the Discovery Agreement joint research committee. The Company concluded that AbbVie ’ The Company determined that the research, development and commercialization licenses for CD71 and discovery targets are not distinct from the Company ’ ’ ’ ’ ’ (1) the CD71 Agreement performance obligation consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and (2) the Discovery Agreement performance obligation consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. The total transaction price upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) less $4.2 million of estimated sublicense fees. The upfront payments under the AbbVie Agreements are allocated between the two performance obligations based on the estimated relative standalone selling prices. The $30.0 million of upfront payments is allocated $20.0 million to the CD71 Agreement, with the remaining $10.0 million allocated to the Discovery Agreement. The $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) and estimated sublicense fees of $4.2 million are allocated to the CD71 Agreement performance obligation as they are directly related to the development of the CD71 Probody therapeutic. In May 2018, the Company earned a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN) under the CD71 Agreement. The $21.0 million milestone payment was included as part of the transaction price in May 2018 and a revenue adjustment of $9.9 million was recognized in the second quarter of 2018 reflecting the percentage completed to-date on the project related to this milestone. The Company determined that the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company ’ The Company is obligated to make sublicense payments under the license agreement with the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), as amended, equal to 7.5% of certain upfront and milestone payments owed to or received by the Company. As of December 31, 2019 and 2018, the Company recorded accrued sublicense fees of zero and $1.1 million, respectively, under the UCSB Agreement. Of the $39.8 million total initial transaction price, the Company allocated $29.8 million to the CD71 Agreement performance obligation and recognized revenue using a cost-based input measure, the common measure of progress for the performance obligation. In applying the cost-based input method, revenue is recognized based on actual full-time employee (“FTE”) hours incurred as a percentage of total estimated FTE hours for completing its combined performance obligation over the estimated service period. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. During 2019, as a result of ongoing dose escalation in the continued development program, there has been a change in estimates of the research service period as well as an increase in the projected FTE hours-to-completion. The research service period for the CD71 Agreement performance obligation is extended from April 2021 to March 2022. The remaining $10.0 million of the total initial transaction price of $39.8 million allocated to the Discovery Agreement performance obligation represents an obligation to continuously make the Company ’ In June 2019, the Company earned a $ 10.0 million milestone payment for the second target selected by AbbVie under the Discovery Agreement. It is recognized also using the common measure of progress of the related obligation over the estimated research service period of five years through June 2024. The Company recognized revenue of $5.9 million, $19.0 million and $19.4 million for 2019, 2018 and 2017, respectively, related to the AbbVie Agreements. As of December 31, 2019 and 2018, deferred revenue related to the CD71 Agreement performance obligation was $20.0 million and $23.2 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $11.6 million and $4.7 million, respectively. As of both December 31, 2019 and 2018, no amount was due from AbbVie under the CD71 Agreement and the Discovery Agreement. Amgen, Inc. On September 29, 2017, the Company and Amgen, Inc. ( “ ” “ ” “ ” ’ six-month Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a Probody T-cell engaging bi-specific therapeutic targeting epidermal growth factor receptor (the “ ” “ ” Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of Probody T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “ ” “ ” “ ” ’ At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing pre-clinical stage T-cell engaging bispecific product from the Amgen pre-clinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a Probody T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product ( “ ” The Company considered the criteria for combining contracts in ASC 606 and determined that the Amgen Agreement and the Purchase Agreement should be combined into one contract. The Company accounted for the Amgen Agreement based on the fair values of the assets and services exchanged. The Company identified the following performance obligations at the inception of the Amgen Agreement: (1) the research, development and commercialization license, (2) the research and development services for the EGFR Products and the Amgen Other Product, and (3) the obligation to participate in the joint steering committee ( “ JSC ” ) and the joint research committee ( “ JRC ” ). The Company determined that research, development and commercialization license and the participation in the JSC and JRC are not distinct from the research and development services and therefore those performance obligations were combined into one combined performance obligation. The Amgen Other Products are accounted for as a separate performance obligation from the EGFR Products as the nature of the services being performed is not the same and the value that Amgen can derive from one program is not dependent on the success of the other. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Concurrent with the execution of the Amgen Agreement, the Company entered into a sublicense agreement whereby the Company granted Amgen a sublicense of its rights to one patent family that it co-owns with UCSB, that is exclusively licensed to the Company under the UCSB Agreement covering Probody antibodies and other pro-proteins in the fields of therapeutics, in vivo diagnostics and prophylactics. This sublicense was incremental to the patents, patent applications and know-how covering T-cell engaging bispecific Probody molecules that were developed and owned by the Company and licensed to Amgen. Under the UCSB Agreement, as amended, the Company is obligated to make a sublicense payment to UCSB equal to 7.5% of certain upfront and milestone payments owed to or received by the Company. As of December 31, 2019 and 2018, the Company recorded liabilities of zero and $2.1 million, respectively, representing the sublicense fee payable to UCSB. The total transaction price of $51.2 million, consisting of the $40.0 million upfront payment, an estimated fair value of $10.7 million for the CytomX Product and $0.5 million of premium on the sale of the Company ’ ’ Of the $51.2 million total transaction price, the Company allocated $46.4 million to the EGFR Products performance obligation and $4.8 million to the Amgen Other Product performance obligations. The transaction price of the EGFR Product performance obligation was recognized using a cost-based input measure. In applying the cost-based input method of revenue recognition, the Company uses actual FTE hours incurred relative to estimated FTE hours expected to be incurred for the combined performance obligation over the research service period. In the fourth quarter of 2018, the JSC officially terminated any further work on two molecules that did not meet required research criteria. Pre-clinical evaluation of additional molecules has been ongoing as part of the candidate identification phase of the EGFR project, and as a result, there was a change in estimate of the FTE hours-to-completion and research service period to seven years during the fourth quarter of 2018. At the end of the second quarter of 2019, the Company determined it will undertake additional testing and assessment of the molecules being evaluated under the EGFR project. As a result, the estimated FTE hours-to-completion and research service period was further increased to eight years. The $4.8 million transaction price allocated to the Amgen Other Product performance obligation represents an obligation to continuously make the Probody therapeutic technology platform available to Amgen, which is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of six years. The Company recognized revenue of $3.9 million, $4.9 million and $1.3 million for the years ended December 31, 2019, 2018 and 2017, respectively, related to the Amgen Agreement. As of December 31, 2019 and 2018, deferred revenue related to the EGFR Products performance obligation was $37.6 million and $40.7 million, respectively. As of December 31, 2019 and 2018, deferred revenue related to the Amgen Other Products performance obligation was $3.0 million and $3.8 million, respectively. As of December 31, 2019 and 2018, no amount was due from Amgen under the Amgen Agreement. Bristol-Myers Squibb Company On May 23, 2014, the Company and Bristol-Myers Squibb Company ( “ ” “ ” ’ Under the terms of the BMS Agreement, the Company granted Bristol-Myers Squibb exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. Bristol-Myers Squibb had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target had a two-year five years of the effective date of the BMS Agreement. The research term for each collaboration target c ould be extended in one year increments up to three times. Pursuant to the BMS Agreement, the financial consideration from Bristol-Myers Squibb was comprised of an upfront payment of $50.0 million and the Company was initially entitled to receive contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid-single digits to low double-digit percentages from potential future sales. The Company also receives research and development service fees based on a prescribed FTE rate that is capped. The Company identified the following performance obligations at the inception of the BMS Agreement: (1) the exclusive research, development and commercialization license, (2) the research and development services and (3) the obligation to participate in the joint research committee. The Company determined that the license, the Company ’ The Company received an upfront payment of $50.0 million from Bristol-Myers Squibb in July 2014. In January and December 2016, Bristol-Myers Squibb selected the third and fourth targets, respectively, and paid the Company $10.0 million and $15.0 million, respectively, pursuant to the terms of the BMS Agreement. In December 2016, Bristol-Myers Squibb selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. In November 2017, the Company recognized a $10.0 million milestone payment from Bristol-Myers Squibb upon approval of the investigational new drug application for the CTLA-4-directed Probody therapeutic. On March 17, 2017, the Company and Bristol-Myers Squibb entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “BMS ” “ ” Pursuant to the BMS Amendment, the financial consideration from Bristol-Myers Squibb is comprised of an upfront payment of $200.0 million and the Company is eligible to receive up to an aggregate of $3,586.0 million as follows: (i) up to $116.0 million in development milestone payments per target or up to $928.0 million if the maximum of eight targets are selected for the first product modality; (ii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $992.0 million if the maximum of eight targets are selected for the first product modality; (iii) up to $60.0 million in sales milestone payments per target or up to $480.0 million if maximum of eight targets are selected for the first product modality; and (iv) up to $56.3 million in development milestone payments or up to $450.0 million if the maximum of eight targets are selected for the second product modality; (v) up to $62.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $496.0 million if the maximum of eight targets are selected for the second product modality; (vi) up to $30.0 million in sales milestone payments per target or up to $240.0 million if maximum of eight targets are selected for the second product modality. The Company is also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. The BMS Amendment does not change the term of the Bristol-Myers Squibb’s royalty obligation under the BMS Agreement. Bristol-Myers Squibb ’ The initial transaction price is $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company ’ During the first quarter of 2019, Bristol-Myers Squibb terminated pre-clinical activities on three of the first four collaboration targets selected under the initial original 2014 BMS Agreement. The first and second targets under the BMS Agreement were combined into a single performance obligation. The Company determined that termination of pre-clinical activities on the second target does not impact the Company’s continuing obligation to Bristol-Myers Squibb for the first target, CTLA-4, as it is still being actively developed by Bristol-Myers Squibb. Therefore, the Company concluded that it will continue to amortize the related deferred revenue over the original performance period. The Company has determined that upon the termination of pre-clinical activities on the third and the fourth collaboration targets selected by Bristol-Myers Squibb in January and December of 2016, respectively, under the BMS Agreement, it has no further obligations and is no longer eligible to receive any further proceeds from milestones, royalties or research and development fees for such targets. As a result, the Company accelerated recognition of all of the related deferred revenue of the third and the fourth targets upon the effective date of termination and recognized $17.4 million in the first quarter of 2019. Research work under the BMS Amendment executed in March 2017 continues. The Company recognized revenue of $47.7 million, $32.8 million and $36.5 million for the years ended December 31, 2019, 2018, and 2017, respectively. As of December 31, 2019 and 2018, deferred revenue related to the BMS Agreement was $158.0 million and $205.6 million, respectively. The amount due from Bristol-Myers Squibb under the BMS Agreement was $13,000 and $97,000 as of December 31, 2019 and 2018, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. ( “ ” “ ” ’ “ ” ’ ’ ’ “ ” “ ” Under the terms of the ImmunoGen Research Agreement, both the Company and ImmunoGen performed research activities on behalf of the other party for no monetary consideration through January 2018 and the arrangement was extended to June 2018, as discussed below. Each party is solely responsible for the development, manufacturing and commercialization of any products resulting from the exclusive development and commercialization license obtained by such party under the agreement. In consideration for the ImmunoGen 2017 License, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments, up to $50.0 million in sales milestone payments and royalties in the mid-single digits percentage on the commercial sales of any resulting product. For the CX-2009 License, the Company is obligated to pay ImmunoGen up to $60.0 million in development and regulatory milestone payments and up to $100.0 million in sales milestone payments and royalties in the mid to high single digits percentage on the commercial sales of any resulting product. In August 2017, the Company made a milestone payment of $1.0 million to ImmunoGen for the first patient dosing with CX-2009. No milestone payments were payable to the Company under the ImmunoGen 2017 License while it was in effect. The Company accounted for the ImmunoGen Research Agreement based on the fair value of the assets and services exchanged. The Company identified the following performance obligations at the inception of the ImmunoGen Research Agreement: (1) the research license, (2) the research services, (3) the obligation to participate in the joint research committee, (4) the exclusive research, development and commercialization license and (5) the obligation to provide future technology improvements, when available. The Company determined that the research license, the research services, the participation in the joint steering committee, and the technology improvements are not distinct from the development and commercialization license and therefore those performance obligations were combined into one combined performance obligation. The Company considered factors such as the limited economic benefits to ImmunoGen if the development and commercialization license was not obtained and the lack of sublicensing rights in the research license. The estimated total fair value of the consideration of $13.2 million was recorded as deferred revenue at the inception of the ImmunoGen Research Agreement. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company ’ The estimated total fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million allocated to the research services, joint research committee participation and technology improvements, which was expensed over the period of services provided. The Company recognized no revenue for the year ended December 31, 2019 and revenue of $1.5 million and $12.5 million for the years ended December 31, 2018 and 2017, respectively. As of December 31, 2019 and 2018, there was no deferred revenue relating to the ImmunoGen Research Agreements, respectively. As of both December 31, 2019 and 2018, no amount was due from ImmunoGen under the ImmunoGen Research Agreement. MD Anderson In November 2015, the Company entered into a research collaboration agreement with MD Anderson to research Probody-enabled chimeric antigen receptor killer (CAR-NK) cell therapies, known as ProCAR-NK cell therapies. Under this collaboration, MD Anderson used the Company ’ Pfizer Inc. In May 2013, the Company and Pfizer Inc. ( “ ” “ ” “ ” Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6.0 million and research and development service fees based on a prescribed FTE rate per year that is capped. The Company identified the following performance obligations at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license was not distinct from the research services and participation in the joint research committee due to the specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a combined performance obligation. The Company concluded that, at the inception of the agreement, Pfizer ’ As the combined performance obligation represented an obligation to continuously make the Probody therapeutic technology platform available to Pfizer, the initial transaction price was recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five and a half years The Company recognized no revenue for the year ended December 31, 2019, and recognized revenue of $1.4 million and $1.9 million for the years ended December 31, 2018 and 2017, respectively. As of December 31, 2019 and 2018, there was no deferred revenue relating to the Pfizer Agreement. As of December 31, 2019 and 2018, there was no amount due from Pfizer under the Pfizer Agreement, respectively. Contract Liabilities The following table presents changes in the Company ’ Balance at 12/31/2018 Additions Deductions Balance at 12/31/2019 (in thousands) Contract liabilities: Deferred revenue $ 277,980 $ 10,000 $ 57,741 $ 230,239 There was a $10.0 million addition to deferred revenue related to the milestone payment triggered by selection of the second target by AbbVie during the year ended December 31, 2019. Deductions of $57.7 million represent primarily revenue recognized, including the accelerated recognition of deferred revenue of $17.4 million due to termination of certain targets by Bristol-Myers Squibb in the first quarter of 2019 that was included in the contract liability balance at the beginning of the period. The Company estimates that the $230.2 million of deferred revenue related to the following contracts as of December 31, 2019 to be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners or other factors outside of the Company ’ • The $20.0 million of deferred revenue related to the CD71 Agreement as of December 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately March 2022. • The $2.6 million of deferred revenue related to the first target under Discovery Agreement as of December 31, 2019 is expected to be recognized ratably until approximately April 2021. The $9.0 million of deferred revenue related to the second target under the Discovery Agreement as of December 31, 2019 is expected to be recognized ratably until approximately June 2024. • The $37.6 million of deferred revenue related to the Amgen EGFR Products as of December 31, 2019 is expected to be recognized based on actual FTE effort and program progress until approximately September 2025. • The $3.0 million of deferred revenue related to the Amgen Other Products as of December 31, 2019 is expected to be recognized ratably until approximately September 2023. • The $158.0 million of deferred revenue related to the BMS Agreement as of December 31, 2019 is expected to be recognized ratably until approximately April 2025.

License Agreement

License Agreement	12 Months Ended
	Dec. 31, 2019
Research And Development [Abstract]
License Agreement	10. License Agreement UCSB The Company has an exclusive, worldwide license agreement with UCSB (the “ ” Pursuant to the UCSB Agreement, the Company is obligated to (i) make royalty payments to UCSB on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to UCSB upon the occurrence of certain events, (iii) make a milestone payment to UCSB upon occurrence of an IPO or change of control, and (iv) reimburse UCSB for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UCSB Agreement, it is obligated to pay UCSB a percentage of the total sublicense revenue received, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company and other permitted deductions. As part of the UCSB Agreement, the Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights. The royalty obligations are cancellable any time by giving notice to the licensor, with the termination being effective 60 days after giving notice. In 2013, the Company amended the UCSB Agreement to reduce certain amounts due to UCSB upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to UCSB 157,332 shares of common stock. The UCSB Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents. In April 2019, the Company entered into Amendment No.3 to the UCSB Agreement to adjust and clarify certain sublicense terms (“Amendment No.3”). In connection with the amendment, the Company issued to UCSB 150,000 shares of CytomX common stock with a fair value of $10.68 per share. Under the terms of Amendment No.3, the Company and UCSB agreed to modify the determination of sublicense revenues payable by the Company to UCSB on certain existing collaboration agreements and on collaboration agreements executed subsequent to Amendment No.3. In exchange, the Company agreed to make an upfront payment of $1.0 million as well as additional annual license maintenance fees of $0.8 million through 2031. In the event that the Company terminates the agreement due to material concern of the safety or efficacy of the related technology, 50% of all remaining maintenance fees will become due immediately. Otherwise, all remaining maintenance fees will become due immediately upon early termination of the agreement unless there is a material breach by UCSB. Pursuant to Amendment No.3, the Company recorded in research and development expense a charge of $3.4 million relating to sublicense and maintenance fees representing the 150,000 shares issued with a fair value of $1.6 million, the upfront payment of $1.0 million and the additional annual maintenance fee of $0.8 million during the second quarter of 2019. In June 2019, the Company incurred an additional $0.8 million of sublicense fees related to the $10.0 million milestone payment for the second target selected by AbbVie under the Discovery Agreement. During the years ended December 31, 2019, 2018 and 2017, the Company incurred sublicense expenses of $4.3 million, $0.6 million and $13.5 million, respectively, under the provisions of the UCSB Agreement. As of December 31, 2019 and 2018, the Company recorded a liability of $0.2 million and $3.2 million, representing sublicense fee payable to UCSB, respectively. ImmunoGen In December 2019, the Company entered into a License Agreement (the “ImmunoGen 2019 License”) with ImmunoGen, Inc. to obtain an exclusive license with respect to epithelial cell adhesion molecule (“EPCAM”). The upfront license fee of $7.5 million was recorded as research and development expense in December 2019 and included in accrued liabilities as of December 31, 2019. The upfront license fee was paid in January 2020.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Legal Proceedings The Company is subject to claims and assessments from time to time in the ordinary course of business but is not aware of any such matters, individually or in the aggregate, that will have a material adverse effect on the Company’s financial position, results of operations or cash flows. Indemnifications In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	12. Leases Operating Lease On December 10, 2015, the Company entered into a lease (the “2016 Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters. The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease. The Lease provided for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months was approximately $4.3 million, which will increase on an annual basis beginning from the 25 th In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the 2016 Lease. The Company has recorded the $0.9 million Letter of Credit as non-current restricted cash on its balance sheet as at both December 31, 2019 and 2018. Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense against the operating lease ROU asset. Rent expense during the years ended December 31, 2019, 2018 and 2017 was $4.8 million, $4.2 million and $4.2 million, respectively. Supplemental information related to leases are as follows: Year Ended December 31, 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 4,855 December 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 25,382 Current operating lease liabilities 2,810 Non-current operating lease liabilities 24,871 Total operating lease liabilities $ 27,681 December 31, 2019 Weighted-average remaining lease term (in years) Operating lease 6.85 Weighted-average discount rate Operating lease 8.25 % December 31, 2019 (in thousands) Maturity of operating lease liabilities 2020 4,990 2021 5,129 2022 5,273 2023 5,420 2024 and beyond 15,689 Total lease payments 36,501 Less imputed interest (8,820 ) Present value of lease liabilities $ 27,681

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Common Stock	13. Common Stock In October 2015, the Company’s board of directors and stockholders approved the amended and restatement of the Company’s certificate of incorporation. The Amended and Restated Certificate of Incorporation was effective as of October 14, 2015, which provides for 75,000,000 authorized shares of common stock with par value of $0.00001 per share and 10,000,000 shares of preferred stock with a par value of $0.00001 per share. Common stockholders are entitled to dividends if and when declared by the Board of Directors subject to the prior rights of the preferred stockholders. As of December 31, 2019 and 2018, no dividends on common stock had been declared by the Board of Directors. In July 2018, the Company completed an underwritten public offering of 5,867,347 shares of common stock at a price of $24.50 per share, which included 765,306 shares issued pursuant to the underwriters ’ The Company had reserved shares of common stock for issuance, as follows: December 31, 2019 2018 Options issued and outstanding 9,936,168 7,803,773 Shares available for future stock option grants 3,145,266 1,884,494 Shares available for future employee stock purchase plan 1,609,137 1,301,254 Total 14,690,571 10,989,521

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	14. Stock-based Compensation The 2010 Plan and 2011 Plan In 2010, the Company adopted its 2010 Stock Incentive Plan (the “2010 Plan”) which provided for the granting of stock options to employees, directors and consultants of the Company. Options granted under the 2010 Plan were either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). In February 2012, the Company adopted its 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan is divided into two separate equity programs, an option and stock appreciation rights grant program and a stock award program. In conjunction with adopting the 2011 Plan, the Company discontinued the 2010 Plan and released the shares reserved and still available under that plan. In connection with the consummation of the IPO in October 2015, the board of directors adopted the Company’s 2015 Equity Incentive Plan (the “2015 Plan” and collectively with the 2010 Plan and 2011 Plan, the “Plans”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards. The 2015 Plan Options under the 2015 Plan may be granted for periods of up to ten years. All options issued to date have had a 10-year life. Under the terms of the 2015 Plan, options may be granted at an exercise price not less than the estimated fair value of the Company’s common stock on the date of grant, as determined by the Company’s board of directors. For employees holding more than 10% of the voting rights of all classes of stock, the exercise price of ISOs and NSOs may not be less than 110% of the estimated fair value of the shares on the date of grant, as determined by the board of directors. To date, options granted under the 2015 Plan generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/48th The initial number of shares of common stock available for future issuance under the 2015 Plan was 2,444,735. Beginning on January 1, 2016 and continuing until the expiration of the 2015 Plan, the total number of shares of common stock available for issuance under the 2015 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of January 1 of the same year. As of December 31, 2019, 1,808,066 shares of common stock were available for future issuance under the 2015 Plan. The 2019 Plan In September 2019, the Board of Directors adopted the 2019 Employment Inducement Incentive Plan (the “2019 Plan”) which provides for the grant of stock options and other equity awards to any employee who has not previously been an employee or director of the Company or who is commencing employment with the Company following a bona fide period of nonemployment by the Company. Awards granted under the 2019 Plan are intended to constitute “employment inducement awards” under Nasdaq Listing Rule 5635(c)(4). Options granted under the 2019 Plan are nonqualified stock options (“NSOs”) which may be exercisable for periods of up to ten years and the options shall be granted at an exercise price of not less than 100% of the fair market value of the Company’s common stock on the date of grant. The initial number of shares of common stock available for future issuance under the 2019 Plan was 1,815,000. As of December 31, 2019, 1,337,200 shares of common stock were available for future issuance under the 2019 Plan. Activity under the Company’s stock option plans is set forth below: Options Outstanding Options Available for Grant Number of Options Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2016 2,493,188 6,158,746 $ 3.69 Options authorized 1,459,606 — — Options granted (2,138,620 ) 2,138,620 13.57 Options exercised — (764,576 ) 4.14 Options forfeited 510,619 (1,029,332 ) 9.12 Balances at December 31, 2017 2,324,793 6,503,458 8.16 Options authorized 1,539,142 — Options granted (2,127,400 ) 2,127,400 24.65 Options exercised — (673,382 ) 6.17 Options forfeited 147,959 (153,703 ) 14.29 Balances at December 31, 2018 1,884,494 7,803,773 12.62 Options authorized 3,618,328 — Options granted (3,269,683 ) 3,269,683 12.40 Options exercised — (146,930 ) 4.30 Options forfeited 912,127 (990,358 ) 16.75 Balances at December 31, 2019 3,145,266 9,936,168 $ 12.26 6.5 $ 14,220 Options Exercisable—December 31, 2019 6,023,372 $ 10.50 6.5 $ 13,310 Options vested and expected to vest—December 31, 2019 9,936,168 $ 12.26 5.0 $ 14,220 The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the underlying common stock as of December 31, 2019, 2018 and 2017, respectively. The aggregate intrinsic value of stock options exercised in the years ended December 31, 2019, 2018 and 2017 was $0.9 million, $14.6 million and $10.5 million, respectively. The options granted in the years ended December 31, 2019, 2018 and 2017 had weighted-average per share grant-date fair values of $7.03, $14.21 and $8.21, respectively. At December 31, 2019 and 2018, the unrecognized compensation expense with respect to options granted to employees was $31.1 million and $34.6 million, respectively, and is expected to be recognized over 2.6 years and 2.4 years, respectively. Early Exercise of Employee Options Certain stock options granted under the Plans provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. Such unvested restricted shares are subject to a repurchase right held by the Company at the original issuance price in the event the optionee’s service to the Company is terminated either voluntarily or involuntarily. The right usually lapses 25% on the first anniversary of the vesting start date and in 36 equal monthly amounts thereafter. These repurchase terms are considered to be a forfeiture provision. The cash or full recourse notes received from employees for exercise of unvested options is treated as a refundable deposit and is classified as a liability on the balance sheets. Employee Stock Purchase Plan Concurrent with the completion of the IPO in October 2015, the Company’s Employee Stock Purchase Plan (“ESPP”) became effective. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for six-month offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company issued 142,949 shares and 63,920 shares of common stock under the ESPP in 2019 and 2018, respectively. Shares available for future purchase under the ESPP were 1,609,137 shares and 1,301,254 shares at December 31, 2019 and 2018, respectively. The compensation expense related to the ESPP was $0.6 million, $0.5 million and $0.2 million for the years ended December 31, 2019, 2018 and 20167 respectively. As of December 31, 2019 and 2018, there was $0.2 million and $0.3 million, respectively, of unrecognized compensation cost related to the ESPP, which the Company expects to recognize over 5 months. Stock Based Compensation Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 9,226 $ 8,313 $ 5,161 General and administrative 9,874 8,565 6,126 Total stock-based compensation expense $ 19,100 $ 16,878 $ 11,287 Stock-based compensation expense for employees was $18.9 million, $16.7 million and $11.0 million for the years ended December 31, 2019, 2018 and 2017, respectively. Stock-based compensation expense for non-employees was $0.2 million, $0.2 million and $0.3 million for the years ended December 31, 2019, 2018 and 2017, respectively. The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions: Options ESPP Year Ended December 31, Year Ended December 31, 2019 2018 2017 2019 2018 2017 Expected volatility 64.4% - 68.6% 65.6%-69.3% 69.1%-72.4% 60.81%-71.89% 46.4%-70.5% 52.3%-63.8% Risk-free interest rate 1.4% - 2.5% 2.5%-3.0% 1.7%-2.2% 1.62%-2.35% 2.1%-2.6% 1.1%-1.5% Dividend yield — % — % — % — % — % — % Expected term (in years) 4.9 - 5.0 4.7-4.9 4.9-5.3 0.5 0.5 0.5 Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period. Expected volatility. The expected stock price volatility for the Company’s stock options was derived from the average historical volatilities of the Company’s stock price and the stock price of several comparable publicly traded companies within the biotechnology and pharmaceutical industry. The Company will continue to apply this process until a sufficient amount of historical information on the Company’s own stock price becomes available. Volatility for ESPP shares is equal to the Company’s historical volatility over a six-month period. Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant. Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands): Years Ended December 31, 2019 2018 2017 Current: Federal $ (390 ) $ 14,302 $ — State — 1 1 Total current (390 ) 14,303 1 Deferred: Federal (37 ) — (514 ) State — — — Total deferred (37 ) — (514 ) Provision for (benefit from) income taxes $ (427 ) $ 14,303 $ (513 ) A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: Years Ended December 31, 2019 2018 2017 U.S. federal taxes at statutory rate 21.0 % 21.0 % 34.0 % State tax, net of federal benefit 1.3 % 0.7 % 7.6 % Stock compensation (1.2 )% 1.7 % 2.3 % Tax attributes subject to 382 limitation 0.0 % 0.0 % 27.5 % Tax credits 2.5 % 6.2 % 2.7 % Change in valuation allowance (22.8 )% (49.5 )% (9.3 )% Change in deferreds due to rate change 0.0 % 0.0 % (58.6 )% Return to provision adjustment (0.3 )% 0.0 % 0.0 % Other (0.1 )% (0.5 )% (5.0 )% Total 0.4 % (20.4 )% 1.2 % The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands): Year Ended December 31, 2019 2018 2017 Net operating loss carryforwards $ 38,967 $ 15,468 $ 24,682 Research and development credits 10,940 7,514 5,757 Lease liability 5,813 — — Intangible assets 3,320 — — Deferred revenue 47,180 56,881 15,631 Accruals and deferred rent 1,296 1,955 1,256 Stock-based compensation 7,837 5,746 3,831 Other — 39 32 Total gross deferred income tax assets 115,353 87,603 51,189 Less: valuation allowance (109,174 ) (86,466 ) (50,791 ) Deferred tax assets, net of valuation allowance 6,179 1,137 398 Fixed assets (591 ) (854 ) (282 ) Right-of-use assets (5,330 ) — — Intangible assets — (108 ) (116 ) Prepaid expenses (243 ) (175 ) — Other (15 ) — — Deferred tax liabilities (6,179 ) (1,137 ) (398 ) Net deferred income tax liabilities $ — $ — $ — The Company has established a valuation allowance against all of its net deferred tax assets. Management considered all available evidence, both positive and negative, including but not limited to our historical operating results, income or loss in recent periods, cumulative losses in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts, and concluded the deferred tax assets are not more likely than not to be realized. The net change in the total valuation allowance for the years ended December 31, 2019, 2018 and 2017 was an increase of $22.7 million, $35.7 million and $4.7 million, respectively. The Company had net operating loss carryforwards for federal and state income tax purposes of approximately $177.5 million and $24.2 million, respectively, as of December 31, 2019, available to reduce future taxable income. Of the federal net operating loss carryforwards, $65.6 million will begin to expire in 2034, if not utilized and $111.9 million will carryforward indefinitely. The state net operating loss carryforwards will begin to expire in 2032, if not utilized. The Company also has federal and state research and development tax credits carryforwards of $9.3 million and $7.1 million, respectively, as of December 31, 2019 available to reduce future income taxes. The federal research and development tax credits will begin to expire in 2031 if not utilized. The state research and development tax credits have no expiration date. Internal Revenue Code section 382 (“IRC Section 382”) places a limitation (the “Section 382 Limitation”) on the amount of taxable income that can be offset by net operating loss (“NOL”) carryforwards after a change in control (generally greater than 50% change in ownership) of a loss corporation. California has similar rules. The Company has performed an IRC Section 382 analysis and determined there was an ownership change in 2017 that resulted in 382 limitations. When an ownership change occurs, IRC Section 382 limits the use of NOLs and credits in subsequent periods based on the annual 382 limitations. The annual 382 limitations may limit the full use of available tax attributes in one year but the identified ownership changes may not result in expiration of tax attributes for use prior to expiration of their respective carryforward periods. Accordingly, none of the tax attributes have been reduced but limited the full use in 2018. The Company has determined that, while an ownership change has occurred, the applicable limits would not impair the value or anticipated use of the Company’s federal and state net operating losses. Although realization is not assured, management believes it is more likely than not that any limitation under IRC Section 382 will not impair the realizability of the deferred income tax assets related to federal and state net operating loss carryforwards. The Company had approximately $5.2 million and $3.8 million of unrecognized tax benefits as of December 31, 2019 and 2018, respectively, and approximately $0.9 million and $1.0 million, respectively, would affect the Company’s effective tax rate if recognized. A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2019 2018 2017 Balance at the beginning of the year $ 3,756 $ 4,320 $ 1,182 Additions based on tax positions related to current year 1,403 1,166 521 Adjustment based on tax positions related to prior years 90 (1,730 ) 2,617 Balance at end of the year $ 5,249 $ 3,756 $ 4,320 The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. To the extent accrued interest and penalties do not ultimately become payable, amounts accrued will be reduced and reflected as a reduction of the provision for income taxes in the period that such determination is made. Interest and penalties have not been accrued for 2019, 2018 and 2017. The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2018 remains open to U.S. federal and state examination to the extent of the utilization of net operating loss and credit carryovers. As of December 31, 20

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	16. Defined Contribution Plan The Company sponsors a defined contribution plan under Section 401(k) of the Internal Revenue Code covering substantially all full-time U.S. employees. Employee contributions are voluntary and are determined on an individual basis subject to the maximum allowable under federal tax regulations. During the years ended December 31, 2019, 2018 and 2017, the Company made contributions to the plan of $0.8 million, $0.6 million and $0.3 million, respectively.

Supplementary Data - Quarterly

Supplementary Data - Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Supplementary Data – Quarterly Financial Data (Unaudited)	17. Supplementary Data – Quarterly Financial Data (Unaudited) The following table represents certain unaudited financial information for each of the quarters in the twelve-month periods ended December 31, 2019 and 2018: Three Months Ended (in thousands, except per share data) December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Revenue $ 8,279 $ 10,712 $ 9,013 $ 29,485 Net income (loss) $ (35,455 ) $ (23,699 ) $ (28,960 ) $ (14,124 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.78 ) $ (0.52 ) $ (0.64 ) $ (0.31 ) Three Months Ended (in thousands, except per share data) December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 11,471 $ 12,509 $ 21,338 $ 14,184 Net income (loss) $ (32,233 ) $ (23,431 ) $ (13,447 ) $ (15,493 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.72 ) $ (0.53 ) $ (0.35 ) $ (0.40 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	18. Subsequent Events In February 2020, Bristol-Myers Squibb initiated a randomized Phase 2 cohort expansion in its Phase 1/2a trial of the anti-CTLA-4 Probody BMS-986249 and t riggered a $10.0 million milestone payment to the Company In February 2020, the Company initiated a first dosing of a patient in the CX-2009 Phase 2 clinical trial and triggered a $3.0 million milestone payment to ImmunoGen pursuant to the ImmunoGen Research Agreement.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ( “
Use of Estimates	Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability. Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments in highly rated money market funds and its short-term investments in U.S. Government Bonds. Customers and collaboration partners who represent 10% or more of the Company’s total revenue during each period presented or accounts receivable balance at each respective balance sheet date are as follows (in thousands): Revenue Accounts Receivable, net Year Ended December 31, December 31, 2019 2018 2017 2019 2018 AbbVie Ireland Unlimited Company $ 5,878 $ 18,997 $ 19,434 $ — $ — Bristol-Myers Squibb Company 47,740 32,780 36,492 13 97 ImmunoGen, Inc. — * 12,503 — — Total revenue from customers who represent 10% or more of the Company's total revenue $ 53,618 $ 51,777 $ 68,429 $ 13 $ 97 * Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented. All of the Company’s customers are located in the United States of America.
Segments	Segments Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer and principal financial officer, who are the Company’s chief operating decision makers. All long-lived assets are maintained in the United States of America.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the amounts shown in the statement of cash flows (in thousands): December 31 2019 2018 Cash and cash equivalents $ 188,425 $ 247,577 Restricted cash 917 917 $ 189,342 $ 248,494
Short-term Investments	Short-term Investments All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities greater than 12 months are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax. A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold.
Property and Equipment, net	Property and Equipment, net Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development. The Company assesses impairment indicators annually as of December 31 or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2019, 2018 and 2017.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the years ended December 31, 201 9 , 201 8 and 201 7 .
Revenue Recognition	Revenue Recognition On January 1, 2018, the Company adopted Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue Recognition (ASC 605) The Company ’ ’ The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company ’ The Company ’ The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, t he Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’
Comprehensive Loss	Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments and impact of adoption of new accounting pronouncements during the period represents the components of other comprehensive income (loss) that is excluded from the reported net loss.
Contract Balances	Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company ’
Stock-Based Compensation	Stock-Based Compensation The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the ratable method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. Forfeitures are recognized as they occur. On January 2019, the Company adopted ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. related Prior to the adoption of ASU 2018-07, such stock-based compensation expense was measured on the date of performance at the fair value of the consideration received or the fair value
Income Taxes	Income Taxes The Company accounts for income taxes using an asset and liability approach. Deferred tax assets and liabilities reflect the net tax effects of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The Company records a valuation allowance to reduce its deferred tax assets to reflect the net amount that it believes as more likely than not to be realized. Realization of the deferred tax assets is dependent on the generation of future taxable income, the amount and timing of which are uncertain. The valuation allowance requires an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable. Based upon the weight of available evidence at December 31, 2019, the Company continues to maintain a full valuation allowance against all of its deferred tax assets after management considered all available evidence, both positive and negative, including but not limited to its historical operating results, income or loss in recent periods, cumulative income in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts. The Compa n y recognize s the tax effects of an uncertain tax position only if it is more likely than not that it will be sustained based solely on its technical merits as of the reporting date and only in an amount more likely than not that it will be sustained upon review by the tax authorities. The Company evaluate s uncertain tax positions on a quarterly basis and adjust the liability for changes in facts and circumstances, such as new regulations or interpretations by the taxing authorities, new information obtained during a tax examination, significant amendment to an existing tax law, or resolution of an examination. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences will impact the income tax provision in the period in which such determination is made. The resolution of its uncertain income tax positions is dependent on uncontrollable factors such as law changes, new case law, and the willingness of the income tax authorities to settle, including the timing thereof and other factors. Although the Company does not anticipate significant changes to its uncertain income tax positions in the next twelve months, items outside of its control could cause its uncertain income tax positions to change in the future, which would be recorded in its statements of operations. Interest and/or penalties related to income tax matters are recognized as a component of income tax expense.
Research and Development Expenses	Research and Development Expenses The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred. In January 2019, the Company acquired certain technology know-how that is complementary to the Company’s proprietary Probody technology from a third party for $5.0 million. The Company plans to use this technology in certain of the Company’s discovery stage projects, and has concluded that the technology acquired does not have an alternative future use. Accordingly, the $5.0 million has been recorded as research and development expense for 2019.
Leases	Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company elected the short-term lease recognition exemption .
Adopted Accounting Pronouncements and Recent Accounting Pronouncements	Adopted Accounting Pronouncements Leases The Company adopted the Accounting Standard Update (“ASU”) No. 2016-02, Leases (Topic 842) lessees recognize on the balance sheet the assets and liabilities that arise from leases, including leases classified as operating leases under current GAAP, and disclose qualitative and quantitative information about leasing arrangements. I n July 2018, the Financial Accounting Standard Board (“FASB”) further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company has elected such option and accordingly the comparative periods are not recast. The Company has also elected the package of practical expedients permitted under the new standard (“ASC 842”). Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company recorded a right-of-use asset of $28.0 million and a lease liability $30.1 million on January 1, 2019. There was no impact on the Company’s accumulated deficit as of the adoption date. This standard does not have material impact on the Company’s results of operations or cash flows. Reclassification of Certain Tax Effects In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Nonemployee Share-Based Payment In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) periods presented upon their effective date. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this ASU and delay adoption of the additional disclosures until their effective date. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40) In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606 In May 2019, the FASB issued ASU 2019-05, Financial Instruments - Credit Losses (Topic 326): Targeted Transition Relief , to provide entities with an option to irrevocably elect the fair value option in Subtopic 825-10, applied on an instrument-by-instrument basis for eligible instruments, that are within the scope of Subtopic 326-20, upon adoption of Topic 326. The fair value option election does not apply to held-to-maturity debt securities. This targeted transition relief is intended to increase comparability of financial statement information for some entities that otherwise would have measured similar financial instruments using different measuring methodologies. The effective date of this ASU is the same as ASU No. 2016-13, for the Company on January 1, 2020. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In November 2019, the FASB issued ASU 2019-08, Compensation - Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606): Codification Improvements – Share-Based Consideration Payable to a Customer. The amendments in this ASU require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. Under ASC 606, these awards are considered a reduction of the transaction price, unless the awards are payment for a distinct good or service received from the customer and should be recorded as a reduction of the transaction price. However, the ASU requires these awards to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718 and should be recognized at the later of when the award is promised and when the entity recognizes revenue for the transfer of the related goods or services in accordance with ASC 606. The grant date is the date at which a grantor (supplier) and a grantee (customer) reach a mutual understanding of the key terms and conditions of a share-based payment award. The classification and subsequent measurement of the award are subject to the guidance in Topic 718 unless the share-based payment award is subsequently modified and the grantee is no longer a customer. The ASU is effective for the Company on January 1, 2020, and interim periods within those fiscal years. The Company does not expect the adoption of this ASU will have a material impact on its financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance	Customers and collaboration partners who represent 10% or more of the Company’s total revenue during each period presented or accounts receivable balance at each respective balance sheet date are as follows (in thousands): Revenue Accounts Receivable, net Year Ended December 31, December 31, 2019 2018 2017 2019 2018 AbbVie Ireland Unlimited Company $ 5,878 $ 18,997 $ 19,434 $ — $ — Bristol-Myers Squibb Company 47,740 32,780 36,492 13 97 ImmunoGen, Inc. — * 12,503 — — Total revenue from customers who represent 10% or more of the Company's total revenue $ 53,618 $ 51,777 $ 68,429 $ 13 $ 97 * Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented.
Schedule of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the amounts shown in the statement of cash flows (in thousands): December 31 2019 2018 Cash and cash equivalents $ 188,425 $ 247,577 Restricted cash 917 917 $ 189,342 $ 248,494
Schedule of Useful Lives of Property and Equipment	The useful lives of property and equipment are as follows: Machinery and equipment 5 years Computer equipment and software 3 years Furniture and fixtures 3 years Leasehold improvements Shorter of remaining lease term or estimated life of the assets

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share	The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented, because including them would have been anti-dilutive: Year Ended December 31, 2019 2018 2017 Options to purchase common stock 9,687,844 7,478,755 6,891,123

Fair Value Measurements and S_2

Fair Value Measurements and Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis	The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands): December 31, 2019 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets Money market funds Level I $ 170,757 $ — $ — $ 170,757 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 107,610 110 — 107,720 Total Securities $ 279,284 $ 110 $ — $ 279,394 December 31, 2018 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets Money market funds Level I $ 226,979 $ — $ — $ 226,979 Restricted cash (money market funds) Level I 917 — — 917 U.S. Government bonds Level I 188,616 — (66 ) 188,550 Total Securities $ 416,512 $ — $ (66 ) $ 416,446 No securities have contractual maturities of longer than one year

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31 2019 2018 Machinery and equipment $ 12,124 $ 10,498 Computer equipment and software 1,555 955 Furniture and fixtures 1,024 749 Leasehold improvements 1,483 893 Construction in progress 236 785 16,422 13,880 Less: accumulated depreciation and amortization (9,050 ) (6,946 ) $ 7,372 $ 6,934

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Goodwill and Intangible Assets	Goodwill and intangible assets consisted of the following (in thousands): Goodwill December 31, 2019 2018 Goodwill $ 949 $ 949 Intangible assets December 31, 2019 2018 In-process research and development $ 1,750 $ 1,750 Less accumulated amortization (438 ) (292 ) $ 1,312 $ 1,458

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Research and clinical expenses $ 19,006 $ 18,520 Payroll and related expenses 6,721 6,585 Legal and professional expenses 1,062 830 Operating lease liabilities - short term 2,810 — Other accrued expenses 452 789 Total $ 30,051 $ 26,724

Research and Collaboration Ag_2

Research and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Revenue by Collaboration Partner	The following table summarizes the revenue by collaboration partner (in thousands): Year Ended December 31, 2019 2018 2017 AbbVie $ 5,878 $ 18,997 $ 19,434 Amgen 3,871 4,899 1,311 Bristol-Myers Squibb 47,740 32,780 36,492 ImmunoGen - 1,471 12,503 Pfizer - 1,355 1,883 Total Revenue $ 57,489 $ 59,502 $ 71,623
Summary of Contract Liabilities	The following table presents changes in the Company ’ Balance at 12/31/2018 Additions Deductions Balance at 12/31/2019 (in thousands) Contract liabilities: Deferred revenue $ 277,980 $ 10,000 $ 57,741 $ 230,239

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Summary of Supplemental Information Related to Leases	Supplemental information related to leases are as follows: Year Ended December 31, 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 4,855 December 31, 2019 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 25,382 Current operating lease liabilities 2,810 Non-current operating lease liabilities 24,871 Total operating lease liabilities $ 27,681 December 31, 2019 Weighted-average remaining lease term (in years) Operating lease 6.85 Weighted-average discount rate Operating lease 8.25 % December 31, 2019 (in thousands) Maturity of operating lease liabilities 2020 4,990 2021 5,129 2022 5,273 2023 5,420 2024 and beyond 15,689 Total lease payments 36,501 Less imputed interest (8,820 ) Present value of lease liabilities $ 27,681

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Schedule Reserved Shares of Common Stock Issuance on Converted Basis	The Company had reserved shares of common stock for issuance, as follows: December 31, 2019 2018 Options issued and outstanding 9,936,168 7,803,773 Shares available for future stock option grants 3,145,266 1,884,494 Shares available for future employee stock purchase plan 1,609,137 1,301,254 Total 14,690,571 10,989,521

Stock-based Compensation (Table

Stock-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Activity Under Company's Stock Option Plans	Activity under the Company’s stock option plans is set forth below: Options Outstanding Options Available for Grant Number of Options Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in thousands) Balances at December 31, 2016 2,493,188 6,158,746 $ 3.69 Options authorized 1,459,606 — — Options granted (2,138,620 ) 2,138,620 13.57 Options exercised — (764,576 ) 4.14 Options forfeited 510,619 (1,029,332 ) 9.12 Balances at December 31, 2017 2,324,793 6,503,458 8.16 Options authorized 1,539,142 — Options granted (2,127,400 ) 2,127,400 24.65 Options exercised — (673,382 ) 6.17 Options forfeited 147,959 (153,703 ) 14.29 Balances at December 31, 2018 1,884,494 7,803,773 12.62 Options authorized 3,618,328 — Options granted (3,269,683 ) 3,269,683 12.40 Options exercised — (146,930 ) 4.30 Options forfeited 912,127 (990,358 ) 16.75 Balances at December 31, 2019 3,145,266 9,936,168 $ 12.26 6.5 $ 14,220 Options Exercisable—December 31, 2019 6,023,372 $ 10.50 6.5 $ 13,310 Options vested and expected to vest—December 31, 2019 9,936,168 $ 12.26 5.0 $ 14,220
Total Stock-based Compensation Recognized	Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 9,226 $ 8,313 $ 5,161 General and administrative 9,874 8,565 6,126 Total stock-based compensation expense $ 19,100 $ 16,878 $ 11,287
Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model	The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions: Options ESPP Year Ended December 31, Year Ended December 31, 2019 2018 2017 2019 2018 2017 Expected volatility 64.4% - 68.6% 65.6%-69.3% 69.1%-72.4% 60.81%-71.89% 46.4%-70.5% 52.3%-63.8% Risk-free interest rate 1.4% - 2.5% 2.5%-3.0% 1.7%-2.2% 1.62%-2.35% 2.1%-2.6% 1.1%-1.5% Dividend yield — % — % — % — % — % — % Expected term (in years) 4.9 - 5.0 4.7-4.9 4.9-5.3 0.5 0.5 0.5

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Components of Provision for (Benefit from) Income Taxes	The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands): Years Ended December 31, 2019 2018 2017 Current: Federal $ (390 ) $ 14,302 $ — State — 1 1 Total current (390 ) 14,303 1 Deferred: Federal (37 ) — (514 ) State — — — Total deferred (37 ) — (514 ) Provision for (benefit from) income taxes $ (427 ) $ 14,303 $ (513 )
Schedule of Effective Tax Rate Reconciliation	A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows: Years Ended December 31, 2019 2018 2017 U.S. federal taxes at statutory rate 21.0 % 21.0 % 34.0 % State tax, net of federal benefit 1.3 % 0.7 % 7.6 % Stock compensation (1.2 )% 1.7 % 2.3 % Tax attributes subject to 382 limitation 0.0 % 0.0 % 27.5 % Tax credits 2.5 % 6.2 % 2.7 % Change in valuation allowance (22.8 )% (49.5 )% (9.3 )% Change in deferreds due to rate change 0.0 % 0.0 % (58.6 )% Return to provision adjustment (0.3 )% 0.0 % 0.0 % Other (0.1 )% (0.5 )% (5.0 )% Total 0.4 % (20.4 )% 1.2 %
Schedule of Deferred Income Tax Liabilities	The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands): Year Ended December 31, 2019 2018 2017 Net operating loss carryforwards $ 38,967 $ 15,468 $ 24,682 Research and development credits 10,940 7,514 5,757 Lease liability 5,813 — — Intangible assets 3,320 — — Deferred revenue 47,180 56,881 15,631 Accruals and deferred rent 1,296 1,955 1,256 Stock-based compensation 7,837 5,746 3,831 Other — 39 32 Total gross deferred income tax assets 115,353 87,603 51,189 Less: valuation allowance (109,174 ) (86,466 ) (50,791 ) Deferred tax assets, net of valuation allowance 6,179 1,137 398 Fixed assets (591 ) (854 ) (282 ) Right-of-use assets (5,330 ) — — Intangible assets — (108 ) (116 ) Prepaid expenses (243 ) (175 ) — Other (15 ) — — Deferred tax liabilities (6,179 ) (1,137 ) (398 ) Net deferred income tax liabilities $ — $ — $ —
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2019 2018 2017 Balance at the beginning of the year $ 3,756 $ 4,320 $ 1,182 Additions based on tax positions related to current year 1,403 1,166 521 Adjustment based on tax positions related to prior years 90 (1,730 ) 2,617 Balance at end of the year $ 5,249 $ 3,756 $ 4,320

Supplementary Data - Quarterl_2

Supplementary Data - Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Unaudited Financial Information	The following table represents certain unaudited financial information for each of the quarters in the twelve-month periods ended December 31, 2019 and 2018: Three Months Ended (in thousands, except per share data) December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Revenue $ 8,279 $ 10,712 $ 9,013 $ 29,485 Net income (loss) $ (35,455 ) $ (23,699 ) $ (28,960 ) $ (14,124 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.78 ) $ (0.52 ) $ (0.64 ) $ (0.31 ) Three Months Ended (in thousands, except per share data) December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 11,471 $ 12,509 $ 21,338 $ 14,184 Net income (loss) $ (32,233 ) $ (23,431 ) $ (13,447 ) $ (15,493 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.72 ) $ (0.53 ) $ (0.35 ) $ (0.40 )

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Revenue	$ 8,279	$ 10,712	$ 9,013	$ 29,485	$ 11,471	$ 12,509	$ 21,338	$ 14,184	$ 57,489	$ 59,502	$ 71,623
Accounts Receivable, net	13				97				13	97
AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue									5,900	19,000	19,400
Major Customer
Concentration Risk [Line Items]
Revenue									53,618	51,777	68,429
Accounts Receivable, net	13				97				13	97
Major Customer | AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue									5,878	18,997	19,434
Major Customer | Bristol Myers Squibb Company
Concentration Risk [Line Items]
Revenue									47,740	32,780	36,492
Accounts Receivable, net	$ 13				$ 97				$ 13	$ 97
Major Customer | Immuno Gen Inc
Concentration Risk [Line Items]
Revenue											$ 12,503

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2019USD ($)	Dec. 31, 2019USD ($)ActivitySegment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jan. 01, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of business activities | Activity		1
Number of operating segments | Segment		1
Impairment of goodwill		$ 0	$ 0	$ 0
Impairment of intangible assets		0	0	0
Impairment of long-lived assets		0	0	0
Increase in accumulated deficit balance		(417,230,000)	(314,981,000)
Research and development expense		131,619,000	$ 103,866,000	$ 92,277,000
Probody technology
Summary Of Significant Accounting Policies [Line Items]
Acquired, technology know-how	$ 5,000,000
Research and development expense		$ 5,000,000
Adjustments | ASC 606
Summary Of Significant Accounting Policies [Line Items]
Increase in accumulated deficit balance					$ (10,900,000)

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 188,425	$ 247,577
Restricted cash	917	917
Cash, cash equivalents, and restricted cash	$ 189,342	$ 248,494

Basis of Presentation and Sum_7

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Machinery and equipment
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years
Computer equipment and software
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated useful life of leasehold improvements, description	Shorter of remaining lease term or estimated life of the assets

Adopted and Recent Accounting_2

Adopted and Recent Accounting Pronouncements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset	$ 25,382,000
Lease liability	27,681,000
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset		$ 28,000,000
Lease liability		$ 30,100,000
ASU 2018-02
Summary Of Significant Accounting Policies [Line Items]
Increase in other comprehensive income	$ 11,000

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	9,687,844	7,478,755	6,891,123

Fair Value Measurements and S_3

Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 279,284	$ 416,512
Gross Unrealized Holding Gains	110
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	279,394	416,446
Level I | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	170,757	226,979
Aggregate Fair Value	170,757	226,979
Level I | Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	917	917
Aggregate Fair Value	917	917
Level I | U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	107,610	188,616
Gross Unrealized Holding Gains	110
Gross Unrealized Holding Losses		(66)
Aggregate Fair Value	$ 107,720	$ 188,550

Fair Value Measurements and S_4

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Dec. 31, 2019USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of longer than one year	$ 0

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Machinery and equipment	$ 12,124	$ 10,498
Computer equipment and software	1,555	955
Furniture and fixtures	1,024	749
Leasehold improvements	1,483	893
Construction in progress	236	785
Total property and equipment	16,422	13,880
Less: accumulated depreciation and amortization	(9,050)	(6,946)
Property and equipment, net	$ 7,372	$ 6,934

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 2.5	$ 1.7	$ 1.5

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Amortization expense of intangible assets	$ 146	$ 146	$ 146
IPR&D [Member]
Finite Lived Intangible Assets [Line Items]
Average estimated lives of patents	12 years
Amortization expense of intangible assets	$ 100	$ 100	$ 100

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Goodwill
Goodwill	$ 949	$ 949
In-Process Research and Development
Intangible assets
Intangible assets, gross	1,750	1,750
Less accumulated amortization	(438)	(292)
Intangible assets, net	$ 1,312	$ 1,458

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 19,006	$ 18,520
Payroll and related expenses	6,721	6,585
Legal and professional expenses	1,062	830
Operating lease liabilities - short term	2,810
Other accrued expenses	452	789
Total	$ 30,051	$ 26,724

Research and Collaboration Ag_3

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partner (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 57,489	$ 59,502	$ 71,623
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	5,878	18,997	19,434
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	3,871	4,899	1,311
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 47,740	32,780	36,492
ImmunoGen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		1,471	12,503
Pfizer
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 1,355	$ 1,883

Research and Collaboration Ag_4

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)	1 Months Ended			3 Months Ended								12 Months Ended
	May 31, 2018USD ($)	Jul. 31, 2017USD ($)	Apr. 30, 2016USD ($)AgreementTargetAccountingUnit	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Mar. 31, 2017Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities				$ 30,051,000				$ 26,724,000				$ 30,051,000	$ 26,724,000
Revenue recognized from collaborative arrangement				8,279,000	$ 10,712,000	$ 9,013,000	$ 29,485,000	11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 57,489,000	$ 59,502,000	$ 71,623,000
Type of Revenue [Extensible List]												us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				230,239,000				277,980,000				$ 230,239,000	$ 277,980,000
UCSB
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities				200,000				3,200,000				200,000	3,200,000
Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid	$ 4,000,000
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received	21,000,000
Revenue adjustment of milestone payment recognized										$ 9,900,000
Revenue recognized from collaborative arrangement												5,900,000	19,000,000	$ 19,400,000
AbbVie Ireland Unlimited Company | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 30,000,000
Milestone payment received			14,000,000
Total transaction price			39,800,000
Estimated sublicense fees			$ 4,200,000
Number of accounting units | AccountingUnit			2
AbbVie Ireland Unlimited Company | Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements | Agreement			2
Percentage of net profits or net losses related to development costs			35.00%
Amount due from agreement				0				0				0	0
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 20,000,000
Milestone payment received	21,000,000	$ 14,000,000
Sublicense fees paid		$ 1,000,000
Milestone payment received	21,000,000
Deferred revenue				20,000,000				23,200,000				20,000,000	23,200,000
AbbVie Ireland Unlimited Company | CD71 Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			20,000,000
Total transaction price			29,800,000
Estimated sublicense fees			4,200,000
Milestone payment received			$ 14,000,000
AbbVie Ireland Unlimited Company | CD71 Agreement | U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales and royalties on ex U.S. sales			35.00%
AbbVie Ireland Unlimited Company | CD71 Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones			$ 470,000,000
AbbVie Ireland Unlimited Company | Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid	$ 4,000,000
AbbVie Ireland Unlimited Company | Seattle Genetics Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid			1,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			10,000,000
Number of targets selected | Target															1
Revenue recognition upon performance obligation			10,000,000
Deferred revenue				11,600,000				4,700,000				11,600,000	4,700,000
AbbVie Ireland Unlimited Company | Discovery Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			10,000,000
Total transaction price			10,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement | Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments received			$ 10,000,000
Deferred revenue				9,000,000								$ 9,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement | Second Target | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price						$ 10,000,000
Estimated research service period												5 years
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets | Target			2
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum | Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 275,000,000
AbbVie Ireland Unlimited Company | License Agreement | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License payment percentage			7.50%
AbbVie Ireland Unlimited Company | License Agreement | UCSB
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued liabilities				$ 0				$ 1,100,000				$ 0	$ 1,100,000

Research and Collaboration Ag_5

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)	Dec. 31, 2019
AbbVie Ireland Unlimited Company | CD71 Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2016-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition upon performance obligation, service period	5 years

Research and Collaboration Ag_6

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)	Sep. 29, 2017USD ($)Target	Oct. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, value of shares issued in connection with agreement												$ 134,596,000	$ 19,957,000
Accrued liabilities			$ 30,051,000				$ 26,724,000				$ 30,051,000	26,724,000
Revenue recognized from collaborative arrangement			8,279,000	$ 10,712,000	$ 9,013,000	$ 29,485,000	11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 57,489,000	$ 59,502,000	$ 71,623,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			230,239,000				277,980,000				$ 230,239,000	$ 277,980,000
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			37,600,000								37,600,000
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			3,000,000								3,000,000
Collaboration and License Agreement | Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 40,000,000	$ 40,000,000
Common stock, shares issuable under agreement | shares		1,156,069
Common stock, shares issuable under agreement, price per share | $ / shares		$ 17.30
Common stock, value of shares issued in connection with agreement		$ 20,000,000
Period used to calculate weighted average price per share		20 days
Lock-up period for share disposal		6 months
Estimated premium on the stock sold		$ 500,000
Number of targets selected | Target	1
Number of additional collaboration target | Target	2
Total transaction price	$ 51,200,000
Portion of transaction price allocated to premium on sale of common stock	500,000
Estimated fair value of products	10,700,000
Revenue recognized from collaborative arrangement											$ 3,900,000	$ 4,900,000	$ 1,300,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			0				$ 0				$ 0	$ 0
Collaboration and License Agreement | Amgen Inc | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 950,000,000
Number of targets | Target	3
Contingent payments payable	$ 203,000,000
Collaboration and License Agreement | Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price			46,400,000								46,400,000
Estimated FTE hours to completion and research service period					8 years		7 years
Deferred revenue			37,600,000				$ 40,700,000				37,600,000	40,700,000
Collaboration and License Agreement | Amgen Inc | EGFR Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 455,000,000
Percentage share of profit and losses	50.00%
Collaboration and License Agreement | Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price			4,800,000								4,800,000
Revenue recognition upon performance obligation			4,800,000								$ 4,800,000
Estimated research service period											6 years
Deferred revenue			3,000,000				3,800,000				$ 3,000,000	3,800,000
Sublicense Agreement | Amgen Inc | UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License payment percentage	7.50%
Accrued liabilities			$ 0				$ 2,100,000				$ 0	$ 2,100,000

Research and Collaboration Ag_7

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)

Apr. 25, 2017USD ($)TargetSaleIndicator

Jul. 07, 2014USD ($)TargetTermResearchTargetSaleIndicator

May 23, 2014USD ($)

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Nov. 30, 2017USD ($)

Mar. 17, 2017USD ($)Target

Dec. 31, 2016USD ($)

Jan. 31, 2016USD ($)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized from collaborative arrangement

$ 8,279,000

$ 10,712,000

$ 9,013,000

$ 29,485,000

$ 11,471,000

$ 12,509,000

$ 21,338,000

$ 14,184,000

$ 57,489,000

$ 59,502,000

$ 71,623,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

Deferred revenue

230,239,000

277,980,000

$ 230,239,000

$ 277,980,000

Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

158,000,000

$ 158,000,000

Collaborative Arrangement | Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Collaboration target research term

2 years

Number of additional collaboration target | Target

2

Research terms

Each collaboration target had a two-year research term and the two additional targets had to be nominated by Bristol-Myers Squibb within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.

Extension of research term for each collaboration target

1 year

Upfront payment received

$ 200,000,000

$ 50,000,000

$ 250,000,000

Contingent milestone payments receivable

$ 15,000,000

$ 10,000,000

Revenue recognition upon performance obligation

272,800,000

Upfront payment received

10,800,000

Milestone payment received

$ 12,000,000

Estimated research service termination date

Apr. 25, 2025

Revenue recognized from collaborative arrangement

$ 47,700,000

$ 32,800,000

$ 36,500,000

Type of Revenue [Extensible List]

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

us-gaap:LicenseAndServiceMember

Deferred revenue

158,000,000

205,600,000

$ 158,000,000

$ 205,600,000

Amount due from agreement

$ 13,000

$ 97,000

13,000

$ 97,000

Collaborative Arrangement | Bristol Myers Squibb Company | Pre Clinical Candidate

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 2,000,000

Collaborative Arrangement | Bristol Myers Squibb Company | New Drug Application for CTLA-4

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 10,000,000

Collaborative Arrangement | Bristol Myers Squibb Company | Third And Fourth Immuno-Oncology Targets

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized from collaborative arrangement

$ 17,400,000

$ 17,400,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of oncology target | Target

4

Number of collaboration target | Target

2

Period of nomination of additional target from effective date

5 years

Times of increments for extended collaboration target research time | Term

3

Contingent milestone payments receivable

$ 1,217,000,000

$ 3,586,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

25,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones For Each Research Target Program

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

116,000,000

114,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones If Four Research Targets Selected By Counterparty

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 456,000,000

Number of research targets selected | ResearchTarget

4

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale In Various Territories

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 124,000,000

$ 124,000,000

Number of sales indicators | SaleIndicator

3

3

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale If Four Research Targets Selected By Counterparty

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 496,000,000

Number of research targets selected | ResearchTarget

4

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones For Each Research Target Program

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 60,000,000

$ 60,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones If Four Research Targets Selected By Counterparty

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 240,000,000

Number of research targets selected | ResearchTarget

4

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement of Development Milestones if Eight Research Targets Selected by Counterparty

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 928,000,000

Number of research targets selected | Target

8

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Eight Targets Selected for the First Product Modality

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 992,000,000

Number of research targets selected | Target

8

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones if Eight Research Targets Selected for First Product Modality

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 480,000,000

Number of research targets selected | Target

8

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 56,300,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 450,000,000

Number of research targets selected | Target

8

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Milestone Payments for First Commercial Sale in Various Territories for Up to Three Indication

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 62,000,000

Number of sales indicators | SaleIndicator

3

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 496,000,000

Number of research targets selected | Target

8

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payments Per Target

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 30,000,000

Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payment if Eight Research Target are selected for Second Product Modality

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contingent milestone payments receivable

$ 240,000,000

Number of research targets selected | Target

8

Collaboration and License Agreement | Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of non-oncology target | Target

2

Collaboration and License Agreement | Maximum | Bristol Myers Squibb Company

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of oncology target | Target

6

Research and Collaboration Ag_8

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details)	1 Months Ended		3 Months Ended								12 Months Ended
	Aug. 31, 2017USD ($)	Jan. 31, 2014USD ($)Target	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			$ 230,239,000				$ 277,980,000				$ 230,239,000	$ 277,980,000
Revenue recognized from collaborative arrangement			8,279,000	$ 10,712,000	$ 9,013,000	$ 29,485,000	11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 57,489,000	$ 59,502,000	$ 71,623,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Collaborative Arrangement | Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		$ 0
Number of research targets selected | Target		2
Milestones payments made during period	$ 1,000,000
Milestone payments payable			0								$ 0
Estimated total fair value consideration recorded as deferred revenue		$ 13,200,000	0				0				0	$ 0
Estimated fair value of assets and services		13,200,000
Revenue recognized from collaborative arrangement											$ 0	$ 1,500,000	$ 12,500,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			$ 0				$ 0				$ 0	$ 0
Collaborative Arrangement | Immuno Gen Inc | Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services		12,700,000
Collaborative Arrangement | Immuno Gen Inc | Research Services, Joint Research Committee Participation and Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services		500,000
Collaborative Arrangement | Immuno Gen Inc | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development and regulatory milestones		30,000,000
Contingent payments receivable upon achieving sales milestones		50,000,000
Collaborative Arrangement | Immuno Gen Inc | Maximum | CX-2009 License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments payable upon achieving development and regulatory milestones		60,000,000
Contingent payments payable upon achieving sales milestones		$ 100,000,000

Research and Collaboration Ag_9

Research and Collaboration Agreements - MD Anderson - Additional Information (Details)	Nov. 02, 2015
Collaborative Arrangement | MD Anderson
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option expiration date	Nov. 2, 2019

Research and Collaboration A_10

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details)	1 Months Ended		3 Months Ended								12 Months Ended			80 Months Ended
	Dec. 31, 2014USD ($)	May 31, 2013USD ($)Target	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement			$ 8,279,000	$ 10,712,000	$ 9,013,000	$ 29,485,000	$ 11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 57,489,000	$ 59,502,000	$ 71,623,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			230,239,000				277,980,000				$ 230,239,000	$ 277,980,000		$ 230,239,000
Collaborative Arrangement | Pfizer Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets | Target		2
Nominated number of additional research targets | Target		2
Additional target as deferred revenue	$ 1,500,000
Upfront payment received		$ 6,000,000
Adjusted amortization period of deferred revenue														5 years 6 months
Revenue recognized from collaborative arrangement											$ 0	$ 1,400,000	$ 1,900,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			0				0				$ 0	$ 0		$ 0
Amount due from agreement			$ 0				$ 0				$ 0	$ 0		$ 0

Research and Collaboration A_11

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Revenue From Contract With Customer [Abstract]
Balance at 12/31/2018	$ 277,980
Additions	10,000
Deductions	57,741
Balance at 12/31/2019	$ 230,239

Research and Collaboration A_12

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deductions to deferred revenue recognized during the period									$ 57,741
Revenue recognized from collaborative arrangement	$ 8,279	$ 10,712	$ 9,013	$ 29,485	$ 11,471	$ 12,509	$ 21,338	$ 14,184	57,489	$ 59,502	$ 71,623
Deferred revenue	230,239				277,980				230,239	277,980
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement									5,900	19,000	19,400
AbbVie Ireland Unlimited Company | Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additions to deferred revenue									10,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	158,000								$ 158,000
Deferred revenue recognition maturity month and year									2025-04
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	37,600								$ 37,600
Deferred revenue recognition maturity month and year									2025-09
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	3,000								$ 3,000
Deferred revenue recognition maturity month and year									2023-09
Collaborative Arrangement | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement									$ 47,700	32,800	$ 36,500
Deferred revenue	158,000				205,600				158,000	205,600
Collaborative Arrangement | Bristol Myers Squibb Company | Third And Fourth Immuno-Oncology Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 17,400					17,400
CD71 Agreement | AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	20,000				23,200				$ 20,000	23,200
Deferred revenue recognition maturity month and year									2022-03
Discovery Agreement | AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	11,600				$ 4,700				$ 11,600	$ 4,700
Discovery Agreement | AbbVie Ireland Unlimited Company | Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	9,000								$ 9,000
Deferred revenue recognition maturity month and year									2024-06
Discovery Agreement | AbbVie Ireland Unlimited Company | First Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 2,600								$ 2,600
Deferred revenue recognition maturity month and year									2021-04

License Agreement - Additional

License Agreement - Additional Information (Details) - USD ($)	Apr. 02, 2019	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013
License Agreement [Line Items]
Annual minimum royalty obligations				$ 150,000
License termination period				60 days
Common stock, shares issued				45,523,088	45,083,209
Research and development				$ 131,619,000	$ 103,866,000	$ 92,277,000
Accrued liabilities				30,051,000	26,724,000
AbbVie | Discovery Agreement | Second Target
License Agreement [Line Items]
Additional sublicense fees		$ 800,000
Milestone payment receivable		$ 10,000,000
Immuno Gen Inc | Collaborative Arrangement
License Agreement [Line Items]
Upfront license payment				7,500,000
UCSB
License Agreement [Line Items]
Common stock, shares issued	150,000						157,332
Fair value of common stock price per share	$ 10.68
Payment of upfront fees	$ 1,000,000
Annual license maintenance fees	$ 800,000		$ 800,000
License payment term	2031
Percentage of remaining maintenance fees outstanding	50.00%
Fair value of common stock issued	$ 1,600,000
Sublicense fees				4,300,000	600,000	$ 13,500,000
Accrued liabilities				$ 200,000	$ 3,200,000
UCSB | Sublicense and Maintenance Fees
License Agreement [Line Items]
Research and development	$ 3,400,000

Leases - Additional Information

Leases - Additional Information (Details)	Dec. 10, 2015USD ($)ft²	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Operating Leased Assets [Line Items]
Restricted cash		$ 917,000	$ 917,000
Rent expense		4,800,000	4,200,000	$ 4,200,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000
Restricted cash		$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space | ft²	76,000
Lease term description		The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.
Initial lease term		10 years
Extended lease term		5 years
Maximum one-time improvement allowance	$ 12,600,000
Improvement allowance from recoverable rent	2,300,000
New Lease Agreement | First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	3,100,000
New Lease Agreement | Second twelve months of lease term
Operating Leased Assets [Line Items]
Annual lease rent	4,300,000
New Lease Agreement | Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	$ 5,500,000

Leases - Summary of Supplementa

Leases - Summary of Supplemental Information Related to Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 4,855
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	25,382
Current operating lease liabilities	2,810
Non-current operating lease liabilities	$ 24,871
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 27,681
Weighted-average remaining lease term (in years)
Operating lease	6 years 10 months 6 days
Weighted-average discount rate
Operating lease	8.25%
Maturity of operating lease liabilities
2020	$ 4,990
2021	5,129
2022	5,273
2023	5,420
2024 and beyond	15,689
Total lease payments	36,501
Less imputed interest	(8,820)
Present value of lease liabilities	$ 27,681

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 14, 2015
Class Of Stock [Line Items]
Common stock, shares authorized		75,000,000	75,000,000		75,000,000
Common stock, par value		$ 0.00001	$ 0.00001		$ 0.00001
Preferred stock, shares authorized					10,000,000
Preferred stock, par value					$ 0.00001
Dividends on common stock		$ 0	$ 0
Net proceeds from sale of common stock			$ 134,596,000	$ 19,957,000
Common Stock
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares		150,000	5,867,347
Net proceeds from sale of common stock	$ 134,600,000
Underwriting discounts and commissions and offering expenses	$ 9,200,000
Common Stock | Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	5,867,347
Common stock price, per share	$ 24.50
Common Stock | Underwriters
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	765,306

Common Stock - Reserved Shares

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Options issued and outstanding	9,936,168	7,803,773
Shares available for future stock option grants	3,145,266	1,884,494
Shares available for future employee stock purchase plan	1,609,137	1,301,254
Total	14,690,571	10,989,521

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		14,690,571	10,989,521
Stock option expiration period		10 years
Aggregate intrinsic value of stock options exercised		$ 900	$ 14,600	$ 10,500
Weighted average grant date fair value, options granted		$ 7.03	$ 14.21	$ 8.21
Unrecognized compensation expense, options granted to employees		$ 31,100	$ 34,600
Number of shares issued during period		142,949	63,920
Stock-based compensation expense		$ 19,100	$ 16,878	$ 11,287
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost, period for recognition		2 years 7 months 6 days	2 years 4 months 24 days
Stock-based compensation expense		$ 18,900	$ 16,700	11,000
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		1,609,137	1,301,254
Estimated fair value of the shares on the date of grant		85.00%
Unrecognized stock-based compensation cost, period for recognition		5 months
Plan maximum percentage of eligible compensation to purchase shares		15.00%
Stock-based compensation expense		$ 600	$ 500	200
Unrecognized stock-based compensation cost		200	300
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 200	$ 200	$ 300
Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		2.08%
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		1,808,066				2,444,735
Increase in number of shares available for issuance, as proportion of issued and outstanding shares of common stock, percentage	4.00%
Stock option expiration period		10 years
Employees holding voting rights of all classes of stock, percentage		10.00%
2015 Plan | Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		4 years
Options granted generally vest percent		25.00%
2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		1,337,200			1,815,000
Early Exercise of Employee Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		36 months
Early Exercise of Employee Options | Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		25.00%
Minimum | 2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		110.00%
Minimum | 2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		100.00%
Maximum | 2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period		10 years

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Activity Under Company's Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options Available for Grant
Balances, beginning of the period	1,884,494
Balances, end of the period	3,145,266	1,884,494
Number of Options
Balances, beginning of the period	7,803,773	6,503,458	6,158,746
Options granted	3,269,683	2,127,400	2,138,620
Options exercised	(146,930)	(673,382)	(764,576)
Options forfeited	(990,358)	(153,703)	(1,029,332)
Balances, end of the period	9,936,168	7,803,773	6,503,458
Options Exercisable, end of the period	6,023,372
Options vested and expected to vest, end of the period	9,936,168
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 12.62	$ 8.16	$ 3.69
Options granted	12.40	24.65	13.57
Options exercised	4.30	6.17	4.14
Options forfeited	16.75	14.29	9.12
Balances, end of the period	12.26	$ 12.62	$ 8.16
Options Exercisable, end of the period	10.50
Options vested and expected to vest, end of the period	$ 12.26
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	6 years 6 months
Options Exercisable, end of the period	6 years 6 months
Options vested and expected to vest, end of the period	5 years
Aggregate Intrinsic Value
Balances, end of the period	$ 14,220
Options Exercisable—December 31, 2019	13,310
Options vested and expected to vest—December 31, 2019	$ 14,220
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	1,884,494	2,324,793	2,493,188
Options authorized	3,618,328	1,539,142	1,459,606
Options granted	(3,269,683)	(2,127,400)	(2,138,620)
Options forfeited	912,127	147,959	510,619
Balances, end of the period	3,145,266	1,884,494	2,324,793

Stock-based Compensation - Tota

Stock-based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based compensation expense:
Stock-based compensation expense	$ 19,100	$ 16,878	$ 11,287
Research and development
Stock-based compensation expense:
Stock-based compensation expense	9,226	8,313	5,161
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 9,874	$ 8,565	$ 6,126

Stock-based Compensation - Sc_2

Stock-based Compensation - Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	64.40%	65.60%	69.10%
Expected volatility, maximum	68.60%	69.30%	72.40%
Risk-free interest rate, minimum	1.40%	2.50%	1.70%
Risk-free interest rate, maximum	2.50%	3.00%	2.20%
Employee Stock Option | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	4 years 10 months 24 days	4 years 8 months 12 days	4 years 10 months 24 days
Employee Stock Option | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years	4 years 10 months 24 days	5 years 3 months 18 days
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	60.81%	46.40%	52.30%
Expected volatility, maximum	71.89%	70.50%	63.80%
Risk-free interest rate, minimum	1.62%	2.10%	1.10%
Risk-free interest rate, maximum	2.35%	2.60%	1.50%
Expected term (in years)	6 months	6 months	6 months

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ (390)	$ 14,302
State		1	$ 1
Total current	(390)	14,303	1
Deferred:
Federal	(37)		(514)
Total deferred	(37)		(514)
Provision for (benefit from) income taxes	$ (427)	$ 14,303	$ (513)

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
U.S. federal taxes at statutory rate	21.00%	21.00%	34.00%
State tax, net of federal benefit	1.30%	0.70%	7.60%
Stock compensation	(1.20%)	1.70%	2.30%
Tax attributes subject to 382 limitation	0.00%	0.00%	27.50%
Tax credits	2.50%	6.20%	2.70%
Change in valuation allowance	(22.80%)	(49.50%)	(9.30%)
Change in deferreds due to rate change	0.00%	0.00%	(58.60%)
Return to provision adjustment	(0.30%)	(0.00%)	(0.00%)
Other	(0.10%)	(0.50%)	(5.00%)
Total	0.40%	(20.40%)	1.20%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Income Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 38,967	$ 15,468	$ 24,682
Research and development credits	10,940	7,514	5,757
Lease liability	5,813
Intangible assets	3,320
Deferred revenue	47,180	56,881	15,631
Accruals and deferred rent	1,296	1,955	1,256
Stock-based compensation	7,837	5,746	3,831
Other		39	32
Total gross deferred income tax assets	115,353	87,603	51,189
Less: valuation allowance	(109,174)	(86,466)	(50,791)
Deferred tax assets, net of valuation allowance	6,179	1,137	398
Fixed assets	(591)	(854)	(282)
Right-of-use assets	(5,330)
Intangible assets		(108)	(116)
Prepaid expenses	(243)	(175)
Other	(15)
Deferred tax liabilities	$ (6,179)	$ (1,137)	$ (398)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Deferred tax asset, net change in total valuation allowance increase during period	$ 22,700,000	$ 35,700,000	$ 4,700,000
Unrecognized tax benefits	5,249,000	3,756,000	4,320,000	$ 1,182,000
Unrecognized tax benefits would affect company's effective tax rate	900,000	1,000,000
Interest and penalties accrued	0	$ 0	$ 0
Minimum
Income Tax Contingency [Line Items]
Percentage of change in ownership			50.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 177,500,000
Operating loss carryforwards expiration year	2034
Operating loss carryforwards to expire if not utilized	$ 65,600,000
Operating loss carryforwards not subject to expiration	$ 111,900,000
Open tax year	2010 2011 2012 2013 2014 2015 2016 2017 2018
Federal | Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 9,300,000
Tax credit carryforward, expiration	2031
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 24,200,000
Operating loss carryforwards expiration year	2032
Open tax year	2010 2011 2012 2013 2014 2015 2016 2017 2018
State | Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 7,100,000

Income Taxes- Schedule of Recon

Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at the beginning of the year	$ 3,756	$ 4,320	$ 1,182
Additions based on tax positions related to current year	1,403	1,166	521
Adjustment based on tax positions related to prior years	90	(1,730)	2,617
Balance at end of the year	$ 5,249	$ 3,756	$ 4,320

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Company contributions to defined contribution plan	$ 0.8	$ 0.6	$ 0.3

Supplementary Data - Quarterl_3

Supplementary Data - Quarterly Financial Data (Unaudited) - Unaudited Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 8,279	$ 10,712	$ 9,013	$ 29,485	$ 11,471	$ 12,509	$ 21,338	$ 14,184	$ 57,489	$ 59,502	$ 71,623
Net income (loss)	$ (35,455)	$ (23,699)	$ (28,960)	$ (14,124)	$ (32,233)	$ (23,431)	$ (13,447)	$ (15,493)	$ (102,238)	$ (84,604)	$ (43,099)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.78)	$ (0.52)	$ (0.64)	$ (0.31)	$ (0.72)	$ (0.53)	$ (0.35)	$ (0.40)	$ (2.26)	$ (2.03)	$ (1.16)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Collaborative Arrangement - USD ($)	Feb. 27, 2020	Dec. 31, 2019	Dec. 31, 2016	Jan. 31, 2016
Bristol Myers Squibb Company
Subsequent Event [Line Items]
Contingent milestone payments receivable			$ 15,000,000	$ 10,000,000
Immuno Gen Inc
Subsequent Event [Line Items]
Milestone payments payable		$ 0
Subsequent Event | Bristol Myers Squibb Company
Subsequent Event [Line Items]
Contingent milestone payments receivable	$ 10,000,000
Subsequent Event | Immuno Gen Inc
Subsequent Event [Line Items]
Milestone payments payable	$ 3,000,000