Document and Entity Information	9 Months Ended
Document and Entity Information	Sep. 30, 2015	Nov. 19, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		36,024,292

CONDENSED BALANCE SHEETS (USD $)	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 36,191,000	$ 64,396,000
Restricted cash	100,000	100,000
Short-term investments	77,495,000	0
Accounts receivable	406,000	1,875,000
Prepaid expenses and other current assets	878,000	482,000
Total current assets	115,070,000	66,853,000
Property and equipment, net	3,667,000	3,018,000
Intangible assets	1,750,000	1,750,000
Goodwill	949,000	949,000
Other assets	3,343,000	492,000
Total assets	124,779,000	73,062,000
Current liabilities:
Accounts payable	2,700,000	1,919,000
Accrued liabilities	5,143,000	1,695,000
Deferred revenues, current portion	6,130,000	6,130,000
Long-term debt, current portion		1,419,000
Total current liabilities	13,973,000	11,163,000
Long-term debt, net of current portion		1,568,000
Deferred revenue, net of current portion	56,236,000	60,833,000
Convertible preferred stock warrant liability	788,000	186,000
Convertible preferred stock liability		395,000
Deferred tax liability	507,000	499,000
Other long-term liabilities	244,000	249,000
Total liabilities	71,748,000	74,893,000
Commitments and contingencies (Note 11)
Stockholders' deficit
Common stock, $0.00001 par value; 36,200,000 and 28,572,789 shares authorized at September 30, 2015 and December 31, 2014, respectively; 1,094,649 and 996,520 shares issued and outstanding at September 30, 2015 and December 31, 2014, respectively	1,000	1,000
Stockholders notes receivable	(78,000)	(404,000)
Accumulated other comprehensive income	8,000
Accumulated deficit	(105,979,000)	(78,138,000)
Total stockholders' deficit	(106,048,000)	(78,541,000)
Total liabilities, redeemable convertible preferred stock, convertible preferred stock and stockholders' deficit	124,779,000	73,062,000
Redeemable Convertible Preferred Stock
Current liabilities:
Redeemable convertible preferred stock, $0.00001 par value – 26,972,316 and 21,759,654 shares authorized at September 30, 2015 and December 31, 2014, respectively; 26,890,671 and 18,458,289 shares issued and outstanding at September 30, 2015 and December 31, 2014, respectively;	158,605,000	76,236,000
Convertible Preferred Stock
Current liabilities:
Redeemable convertible preferred stock, $0.00001 par value – 26,972,316 and 21,759,654 shares authorized at September 30, 2015 and December 31, 2014, respectively; 26,890,671 and 18,458,289 shares issued and outstanding at September 30, 2015 and December 31, 2014, respectively;	$ 474,000	$ 474,000

CONDENSED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2015	Dec. 31, 2014
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	36,200,000	28,572,789
Common stock, shares issued	1,094,649	996,520
Common stock, shares outstanding	1,094,649	996,520
Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.00001	$ 0.00001
Redeemable convertible preferred stock, shares authorized	26,972,316	21,759,654
Redeemable convertible preferred stock, shares issued	26,890,671	18,458,289
Redeemable convertible preferred stock, shares outstanding	26,890,671	18,458,289
Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.00001	$ 0.00001
Redeemable convertible preferred stock, shares authorized	244,782	244,782
Redeemable convertible preferred stock, shares issued	244,782	244,782
Redeemable convertible preferred stock, shares outstanding	244,782	244,782

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Revenues:	$ 1,939	$ 1,915	$ 5,724	$ 3,216
Operating expenses:
Research and development	9,157	3,916	18,854	23,963
General and administrative	4,051	1,463	8,549	4,359
Total operating expenses	13,208	5,379	27,403	28,322
Loss from operations	(11,269)	(3,464)	(21,679)	(25,106)
Interest income	407	2	874	5
Interest expense	(718)	(117)	(1,356)	(378)
Other income (expense), net	(287)	(10)	(1,718)	(44)
Loss before provision for income taxes	(11,867)	(3,589)	(23,879)	(25,523)
Provision for income taxes	3		8
Net loss	(11,870)	(3,589)	(23,887)	(25,523)
Accretion to redemption value and cumulative dividends on preferred stock	(2,958)	(1,169)	(6,147)	(3,370)
Net loss attributable to common stockholders	(14,828)	(4,758)	(30,034)	(28,893)
Net loss per share attributable to common stockholders, basic and diluted	$ (14.26)	$ (4.86)	$ (29.66)	$ (30.04)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	1,039,567	979,134	1,012,534	961,745
Other comprehensive loss:
Changes in unrealized losses on short-term investments	(7)		(8)
Total other comprehensive loss	(7)		(8)
Comprehensive loss	$ (11,877)	$ (3,589)	$ (23,895)	$ (25,523)

CONDENSED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net loss	$ (23,887)	$ (25,523)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Depreciation and amortization	877	562
Amortization of debt discount	80	30
Accretion of discount on short-term investments	816
Stock-based compensation expense	2,065	409
Deferred income taxes	8
Changes in operating assets and liabilities
Accounts receivable	1,469	(317)
Prepaid expenses and other current assets	(396)	(877)
Other assets	121	(355)
Accounts payable	749	444
Accrued liabilities	1,733	333
Deferred revenue	(4,597)	61,450
Net cash (used in)/provided by operating activities	(19,246)	36,200
Cash flows from investing activities:
Purchases of property and equipment	(1,412)	(999)
Purchases of short-term investments	(129,553)
Maturities of short-term investments	51,250
Net cash used in investing activities	(79,715)	(999)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	74,430	10,316
Proceeds from exercise of stock options	128	8
Proceeds from stockholder notes	326
Repayment of notes payable	(3,027)	(917)
Payment of deferred offering costs	(1,101)
Net cash provided by financing activities	70,756	9,407
Net increase/(decrease) in cash and cash equivalents	(28,205)	44,608
Cash and cash equivalents, beginning of period	64,396	8,703
Cash and cash equivalents, end of period	36,191	53,311
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	114
Accretion to redemption value and cumulative dividends on preferred stock	6,147	3,370
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock, net	1,509	1,303
Issuance costs in accounts payable and accrued liabilities	283
Deferred offering costs in accounts payable and accrued liabilities	1,862
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	1,114	13
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability	$ 602	$ 31

Description of the Business	9 Months Ended
Description of the Business	Sep. 30, 2015
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “Company”) is an oncology-focused biopharmaceutical company focused on developing Probody therapeutics for the treatment of cancer. Probody therapeutics are masked antibodies that remain inert in healthy tissue but are activated specifically in the disease microenvironment. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010. Initial Public Offering On October 7, 2015, the Company’s registration statement on Form S-1 (File No. 333-206658) relating to its initial public offering (“IPO”) of its common stock was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on the NASDAQ Global Select Market on October 8, 2015. The public offering price of the shares sold in the IPO was $12.00 per share. The IPO closed on October 14, 2015, pursuant to which the Company sold 7,666,667 shares of common stock, including the sale of 1,000,000 shares of common stock to the underwriters upon their exercise of their option to purchase additional shares. The Company received net proceeds of approximately $82.4 million, after underwriting discounts, commissions and estimated offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of convertible preferred stock and redeemable convertible preferred stock converted into common stock.

Liquidity	9 Months Ended
Liquidity	Sep. 30, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Liquidity	2. Liquidity The accompanying financial statements have been prepared on a going concern basis that contemplates the realization of assets and discharge of liabilities in the normal course of business. Since inception, the Company has incurred recurring net operating losses. As December 31, 2014 and September 30, 2015, the Company had an accumulated deficit of $78.1 million and $106.0 million, respectively, and expects to incur losses for the next several years. Since its inception, the Company has funded its operations primarily with the net proceeds from private placements of convertible preferred stock and proceeds from borrowings. As of December 31, 2014 and September 30, 2015, the Company had cash, cash equivalents and short-term investments of $64.4 million and $113.7 million, respectively. In May and June 2015, the Company received aggregate net proceeds of $73.2 million from the issuance of its Series C and Series D redeemable convertible preferred stock. In October 2015, the Company consummated its IPO and raised net proceeds of approximately $82.4 million, after deducting underwriting discounts and commissions and offering expenses. The Company believes its current available cash, cash equivalents and short-term investments together with cash received from the IPO will be sufficient to fund its planned expenditures and meet the Company’s obligations through at least the next twelve months.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2015

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

3. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The Company’s functional and reporting currency is the U.S. dollar.

Unaudited Interim Financial Information

The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented.

The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The condensed results of operations for the nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes for the year ended December 31, 2014 included in the Company’s Prospectus dated October 7, 2015 filed pursuant to Rule 424(b)(4) with the SEC.

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 that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds and its short-term investments in U.S. Government Bonds.

Customers who represent 10% of more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows:

	Revenue						Revenue						Accounts Receivable, net
	Three Months Ended September 30,						Nine Months Ended September 30,						September 30,			December 31,
	2015			2014			2015			2014			2015			2014
Customer A		76	%		71	%		77	%		42	%		55	%	*
Customer B		24	%		29	%		23	%		58	%		45	%		92	%

*	Less than 10%.

All of the Company’s customers are located in the United States of America.

Deferred Offering Costs

Deferred offering costs consisted primarily of direct incremental costs related to the Company’s initial public offering of its common stock. Approximately $3.0 million of deferred offering costs are included in other assets on the Company’s condensed balance sheet as of September 30, 2015. Upon consummation of the initial public offering in October 2015, these amounts were offset against the proceeds of the IPO.

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, who is the Company’s chief operating decision maker. 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 amounts related to the security deposit for the Company’s credit card accounts.

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. Those investments with contractual maturities greater than 12 months at the date of purchase 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. The Company did not have any investments as of December 31, 2014.

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. Premiums (discounts) are amortized (accreted) over the life of the related security as an adjustment to yield using the straight-line interest method. 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 (“IPR&D”). The Company assesses impairment indicators annually 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 nine months ended September 30, 2015 and the year ended December 31, 2014.

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 periods presented in these condensed financial statements.

Convertible Preferred Stock Warrant Liability

Freestanding warrants for shares that are contingently redeemable are classified as liabilities on the balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are subject to re-measurement at each balance sheet date and the change in fair value, if any, is included in other income (expense), net. The Company will continue to adjust the liability for changes in fair value until immediately prior to the consummation of its IPO in October 2015, at which time all convertible preferred stock warrants were net exercised into shares of common stock and the related convertible preferred stock warrant liability was reclassified to additional paid-in capital.

Immediately prior to the consummation of the Company’s IPO in October 2015, all of the warrants outstanding as of September 30, 2015 were net exercised, resulting in issuance of an aggregate of 60,640 shares of our common stock.

Convertible Preferred Stock Liability

The obligation to issue additional shares of Series B-1 and Series C redeemable convertible preferred stock at a future date was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the convertible preferred stock liability on the balance sheets at its estimated fair value. The liability is subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net. At the time of each funding, the Company remeasures the liability, with the change in fair value recognized as a component of other income (expense), net and then reclassifies the fair value associated with the convertible preferred stock liability to the applicable series of redeemable convertible preferred stock. Immediately prior to the consummation of the Company’s IPO in October 2015, the convertible preferred stock converted to 27,135,453 shares of common stock.

Comprehensive Loss

Comprehensive loss represents all changes in stockholders’ deficit except those resulting from distributions to stockholders. The Company’s unrealized losses on short-term investments represent the only component of other comprehensive loss that is excluded from the reported net loss.

Revenue Recognition

The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; the price to the customer is fixed or determinable and collectability is reasonably assured.

The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (ii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products.

In arrangements involving the delivery of more than one element, each required deliverable is evaluated to determine whether it qualifies as a separate unit of accounting. The determination is based on whether the deliverable has “standalone value” to the customer. If a deliverable does not qualify as a separate unit of accounting, it is combined with the other applicable undelivered item(s) within the arrangement and these combined deliverables are treated as a single unit of accounting.

The arrangement’s consideration that is fixed or determinable is allocated to each separate unit of accounting based on the relative selling price methodology in accordance with the selling price hierarchy, which includes vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE is not available, or the best estimate of selling price, if neither VSOE nor third-party evidence is available.

Payments or reimbursements for the Company’s research and development efforts for the arrangements where such efforts are considered as deliverables are recognized as the services are performed and are presented on a gross basis. When upfront payments are received and if there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue ratably over the associated period of performance.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones and sales-based milestones. Such payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Each contingent and milestone payment is evaluated to determine whether it is substantive and at risk to both parties. The Company recognizes any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. Any payments that are contingent upon achievement of a non-substantive milestone are recognized as revenue prospectively, when such payments become due and collectible, over the remaining expected performance period under the arrangement, which is generally the remaining period over which the research and development services are expected to be provided.

Research and Development Expenses

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.

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 straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised in subsequent periods if actual forfeitures differ from those estimates.

Stock-based compensation expense for options granted to non-employees as consideration for services received is measured on the date of performance at the fair value of the consideration received or the fair value of the equity instruments issued, using the Black-Scholes option-pricing model, whichever can be more reliably measured. Compensation expense for options granted to non- employees is periodically remeasured as the underlying options vest.

Income Taxes

The Company accounts for income taxes under the liability method which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized.

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

Net Loss per Share Attributable to Common Stockholders

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive.

Reverse Stock Split

On October 2, 2015, the Company effected a one-for-62.997 reverse stock split of the Company’s issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock. The par values of the common stock, redeemable convertible preferred stock and convertible preferred stock were not adjusted as a result of the reverse split. All authorized and issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock and per share amounts contained in the accompanying condensed financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will be effective for the Company on January 1, 2018, which is the effective date for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. The Company has not yet selected a transition method nor has it determined the effect of the standard on its ongoing financial reporting.

In August 2014, the FASB issued Accounting Standards Update No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern. This standard update provides guidance around management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. The new guidance is effective for all annual and interim periods ending after December 15, 2016. The Company does not believe that adopting ASU 2014-15 will have a material impact on its financial statements.

Research and Collaboration Agreements	9 Months Ended
Research and Collaboration Agreements	Sep. 30, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	8. Research and Collaboration Agreements Pfizer Inc. In May 2013, the Company and Pfizer Inc. (“Pfizer”) entered into a Research Collaboration, Option and License Agreement (the “Pfizer Agreement”) to collaborate on the discovery and preclinical research activities related to Probody therapeutics, and Probody drug conjugates (“PDCs”) for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target. The Pfizer Agreement provides Pfizer with an option to acquire an exclusive development and commercialization license for each research project target. Upon exercise of the option, Pfizer (1) will receive an exclusive development and commercialization license for use of the Probody therapeutic during the development, manufacturing and commercialization of the potential product, and (2) will be responsible for the development, manufacturing and commercialization of such potential products. Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6 million and is entitled to contingent payments of up to an aggregate of $626.5 million as follows: (i) $1.5 million for each of the two additional targets; (ii) up to $12.0 million upon exercise of the license options, (iii) up to $25.0 million from the achievement of development milestones for each research target program, or up to $82.0 million if the maximum of four research targets are selected by Pfizer; and (iv) up to $98.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $249.5 million if the maximum of four research targets are selected and (v) up to $100.0 million in sales milestones payments per research target program, or up to $280.0 million if the maximum of four research targets are selected by Pfizer. The Company is entitled to receive royalties in the mid-single digits to low teens on initial targets and mid-single digit royalties on additional targets from potential future sales of product candidates. The Company will also receive research and development service fees based on a prescribed full-time employee (“FTE”) rate per year that is capped. In accordance with ASC 605-25, the Company identified the following deliverables 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 does not have stand-alone value to Pfizer due to 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 single unit of accounting. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target do not represent deliverables of the agreement because they are substantive options and do not contain a significant or incremental discount. The upfront payment of $6.0 million was recorded as deferred revenue and is being recognized on a ratable basis over the estimated performance period of seven years. In December 2014, Pfizer selected an additional target and paid $1.5 million, which was recorded as deferred revenue and will be recognized over the remaining performance period. During the three months ended September 30, 2015 and 2014, the Company recognized revenue of $0.5 million and $0.6 million respectively. During the nine months ended September 30, 2015 and 2014, the Company recognized revenue of $1.3 million and $1.9 million, respectively. As of September 30, 2015 and December 31, 2014, deferred revenue relating to the Pfizer Agreement was $5.3 million and $6.1 million, respectively. The amount due from Pfizer under the Agreement was $0.2 million and $1.7 million as of September 30, 2015 and December 31, 2014, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Agreement”). The ImmunoGen Agreement provides the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody technology to create Probody Drug Conjugates (“PDC”) directed at one specified target under a research license, and to subsequently obtain an exclusive, worldwide development and commercialization license to use ImmunoGen’s ADC technology to develop and commercialize such PDCs. The Company made no upfront cash payment in connection with the execution of the agreement. Instead, the Company provided ImmunoGen with the rights to CytomX’s Probody technology to create PDCs directed at two targets under the research license and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. Under the research licenses, the parties have one replacement right for each target, which needs to be made before the third anniversary of the agreement execution. Under the terms of the agreement, both the Company and ImmunoGen are required to perform research activities on behalf of the other party for no monetary consideration. The research activities for a particular target will last until January 2018 unless they are terminated by one of the parties or when a development and commercialization license is obtained with respect to that target. 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. Each party may be liable to pay annual maintenance fees to the other party if the licensed product candidate covered under each development and commercialization license has not progressed to the clinical stage of development within six years of the exercise of the development and commercialization license. In consideration for the exclusive development and commercialization license that may be obtained by ImmunoGen, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments per the research program target, up to $50.0 million in sales milestone payments per target and royalties in the mid-single digits on the commercial sales of any resulting product. For the development and commercialization license that may be obtained by the Company, ImmunoGen is entitled to receive up to $60.0 million in development and regulatory milestone payments, up to $100.0 million in sales milestone payments and royalties in the mid to high single digits on the commercial sales of any resulting product. The Company accounted for the ImmunoGen Agreement based on the fair value of the assets and services exchanged. The Company identified the following significant deliverables at the inception of the ImmunoGen 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, participation in the joint steering committee and the research services do not have stand-alone value from the development and commercialization license and therefore those deliverables were combined into one unit of accounting. The Company considered factors such the limited economic benefits to ImmunoGen if development and commercialization license is 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, of which $13.0 million was allocated to the unit of accounting comprised of the research license, research services, participation in the joint research committee and the development and commercialization license, and $0.2 million was allocated to the future technological improvements. The Company will recognize $13.0 million upon delivery of development and commercialization licenses and will recognize amount allocated to the future technology improvements over the term of the license. The estimated 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 was allocated to the research services, joint research committee participation and technology improvements, which will be expensed over the period of services to be provided. Bristol-Myers Squibb Company On May 23, 2014, the Company and Bristol-Myers Squibb Company (“BMS”) entered into a Collaboration and License Agreement (the “BMS Agreement”) to discover and develop compounds for use in human therapeutics aimed at multiple immuno- oncology targets using the Company’s Probody technology. The effective date of the BMS Agreement was July 7, 2014. Under the terms of the BMS Agreement, the Company granted BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times. Pursuant to the BMS Agreement, the financial consideration from BMS was comprised of an upfront payment of $50.0 million and 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 to high single digits to low teens from potential future sales. The Company will also receive research and development service fees based on a prescribed FTE rate that is capped. The BMS Agreement also provides the Company to sell to BMS the Company’s common stock upon an IPO. In connection with the IPO in October 2015, BMS purchased 833,333 shares of the Company’s common stock at the initial public offering price and on the same terms as other purchasers in the offering. The Company identified the following deliverables at the inception of the BMS Agreement: (1) the exclusive research, development and commercialization license (“license”), (2) the research and development services and (3) the obligation to participate in the joint research committee. The Company determined that the license does not have stand-alone value to BMS without the Company’s research services and expertise related to the development of the product candidates, and accordingly, it was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company received an upfront payment of $50.0 million from BMS in July 2014. The upfront payment was recorded as deferred revenue and being recognized on a ratable basis over the estimated performance period of ten years. The Company determined that the remaining contingent payments under the Agreement do not constitute substantive milestones and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events solely depends on BMS’s performance. Accordingly, any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment will be recognized as revenue in full upon triggering the event. During the three months ended September 30, 2015 and 2014, the Company recognized revenue of $1.5 million and $1.4 million, respectively. During the nine months ended September 30, 2015 and 2014, the Company recognized revenue of $4.4 million and $1.4 million, respectively, under the BMS Agreement. As of September 30, 2015 and December 31, 2014, deferred revenue relating to the BMS Agreement was $43.9 million and $47.6 million, respectively.

Long-term Debt	9 Months Ended
Long-term Debt	Sep. 30, 2015
Debt Disclosure [Abstract]
Long-term Debt	10. Long-term Debt In May 2012, the Company entered into a Master Loan and Security Agreement (the “Debt Facility”). Under the terms of the agreement, an aggregate of $2.0 million could be drawn down during the initial basic loan term of 42 months. In January and December 2013, the Company amended the Debt Facility to borrow an additional $0.3 million and $3.0 million, respectively, with similar terms. Borrowings under the debt facility bear interest at 11.74% per annum. The Company’s obligations under the Debt Facility are collateralized by a security interest in substantially all of its assets, excluding its intellectual property and certain other assets. The Debt Facility also contains customary conditions related to borrowing, events of default, and covenants, including covenants limiting the Company’s ability to dispose of assets, undergo a change in control, merge with or acquire other entities, incur debt, incur liens, pay dividends or other distributions to holders of its capital stock, repurchase stock and make investments, in each case subject to certain exceptions. The agreement also allows the lender to call the debt in the event there is a material adverse change in the Company’s business or financial condition. In connection with the execution and the amendment of the Debt Facility, the Company issued warrants to the lender to purchase an aggregate of 81,620 shares of the Company’s Series B-1 redeemable convertible preferred stock. The warrants expire at the earlier of (i) the tenth anniversary of issuance, (ii) upon the consummation of an IPO of the Company’s common stock, or (iii) the consummation of certain change of control events. The warrants are exercisable in cash at an exercise price of $3.084396 per share or through a cashless exercise provision. Under the cashless exercise provision, the holder may, in lieu of payment of the exercise price in cash, surrender the warrant and receive a net amount of shares based on the fair market value of the Company’s Series B-1 redeemable convertible preferred stock at the time of exercise of the warrant after deducting the aggregate exercise price. If the warrant has not been previously exercised, the cashless exercise provision is automatically triggered upon expiration if the fair value of the Series B-1 redeemable convertible preferred stock is higher than the exercise price of the warrants. In the event that all of the Company’s Series B-1 redeemable convertible preferred stock have been converted into common stock, the warrants will be exercisable for the same number of shares of common stock at the same exercise price. In connection with the consummation of the IPO in October 2015, all of the warrants outstanding as of September 30, 2015 were net exercised, resulting in issuance of an aggregate of 60,640 shares of our common stock. Upon issuance of the warrants, the Company recorded a preferred stock warrant liability based on its initial fair value estimated using the Black-Scholes model with an offset to debt discount. The debt discount is amortized to interest expense using the effective interest method over the term of the Debt Facility. The warrant liability is subject to remeasurement to fair value at each balance sheet date until the earliest of the exercise or expiration of the convertible preferred stock warrant, and any change in fair value is recognized in other income (expense), net. As of September 30, 2015 and December 31, 2014, the warrants remained outstanding. In September 2015, the Company repaid and terminated the Debt Facility.

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2015
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	16. Related Party Transactions Certain employees of Third Rock Ventures, a stockholder of the Company, provides consulting services to the Company. General and administrative expense for these services of $0 and $13,000 were recorded for the three months ended September 30, 2015 and 2014, and $32,000 and $29,000 were recorded for the nine months ended September 30, 2015 and 2014, respectively. The amounts outstanding and included in accounts payable were $0 and $17,600 as of September 30, 2015 and December 31, 2014, respectively. The Company entered into full recourse loans (“stockholder notes” or “loans”) with current and former executive officers. Principal and interest under these loans are due at the earliest of (i) the fifth anniversary of the related note, (ii) the sale of the shares securing the notes, or (iii) thirty days after the termination of services. The principal loan amount and the accrued interest are reported as a deduction from stockholders’ deficit on the Company’s balance sheets. Loans made to two of the Company’s current and former executive officers were repaid and terminated as of September 30, 2015. The remaining balance of these loans was approximately $0.1 million and $0.4 million at September 30, 2015 and December 31, 2014, respectively. Interest income of $4,000 and $6,000 was recorded in the nine months ended September 30, 2015 and 2014, respectively. Interest income earned on the loans was insignificant during the three months ended September 30, 2015 and 2014.

Defined Contribution Plan	9 Months Ended
Defined Contribution Plan	Sep. 30, 2015
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	17. 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 nine months ended September 30, 2015 and 2014, the Company made contributions to the plan of $22,000 and $11,000, respectively.

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2015
Subsequent Events [Abstract]
Subsequent Events	19. Subsequent Events Reverse Stock Split On October 2, 2015, the Company effected a one-for-62.997 reverse stock split of the Company’s issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock. The par values of the common stock, redeemable convertible preferred stock and convertible preferred stock were not adjusted as a result of the reverse split. All authorized and issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock and per share amounts contained in the accompanying condensed financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented. 2015 Equity Incentive 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”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan and released the shares reserved and still available under the plan. Initial Public Offering On October 14, 2015, the Company consummated its IPO and sold 7,666,667 shares of its common stock at a public offering price of $12.00 per share, which included the exercise of the underwriters’ option to purchase 1,000,000 shares of common stock. The Company received net proceeds of approximately $82.4 million, after deducting underwriting discounts, commissions and estimated offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of the Company’s convertible preferred stock and redeemable convertible preferred stock converted into common stock.

Description of the Business - Additional Information (Detail) (Subsequent Event, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	0 Months Ended
	Oct. 31, 2015	Oct. 14, 2015 Initial Public Offering Common Stock	Oct. 14, 2015 Underwriters Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
IPO price per share		$ 12.00
Common stock shares issued		7,666,667	1,000,000
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses	$ 82.4	$ 82.4

Liquidity - Additional Information (Details) (USD $)	9 Months Ended			1 Months Ended	2 Months Ended
Liquidity - Additional Information (Details) (USD $)	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Oct. 31, 2015 Subsequent Event	Jun. 30, 2015 Series C and Series D Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit	$ (105,979,000)		$ (78,138,000)
Cash, cash equivalents, and short-term investments	113,700,000		64,400,000
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	74,430,000	10,316,000			73,200,000
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses				$ 82,400,000

Summary of Significant Accounting Policies - Schedule of Customers Represent 10% of More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) (Customer Concentration Risk)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2015 Revenue Customer A	Sep. 30, 2014 Revenue Customer A	Sep. 30, 2015 Revenue Customer A	Sep. 30, 2014 Revenue Customer A	Sep. 30, 2015 Revenue Customer B	Sep. 30, 2014 Revenue Customer B	Sep. 30, 2015 Revenue Customer B	Sep. 30, 2014 Revenue Customer B	Sep. 30, 2015 Accounts Receivable, net Customer A	Sep. 30, 2015 Accounts Receivable, net Customer B	Dec. 31, 2014 Accounts Receivable, net Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	76.00%	71.00%	77.00%	42.00%	24.00%	29.00%	23.00%	58.00%	55.00%	45.00%	92.00%

Summary of Significant Accounting Policies - Schedule of Customers Represent 10% of More of Total Revenue or Net Accounts Receivable Balance (Details) (Parenthetical) (Customer Concentration Risk, Customer B)	3 Months Ended		9 Months Ended				12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015 Revenue	Sep. 30, 2014 Revenue	Sep. 30, 2015 Revenue	Sep. 30, 2014 Revenue	Sep. 30, 2014 Revenue Maximum	Sep. 30, 2015 Accounts Receivable, net	Dec. 31, 2014 Accounts Receivable, net	Sep. 30, 2014 Accounts Receivable, net Maximum	Dec. 31, 2014 Accounts Receivable, net Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	24.00%	29.00%	23.00%	58.00%	10.00%	45.00%	92.00%	10.00%	10.00%

Summary of Significant Accounting Policies - Additional Information (Details) (USD $)	9 Months Ended		12 Months Ended	0 Months Ended		1 Months Ended	9 Months Ended
	Sep. 30, 2015 Activity Segment	Sep. 30, 2014	Dec. 31, 2014	Oct. 02, 2015 Subsequent Event	Oct. 31, 2015 Subsequent Event	Oct. 31, 2015 Initial Public Offering Common Stock Subsequent Event	Sep. 30, 2015 Maximum
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs	$ 3,000,000
Number of business activities	1
Number of operating segments	1
Original maturities period							3 months
Short-term investments	77,495,000		0
Impairment of goodwill	0		0
Impairment of intangible assets	0		0
Impairment of long-lived assets	0	0
Issuance of common stock						60,640
Convertible preferred stock, shares issued					27,135,453	27,135,453
Interest or penalties charged in relation to unrecognized tax benefits	$ 0
Reverse stock split				one-for-62.997
Reverse stock split conversion ratio				0.01587

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	9 Months Ended
	Sep. 30, 2015
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

Fair Value Measurements and Short-term Investments - Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis (Details) (Fair Value Measurements Recurring, USD $) In Thousands, unless otherwise specified	Sep. 30, 2015	Dec. 31, 2014
Assets
Assets, fair value	$ 110,762	$ 100
Liabilities
Liabilities, fair value	788	581
Money market funds
Assets
Assets, fair value	33,267	100
U. S. Government bonds
Assets
Assets, fair value	77,495
Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities, fair value	788	186
Convertible Preferred Stock Liability
Liabilities
Liabilities, fair value		395
Level I
Assets
Assets, fair value	33,267	100
Level I \| Money market funds
Assets
Assets, fair value	33,267	100
Level II
Assets
Assets, fair value	77,495
Level II \| U. S. Government bonds
Assets
Assets, fair value	77,495
Level III
Liabilities
Liabilities, fair value	788	581
Level III \| Convertible Preferred Stock Warrant Liability
Liabilities
Liabilities, fair value	788	186
Level III \| Convertible Preferred Stock Liability
Liabilities
Liabilities, fair value		$ 395

Fair Value Measurements and Short-term Investment - Summary of Gross Unrealized Gains on the Company Short-term Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 77,487
Gross Unrealized Holding Gains	10
Gross Unrealized Holding Losses	(2)
Aggregate Fair Value	77,495
U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	77,487
Gross Unrealized Holding Gains	10
Gross Unrealized Holding Losses	(2)
Aggregate Fair Value	$ 77,495

Fair Value Measurements and Short-term Investments - Schedule of Contractual Maturities of Securities Classified as Available-for-sale (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Due within one year	$ 77,495
Total	$ 77,495

Property and Equipment - Summary of Property and Equipment, Net (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 6,535	$ 5,010
Less: accumulated depreciation and amortization	(2,868)	(1,992)
Property and equipment, net	3,667	3,018
Machinery and equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,068	4,059
Computer equipment and software
Property Plant And Equipment [Line Items]
Property and equipment, gross	540	315
Furniture and fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	54	54
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	720	183
Construction in progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 153	$ 399

Property and Equipment - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 877	$ 562

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015	Dec. 31, 2014
Goodwill And Intangible Assets [Line Items]
Goodwill	$ 949	$ 949
Intangible assets	1,750	1,750
In-process research and development
Goodwill And Intangible Assets [Line Items]
Intangible assets	$ 1,750	$ 1,750

Accrued Liabilities - Summary of Accrued Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015	Dec. 31, 2014
Accrued Liabilities Current [Abstract]
Deferred offering costs	$ 1,658
Payroll and related expenses	1,464	859
Research and clinical expenses	1,391	276
Legal and professional expenses	530	418
Other accrued expenses	100	142
Total	$ 5,143	$ 1,695

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) (Collaborative Arrangement, Pfizer Inc, USD $)	1 Months Ended		3 Months Ended		9 Months Ended
	Dec. 31, 2014	May 31, 2013 Target	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets		2
Nominated number of additional research targets		2
Upfront payment received		$ 6,000,000
Estimated performance period of agreement		7 years
Additional target as deferred revenue	1,500,000
Revenue recognized from collaborative arrangement			500,000	600,000	1,300,000	1,900,000
Deferred revenue	6,100,000		5,300,000		5,300,000
Amount due from agreement	1,700,000		200,000		200,000
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment recognized as deferred revenue		6,000,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		626,500,000
Achievement Of Additional Nominated Research Target One
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Achievement Of Additional Nominated Research Target Two
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		1,500,000
Exercise of License Options [Member] \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		12,000,000
Achievement Of Development Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		25,000,000
Achievement Of Development Milestones If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		82,000,000
Number of research targets selected		4
Achieving First Commercial Sale In Various Territories \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		98,000,000
Number of sales indicators		3
Achieving First Commercial Sale If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		249,500,000
Number of research targets selected		4
Achieving Sales Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		100,000,000
Achieving Sales Milestones If Four Research Targets Selected By Counterparty \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 280,000,000
Number of research targets selected		4

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) (Collaborative Arrangement, Immuno Gen Inc, USD $)	1 Months Ended
	Jan. 31, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 0
Deferred revenue	13,200,000
Estimated fair value of assets and services	13,200,000
Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services	12,700,000
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
Research License, Research Services, Joint Research Committee, Commercialization License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	13,000,000
Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	200,000
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
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 Agreements - Bristol-Myers Squibb Company - Additional Information (Details) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended			0 Months Ended								1 Months Ended
	Jul. 07, 2014 Collaborative Arrangement Bristol Myers Squibb Company Target	Sep. 30, 2015 Collaborative Arrangement Bristol Myers Squibb Company	Sep. 30, 2014 Collaborative Arrangement Bristol Myers Squibb Company	Sep. 30, 2015 Collaborative Arrangement Bristol Myers Squibb Company	Sep. 30, 2014 Collaborative Arrangement Bristol Myers Squibb Company	Dec. 31, 2014 Collaborative Arrangement Bristol Myers Squibb Company	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Target Term	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Each Of Two Additional Targets	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achievement Of Development Milestones For Each Research Target Program	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achievement Of Development Milestones If Four Research Targets Selected By Counterparty ResearchTarget	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achieving First Commercial Sale In Various Territories SaleIndicator	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achieving First Commercial Sale If Four Research Targets Selected By Counterparty ResearchTarget	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achieving Sales Milestones For Each Research Target Program	Jul. 07, 2014 Collaborative Arrangement Maximum Bristol Myers Squibb Company Achieving Sales Milestones If Four Research Targets Selected By Counterparty ResearchTarget	Oct. 31, 2015 Bristol Myers Squibb Agreement Initial Public Offering Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target							4
Number of collaboration target							2
Collaboration target research term	2 years
Number of additional collaboration target	2
Research terms				BMS will have additional rights to substitute up to two collaboration targets. Each collaboration target has a two year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times.
Period of nomination of additional target from effective date							5 years
Extension of research term for each collaboration target	1 year
Times of increments for extended collaboration target research time							3
Upfront payment received	$ 50,000,000
Contingent milestone payments receivable							1,217,000,000	25,000,000	114,000,000	456,000,000	124,000,000	496,000,000	60,000,000	240,000,000
Number of research targets selected										4		4		4
Number of sales indicators											3
Common stock, shares issued in connection with agreement															833,333
Estimated performance period of agreement	10 years
Revenue recognized from collaborative arrangement		1,500,000	1,400,000	4,400,000	1,400,000
Deferred revenue		$ 43,900,000		$ 43,900,000		$ 47,600,000

License Agreement - Additional Information (Details) (USD $)			9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2015 UC Regents	Sep. 30, 2014 UC Regents	Dec. 31, 2013 UC Regents
License Agreement [Line Items]
Common stock, shares issued	1,094,649	996,520			157,332
Milestone and minimum annual royalty provision paid			$ 225,000	$ 500,000

License Agreement - Schedule of Future Minimum Royalty Obligations Due Under Terms of Certain Exclusive Licensed Patent Rights (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015
Research And Development [Abstract]
2016	$ 150
2017	150
Total minimum royalty obligations	$ 300

Long-term Debt - Additional Information (Details) (USD $)					1 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2015 Warrants Initial Public Offering	Sep. 30, 2015 Series B-1 Redeemable Convertible Preferred Stock	May 31, 2012 Master Loan and Security Agreement	Dec. 31, 2013 Master Loan and Security Agreement	Jan. 31, 2013 Master Loan and Security Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 2,000,000
Line of credit facility, initial basic loan term					42 months
Line of credit facility, additional borrowing capacity						$ 3,000,000	$ 300,000
Debt facility interest rate					11.74%
Warrants issued to purchase stock	81,620	81,620		81,620
Warrants exercise price	$ 3.084396
Convertible preferred stock, shares issued			60,640

Commitments and Contingencies - Additional Information (Details) (USD $)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Commitments And Contingencies Disclosure [Abstract]
Lease agreement expiration	2019
Rent expense	$ 705,000	$ 602,000

Commitments and Contingencies - Schedule of Minimum Lease Payments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2015
Operating Leases Future Minimum Payments Due [Abstract]
2015 (three months remaining)	$ 245
2016	926
2017	864
2018	894
2019	76
Total	$ 3,005

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Shares Authorized	27,217,098	22,004,436
Shares Issued	27,135,453	18,703,071
Shares Outstanding	27,135,453	18,703,071
Net Carrying Value	$ 159,079	$ 76,710
Series A-1
Class Of Stock [Line Items]
Shares Authorized	33,101	33,101
Shares Issued	33,101	33,101
Shares Outstanding	33,101	33,101
Net Carrying Value	49	49
Series A-2
Class Of Stock [Line Items]
Shares Authorized	211,681	211,681
Shares Issued	211,681	211,681
Shares Outstanding	211,681	211,681
Net Carrying Value	425	425
Series B-1
Class Of Stock [Line Items]
Shares Authorized	14,569,803	14,944,578
Shares Issued	14,488,176	14,488,176
Shares Outstanding	14,488,176	14,488,176
Net Carrying Value	60,664	57,695
Series B-2
Class Of Stock [Line Items]
Shares Authorized	862,412	862,412
Shares Issued	862,412	862,412
Shares Outstanding	862,412	862,412
Net Carrying Value	2,949	2,698
Series C
Class Of Stock [Line Items]
Shares Authorized	4,049,546	5,952,664
Shares Issued	4,049,543	3,107,701
Shares Outstanding	4,049,543	3,107,701
Net Carrying Value	23,569	15,843
Series D
Class Of Stock [Line Items]
Shares Authorized	7,490,555
Shares Issued	7,490,540
Shares Outstanding	7,490,540
Net Carrying Value	$ 71,423

Common Stock - Additional Information (Details) (USD $)	9 Months Ended	12 Months Ended		9 Months Ended	1 Months Ended
Common Stock - Additional Information (Details) (USD $)	Sep. 30, 2015	Dec. 31, 2014	Oct. 31, 2015 Subsequent Event	Sep. 30, 2015 Initial Public Offering Common Stock	Oct. 31, 2015 Initial Public Offering Subsequent Event Common Stock
Class Of Stock [Line Items]
Dividends on common stock	$ 0	$ 0
Convertible preferred stock, shares issued			27,135,453		27,135,453
Convertible preferred stock, conversion basis	one-for-one			one-for-one
Issuance of common stock					60,640

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details)	Sep. 30, 2015	Dec. 31, 2014
Equity [Abstract]
Convertible preferred stock outstanding	27,135,453	18,703,071
Options issued and outstanding	5,303,211	2,147,872
Warrants issued to purchase stock	81,620	81,620
Shares available for future stock option grants	627,250	1,896,617
Total	33,147,534	22,829,180

Stock Option Plans - Additional Information (Details)	9 Months Ended
Stock Option Plans - Additional Information (Details)	Sep. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option expiration period	10 years
2011 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employees holding voting rights of all classes of stock, percentage	10.00%
Estimated fair value of the shares on the date of grant	110.00%
2011 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%
Monthly vesting percentage after first anniversary of the issuance date	2.08%