OncoMed Pharmaceuticals 10-K 12 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Mar. 06, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	OMED
Entity Registrant Name	OncoMed Pharmaceuticals Inc
Entity Central Index Key	1302573
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		29,941,886
Entity Public Float			$441,941,375

Balance_Sheets

Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$28,138	$208,931
Short-term investments	203,828	107,263
Receivables-related parties	21	23
Tax receivables	7,102
Prepaid and other current assets	1,721	2,527
Total current assets	240,810	318,744
Property and equipment, net	5,104	4,641
Other assets	1,928	10,300
Total assets	247,842	333,685
Current liabilities:
Accounts payable	4,428	5,706
Income tax payable		10,758
Accrued liabilities	14,683	9,860
Current portion of deferred revenue	18,747	35,059
Current portion of deferred rent	678	624
Liability for shares issued with repurchase rights	10	10
Total current liabilities	38,546	62,017
Deferred revenue, less current portion	130,123	148,871
Deferred rent, less current portion	2,468	3,146
Non-current income tax payable	334	1,515
Liability for shares issued with repurchase rights, less current portion	4	14
Total liabilities	171,475	215,563
Stockholders' equity:
Commitments and contingencies (Note 7)
Preferred stock, $0.001 par value; 5,000,000 shares authorized at December 31, 2014 and 2013, respectively; no shares issued and outstanding at December 31, 2014 and 2013
Common stock, $0.001 par value; 145,000,000 shares authorized at December 31, 2014 and 2013, respectively; 29,847,577 shares and 29,397,964 shares issued and outstanding at December 31, 2014 and 2013, respectively	30	29
Additional paid-in capital	300,790	292,505
Accumulated other comprehensive income (loss)	-17	14
Accumulated deficit	-224,436	-174,426
Total stockholders' equity	76,367	118,122
Total liabilities and stockholders' equity	$247,842	$333,685

Balance_Sheets_Parenthetical

Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	145,000,000	145,000,000
Common stock, shares issued	29,847,577	29,397,964
Common stock, shares outstanding	29,847,577	29,397,964

Statements_of_Operations

Statements of Operations (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue:
Collaboration revenue-related party			$15,970
Collaboration revenue	39,559	37,779	8,689
Grant revenue			22
Total revenue	39,559	37,779	24,681
Operating expenses:
Research and development	76,430	50,048	39,893
General and administrative	13,753	11,630	7,157
Total operating expenses	90,183	61,678	47,050
Loss from operations	-50,624	-23,899	-22,369
Interest and other income (expense), net	105	-228	134
Loss before income taxes	-50,519	-24,127	-22,235
Income tax (provision) benefit	509	-1,944	0
Net loss	($50,010)	($26,071)	($22,235)
Net loss per common share, basic and diluted	($1.69)	($1.93)	($21.30)
Shares used to compute net loss per common share, basic and diluted	29,664,326	13,530,239	1,044,059

Statements_of_Comprehensive_Lo

Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net loss	($50,010)	($26,071)	($22,235)
Other comprehensive loss:
Unrealized gains (losses) on available-for-sale securities, net of tax	-31	-1	-34
Total comprehensive loss	($50,041)	($26,072)	($22,269)

Statements_of_Convertible_Pref

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $)	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Convertible Preferred Stock [Member]
In Thousands, except Share data
Beginning balance at Dec. 31, 2011	($122,934)	$1	$3,136	$49	($126,120)	$182,773
Beginning balance, shares at Dec. 31, 2011		1,005,104				21,180,280
Issuance of common stock upon exercise of options	118		118
Issuance of common stock upon exercise of options, shares	76,000	67,328
Vesting of restricted stock	22		22
Vesting of restricted stock, shares		11,002
Stock-based compensation	836		836
Unrealized loss on available for sale securities	-34			-34
Net loss	-22,235				-22,235
Ending balance at Dec. 31, 2012	-144,227	1	4,112	15	-148,355	182,773
Ending balance, shares at Dec. 31, 2012		1,083,434				21,180,280
Issuance of common stock in connection with initial public offering, net of offering costs	82,668	6	82,662
Issuance of common stock in connection with initial public offering, net of offering costs, shares		5,520,000
Conversion of convertible preferred stock to common stock in connection with initial public offering	182,773	21	182,752			-182,773
Conversion of convertible preferred stock to common stock in connection with initial public offering, shares		21,180,280				-21,180,280
Conversion of preferred stock warrants to common stock warrants in connection with initial public offering	462		462
Conversion of preferred stock warrants to common stock warrants in connection with initial public offering, shares	0	0	0	0	0	0
Issuance of common stock in connection with a research and collaboration agreement	20,519	1	20,518
Issuance of common stock in connection with a research and collaboration agreement, shares		1,470,588
Issuance of common stock upon exercise of warrants	0	0	0	0	0	0
Issuance of common stock upon exercise of warrants, shares		26,898
Issuance of common stock upon exercise of options	250		250
Issuance of common stock upon exercise of options, shares	113,000	113,519
Vesting of restricted stock	13		13
Vesting of restricted stock, shares		3,245
Stock-based compensation	1,736		1,736
Unrealized loss on available for sale securities	-1			-1
Net loss	-26,071				-26,071
Ending balance at Dec. 31, 2013	118,122	29	292,505	14	-174,426
Ending balance, shares at Dec. 31, 2013		29,397,964
Issuance of common stock upon exercise of options	1,080	1	1,079
Issuance of common stock upon exercise of options, shares	382,000	381,599
Issuance of common stock for ESPP purchase	1,002		1,002
Issuance of commons stock for ESPP purchase, shares		65,909
Vesting of restricted stock	10		10
Vesting of restricted stock, shares		2,105
Stock-based compensation	6,194		6,194
Unrealized loss on available for sale securities	-31			-31
Net loss	-50,010				-50,010
Ending balance at Dec. 31, 2014	$76,367	$30	$300,790	($17)	($224,436)
Ending balance, shares at Dec. 31, 2014		29,847,577

Statements_of_Cash_Flows

Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net loss	($50,010)	($26,071)	($22,235)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	1,430	1,378	1,407
Gain on disposal of equipment	-113
Stock-based compensation	6,194	1,736	836
Revaluation of convertible preferred stock warrant liability		280	-17
Prepaid convertible preferred stock warrant expense		17	5
Amortization of discount on short-term investments	-14	-50	-112
Changes in operating assets and liabilities:
Accounts receivable-related parties	2	4,000	-4,023
Tax receivable	-7,102
Prepaid and other current assets	-372	-229	-540
Other assets	-781	-1,096	-2,871
Deferred tax assets	10,331	-10,331
Accounts payable	-1,278	4,857	-2,562
Accrued liabilities and Other	3,643	7,952	-622
Deferred revenue	-35,059	151,884	-2,165
Deferred rent	-624	-541	-446
Income tax payable	-10,758	10,758
Net cash provided by (used in) operating activities	-84,511	144,544	-33,345
Investing activities
Purchases of property and equipment	-1,809	-557	-714
Proceeds from sale of equipment	30
Purchases of short-term investments	-436,856	-132,537	-73,939
Maturities of short-term investments	340,271	75,300	112,666
Net cash provided by (used in) investing activities	-98,364	-57,794	38,013
Financing activities
Proceeds from issuance of common stock upon initial public offering, net		85,149
Proceeds from issuance of common stock related to a collaboration agreement, net		20,519
Proceeds from issuance of common stock related to the exercise of options and employee stock plan purchases	2,082	250	156
Repayments on notes payable			-346
Net cash provided by (used in) financing activities	2,082	105,918	-190
Net increase (decrease) in cash and cash equivalents	-180,793	192,668	4,478
Cash and cash equivalents at beginning of period	208,931	16,263	11,785
Cash and cash equivalents at end of period	28,138	208,931	16,263
Supplemental cash flow information:
Cash paid (refund) for income taxes, net	8,200
Non-cash financing activities:
Conversion of preferred stock warrants to common stock warrants		$462

Organization

Organization	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Organization	1. Organization
	OncoMed Pharmaceuticals, Inc. (“OncoMed” or the “Company”) is a clinical development-stage biotechnology company focused on discovering and developing first-in-class protein therapeutics targeting cancer stem cells (“CSCs”). The Company was originally incorporated in July 2004 in Delaware. The Company’s operations are based in Redwood City, California and it operates in one segment.
	The Company has six anti-CSC product candidates in clinical development and has a seventh anti-CSC product candidate in preclinical development. The first candidate currently in clinical development, demcizumab (anti-DLL4, OMP-21M18), has completed a single-agent Phase Ia safety and dose escalation trial. It is currently in Phase Ib combination therapy trials in patients with non-small cell lung cancer (with carboplatin and pemetrexed) and pancreatic cancer (with gemcitabine and Abraxane®) and a Phase Ib/II trial combining demcizumab with paclitaxel in ovarian cancer. The Company initiated a Phase II randomized trial of demcizumab in non-small cell lung cancer (with carboplatin and pemetrexed) in February 2015. The Company also initiated a Phase II randomized trial of demcizumab in pancreatic cancer and is working to enroll patients. The second candidate, tarextumab (anti-Notch2/3, OMP-59R5), completed a Phase Ia safety and dose escalation trial. It is currently in the Phase II portion of a Phase Ib/II trial in pancreatic cancer (with gemcitabine and Abraxane®) and also in the Phase II portion of a Phase Ib/II trial in small cell lung cancer (with etoposide and platinum chemotherapy). The third candidate, vantictumab (anti-Fzd7, OMP-18R5), has completed a single-agent Phase Ia trial and is currently in three separate Phase Ib combination trials, one trial each in patients with breast cancer (with paclitaxel), pancreatic cancer (with gemcitabine and Abraxane®) and non-small cell lung cancer (with docetaxel). The fourth candidate, ipafricept (Fzd8-Fc, OMP-54F28), is in a single-agent Phase Ia safety and dose escalation trial in solid tumor malignancies and is currently in three separate Phase Ib combination trials, one trial each in patients with ovarian cancer (with carboplatin and paclitaxel), pancreatic cancer (with gemcitabine and Abraxane®) and hepatocellular carcinoma (with sorafenib). The fifth candidate, brontictuzumab (anti-Notch1, OMP-52M51), is in two single-agent Phase Ia safety and dose escalation trials in hematologic and solid tumor malignancies. In 2014 the Company filed an IND for a sixth product candidate, anti-DLL4/VEGF bispecific (OMP-305B83), and are currently enrolling patients in a single-agent Phase Ia trial in advanced solid tumor patients.
	Initial Public Offering
	On July 17, 2013, the Company’s registration statement on Form S-1 (File No. 333-181331) relating to the initial public offering (the “IPO”) of its common stock was declared effective by the SEC. The IPO closed on July 23, 2013 at which time the Company sold 5,520,000 shares of its common stock, which included 720,000 shares of common stock purchased by the underwriters upon the full exercise of their option to purchase additional shares of common stock to cover over-allotments. The Company received net cash proceeds of $82.7 million from the IPO, net of underwriting discounts and commissions and expenses paid by the Company.
	On July 23, 2013, prior to the closing of the IPO, all outstanding shares of convertible preferred stock converted into 21,180,280 shares of common stock with the related carrying value of $182.8 million reclassified to common stock and additional paid-in capital. In addition, all convertible preferred stock warrants were also thereby converted into common stock warrants. Additionally, all shares of Class B common stock were converted into Class A common stock, and the Class A common stock was redesignated “common stock”.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies
	Basis of Presentation
	The Company’s financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”).
	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to revenue recognition, preclinical study and clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results may differ from those estimates.
	Cash and Cash Equivalents
	The Company considers all highly liquid investments with original maturities of 90 days or less at the date of purchase to be cash and cash equivalents.
	Short-Term Investments
	Short-term investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than 365 days from the date of acquisition. Short-term investments are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale securities are excluded from earnings and were reported as a component of accumulated other comprehensive income (loss). The cost of available-for-sale securities sold is based on the specific-identification method.
	Concentrations of Credit Risk
	Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. Cash and cash equivalents and short-term investments are invested through banks and other financial institutions in the United States. Such deposits may be in excess of insured limits. The Company maintains cash and cash equivalents and investments with various high credit quality and capitalized financial institutions.
	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which range from three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the remaining life of the lease at the time the asset is placed into service.
	Impairment of Long-Lived Assets
	The carrying value of long-lived assets, including property and equipment, are reviewed for impairment whenever events or changes in circumstances indicate that the asset may not be recoverable. An impairment loss is recognized when the total of estimated future undiscounted cash flows, expected to result from the use of the asset and its eventual disposition, are less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. Through December 31, 2014, there have been no such impairment losses.
	Revenue Recognition
	The Company generates substantially all its revenue from collaborative research and development agreements with pharmaceutical companies. The terms of the agreements may include nonrefundable upfront payments, milestone payments, other contingent payments and royalties on any product sales derived from collaborations. These multiple element arrangements are analyzed to determine whether the deliverables can be separated or whether they must be accounted for as a single unit of accounting.
	The determination of stand-alone value is generally based on whether any deliverable has stand-alone value to the customer. The Company determines how to allocate arrangement consideration to identified units of accounting based on the selling price hierarchy provided under the relevant guidance. The selling price used for each unit of accounting is based on vendor-specific objective evidence, if available, third-party evidence if vendor-specific objective evidence is not available or estimated selling price if neither vendor-specific nor third-party evidence is available. Management may be required to exercise considerable judgment in determining whether a deliverable is a separate unit of accounting and in estimating the selling prices of identified units of accounting for new agreements.
	Typically, the Company has not granted licenses to collaborators at the beginning of its arrangements and thus there are no delivered items separate from the research and development services provided. As such, upfront payments are recorded as deferred revenue in the balance sheet and are recognized as collaboration revenue over the estimated period of performance that is consistent with the terms of the research and development obligations contained in the collaboration agreement. The Company regularly reviews the estimated period of performance based on the progress made under each arrangement.
	Payments that are contingent upon achievement of a substantive milestone are recognized in their entirety in the period in which the milestone is achieved. Milestones are defined as an event that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones. Further, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables and payment terms within the agreement and commensurate with the Company’s performance to achieve the milestone after commencement of the agreement. Other contingent payments received for which payment is contingent solely on the results of a collaborative partner’s performance (bonus payments) are not accounted for using the milestone method. Such bonus payments will be recognized as revenue when collectability is reasonably assured.
	Payments related to options to license the Company’s program candidates are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. For arrangements under which an option is considered substantive, the Company does not consider the item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration, assuming the option is not priced at a significant and incremental discount. Conversely, for arrangements under which an option is not considered substantive or if an option is priced at a significant and incremental discount, the Company would consider the item underlying the option to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration.
	Customer Concentration
	Customers whose collaboration revenue accounted for 10% or more of total revenues were as follows:
			Year Ended December 31,
			2014			2013			2012
	GlaxoSmithKline LLC (“GSK”)			31	%		*			65	%
	Bayer Pharma AG (“Bayer”)			30	%		92	%		35	%
	Celgene Corporation (“Celgene”)			39	%		*			—
	*	less than 10%
	Research and Development Expenses
	Research and development costs are expensed as incurred. Research and development costs consist of salaries and other personnel-related expenses, including associated stock-based compensation, consulting fees, lab supplies, and facility costs, as well as fees paid to other entities that conduct certain research, development and manufacturing activities on behalf of the Company.
	Clinical Trial Accruals
	Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with clinical research organizations and clinical sites. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services.
	Nonrefundable advance payments for goods and services that will be used or rendered in future research and development activities, are deferred and recognized as expense in the period that the related goods are delivered or services are performed.
	Stock-Based Compensation
	The Company recognizes compensation expense for all share-based payment awards made to employees and directors based on estimated fair values. For employee stock options, the Company determines the grant date fair value of the awards using the Black-Scholes option-pricing model and generally recognizes the fair value as stock-based compensation expense on a straight-line basis over the vesting period of the respective awards. Stock-based compensation expense 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, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For restricted stock, the compensation cost for these awards is based on the closing price of the Company’s common stock on the date of grant and recognized as compensation expense on a straight-line basis over the requisite service period.
	The Company accounts for equity instruments issued to nonemployees based on their fair values on the measurement dates using the Black-Scholes option-pricing model. The estimated fair values of the options granted to nonemployees are remeasured as they vest. As a result, the noncash charge to operations for nonemployee options with vesting conditions is affected each reporting period by changes in the fair value of the Company’s common stock.
	Income Taxes
	The Company accounts for income taxes using the liability method under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are established when necessary to reduce deferred tax assets to the amount which is more likely than not to be realizable.
	The recognition, derecognition and measurement of a tax position is based on management’s best judgment given the facts, circumstances and information available at each reporting date. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
	Net Loss per Common Share
	Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per common share is computed by dividing the net loss by the weighted-average number of common shares and common share equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, potentially dilutive securities consisting of convertible preferred stock, common stock subject to repurchase, stock options and warrants are considered to be common stock equivalents and were excluded in the calculation of diluted net loss per common share because their effect would be anti-dilutive for all periods presented.
	Newly Adopted and Recent Accounting Pronouncements
	In June 2014, the Financial Accounting Standards Board (“FASB”) issued final guidance that a performance target that affects vesting of a share-based payment and that could be achieved after the requisite service period is a performance condition under Accounting Standards Codification (“ASC”) 718, Compensation—Stock Compensation. As a result, the target is not reflected in the estimation of the award’s grant date fair value. Compensation cost for such an award would be recognized over the required service period, if it is probable that the performance condition will be achieved. The guidance is effective for all entities for annual periods beginning after December 15, 2015 and interim periods within those annual periods. The guidance should be applied on a prospective basis to awards that are granted or modified on or after the effective date. The Company does not believe the adoption of this guidance will have a material impact on its financial statements.
	In May 2014, the FASB and the International Accounting Standards Board issued Accounting Standards Update (“ASU”) No. 2014-09 (Topic 606)—Revenue from Contracts with Customers (“ASU 2014-09”). This ASU affects any entity that either enters into contracts with customers to transfer goods and services or enters into contracts for the transfer of nonfinancial assets. This ASU will supersede the revenue recognition requirements in Topic 605, and most industry specific guidance. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under today’s guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is effective for the Company for annual periods beginning after December 15, 2016, including interim periods within that period. Entities can choose to apply the standard using either the full retrospective approach or a modified retrospective approach. Entities electing the full retrospective adoption will apply the standard to each period presented in the financial statements. This means that entities will have to apply the new guidance as if it had been in effect since the inception of all its contracts with customers presented in the financial statements. Entities that elect the modified retrospective approach will apply the guidance retrospectively only to the most current period presented in the financial statements. This means that entities will have to recognize the cumulative effect of initially applying the new standard as an adjustment to the opening balance of retained earnings at the date of initial application. Early adoption is not permitted under U.S. GAAP. The Company is currently evaluating the impact of adoption of this accounting standards update on its financial statements.
	In August 2014, the FASB issued ASU No. 2014-15 (Subtopic 205-40)—Presentation of Financial Statements—Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (“ASU 2014-15”) which provides guidance about management’s responsibility to evaluate whether or not there is substantial doubt about the Company’s ability to continue as a going concern and to provide related footnote disclosure. ASU 2014-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early application is permitted. The adoption of this standard is not expected to have an impact on the Company’s Financial Statements.

Cash_Equivalents_and_Investmen

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2014
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Investments	3. Cash Equivalents and Investments
	The fair value of securities, not including cash at December 31, 2014 and 2013, were as follows (in thousands):
		December 31, 2014
		Amortized		Gross Unrealized					Fair Value
		Cost		Gains		Losses
	Money market funds	$	8,460	$	—	$	—		$	8,460
	U.S. treasury bills		203,845		37		(54	)		203,828
	Total available-for-sale securities	$	212,305	$	37	$	(54	)	$	212,288
	Classified as:
	Cash equivalents								$	8,460
	Short-term investments									203,828
	Total cash equivalents and investments								$	212,288
	As of December 31, 2014, the Company had a total of $232.0 million in cash, cash equivalents, and short-term investments, which includes $19.7 million in cash and $212.3 million in cash equivalents and short-term investments.
		December 31, 2013
		Amortized		Gross Unrealized					Fair Value
		Cost		Gains		Losses
	Money market funds	$	7,980	$	—	$	—		$	7,980
	U.S. treasury bills		267,242		14		—			267,256
	Total available-for-sale securities	$	275,222	$	14	$	—		$	275,236
	Classified as:
	Cash equivalents								$	167,973
	Short-term investments									107,263
	Total cash equivalents and investments								$	275,236
	As of December 31, 2013, the Company had a total of $316.2 million in cash, cash equivalents, and short-term investments, which includes $41.0 million in cash and $275.2 million in cash equivalents and short-term investments.
	All available-for-sale securities held as of December 31, 2014 and 2013 had contractual maturities of less than one year. There have been no significant realized gains or losses on available-for-sale securities for the periods presented.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements
	The Company records its financial assets and liabilities at fair value. The carrying amounts of certain of the Company’s financial instruments, including cash and cash equivalents, short-term investments, and accounts payable, approximate their fair value due to their short maturities. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value, and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows:
		•		Level 1: Inputs which include quoted prices in active markets for identical assets and liabilities.
		•		Level 2: Inputs other than Level 1 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 3: 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 Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):
			December 31, 2014
			Level 1		Level 2		Level 3		Total
	Assets:
	Money market funds		$	8,460	$	—	$	—	$	8,460
	U.S. treasury bills			—		203,828		—		203,828
	Total		$	8,460	$	203,828	$	—	$	212,288
			December 31, 2013
			Level 1		Level 2		Level 3		Total
	Assets:
	Money market funds		$	7,980	$	—	$	—	$	7,980
	U.S. treasury bills			—		267,256		—		267,256
	Total		$	7,980	$	267,256	$	—	$	275,236
	Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds as Level 1. When quoted market prices are not available for the specific security, then the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes, and issuer spreads. The Company classifies U.S. Treasury securities as Level 2. There were no transfers between Level 1 and Level 2 during the periods presented.

Property_and_Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and Equipment	5. Property and Equipment
	Property and equipment consist of the following (in thousands):
		December 31,
		2014			2013
	Computer equipment and software	$	1,715		$	1,632
	Furniture and fixtures		443			392
	Laboratory equipment		9,739			9,025
	Leasehold improvements		8,826			8,624
			20,723			19,673
	Less accumulated depreciation and amortization		(15,619	)		(15,032	)
	Property equipment, net	$	5,104		$	4,641

Accrued_Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued Liabilities	6. Accrued Liabilities
	Accrued liabilities consist of the following (in thousands):
		December 31,
		2014		2013
	Research and development related	$	9,965	$	5,949
	Compensation related		4,208		3,446
	Other		510		465
	Total accrued liabilities	$	14,683	$	9,860

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies
	Operating Leases
	The Company leases office and laboratory facilities in Redwood City, California under a lease agreement that expires in January 2019 and includes lease extension options for two additional three-year terms.
	The operating lease agreement contains rent escalation provisions and tenant improvement allowances. The total rent obligation is being expensed ratably over the term of the agreement.
	Minimum annual rental commitments under the lease agreement are as follows (in thousands):
	Year ending December 31:
	2015	$	1,942
	2016		2,002
	2017		2,062
	2018		2,122
	Thereafter		177
	Total	$	8,305
	In 2006, the landlord provided the Company a tenant improvement allowance of $5.7 million in order for the Company to complete an office and lab build out. In accordance with the lease amendment in December 2010, the landlord funded $1.6 million of the provided tenant improvement allowance for the Company to complete the lab expansion. The Company has recorded the aggregate tenant improvement allowance received as a leasehold improvement asset and a deferred rent liability on the accompanying balance sheets. The tenant improvement allowance asset is being amortized as depreciation expense and the deferred rent liability balance as a credit to rent expense over the period from which the improvements were placed in service until the end of their useful life, which is the end of the lease term. As of December 31, 2014 and 2013, the Company has an unamortized tenant improvement allowance of $2.1 million and $2.7 million, respectively.
	Rent expense for years ended December 31, 2014, 2013 and 2012, was $1.3 million, $1.3 million and $1.3 million, respectively.
	Guarantees and Indemnifications
	The Company, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws, and pursuant to indemnification agreements with certain directors, indemnifies its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity.
	The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance limits the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

License_Agreement

License Agreement	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
License Agreement	8. License Agreement
	In 2004, the Company assumed an exclusive, worldwide license agreement with the University of Michigan relating to the use of certain patents and technology relating to its cancer stem cell technology for which an up-front fee of $10,000 had been paid and the Company issued 7,796 shares of its common stock. Pursuant to the agreement, the Company is obligated to make low single-digit royalty payments to the University of Michigan on net sales of its or its licensees products and processes covered under the agreement, pay an annual license maintenance fee, and reimburse the University of Michigan for costs of prosecution and maintenance of the licensed patents which reduces future royalty obligations. With respect to one family of licensed patent applications that does not relate to any of the Company’s lead therapeutic programs, the Company is also required to pay a tiered, single-digit percentage of any sublicense revenues, including any upfront or milestone payments, received from any sublicensees under such family of patents. Once the University of Michigan has received $10.0 million in royalties, the Company may at its option convert the license to a fully paid-up license provided the Company transfers additional shares of nonvoting common stock equal to 0.25% of the fully diluted shares then outstanding to the University of Michigan. The amounts incurred for patent legal costs amounted to $158,000, $151,000 and $175,000 for the years ended December 31, 2014, 2013 and 2012, respectively, all of which has been recorded as general and administrative expense in the statements of operations.

Supply_and_License_Agreements

Supply and License Agreements	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Supply and License Agreements	9. Supply and License Agreements
	In June 2006, the Company entered into a subscription and license agreement with MorphoSys AG to obtain a research license and access to certain technology libraries, antibodies and support services. The Company paid technology access and subscription fees from the date of signing through 2008 of €650,000 (approximately $919,000) and €250,000 (approximately $350,000) in 2009. The Company also exercised an option to extend an extended research license under the agreement for five years (through 2015). The Company may choose to exercise an annual option to extend the extended research license for up to an additional five years (through 2020), and, upon exercise of such option, must pay an annual fee of €20,000 (approximately $24,000). These annual fees are capitalized in prepaid and other current assets, and amortized over the relevant one year period. Additionally, the Company may owe MorphoSys AG up to €5.8 million (approximately $7.1 million) in future milestone payments for each product developed using the licensed antibodies if all milestone events are achieved, primarily in Phase III clinical trials and later development. GSK would reimburse the Company for 50% of such payments for tarextumab (Anti-Notch2/3, OMP-59R5).
	The amortization expense for the years ended December 31, 2014, 2013 and 2012 was $27,000, $26,000 and $27,000, respectively, and was recorded as research and development expense in the statements of operations.
	In April/May 2008, the Company obtained commercial licenses to two antibodies identified from the licensor’s library of antibodies, and must make additional payments to MorphoSys AG for these licenses. GSK reimburses the Company for 50% of the license payments for the first antibody, which is used in the tarextumab program under the Company’s collaboration with GSK, while the Company is responsible for all the license payments for the second antibody, which is used in the vantictumab (anti-Fzd7, OMP-18R5) program under its collaboration with Bayer. There were no payments made for the licensed antibodies nor reimbursements received from GSK during the years ended December 31, 2014, 2013 and 2012.

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	10. Collaborations
	The Company has entered into three collaboration arrangements, each having multiple deliverables under which it received non-refundable upfront payments. For collaborations where the Company has determined that there is a single unit of accounting the Company recognizes revenue related to the upfront payments ratably over its estimated period of performance for each collaboration.
	The Company has entered into collaborations arrangements that include contractual milestones, which relate to the achievement of pre-specified research, development, regulatory and commercialization events. The milestone events contained in the Company’s alliances coincide with the progression of the Company’s product candidates from research and development, to regulatory approval and through to commercialization. The process of successfully discovering a new product candidate, having it selected by the alliance partner for development, having it approved and ultimately sold for a profit is highly uncertain. As such, the milestone payments that the Company may earn from its collaborators involve a significant degree of risk to achieve.
	Research and development milestones in the Company’s strategic alliances may include the following types of events:
		•		Completion of pre-clinical research and development work leading to selection of product clinical candidates.
		•		Advancement of candidates into clinical development, which may include filing of investigational new drug (“IND”) applications.
		•		Initiation of a Phase I or Phase II clinical trials.
		•		Achievement of certain scientific or development events.
	Regulatory milestones may include the following types of events:
		•		Filing of regulatory applications for marketing approval such as a New Drug Application in the United States, or a Marketing Authorization Application in Europe.
		•		Marketing approval in a major market, such as the United States, Europe or Japan.
	Commercialization milestones may include the following types of events:
		•		Product sales in excess of pre-specified thresholds.
	Summary of Collaboration Related Revenue
	The Company has recognized the following revenues from its collaboration agreements during the years ended December 31, 2014, 2013 and 2012 (in thousands):
			Year Ended December 31,
			2014		2013		2012
	GSK:
	Recognition of upfront payment		$	1,248	$	1,971	$	1,970
	Milestone revenue			11,000		—		14,000
	GSK total			12,248		1,971		15,970
	Bayer:
	Recognition of upfront payments			9,756		9,756		8,689
	Milestone revenue			2,000		25,000		—
	Bayer total			11,756		34,756		8,689
	Celgene:
	Recognition of upfront payment			13,055		1,052		—
	Milestone revenue			2,500		—		—
	Celgene total			15,555		1,052		—
	Total collaboration related revenue		$	39,559	$	37,779	$	24,659
	GSK Strategic Alliance
	On December 7, 2007, the Company entered into a Collaboration and Option Agreement with GSK. The agreement was formed to discover, develop and market novel antibody therapeutics to target cancer stem cells (“CSCs”). The agreement gives GSK the option to obtain an exclusive license for product candidates targeting the Notch signaling pathway.
	Under the original agreement, the Company had a research obligation to provide GSK four antibodies that meet specified selection criteria and was responsible for the development of two antibodies through clinical proof-of-concept, or “POC”, generally considered to be at the end of certain Phase II clinical trials. There is no further obligation by the Company to perform development once these are achieved. Upon exercise of its option, GSK obtains an exclusive worldwide license to the antibody, assume full financial responsibility for funding further clinical development and commercialization and will be obligated to make payments to the Company for further development and commercialization milestones and royalties on product sales.
	The Company received an initial payment of $35.0 million, with half in the form of an equity investment by GSK in the Company’s Series B-2 convertible preferred stock and the other half as an up-front cash payment which was initially recorded as deferred revenue. The Company is eligible to earn milestone payments in connection with research and development activities, and contingent consideration in connection with further development, regulatory approval and commercialization activities. In addition, the Company can earn royalty payments on all future collaboration product sales, if any.
	The 1,441,396 shares of Series B-2 convertible preferred stock sold by the Company to GSK were issued at a premium of $4.3 million above the estimated fair value of convertible preferred stock at the time of issuance. This premium was considered an additional up-front payment and was added to the $17.5 million deferred revenue and was being recognized on a ratable basis over the estimated period of performance of five years.
	In July 2011, the Company amended the terms of its development agreement with GSK to focus the collaboration entirely on the development of two product candidates, tarextumab (anti-Notch 2/3, OMP-59R5) and brontictuzumab (anti-Notch1, OMP-52M51). The Company will receive additional funding from GSK to support certain of its development activities conducted in relation to one of these product candidates, up to a maximum of $2.0 million. There is no further obligation for the Company to continue to progress or fund development once specified Phase II clinical trials are completed for each of the two product candidates. Following exercise of its option for a product candidate, GSK will have an exclusive license to progress further clinical development and commercialization of such product candidate, and will assume full financial responsibility for funding such activities. After exercising its option, GSK will be obligated to make payments to the Company in the form of contingent consideration related to further development and commercialization of such product candidate, as well as royalties on product sales.
	The Company evaluated the terms of the July 2011 amendment relative to the entire arrangement and determined the amendment to be a material modification to the original agreement for financial reporting purposes. As a result, the Company evaluated the entire arrangement under the multiple-element accounting guidance. The Company determined that the only undelivered elements were providing certain development services it is obligated to provide for the second product candidate and the continued development work to progress the first product candidate through certain Phase II POC clinical trials. The Company has determined that the undelivered elements are a single unit of accounting. Accordingly, the $7.9 million deferred revenue balance at the modification date was being recognized as revenue ratably over the estimated period of performance over four years beginning on the date of the material modification of the original agreement. The Company determined that the clinical data it has obtained to date supported a change in study design which extended the estimated completion of the deliverable, certain Phase II POC clinical trials to June 2016. In January 2014, the Company changed its estimate of the period of performance from four years to five years. Accordingly, the Company is recognizing the remaining unamortized portion of the up-front payment over the revised estimated period of performance on a prospective basis.
	In July 2012, the Company and GSK entered into the Second Amendment Regarding Payment of Certain Milestone Payments for the tarextumab program. This amendment modified one of the development milestones for advancing the tarextumab program. The restructuring of the milestone payment did not alter the aggregate amount of development milestone payments or the aggregate amount of contingent consideration payments that the Company is eligible to receive in its collaboration with GSK. The Company received a $3.0 million payment upon the initiation of the Phase Ib portion of the tarextumab program in October 2012, which was not considered to be the achievement of a substantive milestone for accounting purposes. Rather it is an advance payment on a future substantive milestone. Accordingly, the $3.0 million was recorded as deferred revenue to be recognized upon the achievement of the underlying substantive milestone. This modification was not material to the collaboration taken as a whole.
	During the year ended December 31, 2012, the Company recognized a $5.0 million proof-of-principle development milestone for the tarextumab program, a $5.0 million development milestone for the IND acceptance for its brontictuzumab program and a $4.0 million development milestone for the first patient enrollment for its brontictuzumab clinical trial.
	In June 2013, the Company received an $8.0 million advance payment from GSK pursuant to the terms of its tarextumab program. The $8.0 million was recorded as deferred revenue to be recognized as collaboration revenue upon the achievement of the underlying substantive milestone.
	In July 2014 the Company began dosing patients in the randomized, placebo-controlled Phase II portion of its clinical trial of tarextumab in pancreatic cancer. As a result, payments of $8.0 million and $3.0 million previously recorded as deferred revenue were recognized as collaboration revenue upon the achievement of this underlying substantive milestone.
	As of December 31, 2014, the Company was eligible to receive in its collaboration with GSK up to $81.0 million in future development milestone payments prior to the completion of certain Phase II POC clinical trials. These remaining potential development milestones include up to $5.0 million for the advancement into specified clinical testing related to the brontictuzumab program, up to $16.0 million for the start of certain Phase II clinical trials, including a $5.0 million bonus payment, and up to $60.0 million if GSK exercises its options for the two programs, including a $10.0 million bonus payment. GSK has the option to license the brontictuzumab program as early as the end of Phase Ia or both programs at Phase II POC, and will be responsible for all further development and commercialization following such option exercise. If GSK successfully develops and commercializes both candidates for more than one indication, the Company could receive contingent consideration payments of up to $309.0 million for the achievement of regulatory events and up to $280.0 million upon the achievement of certain levels of worldwide net sales, for a total of $670.0 million of potential future payments. In addition, the Company can earn royalty payments on all future collaboration product sales, if any. As all contingent consideration payments are based solely on the performance of GSK, the milestone method of accounting will not be applied to such amounts.
	As of December 31, 2013, GSK was no longer considered a related party. Previously, GSK was deemed a related party by ownership of more than 10% of the voting common stock of the Company. Accordingly, related party transactions were reported as collaboration revenue—related party in the Company’s statement of operations for the year ended December 31, 2012.
	Bayer Strategic Alliance
	On June 15, 2010, the Company entered into a Collaboration and Option Agreement with Bayer. The agreement sets forth an alliance to discover, develop and market novel antibody, protein and small molecule therapeutics targeting CSCs. Specifically, the alliance efforts are directed toward therapeutics affecting targets within the Wnt signaling pathway.
	Under the terms of the agreement, Bayer has an exclusive option to license biologic therapeutic product candidates discovered and developed by the Company over a biologics research term that will extend until the later of five years after the effective date of the agreement or the occurrence of certain specified events. In addition, the Company will assist Bayer with its advancement of a specified number of small molecule candidates discovered and developed at Bayer to certain development stages. The Company estimates its period of performance to be approximately five years. The Company is obligated to use commercially reasonable efforts to advance three biologics through to a specified stage of research and two biologics to a specified early clinical development stage.
	The option for Bayer to obtain an exclusive license to any of the biologics therapeutic product candidates commences on the effective date of the agreement and extends for a specified time period. The Company is responsible for all preclinical and development costs for each biologic product candidate up to the end of a defined early clinical development stage. Once Bayer exercises its option to obtain an exclusive license to a class of biologic therapeutic products, they assume full financial responsibility for funding further clinical development and commercialization of such product candidates.
	The Company received an upfront payment of $40.0 million and is eligible to receive development, regulatory approval and commercialization milestone or post-option contingent consideration payments up to $387.5 million for each biologic and $112.0 million for each small molecule candidate. The upfront payment was recorded as deferred revenue and is being amortized to revenue over the Company’s estimated period of performance. Upon product sales, the Company is eligible to receive royalties that adjust depending on sales volume.
	The Company achieved a $20.0 million development milestone related to the acceptance of the IND for the vantictumab (OMP-18R5) product candidate during the year ended December 31, 2011 that was determined to be substantive and at risk at the inception of the arrangement and, as such, was recognized in the period the milestone was achieved.
	In April 2011, the Company entered into a clinical manufacturing agreement which expanded its alliance with Bayer. Pursuant to this agreement, Bayer HealthCare LLC agreed to manufacture ipafricept (Fzd8-Fc, OMP-54F28) at its Berkeley, California site to support the Company’s clinical development activities.
	In August 2012, the Company and Bayer entered into Amendment 1 to the Collaboration and Option Agreement. This amendment modified the timing of payments under the Company’s ipafricept program and another biologic therapeutic product program under the agreement. The modification did not alter the aggregate amount of development milestone payments or the aggregate amount of contingent consideration payments the Company is eligible to receive in its collaboration with Bayer. This modification was not material to the collaboration taken as a whole.
	During the year ended December 31, 2012, the Company received a $5.0 million payment related to the ipafricept program. As the payment was not deemed to be for a substantive milestone under Amendment 1, the Company is recognizing the $5.0 million payment over the remaining estimated period of performance under the agreement.
	In August 2013, the Company and Bayer entered into Amendment 2 to the Collaboration and Option Agreement. The amendment confirms the achievement of a development milestone of $10.0 million for dose escalation of vantictumab (anti-Fzd7, OMP-18R5) in Phase Ia as well as agreement on the Phase Ib trial design. In addition, the amendment excludes a target that is not being developed under the collaboration. This amendment was not considered a material modification for accounting or reporting purposes. In October 2013, the Company achieved a $15.0 million development milestone related to Phase I dose escalation in its Fzd8-Fc program under its agreement with Bayer. The two milestones totaling $25.0 million were determined to be substantive and at risk at the inception of the agreement and, as such, were recognized in the period the milestones were achieved.
	In September 2014, the Company earned $2.0 million in milestone payments from Bayer as a result of the commencement of preclinical development of a small molecule product candidate. The $2.0 million milestone was determined to be substantive and at risk at the inception of the agreement and, as such, was recognized in the period the milestone was achieved.
	As of December 31, 2014, the Company was eligible to receive up to $10.0 million in future development milestone payments in its collaboration with Bayer for the Company’s development of biologic product candidates, prior to the point that Bayer exercises its options. The Company is eligible to receive up to $55.0 million if Bayer exercises its options for biologic product candidates. Bayer will be responsible for all further development and commercialization following the exercise of an option for a product candidate. The Company is eligible to receive up to $22.0 million in development milestone payments for the small molecule candidates. If Bayer successfully develops and commercializes all of the product candidates for more than one indication, the Company could receive contingent consideration payments of up to $185.0 million for the achievement of regulatory events (up to $135.0 million for biologics and $50.0 million for small molecules) and up to $1.0 billion upon the achievement of specified future product sales (up to $862.5 million for biologics and $140.0 million for small molecules). As all contingent consideration is based solely on the performance of Bayer, the Company would recognize the contingent payments upon receipt immediately as collaboration revenue if the Company had no further performance obligations under the agreement with Bayer.
	Celgene Strategic Alliance
	In December 2013, the Company entered into a Master Research and Collaboration Agreement (the Agreement) with Celgene pursuant to which the Company and Celgene will collaborate on research and development programs directed to the discovery and development of novel biologic therapeutic programs to target CSCs, and, if Celgene exercises an option to do so, the discovery, development and commercialization of novel small molecule therapeutic programs to target CSCs. Pursuant to the biologic therapeutic programs, the Company will conduct further development of demcizumab (anti-DLL4, OMP-21M18), anti-DLL4/VEGF bispecific antibodies, biologic therapeutics directed to targets in the RSPO-LGR signaling pathway, including anti-RSPO3 (OMP131R10), and biologic therapeutics directed to targets in an undisclosed pathway. Celgene has options to obtain exclusive licenses to develop further and commercialize biologic therapeutics in specified programs, which may be exercised during time periods specified in the agreement through completion of certain clinical trials, provided that such option exercise occurs within the contractual Option Period of 12 years. The Company at its option may enter into co-commercialization and co-development agreements for five of the six biologic programs. During the Option Period, the Company will provide research and development services and the resultant data to Celgene for analysis in order for Celgene to determine whether or not to exercise its options.
	Pursuant to the Agreement, the Company leads the discovery and development of biologic therapeutic products prior to Celgene’s exercise of its option. With respect to biologic therapeutics targeting the RSPO-LGR signaling pathway and the undisclosed pathway, prior to Celgene’s exercise of its option for a given program, Celgene is required to designate each program for which it wishes to retain the right to exercise its option, based on data generated by the Company, for up to a maximum of four programs. The Company is entitled to receive certain fees for each program that Celgene designates. Celgene has the right to designate programs until December 2, 2017, with an option to extend for another two years upon payment of an extension fee. Following such designation(s), Celgene will have the right to exercise its option for each such program within the Option Period.
	With respect to biologic therapeutic programs, with the exception of one program targeting either the RSPO-LGR signaling pathway or the undisclosed pathway, and any program for which the Company elects not to exercise its co-development and co-commercialization right, following Celgene’s exercise of its option, the Company and Celgene will enter into an agreed form of co-development and co-commercialization agreement for such program. The Company will have the right to co-develop and to co-commercialize products arising out of such program in the United States, and Celgene will have the exclusive right to develop and commercialize products arising out of such program outside of the United States. The Company’s involvement in co-commercialization will include participation in specified promotion activities by means of a dedicated sales force of up to half of the overall sales force for the applicable program products, as well as marketing and other commercial activities, with Celgene recording all product sales. The Company will also bear a one-third share of all development costs, with Celgene bearing the remaining two-thirds. However, for one program targeting either the RSPO-LGR signaling pathway or the undisclosed pathway, and any program for which the Company elects not to co-develop and co-commercialize products arising from such program, the Company and Celgene will instead enter into an agreed form of a license agreement, pursuant to which Celgene retains all rights to develop further and commercialize biologic therapeutic products arising from such program on a worldwide basis, with certain support for development from the Company. The Company may elect not to co-develop and co-commercialize any products arising under such programs at any time, either prior to, or following Celgene’s option exercise, with the exception of a defined period of time near commercial launch of a product under a program. If the Company opts out of its co-development and co-commercialization rights with respect to a program, Celgene will have the exclusive right to develop and commercialize products arising out of such program, at Celgene’s expense.
	With respect to small molecule therapeutics targeting an undisclosed pathway, following Celgene’s exercise of its option, the Company will collaborate with Celgene on the discovery of and research on small molecule therapeutics, but Celgene will be solely responsible for development and commercialization of such therapeutics.
	Under the terms of the Agreement, the Company received an upfront cash payment of $155.0 million. In addition, Celgene purchased 1,470,588 shares of the Company’s common stock at a price of $15.13 per share, resulting in gross proceeds of $22.2 million. The price paid by Celgene for the common stock represented a premium over the closing price of the Company’s common stock on the date of the Agreement. The Company accounted for the $1.7 million premium as additional consideration under the Agreement and the common stock was recorded at its fair market value of $20.5 million. The Company is also eligible to receive option fees upon Celgene’s exercise of the option for each biologic therapeutic program. The collaboration also includes milestone payments for achievement of specified development, regulatory and commercial milestones, paid on a per-product and per-program basis. The payments for option exercise, program designation and achievement of development, regulatory and commercial milestones may total up to (1) $791.0 million for products in the demcizumab program, including a $70.0 million payment upon the achievement of certain pre-determined safety criteria in Phase II clinical trials with respect to demcizumab, (2) $505.0 million for products in the anti-DLL4/VEGF bispecific program, (3) up to $442.8 million for each of the four products achieving regulatory approval that are directed to targets in each of the RSPO-LGR signaling pathway and the undisclosed pathway programs for which Celgene exercises its option, and (4) $107.0 million for products in the small molecule therapeutic program.
	For programs in which the Company is co-developing and co-commercializing biologic therapeutic products in the United States, the Company is also entitled to share 50% of all product profits and losses in the United States. For such programs outside the United States, the Company is eligible to receive tiered royalties equal to a percentage of net product sales outside of the United States. If the Company elects not to co-develop or co-commercialize biologic therapeutic products or does not have the right to do so for a given program, Celgene is required to pay the Company tiered royalties equal to a percentage of net product sales worldwide, with such royalties being increased where the Company had the right to co-develop and co-commercialize such biologic therapeutic products under such program but elected not to do so. The Company is responsible for funding all research and development activities for biologic therapeutics under the collaboration prior to Celgene’s exercise of the option for such program.
	In addition to the development and regulatory milestone payments the Company will be entitled to receive if Celgene exercises its option for the small molecule program, the Company may also receive royalties equal to a percentage of worldwide net sales of small molecule products in the low- to mid-single digits.
	The Agreement will terminate upon the expiration of all of Celgene’s payment obligations under all license or co-development and co-commercialization agreements entered into with respect to programs following Celgene’s exercise of an option for a given program, or if Celgene fails to exercise any of its options within the Option Period. The Agreement will also terminate, on a program-by-program basis, on the expiration of the option term, if Celgene fails to exercise its option for such program. The Company may also terminate the Agreement with respect to one or more programs in the event that Celgene challenges the licensed patents with respect to such program.
If Celgene does not exercise its option with respect to a biologic therapeutic program within the Option Period, the Company retains worldwide rights to such program(s), except that if Celgene exercises its option to obtain a license for either the demcizumab program or the anti-DLL4/VEGF bispecific program, then for so long as such license is in effect, the Company cannot develop or commercialize products under the other of such two programs. In addition, under certain termination circumstances, the Company would also have worldwide rights to the terminated biologic therapeutic programs.
The Company’s deliverables under the arrangement with Celgene are research and development services, including the obligation that the Company provides the resultant data to Celgene, which are accounted for as a single unit of accounting. The Company has determined that the options to license programs are substantive options. Additionally, as a result of the uncertain outcome of the discovery, research and development activities, the Company is at risk with regard to whether Celgene will exercise the options. Accordingly, the options are not considered deliverables at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration. The Company has identified the initial arrangement consideration to be approximately $156.7 million which will be recognized on a straight-line basis over the estimated period of performance of 12 years. Due to the uncertain timeline associated with the deliverables at the outset of the Agreement, the Company determined it will use 12 years, which is the maximum period under the Agreement for Celgene to exercise its options. The Company will reevaluate the estimated performance period at each reporting period.
In November 2014, the Company received a designation notice from Celgene relating to the Company’s anti-RSPO3 product candidate, OMP-131R10, which is currently in preclinical testing. This designation triggered a $2.5 million payment due to the Company from Celgene under the Celgene Agreement.
As of December 31, 2014, the Company was eligible to receive in its collaboration with Celgene up to $87.5 million in future development milestones across all programs, prior to the point that Celgene exercises its options. The Company is also eligible to receive up to $240.0 million of contingent consideration if Celgene exercises all its options for the biologic and small molecule therapeutic programs. Celgene will be responsible for all further development and commercialization following the exercise of the options for specified programs. If Celgene successfully develops and commercializes all of the product candidates, the Company could receive additional contingent consideration of up to $2.8 billion for the achievement of regulatory events (up to $2.7 billion for biologics and $95.0 million for small molecules). As all contingent consideration is based solely on the performance of Celgene, the Company would recognize the contingent payments upon receipt immediately as collaboration revenue if the Company had no further performance obligations under the Agreement.

Lonza_Sales_AG_Agreement

Lonza Sales AG Agreement	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Lonza Sales AG Agreement	11. Lonza Sales AG Agreement
	In August 2012, the Company entered into a multi-product license agreement and a collaboration agreement with Lonza Sales AG (“Lonza”). These agreements cover the process development and manufacturing of the Company’s biologics portfolio with Lonza. Under the collaboration agreement, Lonza will be the Company’s preferred supplier of process development and manufacturing services for the Company’s bulk drug substances in Lonza’s Slough, United Kingdom facility. The parties also agreed to establish a joint steering committee to oversee, coordinate and expedite the performance of the collaboration agreement, resolve disputes arising between the parties; monitor the progress of the relevant services; and plan and assess needs for future services. Under the multi-product license agreement, the Company receives licenses to utilize Lonza’s glutamine synthetase gene expression system and related technologies for commercial production of the Company’s product candidates. Under this license agreement, the Company paid an upfront payment of $488,000 which was recorded to research and development expense during 2012 and is obligated to pay Lonza certain payments up to up to £1.4 million (approximately $2.3 million) on achievement of specified events for each licensed product, and royalties up to the very low single digits on sales of its licensed products. There has been no further payment made by the Company to Lonza pursuant to either the license agreement or collaboration agreement as of the year ended December 31, 2013 or 2014.
	The multi-product license agreement shall remain in force on a product by product and country by country basis until expiration of the Company’s obligation to make payments to Lonza with respect to such product in such country. The agreement can otherwise be terminated by the Company for any reason or no reason upon advance written notice to Lonza, or by either the Company or Lonza upon the other party’s material breach of the agreement, or if the other party ceases to carry on business. Lonza may also terminate the licenses granted under the agreement if the Company challenges any of the Lonza patent rights.
	The collaboration agreement shall remain in force for five years, unless either party elects to terminate the collaboration agreement by prior written notice effective on the date that is two and a half years after the effective date of the agreement, or unless the parties mutually agree in writing to extend the term of the collaboration agreement prior to the end of the five year term for an additional two years. Either the Company or Lonza may elect to terminate the collaboration agreement upon advance written notice for material breach by the other party, or if the other party ceases to carry on business. The collaboration agreement can also be terminated by either party by advance written notice in the event that Lonza is no longer obligated to provide manufacturing services to the Company.

Stock_Incentive_Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Incentive Plans	12. Stock Incentive Plans
	2004 Plan
	The Company granted options under its 2004 Stock Incentive Plan (the “2004 Plan”) until July 2013 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2004 Plan. The 2004 Plan provided for the award of restricted shares, grants of incentive and nonstatutory stock options, and sales of shares of the Company’s common stock. Awards can be made to employees, outside directors, and consultants of the Company. Stock options granted generally vest over a period of five years from the date of grant, with 20% of the total grant vesting on the first anniversary of the option vesting commencement date and 1/48 of the remaining grant vesting each month thereafter. Restricted stock issuances and early exercise of stock options were subject to the Company’s right of repurchase at the original issuance price, which right lapses over the vesting period of the stock. In connection with the Board of Directors and stockholders approval of the 2013 Plan, all remaining shares available for future award under the 2004 Plan were transferred to 2013 Plan, and the 2004 Plan was terminated as to future awards.
	2013 Plan
	In July 2013, the Company’s Board of Directors and stockholders approved the 2013 Equity Incentive Award Plan (the “2013 Plan”). Under the 2013 Plan, the Company initially reserved 500,000 shares of common stock for issuance as of its effective date of July 17, 2013, plus 90,125 shares which were then available for issuance under the Company’s 2004 Plan. The number of shares reserved for issuance under the 2013 Plan will increase by the number of shares represented by awards outstanding under the 2004 Plan that are forfeited or lapse unexercised and which following July 17, 2013 are not issued under the 2004 Plan. Additionally, on the first day of each calendar year, beginning in 2014 and ending in 2023, the number of shares in the reserve will increase by the least of 1,500,000 shares, 4% of the shares of the Company’s common stock outstanding (on an as-converted basis) on the last day of the immediately preceding fiscal year or such smaller number of shares of stock as determined by the Company’s Board of Directors. The 2013 Plan authorizes discretionary grants of incentive stock options, nonqualified stock options, restricted stock, restricted stock units, performance awards, dividend equivalents, stock payments, deferred stock, deferred stock units, and stock appreciation rights to employees and consultants of the Company, or any of its qualifying affiliates, and to members of the Board of Directors. The exercise price per share subject to each option shall not be less than 100% of the fair value of the common stock on the date of grant. In addition, in the case of incentive stock options granted to a greater than 10% stockholder, such price shall not be less than 110% of the fair value on the date the option is granted. The term of the options shall not be more than 10 years from the grant date, or 5 years from the date an incentive stock option is granted to a greater than 10% stockholder. Stock options granted generally vest over a period of four years from the date of grant, with 25% of the total grant vesting on the first anniversary of the option vesting commencement date and 1/48th of the original grant vesting each month thereafter for stock options granted upon hiring, and with 1/48th of the total grant vesting each month after the option vesting commencement date for any stock options granted after the hiring date.
	As of December 31, 2014, a total of 1,768,645 shares of common stock have been authorized under the 2013 Plan, including the additional 1,175,918 shares of common stock that became available for future issuance under the 2013 Plan as of January 1, 2014 as a result of an annual automatic increase provision in the 2013 Plan. As of December 31, 2014, a total of 1,743,484 shares are subject to options outstanding under the 2013 Plan. There are 2,078,526 shares subject to options outstanding under the 2004 Plan as of December 31, 2014, which will become available for issuance under the 2013 Plan to the extent the options are forfeited or lapse unexercised without issuance of such shares under the 2004 Plan. As of January 1, 2015 an additional 1,193,903 shares of our common stock became available for future issuance as a result of the annual increase provision in the 2013 Plan.
	Summary of Activity
	The following table summarizes activity under 2004 Plan and 2013 Plan during the twelve months ended December 31, 2014, including grants to nonemployees and restricted stock units (“RSUs”) granted:
	(In thousands, except per share amounts)	Shares Available			Options and			Weighted			Weighted		Aggregate
		for Grant of			Awards			Average			Average		Intrinsic
		Options and			Outstanding			Exercise Price			Remaining		Value
		Awards						per Share			Contractual Life
	Balances at December 31, 2011		207			2,529		$	2.91
	Options granted		(129	)		129			8.41
	Options exercised		—			(76	)		2.06
	Options forfeited		133			(133	)		2.29
	Balances at December 31, 2012		211			2,449			3.48
	Options authorized		500			—			—
	Options granted		(597	)		597			15.1
	Options exercised		—			(113	)		2.19
	Options forfeited		3			(3	)		6.75
	Balances at December 31, 2013		117			2,930			5.9
	Options authorized		1,176			0			—
	Options granted		(1,027	)		1,027			22.5
	RSUs granted		(294	)		294			—
	Options exercised		—			(382	)		2.83
	Options forfeited		47			(47	)		22.28
	Balances at December 31, 2014		19			3,822		$	10.84			6.29	$	46,039
	Vested and Expected to vest—December 31, 2014					3,773		$	10.72			6.25	$	45,904
	Vested—December 31, 2014					2,011		$	4.66			4.65	$	34,392
	The intrinsic value of options exercised was $8.9 million, $3.1 million and $490,000 for the years ended December 31, 2014, 2013, and 2012. The intrinsic value of outstanding options was calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock of $21.76 per share, $29.52 per share and $1.50 per share as of December 31, 2014, 2013 and 2012. The weighted-average grant date estimated fair values of options and RSUs granted during the year ended December 31, 2014 were $13.90 per share and $31.03 per share, respectively. RSUs were only granted in 2014. The weighted-average grant date estimated fair value of options was $9.48 per share for the year ended December 31, 2013 and $5.14 per share for the year ended December 31, 2012.
	Liability for Shares with Repurchase Rights
	The 2004 Plan allowed for the granting of options that may be exercised before the options have vested. Shares issued as a result of early exercise that have not vested are subject to repurchase by the Company upon termination of the purchaser’s employment or services, at the price paid by the purchaser. The amounts received in exchange for these shares have been recorded as a liability on the accompanying balance sheets and will be reclassified into common stock and additional paid-in-capital as the shares vest.
	At December 31, 2014 and 2013, there were 2,983 shares and 5,088 shares of common stock outstanding, respectively, subject to the Company’s right of repurchase at a price of $4.56 per share. At December 31, 2014 and 2013, the Company recorded $14,000 and $24,000, respectively, as liabilities associated with shares issued with repurchase rights.
	Employee Stock Purchase Plan
	As of December 31, 2014, a total of 593,979 shares of common stock have been authorized and 528,070 shares of common stock are available for future issuance under the Company’s Employee Stock Purchase Plan (the “ESPP”). This authorized number includes the additional 293,979 shares of common stock that became available for future issuance under the ESPP as of January 1, 2014 as a result of an annual automatic increase provision in the ESPP. 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 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 day of the offering period.
	There were no shares issued during the year ended December 31, 2013 under the ESPP. During the year ended December 31, 2014, the Company issued 65,909 shares under the ESPP. The Company used the following assumptions to estimate the fair value of the ESPP offered for the twelve months ended December 31, 2014: expected term of 0.5 years, weighted-average volatility from 65.7% to 115.4%, risk-free interest rate from 0.05% to 0.08% and expected dividend yield of zero. As of January 1, 2015 an additional 298,475 shares of our common stock became available for future issuance as a result of the annual increase provision in the ESPP.
	Restricted Stock Units
	In March 2014, the Company awarded 293,980 RSUs under the 2013 Plan. Each vested RSU represents the right to receive one share of common stock. The fair value of the RSU awards was calculated based on the NASDAQ quoted stock price on the date of the grant with the expense being recognized over the vesting period. The RSUs are generally scheduled to vest at the end of three years at March 31, 2017. However, the vesting will be accelerated to 25% of the awarded RSUs upon the payment by Celgene of a designated milestone payment related to Phase II clinical trials of demcizumab (anti-DLL4, OMP-21M18). The stock-based compensation expense for these RSUs is being amortized on the straight-line basis over the three-year vesting period. The Company continues to assess at each reporting date whether achievement of any performance condition is probable and would begin recognizing compensation costs based on the accelerated vesting if and when achievement of the performance condition becomes probable. The Company has recognized the stock-based compensation expense of $2.3 million related to these RSUs for the year ended December 31, 2014.
	Stock-Based Compensation
	Employee stock-based compensation expense was calculated based on awards expected to vest and has been reduced for estimated forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.
	Stock-based compensation expense recognized was as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Research and development	$	3,600		$	957		$	497
	General and administrative		2,594			779			339
	Total	$	6,194		$	1,736		$	836
	As of December 31, 2014, the Company had $15.9 million and $6.8 million of unrecognized compensation expense related to unvested stock options and RSUs, respectively, which are expected to be recognized over an estimated weighted-average period of 3.16 years and 2.25 years, respectively.
	The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following assumptions:
		Year Ended December 31,
		2014			2013			2012
	Weighted-average volatility		66.3	%		68.3	%		66.9	%
	Weighted-average expected term (years)		6.2			6.2			6.2
	Risk-free interest rate		2.06	%		1.82	%		1.09	%
	Expected dividend yield		—			—			—
	Volatility
	Since the Company has limited information on the volatility of its common stock due to no significant trading history, the expected stock price volatility was calculated based on the average volatility for comparable publicly traded biopharmaceutical companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on their similar size, stage in the life cycle, and financial leverage to the Company.
	Expected Term
	The Company has very limited historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock-option grants. As such, the expected term was estimated using the simplified method.
	Risk-Free Rate
	The risk-free interest rate assumption is based on the zero-coupon U.S. Treasury instruments on the date of grant with a maturity date consistent with the expected term of the Company’s stock option grants.
	Expected Dividend Yield
	To date, the Company has not declared or paid any cash dividends and does not have any plans to do so in the future. Therefore, the Company used an expected dividend yield of zero.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes
	For the year ended December 31, 2014, the Company recorded an income tax benefit of $0.5 million due primarily to the recognition of additional tax attributes that can offset alternative minimum tax as a result of the carryback. For the year ended December 31, 2013, the Company recorded an income tax provision of $1.9 million due primarily to the accelerated recognition of certain upfront payments for tax purposes that could not be fully offset by tax attributes. For the year ended December 31, 2012, the Company did not record an income tax provision on pre-tax income because we incurred taxable losses for both state and federal income tax purposes.
	Income (loss) before income taxes for the years ended December 31, 2014, 2013 and 2012 was from the United States.
	The components of the provision for income taxes are as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Current
	Federal	$	(10,841	)	$	12,275		$	—
	State		1			1			—
	Total		(10,840	)		12,276			—
	Deferred
	Federal		10,331			(10,331	)		—
	State		—			—			—
	Total		10,331			(10,331	)		—
	Income tax provision (benefit)	$	(509	)	$	1,944		$	—
	The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Tax at statutory federal rate		35	%		35	%		34	%
	State tax—net of federal benefit		—			—			—
	Research and development credit		12	%		6	%		—
	Change in valuation allowance		(45	%)		(54	%)		(34	%)
	Federal tax rate differential		—			6	%		—
	Other		(1	%)		(1	%)		—
	Income tax (provision) benefit		1	%		(8	%)		—	%
	Net deferred tax assets as of December 31, 2014 and 2013 consist of the following (in thousands):
		Year Ended December 31,
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	28,414		$	16,678
	Accruals		911			853
	Research and development credits		19,249			4,694
	Deferred revenue		52,054			64,270
	Other		2,378			929
	Gross deferred tax assets		103,006			87,424
	Deferred tax liability		(143	)		—
	Valuation allowance		(102,863	)		(77,093	)
	Net deferred tax assets	$	—		$	10,331
	The valuation allowance increased by $25.8 million and $11.4 million for the year ended December 31, 2014 and 2013, respectively. ASC Topic 740 requires that the tax benefit of deductible temporary differences or carryforwards be recorded as a deferred tax asset to the extent that management assesses the realization is “more likely than not.” Future realization of the tax benefit ultimately depends on the existence of sufficient taxable income within the carryback or carryforward period available under the tax law. At December 31, 2013 the Company had concluded that it was more likely than not that $10.3 million deferred tax assets at the year ended December 31, 2013 would be realizable as a result of carryback potential. At December 31, 2014, the Company has reclassified the carryback potential from deferred tax assets to tax receivable expected to be claimed as result of the carryback. At December 31, 2014, the Company has set up valuation allowances against all federal and state deferred tax assets because based on all available evidence, these deferred tax assets are not more likely than not to be realizable.
	At December 31, 2014, the Company had federal and state net operating loss carryforwards aggregating approximately $69.0 million and $116.5 million, respectively. These federal and California net operating loss carryforwards will begin to expire in 2023 and 2017, respectively, if not utilized. At December 31, 2014, the Company also had federal and California research and development credit carryforwards aggregating approximately $13.1 million and $12.0 million, respectively. The federal credits will expire in 2024, if not utilized. California research and development credits have no expiration date.
	Utilization of the net operating loss and tax credits carryforwards may be limited by “ownership change” rules, as defined in Section 382 of the Internal Revenue Code of 1986, as amended, and similar state provisions. This annual limitation may result in the expiration of the net operating losses and credits before utilization. The Company has performed an analysis to determine whether an “ownership change” has occurred from inception to December 31, 2014. Based on this analysis, management has determined that $0.7 million in federal and $0.7 million in California net operating losses generated during that period will expire without being used.
	On January 1, 2009, the Company adopted the Financial Accounting Standards Board Interpretation No. 48 (ASC 740-10), “Accounting for Uncertainties in Income Taxes” (formerly known as FIN 48). ASC 740 requires the Company to recognize the financial statements effects of a tax position when it is more likely than not, based on technical merits, that the position will be sustained upon examination.
	A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands):
		December 31,
		2014			2013			2012
	Balance at beginning of year	$	4,464		$	3,404		$	2,996
	Increase related to current year tax provision		1,539			708			338
	Increase related to prior year tax provision		—			354			70
	Decrease related to current year tax provision		—			(2	)		—
	Decrease related to prior year tax provision		(569	)		—			—
	Balance at end of year	$	5,434		$	4,464		$	3,404
	The unrecognized tax benefits, if recognized and in absence of full valuation allowance, would impact the income tax provision by $4.6 million and $3.7 million as of December 31, 2014 and 2013, respectively.
	The Company has elected to include interest and penalties as a component of tax expense. The Company accrued approximately $37,000 of interest and penalties during 2014. As of December 31, 2013 and 2014, the Company had recognized a liability for interest and penalties of approximately $0 and $37,000, respectively.
	The Company files federal and state income tax returns in the U.S. Tax years from 2004 forward remain open to examination due to the carryover of net operating losses and other tax attributes.

Net_Loss_per_Common_Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Loss per Common Share	14. Net Loss per Common Share
	The following outstanding common stock equivalents were excluded from the computation of diluted net loss per common share for the periods presented because including them would have been antidilutive:
		Year Ended December 31,
		2014		2013		2012
	Convertible preferred stock		0		—		21,180,280
	Options to purchase common stock		3,528,032		2,930,381		2,449,441
	RSUs		293,980		—		—
	Warrants to purchase convertible preferred stock		0		—		47,859
			3,822,012		2,930,381		23,677,580

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	15. Selected Quarterly Financial Data (Unaudited)
	Selected quarterly results from operations for the years ended December 31, 2014 and 2013 are as follows (in thousands, except per share amounts):
		2014 Quarter End
		March 31			June 30			September 30			December 31
	Total revenue	$	6,015		$	6,015		$	19,015		$	8,514
	Operating expenses		19,922			21,607			24,515			24,139
	Net loss		(13,871	)		(15,597	)		(5,486	)		(15,056	)
	Basic and diluted net loss per common share		(0.47	)		(0.53	)		(0.18	)		(0.50	)
		2013 Quarter End
		March 31			June 30			September 30			December 31
	Total revenue	$	2,932		$	2,932		$	12,932		$	18,983
	Operating expenses		(11,561	)		(12,427	)		(16,301	)		(21,389	)
	Net loss		(8,598	)		(9,644	)		(3,486	)		(4,343	)
	Basic and diluted net loss per common share		(7.92	)		(8.83	)		(0.15	)		(0.15	)

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events
	On January 15, 2015 the Company announced the enrollment of the first biomarker-selected patient in its brontictuzumab (anti-Notch1, OMP-52M51) Phase Ia solid tumor clinical trial. The advancement to the predictive biomarker expansion stage triggered a $5.0 million milestone from OncoMed’s partner, GlaxoSmithKline (GSK).

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The Company’s financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”).
Use of Estimates	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to revenue recognition, preclinical study and clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results may differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers all highly liquid investments with original maturities of 90 days or less at the date of purchase to be cash and cash equivalents.
Short-Term Investments	Short-Term Investments
	Short-term investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than 365 days from the date of acquisition. Short-term investments are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale securities are excluded from earnings and were reported as a component of accumulated other comprehensive income (loss). The cost of available-for-sale securities sold is based on the specific-identification method.
Concentrations of Credit Risk	Concentrations of Credit Risk
	Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. Cash and cash equivalents and short-term investments are invested through banks and other financial institutions in the United States. Such deposits may be in excess of insured limits. The Company maintains cash and cash equivalents and investments with various high credit quality and capitalized financial institutions.
Property and Equipment	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which range from three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the remaining life of the lease at the time the asset is placed into service.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The carrying value of long-lived assets, including property and equipment, are reviewed for impairment whenever events or changes in circumstances indicate that the asset may not be recoverable. An impairment loss is recognized when the total of estimated future undiscounted cash flows, expected to result from the use of the asset and its eventual disposition, are less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. Through December 31, 2014, there have been no such impairment losses.
Revenue Recognition	Revenue Recognition
	The Company generates substantially all its revenue from collaborative research and development agreements with pharmaceutical companies. The terms of the agreements may include nonrefundable upfront payments, milestone payments, other contingent payments and royalties on any product sales derived from collaborations. These multiple element arrangements are analyzed to determine whether the deliverables can be separated or whether they must be accounted for as a single unit of accounting.
	The determination of stand-alone value is generally based on whether any deliverable has stand-alone value to the customer. The Company determines how to allocate arrangement consideration to identified units of accounting based on the selling price hierarchy provided under the relevant guidance. The selling price used for each unit of accounting is based on vendor-specific objective evidence, if available, third-party evidence if vendor-specific objective evidence is not available or estimated selling price if neither vendor-specific nor third-party evidence is available. Management may be required to exercise considerable judgment in determining whether a deliverable is a separate unit of accounting and in estimating the selling prices of identified units of accounting for new agreements.
	Typically, the Company has not granted licenses to collaborators at the beginning of its arrangements and thus there are no delivered items separate from the research and development services provided. As such, upfront payments are recorded as deferred revenue in the balance sheet and are recognized as collaboration revenue over the estimated period of performance that is consistent with the terms of the research and development obligations contained in the collaboration agreement. The Company regularly reviews the estimated period of performance based on the progress made under each arrangement.
	Payments that are contingent upon achievement of a substantive milestone are recognized in their entirety in the period in which the milestone is achieved. Milestones are defined as an event that can only be achieved based on the Company’s performance and there is substantive uncertainty about whether the event will be achieved at the inception of the arrangement. Events that are contingent only on the passage of time or only on counterparty performance are not considered milestones. Further, the amounts received must relate solely to prior performance, be reasonable relative to all of the deliverables and payment terms within the agreement and commensurate with the Company’s performance to achieve the milestone after commencement of the agreement. Other contingent payments received for which payment is contingent solely on the results of a collaborative partner’s performance (bonus payments) are not accounted for using the milestone method. Such bonus payments will be recognized as revenue when collectability is reasonably assured.
	Payments related to options to license the Company’s program candidates are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. For arrangements under which an option is considered substantive, the Company does not consider the item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration, assuming the option is not priced at a significant and incremental discount. Conversely, for arrangements under which an option is not considered substantive or if an option is priced at a significant and incremental discount, the Company would consider the item underlying the option to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration.
Customer Concentration	Customer Concentration
	Customers whose collaboration revenue accounted for 10% or more of total revenues were as follows:
			Year Ended December 31,
			2014			2013			2012
	GlaxoSmithKline LLC (“GSK”)			31	%		*			65	%
	Bayer Pharma AG (“Bayer”)			30	%		92	%		35	%
	Celgene Corporation (“Celgene”)			39	%		*			—
	*	less than 10%
Research and Development Expenses	Research and Development Expenses
	Research and development costs are expensed as incurred. Research and development costs consist of salaries and other personnel-related expenses, including associated stock-based compensation, consulting fees, lab supplies, and facility costs, as well as fees paid to other entities that conduct certain research, development and manufacturing activities on behalf of the Company.
Clinical Trial Accruals	Clinical Trial Accruals
	Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with clinical research organizations and clinical sites. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services.
	Nonrefundable advance payments for goods and services that will be used or rendered in future research and development activities, are deferred and recognized as expense in the period that the related goods are delivered or services are performed.
Stock-Based Compensation	Stock-Based Compensation
	The Company recognizes compensation expense for all share-based payment awards made to employees and directors based on estimated fair values. For employee stock options, the Company determines the grant date fair value of the awards using the Black-Scholes option-pricing model and generally recognizes the fair value as stock-based compensation expense on a straight-line basis over the vesting period of the respective awards. Stock-based compensation expense 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, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For restricted stock, the compensation cost for these awards is based on the closing price of the Company’s common stock on the date of grant and recognized as compensation expense on a straight-line basis over the requisite service period.
	The Company accounts for equity instruments issued to nonemployees based on their fair values on the measurement dates using the Black-Scholes option-pricing model. The estimated fair values of the options granted to nonemployees are remeasured as they vest. As a result, the noncash charge to operations for nonemployee options with vesting conditions is affected each reporting period by changes in the fair value of the Company’s common stock.
Income Taxes	Income Taxes
	The Company accounts for income taxes using the liability method under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are established when necessary to reduce deferred tax assets to the amount which is more likely than not to be realizable.
	The recognition, derecognition and measurement of a tax position is based on management’s best judgment given the facts, circumstances and information available at each reporting date. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Net Loss per Common Share	Net Loss per Common Share
	Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per common share is computed by dividing the net loss by the weighted-average number of common shares and common share equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, potentially dilutive securities consisting of convertible preferred stock, common stock subject to repurchase, stock options and warrants are considered to be common stock equivalents and were excluded in the calculation of diluted net loss per common share because their effect would be anti-dilutive for all periods presented.
Newly Adopted and Recent Accounting Pronouncements	Newly Adopted and Recent Accounting Pronouncements
	In June 2014, the Financial Accounting Standards Board (“FASB”) issued final guidance that a performance target that affects vesting of a share-based payment and that could be achieved after the requisite service period is a performance condition under Accounting Standards Codification (“ASC”) 718, Compensation—Stock Compensation. As a result, the target is not reflected in the estimation of the award’s grant date fair value. Compensation cost for such an award would be recognized over the required service period, if it is probable that the performance condition will be achieved. The guidance is effective for all entities for annual periods beginning after December 15, 2015 and interim periods within those annual periods. The guidance should be applied on a prospective basis to awards that are granted or modified on or after the effective date. The Company does not believe the adoption of this guidance will have a material impact on its financial statements.
	In May 2014, the FASB and the International Accounting Standards Board issued Accounting Standards Update (“ASU”) No. 2014-09 (Topic 606)—Revenue from Contracts with Customers (“ASU 2014-09”). This ASU affects any entity that either enters into contracts with customers to transfer goods and services or enters into contracts for the transfer of nonfinancial assets. This ASU will supersede the revenue recognition requirements in Topic 605, and most industry specific guidance. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under today’s guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is effective for the Company for annual periods beginning after December 15, 2016, including interim periods within that period. Entities can choose to apply the standard using either the full retrospective approach or a modified retrospective approach. Entities electing the full retrospective adoption will apply the standard to each period presented in the financial statements. This means that entities will have to apply the new guidance as if it had been in effect since the inception of all its contracts with customers presented in the financial statements. Entities that elect the modified retrospective approach will apply the guidance retrospectively only to the most current period presented in the financial statements. This means that entities will have to recognize the cumulative effect of initially applying the new standard as an adjustment to the opening balance of retained earnings at the date of initial application. Early adoption is not permitted under U.S. GAAP. The Company is currently evaluating the impact of adoption of this accounting standards update on its financial statements.
	In August 2014, the FASB issued ASU No. 2014-15 (Subtopic 205-40)—Presentation of Financial Statements—Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (“ASU 2014-15”) which provides guidance about management’s responsibility to evaluate whether or not there is substantial doubt about the Company’s ability to continue as a going concern and to provide related footnote disclosure. ASU 2014-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early application is permitted. The adoption of this standard is not expected to have an impact on the Company’s Financial Statements.
Fair Value Measurement Policy	The Company records its financial assets and liabilities at fair value. The carrying amounts of certain of the Company’s financial instruments, including cash and cash equivalents, short-term investments, and accounts payable, approximate their fair value due to their short maturities. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value, and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows:
		•		Level 1: Inputs which include quoted prices in active markets for identical assets and liabilities.
		•		Level 2: Inputs other than Level 1 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 3: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
Fair Value of Financial Instruments Policy	Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds as Level 1. When quoted market prices are not available for the specific security, then the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes, and issuer spreads. The Company classifies U.S. Treasury securities as Level 2. There were no transfers between Level 1 and Level 2 during the periods presented.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Schedule of Collaboration Revenue	Customers whose collaboration revenue accounted for 10% or more of total revenues were as follows:
			Year Ended December 31,
			2014			2013			2012
	GlaxoSmithKline LLC (“GSK”)			31	%		*			65	%
	Bayer Pharma AG (“Bayer”)			30	%		92	%		35	%
	Celgene Corporation (“Celgene”)			39	%		*			—
	*	less than 10%

Cash_Equivalents_and_Investmen1

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Cash and Cash Equivalents [Abstract]
Schedule of Fair Value of Securities, Not Including Cash	The fair value of securities, not including cash at December 31, 2014 and 2013, were as follows (in thousands):
		December 31, 2014
		Amortized		Gross Unrealized					Fair Value
		Cost		Gains		Losses
	Money market funds	$	8,460	$	—	$	—		$	8,460
	U.S. treasury bills		203,845		37		(54	)		203,828
	Total available-for-sale securities	$	212,305	$	37	$	(54	)	$	212,288
	Classified as:
	Cash equivalents								$	8,460
	Short-term investments									203,828
	Total cash equivalents and investments								$	212,288
	As of December 31, 2014, the Company had a total of $232.0 million in cash, cash equivalents, and short-term investments, which includes $19.7 million in cash and $212.3 million in cash equivalents and short-term investments.
		December 31, 2013
		Amortized		Gross Unrealized					Fair Value
		Cost		Gains		Losses
	Money market funds	$	7,980	$	—	$	—		$	7,980
	U.S. treasury bills		267,242		14		—			267,256
	Total available-for-sale securities	$	275,222	$	14	$	—		$	275,236
	Classified as:
	Cash equivalents								$	167,973
	Short-term investments									107,263
	Total cash equivalents and investments								$	275,236

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets and Liabilities Subject to Fair Value Measurements on Recurring Basis	The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):
		December 31, 2014
		Level 1		Level 2		Level 3		Total
	Assets:
	Money market funds	$	8,460	$	—	$	—	$	8,460
	U.S. treasury bills		—		203,828		—		203,828
	Total	$	8,460	$	203,828	$	—	$	212,288
		December 31, 2013
		Level 1		Level 2		Level 3		Total
	Assets:
	Money market funds	$	7,980	$	—	$	—	$	7,980
	U.S. treasury bills		—		267,256		—		267,256
	Total	$	7,980	$	267,256	$	—	$	275,236

Property_and_Equipment_Tables

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consist of the following (in thousands):
		December 31,
		2014			2013
	Computer equipment and software	$	1,715		$	1,632
	Furniture and fixtures		443			392
	Laboratory equipment		9,739			9,025
	Leasehold improvements		8,826			8,624
			20,723			19,673
	Less accumulated depreciation and amortization		(15,619	)		(15,032	)
	Property equipment, net	$	5,104		$	4,641

Accrued_Liabilities_Tables

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consist of the following (in thousands):
		December 31,
		2014		2013
	Research and development related	$	9,965	$	5,949
	Compensation related		4,208		3,446
	Other		510		465
	Total accrued liabilities	$	14,683	$	9,860

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Summary of Minimum Annual Rental Commitments under Lease Agreement	Minimum annual rental commitments under the lease agreement are as follows (in thousands):
	Year ending December 31:
	2015	$	1,942
	2016		2,002
	2017		2,062
	2018		2,122
	Thereafter		177
	Total	$	8,305

Collaborations_Tables

Collaborations (Tables)	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Company Recognized Revenues from Collaboration Agreements	The Company has recognized the following revenues from its collaboration agreements during the years ended December 31, 2014, 2013 and 2012 (in thousands):
		Year Ended December 31,
		2014		2013		2012
	GSK:
	Recognition of upfront payment	$	1,248	$	1,971	$	1,970
	Milestone revenue		11,000		—		14,000
	GSK total		12,248		1,971		15,970
	Bayer:
	Recognition of upfront payments		9,756		9,756		8,689
	Milestone revenue		2,000		25,000		—
	Bayer total		11,756		34,756		8,689
	Celgene:
	Recognition of upfront payment		13,055		1,052		—
	Milestone revenue		2,500		—		—
	Celgene total		15,555		1,052		—
	Total collaboration related revenue	$	39,559	$	37,779	$	24,659

Stock_Incentive_Plans_Tables

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Activity under 2004 Plan and 2013 Stock Plan	The following table summarizes activity under 2004 Plan and 2013 Plan during the twelve months ended December 31, 2014, including grants to nonemployees and restricted stock units (“RSUs”) granted:
	(In thousands, except per share amounts)	Shares Available			Options and			Weighted			Weighted		Aggregate
		for Grant of			Awards			Average			Average		Intrinsic
		Options and			Outstanding			Exercise Price			Remaining		Value
		Awards						per Share			Contractual Life
	Balances at December 31, 2011		207			2,529		$	2.91
	Options granted		(129	)		129			8.41
	Options exercised		—			(76	)		2.06
	Options forfeited		133			(133	)		2.29
	Balances at December 31, 2012		211			2,449			3.48
	Options authorized		500			—			—
	Options granted		(597	)		597			15.1
	Options exercised		—			(113	)		2.19
	Options forfeited		3			(3	)		6.75
	Balances at December 31, 2013		117			2,930			5.9
	Options authorized		1,176			0			—
	Options granted		(1,027	)		1,027			22.5
	RSUs granted		(294	)		294			—
	Options exercised		—			(382	)		2.83
	Options forfeited		47			(47	)		22.28
	Balances at December 31, 2014		19			3,822		$	10.84			6.29	$	46,039
	Vested and Expected to vest—December 31, 2014					3,773		$	10.72			6.25	$	45,904
	Vested—December 31, 2014					2,011		$	4.66			4.65	$	34,392
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense recognized was as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Research and development	$	3,600		$	957		$	497
	General and administrative		2,594			779			339
	Total	$	6,194		$	1,736		$	836
Assumptions Used for Determining Fair Value of Stock Options Using Black-Scholes Valuation Model	The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following assumptions:
		Year Ended December 31,
		2014			2013			2012
	Weighted-average volatility		66.3	%		68.3	%		66.9	%
	Weighted-average expected term (years)		6.2			6.2			6.2
	Risk-free interest rate		2.06	%		1.82	%		1.09	%
	Expected dividend yield		—			—			—

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Components of Provision for Income Taxes	The components of the provision for income taxes are as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Current
	Federal	$	(10,841	)	$	12,275		$	—
	State		1			1			—
	Total		(10,840	)		12,276			—
	Deferred
	Federal		10,331			(10,331	)		—
	State		—			—			—
	Total		10,331			(10,331	)		—
	Income tax provision (benefit)	$	(509	)	$	1,944		$	—
Schedule of Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Tax Rate	The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Tax at statutory federal rate		35	%		35	%		34	%
	State tax—net of federal benefit		—			—			—
	Research and development credit		12	%		6	%		—
	Change in valuation allowance		(45	%)		(54	%)		(34	%)
	Federal tax rate differential		—			6	%		—
	Other		(1	%)		(1	%)		—
	Income tax (provision) benefit		1	%		(8	%)		—	%
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2014 and 2013 consist of the following (in thousands):
		Year Ended December 31,
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	28,414		$	16,678
	Accruals		911			853
	Research and development credits		19,249			4,694
	Deferred revenue		52,054			64,270
	Other		2,378			929
	Gross deferred tax assets		103,006			87,424
	Deferred tax liability		(143	)		—
	Valuation allowance		(102,863	)		(77,093	)
	Net deferred tax assets	$	—		$	10,331
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the Company’s unrecognized tax benefits is as follows (in thousands):
		December 31,
		2014			2013			2012
	Balance at beginning of year	$	4,464		$	3,404		$	2,996
	Increase related to current year tax provision		1,539			708			338
	Increase related to prior year tax provision		—			354			70
	Decrease related to current year tax provision		—			(2	)		—
	Decrease related to prior year tax provision		(569	)		—			—
	Balance at end of year	$	5,434		$	4,464		$	3,404

Net_Loss_per_Common_Share_Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Outstanding Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Common Share	The following outstanding common stock equivalents were excluded from the computation of diluted net loss per common share for the periods presented because including them would have been antidilutive:
		Year Ended December 31,
		2014		2013		2012
	Convertible preferred stock		0		—		21,180,280
	Options to purchase common stock		3,528,032		2,930,381		2,449,441
	RSUs		293,980		—		—
	Warrants to purchase convertible preferred stock		0		—		47,859
			3,822,012		2,930,381		23,677,580

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of Selected Quarterly Results from Operations	Selected quarterly results from operations for the years ended December 31, 2014 and 2013 are as follows (in thousands, except per share amounts):
		2014 Quarter End
		March 31			June 30			September 30			December 31
	Total revenue	$	6,015		$	6,015		$	19,015		$	8,514
	Operating expenses		19,922			21,607			24,515			24,139
	Net loss		(13,871	)		(15,597	)		(5,486	)		(15,056	)
	Basic and diluted net loss per common share		(0.47	)		(0.53	)		(0.18	)		(0.50	)
		2013 Quarter End
		March 31			June 30			September 30			December 31
	Total revenue	$	2,932		$	2,932		$	12,932		$	18,983
	Operating expenses		(11,561	)		(12,427	)		(16,301	)		(21,389	)
	Net loss		(8,598	)		(9,644	)		(3,486	)		(4,343	)
	Basic and diluted net loss per common share		(7.92	)		(8.83	)		(0.15	)		(0.15	)

Organization_Additional_Inform

Organization - Additional Information (Detail) (USD $)	12 Months Ended		0 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jul. 23, 2013
	Segment
Class of Stock [Line Items]
Number of operating segment	1
Incorporation date	31-Jul-04
Initial Public Offering effective date	17-Jul-13
Initial Public Offering closure date	23-Jul-13
Cash proceeds from IPO, net of underwriting discounts and commissions and expenses		$82,668
Conversion of preferred stock into common stock, carrying value		182,773
Initial Public Offering [Member]
Class of Stock [Line Items]
Common stock, shares issued			5,520,000
Cash proceeds from IPO, net of underwriting discounts and commissions and expenses			82,700
Conversion of preferred stock into common stock, shares			21,180,280
Conversion of preferred stock into common stock, carrying value			$182,800
Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock, shares issued			720,000

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Impairment of long-lived assets	$0
Interest or penalties charged in relation to unrecognized tax benefits	$37,000	$0
Leasehold Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Description of property and equipment over estimated useful lives	Shorter of their estimated useful lives or the remaining life of the lease at the time the asset is placed into service
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents, maturity	90 days
Maximum [Member] | Property and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Estimated useful life	5 years
Minimum [Member] | Property and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Estimated useful life	3 years
Minimum [Member] | Revenue [Member] | Customer Concentration [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage of revenue represented by major customers	10.00%
Short-Term Investments [Member]
Summary Of Significant Accounting Policies [Line Items]
Short-term investments maturities, maximum	365 days
Short-term investments maturities, minimum	90 days

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Schedule of Collaboration Revenue (Detail) (Revenue [Member], Customer Concentration [Member], Collaborative Arrangement [Member])	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
GSK [Member]
Concentration Risk [Line Items]
Collaborative research and development revenue, Percentage	31.00%		[1]	65.00%
Bayer [Member]
Concentration Risk [Line Items]
Collaborative research and development revenue, Percentage	30.00%	92.00%		35.00%
Celgene [Member]
Concentration Risk [Line Items]
Collaborative research and development revenue, Percentage	39.00%		[1]
[1]	* less than 10%

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - Schedule of Collaboration Revenue (Parenthetical) (Detail) (Minimum [Member], Customer Concentration [Member], Revenue [Member])	12 Months Ended
	Dec. 31, 2014
Minimum [Member] | Customer Concentration [Member] | Revenue [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue represented by major customers	10.00%

Cash_Equivalents_and_Investmen2

Cash Equivalents and Investments - Schedule of Fair Value of Securities, Not Including Cash (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$212,305	$275,222
Gross Unrealized Gains	37	14
Gross Unrealized Losses	-54
Fair Value	212,288	275,236
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	8,460	167,973
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	203,828	107,263
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,460	7,980
Fair Value	8,460	7,980
U.S. Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	203,845	267,242
Gross Unrealized Gains	37	14
Gross Unrealized Losses	-54
Fair Value	$203,828	$267,256

Cash_Equivalents_and_Investmen3

Cash Equivalents and Investments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total cash, cash equivalents, and short-term investments	$232,000,000	$316,200,000
Cash	19,700,000	41,000,000
Cash equivalents and short-term investments	212,288,000	275,236,000
Realized gains or losses on available-for-sale securities	$0	$0
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, contractual maturity period	1 year	1 year

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Company's Financial Assets and Liabilities Subject to Fair Value Measurements on Recurring Basis (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Assets:
Assets fair value disclosure	$212,288	$275,236
Money Market Funds [Member]
Assets:
Assets fair value disclosure	8,460	7,980
U.S. Treasury Bills [Member]
Assets:
Assets fair value disclosure	203,828	267,256
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Assets fair value disclosure	8,460	7,980
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]
Assets:
Assets fair value disclosure	8,460	7,980
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Assets fair value disclosure	203,828	267,256
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Bills [Member]
Assets:
Assets fair value disclosure	$203,828	$267,256

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Transfers between Level 1 and Level 2	$0

Property_and_Equipment_Schedul

Property and Equipment - Schedule of Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$20,723	$19,673
Less accumulated depreciation and amortization	-15,619	-15,032
Property equipment, net	5,104	4,641
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,715	1,632
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	443	392
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,739	9,025
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$8,826	$8,624

Accrued_Liabilities_Schedule_o

Accrued Liabilities - Schedule of Accrued Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Payables and Accruals [Abstract]
Research and development related	$9,965	$5,949
Compensation related	4,208	3,446
Other	510	465
Total accrued liabilities	$14,683	$9,860

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010	Dec. 31, 2006
	Lease
Commitments and Contingencies Disclosure [Abstract]
Operating lease, expiration date	2019-01
Operating leases options	2
Operating lease, additional period	3 years
Landlord provided for tenant improvement allowance					$5.70
Landlord funded provided tenant improvement				1.6
Lease amendment date	2010-12
Unamortized tenant improvement allowance	2.1	2.7
Rental expense	$1.30	$1.30	$1.30

Commitments_and_Contingencies_2

Commitments and Contingencies - Summary of Minimum Annual Rental Commitments under Lease Agreement (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]
2015	$1,942
2016	2,002
2017	2,062
2018	2,122
Thereafter	177
Total	$8,305

License_Agreement_Additional_I

License Agreement - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2004
License Agreements [Line Items]
Up-front fee				$10,000
Common shares	29,847,577	29,397,964
Royalties payments to licensor				10,000,000
Legal costs	$158,000	$151,000	$175,000
Common Stock [Member]
License Agreements [Line Items]
Common shares				7,796
Nonvoting Common Stock [Member]
License Agreements [Line Items]
Additional common stock				0.25%

Supply_and_License_Agreements_

Supply and License Agreements - Additional Information (Detail)

12 Months Ended

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2009

Dec. 31, 2009

Dec. 31, 2008

Dec. 31, 2008

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

USD ($)

USD ($)

USD ($)

EUR (€)

USD ($)

EUR (€)

USD ($)

EUR (€)

Minimum [Member]

Maximum [Member]

GSK [Member]

GSK [Member]

GSK [Member]

MorphoSys AG [Member]

MorphoSys AG [Member]

Tarextumab Anti-Notch 2/3, OMP-59R5 [Member]

License Agreement [Member]

USD ($)

USD ($)

USD ($)

USD ($)

EUR (€)

Supply And License Agreement Commitment [Line Items]

Technology access and subscription fees paid

$350,000

€ 250,000

$919,000

€ 650,000

Research license agreement extension, term

5 years

Research license agreement additional extension, term

5 years

Annual license fee

24,000

20,000

Research license agreement expiration

2015

2020

Future milestone payments

7,100,000

5,800,000

Reimbursement percentage

50.00%

License agreement year

2006-06

Additional research license agreement expiration

2020

Amortization expense

27,000

26,000

27,000

Payments and reimbursements

$0

$0

$0

Collaborations_GSK_Strategic_A

Collaborations - GSK Strategic Alliance - Additional Information (Detail) (USD $)	12 Months Ended	1 Months Ended		12 Months Ended
	Dec. 31, 2014	Jul. 31, 2014	Jan. 31, 2014	Dec. 31, 2012	Dec. 31, 2013	Jun. 30, 2013	Oct. 31, 2012
	Agreement
Related Party Transaction [Line Items]
Number of collaboration arrangements	3
Deferred revenue	$130,123,000				$148,871,000
GSK [Member]
Related Party Transaction [Line Items]
Estimated revenue recognition period	5 years
GSK [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Estimated amount to be received for development cost	2,000,000
GSK [Member] | Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Deferred revenue						8,000,000
Development milestone payments	81,000,000
Remaining potential development milestones	5,000,000
Contingent consideration payments	309,000,000
Contingent consideration payments for achievement of worldwide net sales	280,000,000
Potential future payments	670,000,000
GSK [Member] | Collaborative Arrangement [Member] | Brontictuzumab Anti-Notch 1, OMP-52M51 [Member]
Related Party Transaction [Line Items]
Remaining potential development milestone payment	16,000,000
Bonus payment	5,000,000
GSK [Member] | Collaborative Arrangement [Member] | Tarextumab Anti-Notch 2/3, OMP-59R5 [Member]
Related Party Transaction [Line Items]
Deferred revenue							3,000,000
Bonus payment	10,000,000
Milestones on completion of exercise options	60,000,000
GSK [Member] | Collaborative Arrangement [Member] | Tarextumab Anti-Notch 2/3, OMP-59R5 [Member] | Milestone payment one [Member]
Related Party Transaction [Line Items]
Deferred revenue, revenue recognized		8,000,000
GSK [Member] | Collaborative Arrangement [Member] | Tarextumab Anti-Notch 2/3, OMP-59R5 [Member] | Milestone payment two [Member]
Related Party Transaction [Line Items]
Deferred revenue, revenue recognized		3,000,000
GSK [Member] | Collaborative Arrangement [Member] | Minimum [Member]
Related Party Transaction [Line Items]
Deemed ownership percentage of related party					10.00%
GSK [Member] | July 2011 Amendment [Member]
Related Party Transaction [Line Items]
Estimated revenue recognition period	4 years		5 years
Deferred revenue	7,900,000
Extended estimated completion of clinical trials deliverable	2016-06
GSK [Member] | Proof of Principle Development Milestone Payments [Member] | Tarextumab Anti-Notch 2/3, OMP-59R5 [Member]
Related Party Transaction [Line Items]
Milestone recognized				5,000,000
GSK [Member] | IND Acceptance [Member] | Brontictuzumab Anti-Notch 1, OMP-52M51 [Member]
Related Party Transaction [Line Items]
Milestone recognized				5,000,000
GSK [Member] | First Patient [Member] | Brontictuzumab Anti-Notch 1, OMP-52M51 [Member]
Related Party Transaction [Line Items]
Milestone recognized				4,000,000
GSK [Member] | Series B-2 Convertible Preferred Stock [Member]
Related Party Transaction [Line Items]
Collaboration and Option Agreement date	7-Dec-07
Initial payment received from collaboration arrangement	35,000,000
Number of shares issued	1,441,396
Premium received on number of shares issued	4,300,000
Deferred revenue	$17,500,000

Collaborations_Company_Recogni

Collaborations - Company Recognized Revenues from Collaboration Agreements (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									$39,559	$37,779	$8,689
Total revenue	8,514	19,015	6,015	6,015	18,983	12,932	2,932	2,932	39,559	37,779	24,681
Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total revenue									39,559	37,779	24,659
Collaborative Arrangement [Member] | GSK [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									12,248	1,971	15,970
Collaborative Arrangement [Member] | GSK [Member] | Recognition of Upfront Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									1,248	1,971	1,970
Collaborative Arrangement [Member] | GSK [Member] | Milestone Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									11,000		14,000
Collaborative Arrangement [Member] | Bayer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									11,756	34,756	8,689
Collaborative Arrangement [Member] | Bayer [Member] | Recognition of Upfront Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									9,756	9,756	8,689
Collaborative Arrangement [Member] | Bayer [Member] | Milestone Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									2,000	25,000
Collaborative Arrangement [Member] | Celgene [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									15,555	1,052
Collaborative Arrangement [Member] | Celgene [Member] | Recognition of Upfront Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									13,055	1,052
Collaborative Arrangement [Member] | Celgene [Member] | Milestone Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue									$2,500

Collaborations_Bayer_Strategic

Collaborations - Bayer Strategic Alliance - Additional Information (Detail) (Bayer [Member], USD $)	12 Months Ended		1 Months Ended		12 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2012	Oct. 31, 2013	Aug. 31, 2013	Dec. 31, 2011	Sep. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration and Option Agreement date	15-Jun-10
Anticipated estimated revenue recognition period	5 years
Recognition of Upfront Payment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments earned	40
Biologic Product [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	387.5
Small Molecules [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	112
ipafricept Fzd8-Fc, OMP-54F28 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment received		5
Deferred revenue		5
Vantictumab [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments earned	25		15	10	20
Number of milestones recognized and achieved	2
Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future development milestone payments	10
Collaborative Arrangement [Member] | Achievement of Regulatory Events [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	185
Collaborative Arrangement [Member] | Achievement of Specified Future Product Sales [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	1,000
Collaborative Arrangement [Member] | Biologic Product [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future development milestone payments	55
Collaborative Arrangement [Member] | Biologic Product [Member] | Achievement of Regulatory Events [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	135
Collaborative Arrangement [Member] | Biologic Product [Member] | Achievement of Specified Future Product Sales [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	862.5
Collaborative Arrangement [Member] | Small Molecules [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments earned						2
Future development milestone payments	22
Collaborative Arrangement [Member] | Small Molecules [Member] | Achievement of Regulatory Events [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	50
Collaborative Arrangement [Member] | Small Molecules [Member] | Achievement of Specified Future Product Sales [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration payments for regulatory events	140

Collaborations_Celgene_Strateg

Collaborations - Celgene Strategic Alliance - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Nov. 30, 2014
		Program
Related Party Transaction [Line Items]
Proceeds from issuance of common stock	$20,519,000
Celgene [Member] | Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Collaboration and Option Agreement date		Dec-13
Contractual option exercised period of clinical trials		12 years
Number of biologic programs		6
Number of biological program for which company can enter into co-commercialization and co-development agreements		5
Collaboration agreement extension period		2 years
Upfront cash payment received		155,000,000
Issuance of common stock in connection with research and collaboration agreement		1,470,588
Common stock price per share		$15.13
Proceeds from issuance of common stock		22,200,000
Premium on closing price of common stock value		1,700,000
Common stock, fair market value		20,500,000
Profits and losses sharing percentage		50.00%
Initial arrangement consideration		156,700,000
Estimated revenue recognition period		12 years
Payment receivable under agreement			2,500,000
Future development milestone payments		87,500,000
Celgene [Member] | Collaborative Arrangement [Member] | Achievement of Regulatory Events [Member]
Related Party Transaction [Line Items]
Additional contingent consideration for regulatory events		2,800,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Demcizumab Program [Member]
Related Party Transaction [Line Items]
Regulatory and commercial milestones payments		791,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Demcizumab Program [Member] | Upon achievement of safety criteria in clinical trials [Member]
Related Party Transaction [Line Items]
Regulatory and commercial milestones payments		70,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Bispecific Antibody Program [Member]
Related Party Transaction [Line Items]
Regulatory and commercial milestones payments		505,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Undisclosed Pathway Programs [Member]
Related Party Transaction [Line Items]
Regulatory and commercial milestones payments		442,800,000
Celgene [Member] | Collaborative Arrangement [Member] | Small Molecules [Member]
Related Party Transaction [Line Items]
Regulatory and commercial milestones payments		107,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Small Molecules [Member] | Achievement of Regulatory Events [Member]
Related Party Transaction [Line Items]
Additional contingent consideration for regulatory events		95,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Biologic and Small Molecule Therapeutic Programs [Member]
Related Party Transaction [Line Items]
Future development milestone payments		240,000,000
Celgene [Member] | Collaborative Arrangement [Member] | Biologic Product [Member] | Achievement of Regulatory Events [Member]
Related Party Transaction [Line Items]
Additional contingent consideration for regulatory events		$2,700,000,000

Lonza_Sales_AG_Agreement_Addit

Lonza Sales AG Agreement - Additional Information (Detail) (Lonza Sales AG [Member])	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
	USD ($)	USD ($)	USD ($)	GBP (£)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
License agreement, upfront payment recorded in research and development expense		$488,000
Obligation to pay certain payments	$0	$2,300,000	$0	£ 1,400,000
Collaboration agreement terms	The collaboration agreement shall remain in force for five years, unless either party elects to terminate the collaboration agreement by prior written notice effective on the date that is two and a half years after the effective date of the agreement, or unless the parties mutually agree in writing to extend the term of the collaboration agreement prior to the end of the five year term for an additional two years.
Collaboration agreement period	5 years
Collaboration agreement prior written notice period	2 years 6 months
Collaboration agreement additional period	2 years

Stock_Incentive_Plans_Addition

Stock Incentive Plans - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Jul. 31, 2013	Jul. 17, 2013	Jan. 02, 2014	Jan. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting original months	6 years 3 months
Intrinsic value of options exercised	$8,900,000	$3,100,000	$490,000
Common stock closing price	$21.76	$29.52	$1.50
Weighted-average grant-date fair value, granted	$13.90	$9.48	$5.14
Repurchase of common stock outstanding	2,983	5,088
Repurchase prices of common stock	$4.56	$4.56
Liability for shares issued with repurchase rights	14,000	24,000
Estimated fair value expected term	6 years 2 months 12 days	6 years 2 months 12 days	6 years 2 months 12 days
Estimated fair value weighted-average volatility rate	66.30%	68.30%	66.90%
Estimated fair value risk-free interest rate	2.06%	1.82%	1.09%
Estimated fair value expected dividend yield	0.00%	0.00%	0.00%
Restricted stock units awarded	294,000
Stock-based compensation expense	6,194,000	1,736,000	836,000
Dividends declared	$0
Dividends paid	0
Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future issuance under plan	528,070						293,979
Common stock shares authorized	593,979
Common stock purchased at discount through payroll deductions	15.00%
Shares purchased lower of the fair market value	85.00%
Employee stock purchase plan, offering period	6 months
Common stock issued to employees under ESPP	65,909	0
Estimated fair value expected term	6 months
Estimated fair value expected dividend yield	0.00%
Unvested Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	15,900,000
Estimated weighted-average period	3 years 1 month 28 days
Maximum [Member] | Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value weighted-average volatility rate	115.40%
Estimated fair value risk-free interest rate	0.08%
Minimum [Member] | Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated fair value weighted-average volatility rate	65.70%
Estimated fair value risk-free interest rate	0.05%
Subsequent Event [Member] | Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future issuance under plan								298,475
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period				3 years
Weighted-average grant-date fair value, granted	$31.03
Restricted stock units awarded				293,980
Restricted stock units vesting date	31-Mar-17
Restricted stock units vesting, percentage	25.00%
Amortization of stock based compensation expense on straight-line basis	3 years
Stock-based compensation expense	2,300,000
Unrecognized compensation expense	$6,800,000
Estimated weighted-average period	2 years 3 months
2004 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period					5 years
Stock options vesting description	Stock options granted generally vest over a period of five years from the date of grant, with 20% of the total grant vesting on the first anniversary of the option vesting commencement date and 1/48 of the remaining grant vesting each month thereafter.
Stock options vesting original months					48 months
Common stock available for future issuance under plan							1,175,918
Common stock subject to options outstanding	2,078,526
2004 Stock Incentive Plan [Member] | First Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting, percentage					20.00%
2013 Equity Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period						4 years
Stock options vesting description	Stock options granted generally vest over a period of four years from the date of grant, with 25% of the total grant vesting on the first anniversary of the option vesting commencement date and 1/48th of the remaining grant vesting each month thereafter.
Stock options vesting original months						48 months
Common stock available for future issuance under plan						500,000
Exercise price of option, percentage						100.00%
2013 Equity Incentive Award Plan [Member] | Greater than 10% Stockholder [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period						5 years
Exercise price of option, percentage						110.00%
Percentage of incentive stock option granted						10.00%
2013 Equity Incentive Award Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period						10 years
2013 Equity Incentive Award Plan [Member] | Stock Plan, Annual Increase of Shares 2014 to 2023 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in common stock						1,500,000
Percentage of common stock outstanding as-converted basis						4.00%
2013 Equity Incentive Award Plan [Member] | First Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting, percentage						25.00%
2004 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future issuance under plan						90,125
2013 Equity Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized	1,768,645
Common stock subject to options outstanding	1,743,484
2013 Equity Incentive Award Plan [Member] | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future issuance under plan								1,193,903

Stock_Incentive_Plans_Summary_

Stock Incentive Plans - Summary of Activity under 2004 Plan and 2013 Stock Plan (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares Available for Grant, Beginning Balance	117	211	207
Options Outstanding,Options authorized	0
Weighted Average Exercise Price per Share, Options authorized	$0	$0
Shares Available for Grant, Options authorized	1,176	500
Shares Available for Grant, Options granted	-1,027	-597	-129
Shares Available for Grant of Options and Awards, RSUs granted	-294
Shares Available for Grant, Options exercised
Shares Available for Grant, Options forfeited	47	3	133
Shares Available for Grant, Ending Balance	19	117	211
Options Outstanding, Beginning Balance	2,930	2,449	2,529
Options Outstanding, Options granted	1,027	597	129
Options and Awards Outstanding, RSUs granted	294
Options Outstanding, Options exercised	-382	-113	-76
Options Outstanding, Options forfeited	-47	-3	-133
Options Outstanding, Ending Balance	3,822	2,930	2,449
Weighted Average Exercise Price per Share, Beginning Balance	$5.90	$3.48	$2.91
Options Outstanding, Vested and Expected to vest - Ending Balance	3,773
Options and Awards Outstanding, Vested	2,011
Weighted Average Exercise Price per Share, Options granted	$22.50	$15.10	$8.41
Weighted Average Exercise Price per Share, RSUs granted	$0
Weighted Average Exercise Price per Share, Options exercised	$2.83	$2.19	$2.06
Weighted Average Exercise Price per Share, Options forfeited	$22.28	$6.75	$2.29
Weighted Average Exercise Price per Share, Ending Balance	$10.84	$5.90	$3.48
Weighted Average Exercise Price per Share, Vested and Expected to vest - Ending Balance	$10.72
Weighted Average Exercise Price per Share, Vested	$4.66
Weighted Average Remaining Contractual Life, Outstanding	6 years 3 months 15 days
Weighted Average Remaining Contractual Life, Vested and Expected to vest	6 years 3 months
Weighted Average Remaining Contractual Life, Vested	4 years 7 months 24 days
Aggregate Intrinsic Value, Ending Balance	$46,039
Aggregate Intrinsic Value, Vested ans Expected to vest - Ending Balance	45,904
Aggregate Intrinsic Value,Vested	$34,392

Stock_Incentive_Plans_Schedule

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$6,194	$1,736	$836
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	3,600	957	497
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$2,594	$779	$339

Stock_Incentive_Plans_Assumpti

Stock Incentive Plans - Assumptions Used for Determining Fair Value of Stock Options Using Black-Scholes Valuation Model (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Weighted-average volatility	66.30%	68.30%	66.90%
Weighted-average expected term (years)	6 years 2 months 12 days	6 years 2 months 12 days	6 years 2 months 12 days
Risk-free interest rate	2.06%	1.82%	1.09%
Expected dividend yield	0.00%	0.00%	0.00%

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Income tax (provision) benefit	$509,000	($1,944,000)	$0
Increase in valuation allowance	25,800,000	11,400,000
Net deferred tax assets		10,300,000
Net operating loss carryforward	116,500,000
Impact on income tax provision of unrecognized tax benefits, if recognized	4,600,000	3,700,000
Accrued interest and penalties related to unrecognized tax benefits	37,000,000
Liability recognized for interest and penalties	37,000	0
California [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration year	2017
Estimated expire amount of operating loss carryforwards	700,000
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	69,000,000
Operating loss carryforwards, expiration year	2023
Estimated expire amount of operating loss carryforwards	700,000
Federal [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Aggregate tax credit carryforwards	13,100,000
Tax credit carryforwards, expiration year	2024
State [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Aggregate tax credit carryforwards	$12,000,000

Income_Taxes_Components_of_Pro

Income Taxes - Components of Provision for Income Taxes (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current
Federal	($10,841)	$12,275
State	1	1
Total	-10,840	12,276
Deferred
Federal	10,331	-10,331
State	0	0	0
Total	10,331	-10,331
Income tax provision (benefit)	($509)	$1,944	$0

Income_Taxes_Schedule_of_Recon

Income Taxes - Schedule of Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax at statutory federal rate	35.00%	35.00%	34.00%
State tax-net of federal benefit	0.00%	0.00%	0.00%
Research and development credit	12.00%	6.00%
Change in valuation allowance	-45.00%	-54.00%	-34.00%
Federal tax rate differential		6.00%
Other	-1.00%	-1.00%
Income tax (provision) benefit	1.00%	-8.00%

Income_Taxes_Schedule_of_Net_D

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred tax assets:
Net operating loss carryforwards	$28,414	$16,678
Accruals	911	853
Research and development credits	19,249	4,694
Deferred revenue	52,054	64,270
Other	2,378	929
Gross deferred tax assets	103,006	87,424
Deferred tax liability	-143
Valuation allowance	-102,863	-77,093
Net deferred tax assets		$10,331

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Balance at beginning of year	$4,464	$3,404	$2,996
Increase related to current year tax provision	1,539	708	338
Increase related to prior year tax provision		354	70
Decrease related to current year tax provision		-2
Decrease related to prior year tax provision	-569
Balance at end of year	$5,434	$4,464	$3,404

Net_Loss_per_Common_Share_Outs

Net Loss per Common Share - Outstanding Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Common Share (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of common stock equivalents	3,822,012	2,930,381	23,677,580
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of common stock equivalents	3,528,032	2,930,381	2,449,441
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of common stock equivalents	293,980
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of common stock equivalents	0		21,180,280
Convertible Preferred Stock [Member] | Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of common stock equivalents	0		47,859

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data - Summary of Selected Quarterly Results from Operations (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Total revenue	$8,514	$19,015	$6,015	$6,015	$18,983	$12,932	$2,932	$2,932	$39,559	$37,779	$24,681
Operating expenses	24,139	24,515	21,607	19,922	-21,389	-16,301	-12,427	-11,561	-90,183	-61,678	-47,050
Net loss	($15,056)	($5,486)	($15,597)	($13,871)	($4,343)	($3,486)	($9,644)	($8,598)	($50,010)	($26,071)	($22,235)
Basic and diluted net loss per common share	($0.50)	($0.18)	($0.53)	($0.47)	($0.15)	($0.15)	($8.83)	($7.92)	($1.69)	($1.93)	($21.30)

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (Subsequent Event [Member], GSK [Member], USD $)	Jan. 15, 2015
In Millions, unless otherwise specified
Subsequent Event [Member] | GSK [Member]
Subsequent Event [Line Items]
Milestone payments from collaboration partner	$5