Xencor (XNCR) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Document and Entity Information
Entity Registrant Name	Xencor Inc
Entity Central Index Key	1,326,732
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,046,791,343
Entity Common Stock, Shares Outstanding		56,292,169
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 26,246	$ 16,528
Marketable securities	268,115	207,603
Accounts receivable	10,187	1,142
Income tax receivable	804
Prepaid expenses and other current assets	10,375	5,606
Total current assets	315,727	230,879
Property and equipment, net	11,813	7,088
Patents, licenses, and other intangible assets, net	11,969	11,148
Marketable securities - long term	236,108	139,198
Income tax receivable	804	1,524
Loan receivable		86
Interest receivable		14
Other assets	311	265
Total assets	576,732	390,202
Current liabilities
Accounts payable	3,797	6,869
Accrued expenses	9,662	5,480
Current portion of deferred rent	315	26
Current portion of deferred revenue	40,079	60,118
Income taxes payable		157
Total current liabilities	53,853	72,650
Deferred rent, less current portion	1,198	1,088
Total liabilities	55,051	73,738
Commitments and contingencies (see note 8)
Stockholders' equity
Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at December 31, 2018 and 2017
Common stock, $0.01 par value: 200,000,000 authorized shares; 56,279,542 issued and outstanding shares at December 31, 2018 and 47,002,488 issued and outstanding at December 31, 2017	563	470
Additional paid-in capital	845,366	570,670
Accumulated other comprehensive loss	(971)	(1,808)
Accumulated deficit	(323,277)	(252,868)
Total stockholders' equity	521,681	316,464
Total liabilities and stockholders' equity	$ 576,732	$ 390,202

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Condensed Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	56,279,542	47,002,488
Common stock, shares outstanding	56,279,542	47,002,488

Statements of Comprehensive Inc

Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaborations, licenses and milestones	$ 11,564	$ 29,039			$ 30,150		$ 12,500	$ 3,500	$ 40,603	$ 46,150	$ 109,020
Operating expenses
Research and development									97,501	71,772	51,872
General and administrative									22,472	17,501	13,108
Total operating expenses									119,973	89,273	64,980
Income (loss) from operations	(21,082)	651	$ (28,290)	$ (30,649)	5,326	$ (23,580)	(8,510)	(16,359)	(79,370)	(43,123)	44,040
Other income (expense)
Interest income									9,102	4,194	2,091
Interest expense									(16)	(13)	(21)
Other income (expense)									(125)	(7)	6
Total other income, net									8,961	4,174	2,076
Income (loss) before income tax									(70,409)	(38,949)	46,116
Income tax expense (benefit)										(463)	991
Net income (loss)	$ (18,197)	$ 3,150	$ (25,869)	$ (29,493)	$ 7,366	$ (22,652)	$ (7,725)	$ (15,475)	(70,409)	(38,486)	45,125
Other comprehensive income (loss)
Net unrealized gain (loss) on marketable securities available-for-sale									837	(367)	(925)
Comprehensive income (loss)									$ (69,572)	$ (38,853)	$ 44,200
Net income (loss) per share attributable to common stockholders:
Basic net income (loss) (in dollars per share)	$ (0.32)	$ 0.06	$ (0.46)	$ (0.62)	$ 0.16	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.09
Diluted net income (loss) (in dollars per share)	$ (0.32)	$ 0.05	$ (0.46)	$ (0.62)	$ 0.15	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.07
Weighted average shares used to compute net income (loss) per share attributable to common stockholders:
Basic (in shares)									53,942,116	46,817,756	41,267,329
Diluted (in shares)									53,942,116	46,817,756	42,388,867

Statement of Stockholders' Equi

Statement of Stockholders' Equity - USD ($) $ in Thousands	Scenario, Previously ReportedCommon Stock	Scenario, Previously ReportedAdditional Paid-in Capital	Scenario, Previously ReportedAccumulated Other Comprehensive Loss	Scenario, Previously ReportedAccumulated Deficit	Scenario, Previously Reported	Restatement AdjustmentAdditional Paid-in Capital	Restatement AdjustmentAccumulated Deficit	Restatement Adjustment	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle | ASU 2014-09							$ 2,479	$ 2,479
Balance, revised									$ 405	$ 424,128	$ (516)	$ (259,106)	$ 164,911
Balance at Dec. 31, 2015	$ 405	$ 424,128	$ (516)	$ (261,585)	$ 162,432
Balance (in shares) at Dec. 31, 2015	40,551,039								40,551,039
Increase (Decrease) in Stockholders' Equity
Sale of common stock, net of issuance cost									$ 53	119,216			119,269
Sale of common stock, net of issuance cost (in shares)									5,272,750
Issuance of common stock upon exercise of stock awards									$ 7	1,153			1,160
Issuance of common stock upon exercise of stock awards (in shares)									699,066
Issuance of common stock under the Employee Stock Purchase Plan									$ 1	544			545
Issuance of common stock under the Employee Stock Purchase Plan (in shares)									45,123
Comprehensive loss											(925)	45,125	44,200
Stock-based compensation expense										7,848			7,848
Balance at Dec. 31, 2016	$ 466	$ 552,889	$ (1,441)	$ (213,981)	$ 337,933				$ 466	552,889	(1,441)	(213,981)	337,933
Balance (in shares) at Dec. 31, 2016	46,567,978								46,567,978
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle | ASU 2016-09						$ 401	$ (401)
Balance, revised									$ 466	553,290	(1,441)	(214,382)	337,933
Issuance of common stock upon exercise of stock awards									$ 4	2,793			2,797
Issuance of common stock upon exercise of stock awards (in shares)									363,603
Issuance of common stock under the Employee Stock Purchase Plan										936			936
Issuance of common stock under the Employee Stock Purchase Plan (in shares)									70,907
Comprehensive loss											(367)	(38,486)	(38,853)
Stock-based compensation expense										13,651			13,651
Balance at Dec. 31, 2017									$ 470	570,670	(1,808)	(252,868)	316,464
Balance (in shares) at Dec. 31, 2017									47,002,488
Increase (Decrease) in Stockholders' Equity
Balance, revised									$ 470	570,670	(1,808)	(252,868)	316,464
Sale of common stock, net of issuance cost									$ 84	245,420			245,504
Sale of common stock, net of issuance cost (in shares)									8,395,000
Issuance of common stock upon exercise of stock awards									$ 8	7,609			7,617
Issuance of common stock upon exercise of stock awards (in shares)									824,731
Issuance of common stock under the Employee Stock Purchase Plan									$ 1	1,119			1,120
Issuance of common stock under the Employee Stock Purchase Plan (in shares)									57,323
Comprehensive loss											837	(70,409)	(69,572)
Stock-based compensation expense										20,548			20,548
Balance at Dec. 31, 2018									$ 563	$ 845,366	$ (971)	$ (323,277)	$ 521,681
Balance (in shares) at Dec. 31, 2018									56,279,542

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income (loss)	$ (70,409)	$ (38,486)	$ 45,125
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	3,251	2,030	1,466
Amortization of premium on marketable securities	(394)	2,845	2,037
Stock-based compensation	20,548	13,651	7,848
Abandonment of capitalized intangible assets	239	396	356
Loss on disposal of assets	102	83
Loss (gain) on sale of marketable securities available-for-sale	74		(5)
Changes in operating assets and liabilities:
Accounts receivable	(9,045)	7,474	(8,572)
Interest receivable	(535)	(307)	(609)
Prepaid expenses and other assets	(4,769)	(2,705)	(1,700)
Income tax receivable	(84)	(1,524)
Other assets	(46)	(161)	(40)
Accounts payable	(3,072)	2,989	(2,520)
Accrued expenses	4,182	(1,212)	3,058
Deferred rent	398	589	(89)
Income taxes payable	(157)	91	65
Deferred revenue	(20,039)	(19,350)	48,818
Net cash provided by (used in) operating activities	(79,756)	(33,597)	95,238
Cash flows from investing activities
Proceeds from sale and maturities of marketable securities available-for-sale	222,125	115,757	105,505
Proceeds from sale of property and equipment	9
Purchase of marketable securities	(377,840)	(76,529)	(316,149)
Purchase of intangible assets	(1,935)	(1,967)	(1,502)
Purchase of property and equipment	(7,212)	(5,311)	(1,507)
Proceeds from repayment of (investment in) loan receivable	86	(86)	(621)
Net cash provided by (used in) investing activities	(164,767)	31,864	(214,274)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	7,617	2,797	1,160
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	1,120	936	545
Proceeds from issuance of common stock	260,245		126,546
Common stock issuance costs	(14,741)		(7,277)
Net cash provided by financing activities	254,241	3,733	120,974
Net increase in cash and cash equivalents	9,718	2,000	1,938
Cash and cash equivalents, beginning of period	16,528	14,528	12,590
Cash and cash equivalents, end of period	26,246	16,528	14,528
Cash paid during the period for:
Interest	16	13	21
Taxes	233	969	936
Supplemental disclosures of non-cash investing activities
Net unrealized gain (loss) on marketable securities available-for-sale	$ 837	$ (367)	$ (925)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	1. Summary of Significant Accounting Policies Description of Business Xencor, Inc. (we, us, our, or the Company) was incorporated in California in 1997 and reincorporated in Delaware in September 2004. We are a clinical‑stage biopharmaceutical company focused on discovering and developing engineered monoclonal antibody and proteins to treat severe and life‑threatening diseases with unmet medical needs. We use our proprietary XmAb technology platform to create next‑generation antibody product candidates designed to treat cancer, autoimmune and allergic diseases, and other conditions. We focus on the portion of the antibody that interacts with multiple segments of the immune system, referred to as the Fc domain, which is constant and interchangeable among antibodies. Our engineered Fc domains, the XmAb technology, are applied to our pipeline of antibody and protein‑based drug candidates to increase immune inhibition, improve cytotoxicity, extend half‑life and most recently create bispecific Fc domain antibody and protein molecules. Our operations are based in Monrovia and San Diego, California. Basis of Presentation The Company’s financial statements as of December 31, 2018, 2017, and 2016 and for the years then ended have been prepared in accordance with accounting principles generally accepted in the United States (U.S.). Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates include useful lives of long-lived assets, the periods over which certain revenues and expenses will be recognized including collaboration revenue recognized from non-refundable upfront licensing payments, the amount of non-cash compensation costs related to share-based payments to employees and non-employees and the period over which these costs are expensed. Recent Accounting Pronouncements Pronouncements adopted in 2018 Effective January 1, 2018, the Company adopted Accounting Standards Codification Topic 606 (ASC 606), Revenue from Contracts with Customers, using the full retrospective transition method. Under this method, the Company is presenting its financial statements for the years ended December 31, 2017 and 2016 as if ASC 606 had been effective for those periods. Under ASC 606 an entity recognizes revenue when its customer obtains control of promised goods or services in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. A five-step model is used to achieve the core principle: (1) identify the customer contract, (2) identify the contract’s performance obligations, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations and (5) recognize revenue when or as a performance obligation is satisfied. The Company applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. The new guidance provides that revenue recognition for performance obligations related to delivery of certain goods or services occurs when control over the good or service is transferred to the customer. In addition, the timing of revenue recognition from licensing of our intellectual property that are functional and are distinct performance obligations changed from being recognized over the term of access to our license or technology to being recognized at a point in time. See Note 12 “Prior Period Financial Statements” for a complete discussion of the impact of adopting the new standard. Effective January 1, 2018, the Company adopted ASU 2016-01, Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Liabilities, which eliminates the available-for-sale classification for equity securities and requires equity securities to be measured at fair value with changes in the fair value recognized through net income. This ASU eliminates the available-for-sale classification for equity investments that recognized changes in the fair value as a component of other comprehensive income. The adoption had no effect on the Company’s financial statements. Effective January 1 2018, the Company adopted ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which addresses eight specific cash flow issues with the objective of reducing the existing diversity in practice. The standard clarifies when cash receipts and cash payments have aspects of more than one class of cash flows and cannot be separated. Classification will depend on the predominant source or use. The adoption did not have an effect on the Company’s statements of cash flow. Effective January 1, 2018, the Company adopted ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) . The standard applies when a company changes the terms of a stock compensation award previously granted to an employee where modification accounting applies. According to the standard, modification accounting is not required if (1) the fair value of the modified award (or the award’s calculated value or intrinsic value as appropriate) is the same as the value immediately prior to its modification, (2) the vesting conditions of the modified award are the same as the vesting conditions of the award immediately prior to its modification; and (3) the award’s classification as an equity or liability is the same after the modification as it was immediately prior to its modification. The Company did not have any modifications upon adopting the new standard; therefore, adoption had no effect on the Company’s financial statements. Pronouncements not yet effective In February 2016, the FASB issued ASU No. 2016-02 Leases (Topic 842) . The new guidance requires lessees to recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term for all leases not considered short term. The new standard will be effective for reporting periods beginning after December 15, 2018. In July 2018, the FASB issued ASU No. 2018-10 Codification Improvements to Topic 842, Leases and ASU No. 2018-11, Lease (Topic 842): Targeted Improvements , which provides narrow aspects of the guidance issued in ASU No. 2016-02 as well as an alternative transition method of adoption, permitting the recognition of cumulative-effect adjustment to retained earnings on the date of adoption. The new standard also provides a number of optional practical expedients in transition, including the “package of practical expedients,” which permits us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct cost. We will adopt the new standard on January 1, 2019 using the cumulative effect adjustment method, with the election of the package of practical expedients and hindsight practical expedient. We have completed a preliminary assessment of the new standard’s impact, which includes a total of five operating leases for facilities in Monrovia and San Diego. Under Topic 842, deferred rent liability under operating lease is no longer tracked separately; instead, it will integrate as part of the right-of-use (ROU) asset. As a result, we expect to adjust the cumulative effect to the beginning balance for deferred rent liability, and adopt the use of right-of-use asset and lease liability. We do not expect that this standard will have a material impact on our financial statements. Upon adoption, we estimate that we will have additional liabilities ranging from $10 million to $12 million with a corresponding ROU assets of a similar amount for lease agreements in effect as of December 31, 2018. This will result in an estimated increase to the beginning balance on both assets and liabilities after the adjustment of between $10 million and $12 million, with no impact on our retained earnings. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which amends the guidance on reporting credit losses for assets held at amortized cost basis and available-for-sale debt securities. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses rather than as a direct write-down to the security. Credit losses on available-for-sale securities will be required when the amortized cost is below the fair market value. The amendment is effective for fiscal years beginning after December 15, 2019 including interim periods within those fiscal years. In November 2018, the FASB issued ASU No. 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses , which clarifies that receivables arising from operating leases are not within the scope of Topic 326. We will apply the standard’s provision as a cumulative effect adjustment to retained earnings as of the beginning of the first effective reporting period. We do not expect the adoption to have a material impact on our results of operations or financial position. In March 2017, the FASB issued ASU No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities, which amends the guidance on the amortization period of premiums on certain purchased callable debt securities by shortening the amortization period of premiums to the earliest call date. The amendment affects all entities that hold investments in callable debt securities that have an amortized cost basis in excess of the amount that is repayable by the issuer at the earliest call date. The amendment is effective for fiscal years after December 31, 2018 with early adoption permitted. The Company will review the requirements of the standard but does not anticipate it will have a significant impact on our financial statements. In February 2018, the FASB issued ASU No. 2018-02. Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income , an amendment which permits companies to reclassify the income tax effects of the 2017 Tax Cuts and Jobs Act (TCJA) on items within accumulated other comprehensive income to retained earnings. The standard also requires new disclosures about these stranded tax effects and is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted and can be applied either in the period of adoption or retrospectively to each period (or periods) in which the effect of the change in the U.S. federal corporate income tax rate in the TCJA is recognized. The Company is currently evaluating the impact the guidance will have on its financial statements. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting , which expands the scope of Topic 718 to include share-based payments issued to nonemployees for goods and services. The standard is effective for fiscal years beginning after December 15, 2018 and interim periods within such fiscal year. The Company does not anticipate that the standard will have a significant impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement , which modifies the disclosures for transfers between Level 1 and Level 2 of the fair value hierarchy, modifies the level 3 disclosure requirements for non-public entities and requires additional disclosure for Level 3 fair value hierarchy. The amendment is effective for fiscal years beginning after December 15, 2019. The Company does not anticipate that the standard will have a significant impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract , which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendment. The amendment is effective for annual periods, including interim periods within those annual periods, beginning after December 15, 2019. The Company does not anticipate that the standard will have a significant impact on its financial statements. In October 2018, the FASB issued ASU No. 2018-17, Consolidation (Topic 810): Targeted Improvements to Related Party Guidance for Variable Interest Entities, which amends the guidance for determining whether a decision-making fee is a variable interest. The amendments require organizations to consider indirect interests held through related parties under common control on a proportional basis rather than as the equivalent of a direct interest in its entirety (as currently required). The Company does not anticipate that the standard will have a significant impact on tis financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606, which provides guidance on how to assess whether certain transactions between collaborative arrangement participants should be accounted for within the revenue recognition standard. The standard is effective for fiscal years beginning after December 15, 2019 and interim period within those years. The Company does not anticipate that the standard will have a significant impact on its financial statements. Revenue Recognition We have, to date, earned revenue from research and development collaborations, which may include research and development services, licenses of our internally-developed technologies, licenses of our internally-developed drug candidates, or combinations of these. The terms of our license and research and development and collaboration agreements generally include non-refundable upfront payments, research funding, co-development reimbursements, license fees and, milestone and other contingent payments to us for the achievement of defined collaboration objectives and certain clinical, regulatory and sales‑based events, as well as royalties on sales of any commercialized products. The terms of our licensing agreements include non‑refundable upfront fees, annual licensing fees, and contractual payment obligations for the achievement of pre‑defined preclinical, clinical, regulatory and sales‑based events by our partners. The licensing agreements also include royalties on sales of any commercialized products by our partners. We recognize revenue through the five-step process in accordance with ASC 606 Revenue Recognition when control of the promised goods or services is transferred to our customers in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. Deferred Revenue Deferred revenue arises from payments received in advance of the culmination of the earnings process. We have classified deferred revenue for which we stand ready to perform within the next 12 months as a current liability. We recognize deferred revenue as revenue in future periods when the applicable revenue recognition criteria have been met. The total amounts reported as deferred revenue were $40.1 million and $60.1 million at December 31, 2018 and 2017, respectively. Research and Development Expenses Research and development expenses include costs we incur for our own and for our collaborators’ research and development activities. Research and development costs are expensed as incurred. These costs consist primarily of salaries and benefits, including associated stock‑based compensation, laboratory supplies, facility costs, and applicable overhead expenses of personnel directly involved in the research and development of new technology and products, as well as fees paid to other entities that conduct certain research development activities on our behalf. We estimate preclinical study and clinical trial expenses based on the services performed pursuant to the contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on our behalf based on the actual time and expenses incurred by them. Further, we accrue expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. We capitalize acquired research and development technology licenses and third‑party contract rights and amortize the costs over the shorter of the license term or the expected useful life. We review the license arrangements and the amortization period on a regular basis and adjust the carrying value or the amortization period of the licensed rights if there is evidence of a change in the carrying value or useful life of the asset. Cash and Cash Equivalents We consider cash equivalents to be only those investments which are highly liquid, readily convertible to cash and which mature within three months from the date of purchase. Marketable Securities The Company has an investment policy that includes guidelines on acceptable investment securities, minimum credit quality, maturity parameters and concentration and diversification. The Company invests its excess cash primarily in marketable debt securities issued by investment grade institutions. The Company considers its marketable debt securities to be “available-for-sale”, as defined by authoritative guidance issued by the FASB. These assets are carried at fair value and the unrealized gains and losses are included in accumulated other comprehensive income (loss). Accrued interest on marketable debt securities is included in marketable securities. Accrued interest was $2.3 million and $1.7 million at December 31, 2018 and 2017, respectively. If a decline in the value of a marketable security in the Company’s investment portfolio is deemed to be other-than-temporary, the Company writes down the security to its current fair value and recognizes a loss as a charge against income. The Company reviews its portfolio of marketable debt securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost are other-than-temporary. Concentrations of Risk Cash, cash equivalents and marketable debt securities are financial instruments that potentially subject the Company to concentrations of risk. We invest our cash in corporate debt securities and U.S. sponsored agencies with strong credit ratings. We have established guidelines relative to diversification and maturities that are designed to help ensure safety and liquidity. These guidelines are periodically reviewed to take advantage of trends in yields and interest rates. Cash and cash equivalents are maintained at financial institutions and, at times, balances may exceed federally insured limits. We have never experienced any losses related to these balances. Amounts on deposit in excess of federally insured limits at December 31, 2018 and 2017 approximated $26.0 million and $16.3 million, respectively. We have payables with four service providers that represent 49% of our total payables and two service providers that represented 40% of our total payables at December 31, 2018 and 2017, respectively. We rely on three critical suppliers for the manufacture of our drug product for use in our clinical trials. While we believe that there are alternative vendors available, a change in manufacturing vendors could cause a delay in the availability of drug product and result in a delay of conducting and completing our clinical trials. No other vendor accounted for more than 10% of total payables at December 31, 2018 or 2017. Fair Value of Financial Instruments Our financial instruments primarily consist of cash and cash equivalents, marketable debt securities, accounts receivable, accounts payable and accrued expenses. Marketable debt securities and cash equivalents are carried at fair value. The fair value of the other financial instruments closely approximate their fair value due to their short maturities. The Company accounts for recurring and non-recurring fair value measurements in accordance with FASB Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures (ASC 820). ASC 820 defines fair value, establishes a fair value hierarchy for assets and liabilities measured at fair value, and requires expanded disclosure about fair value measurements. The ASC 820 hierarchy ranks the quality of reliable inputs, or assumptions, used in the determination of fair value and requires assets and liabilities carried at fair value to be classified and disclosed in one of the following three categories: Level 1— Fair Value is determined by using unadjusted quoted prices that are available in active markets for identical assets or liabilities. Level 2— Fair Value is determined by using inputs other than Level 1 quoted prices that are directly or indirectly observable. Inputs can include quoted prices for similar assets or liabilities in active markets or quoted prices for identical assets or liabilities in markets that are not active. Related inputs can also include those used in valuation or other pricing models, such as interest rates and yield curves that can be corroborated by observable market data. Level 3— Fair value is determined by inputs that are unobservable and not corroborated by market data. Use of these inputs involves significant and subjective judgments to be made by the reporting entity –e.g. determining an appropriate discount factor for illiquidity associated with a given security. The Company measures the fair value of financial assets using the highest level of inputs that are reasonably available as of the measurement date. The assets recorded at fair value are classified within the hierarchy as follows for the periods reported (in thousands): December 31, 2018 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 18,270 $ 18,270 $ — Corporate Securities 104,967 — 104,967 Government Securities 399,256 — 399,256 $ 522,493 $ 18,270 $ 504,223 December 31, 2017 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 5,175 $ 5,175 $ — Corporate Securities 123,270 — 123,270 Government Securities 223,530 — 223,530 $ 351,975 $ 5,175 $ 346,800 Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the assets. Expenditures for repairs and maintenance are charged to expense as incurred while renewals and improvements are capitalized. Useful lives by asset category are as follows: Computers, software and equipment 3 - 5 years Furniture and fixtures 5 - 7 years Leasehold improvements 5 - 7 years or remaining lease term, whichever is less Patents, Licenses, and Other Intangible Assets The cost of acquiring licenses is capitalized and amortized on the straight‑line basis over the shorter of the term of the license or its estimated economic life, ranging from five to 25 years. Third‑party costs incurred for acquiring patents are capitalized. Capitalized costs are accumulated until the earlier of the period that a patent is issued or we abandon the patent claims. Cumulative capitalized patent costs are amortized on a straight‑line basis from the date of issuance over the shorter of the patent term or the estimated useful economic life of the patent, ranging from 13 to 20 years. Our senior management, with advice from outside patent counsel, assesses three primary criteria to determine if a patent will be capitalized initially: i) technical feasibility, ii) magnitude and scope of new technical function covered by the patent compared to the company’s existing technology and patent portfolio, particularly assessing the value added to our product candidates or licensing business, and iii) legal issues, primarily assessment of patentability and prosecution cost. We review our intellectual property on a regular basis to determine if there are changes in the estimated useful life of issued patents and if any capitalized costs for unissued patents should be abandoned. Capitalized patent costs related to abandoned patent filings are charged off in the period of the decision to abandon. During 2018, 2017 and 2016, we abandoned previously capitalized patent and licensing related charges of $0.2 million, $0.4 million and $0.4 million, respectively. The carrying amount and accumulated amortization of patents, licenses, and other intangibles is as follows (in thousands): December 31, 2018 2017 Patents, definite life $ 9,320 $ 8,915 Patents, pending issuance 5,644 4,360 Licenses and other amortizable intangible assets 2,011 2,011 Nonamortizable intangible assets (trademarks) 399 399 Total gross carrying amount 17,374 15,685 Accumulated amortization—patents (4,142) (3,413) Accumulated amortization—licenses and other (1,263) (1,124) Total intangible assets, net $ 11,969 $ 11,148 Amortization expense for patents, licenses, and other intangible assets was $0.9 million, $0.8 million, and $0.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. Future amortization expense for patent, licenses, and other intangible assets recorded as of December 31, 2018, and for which amortization has commenced, is as follows: Year ended December 31, (in thousands) 2019 $ 862 2020 860 2021 757 2022 730 2023 659 Thereafter 1,923 Total $ 5,791 The above amortization expense forecast is an estimate. Actual amounts of amortization expense may differ from estimated amounts due to additional intangible asset acquisitions, impairment of intangible assets, accelerated amortization of intangible assets, and other events. As of December 31, 2018, the Company has $5.6 million of intangible assets which are in‑process and have not been placed in service and, accordingly amortization on these assets has not commenced. Long‑Lived Assets Management reviews long‑lived assets which include fixed assets and amortizable intangibles for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the fair value of the assets. We did not recognize a loss from impairment for the years ended December 31, 2018, 2017 or 2016. Income Taxes We account for income taxes in accordance with accounting guidance which requires an asset and liability approach to financial accounting and reporting for income taxes. Deferred income tax assets and liabilities are computed annually for differences between the financial statement and tax bases of assets and liabilities that will result in taxable or deductible amounts in the future based on enacted tax laws and rates applicable to the periods in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Income tax expense is the tax payable or refundable for the period plus or minus the change during the period in deferred tax assets and liabilities. We assess our income tax positions and record tax benefits for all years subject to examination based upon our evaluation of the facts, circumstances and information available at the reporting date. For those tax positions where there is greater than 50% likelihood that a tax benefit will be sustained, we have recorded the largest amount of tax benefit that may potentially be realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. For those income tax positions where there is a 50% or less likelihood that a tax benefit will be sustained, no tax benefit has been recognized in the financial statements. We did not have any material uncertain tax positions at December 31, 2018 or 2017. Our policy is to recognize interest and penalties on taxes, if any, as a component of income tax expense. The Tax Cuts and Jobs Act (tax reform) was enacted on December 22, 2017 and has several key provisions impacting accounting for and reporting of income taxes. The most significant provisions reduced the U.S. corporate statutory tax rate from 35% to 21%, eliminated the Alternative Minimum Tax (AMT) system, and made changes to the utilization and carryforward of net operating losses beginning on January 1, 2018. The tax reform provided for a refund of unused AMT carryforwards for years beginning after December 31, 2017. We recorded an income tax receivable as of December 31, 2018 of $1.6 million related to federal AMT carryforwards. Stock‑Based Compensation We recognize compensation expense using a fair‑value‑based method for costs related to all share‑based payments, including stock options and shares issued under our Employee Stock Purchase Plan (ESPP). Stock‑based compensation cost related to employees and directors is measured at the grant date, based on the fair‑value-based measurement of the award using the Black‑Scholes method, and is recognized as expense over the requisite service period on a straight‑line basis. We account for forfeitures when they occur. We recorded stock‑based compensation and expense for stock‑based awards to employees, directors and consultants of approximately $20.5 million, $13.7 million, and $7.8 million for the years ended December 31, 2018, 2017 and 2016 respectively. Included in the 2018, 2017, and 2016 balances for total compensation expense is $0.7 million, $0.5 million, and $0.4 million, respectively, relating to our ESPP. Options granted to individual service providers that are not employees or directors are accounted for at estimated fair value using the Black‑Scholes option‑pricing method and are subject to periodic re‑measurement over the period during which the services are rendered. Net Income (Loss) Per Share Basic net income (loss) per common share is computed by dividing the net income or loss by the weighted‑average number of common shares outstanding during the period. Potentially dilutive securities consisting of stock options for 2018 and 2017, and stock purchases under the Employee Stock Purchase Plan were not included in the diluted net loss per common shares calculation because the inclusion of such shares would have had an antidilutive effect as follows: Year Ended December 31, 2018 2017 2016 (in thousands) Options to purchase common stock 1,881 1,291 — Employee stock purchase plan shares 3 — — Total 1,884 1,291 — Year Ended December 31, 2018 2017 2016 (in thousands, except share and per share data) (As Revised) (As Revised) Basic Numerator: Net income (loss) attributable to common stockholders for basic net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted-average common shares outstanding 53,942,116 46,817,756 41,267,329 Basic net income (loss) per common share $ (1.31) $ (0.82) $ 1.09 Diluted Numerator: Net income (loss) attributable to common stockholders for diluted net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted average number of common shares outstanding used in computing basic net income (loss) per common share 53,942,116 46,817,756 41,267,329 Dilutive effect of employee stock options and ESPP — — 1,121,538 Weighted-average number of common shares outstanding used in computing diluted net income (loss) per common share 53,942,116 46,817,756 42,388,867 Diluted net income (loss)

Comprehensive Income (Loss)

Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2018
Comprehensive Income (Loss)
Comprehensive Income (Loss)	2. Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). For the years ended December 31, 2018 and 2017, the only component of other comprehensive loss is net unrealized losses on marketable debt securities. There were no material reclassifications out of accumulated other comprehensive loss during the year ended December 31, 2018.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2018
Marketable Securities
Marketable Securities	3. Marketable Securities The Company’s marketable debt securities held as of December 31, 2018 and 2017 are summarized below: December 31, 2018 Gross Gross Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Money Market Funds $ 18,270 $ — $ — $ 18,270 Corporate Securities 105,311 1 (345) 104,967 Government Securities 399,873 187 (804) 399,256 $ 523,454 $ 188 $ (1,149) $ 522,493 Reported as Cash and cash equivalents $ 18,270 Marketable securities 504,223 Total investments $ 522,493 December 31, 2017 Gross Gross Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Money Market Funds $ 5,175 $ — $ — $ 5,175 Corporate Securities 123,860 — (590) 123,270 Government Securities 224,739 — (1,209) 223,530 $ 353,774 $ — $ (1,799) $ 351,975 Reported as Cash and cash equivalents $ 5,175 Marketable securities 346,800 Total investments $ 351,975 The maturities of the Company’s marketable debt securities as of December 31, 2018 are as follows: Amortized Estimated Cost Fair Value (in thousands) Mature in one year or less $ 269,092 $ 268,115 Mature after one year through five years 236,092 236,108 $ 505,184 $ 504,223 The unrealized losses on available-for-sale investments and their related fair values as of December 31, 2018 and 2017 are as follows: December 31, 2018 Less than 12 months 12 months or greater Fair value Unrealized losses Fair value Unrealized gain (losses) (in thousands) Corporate Securities $ 84,770 $ (310) $ 20,198 $ (34) Government Securities 183,345 (667) 215,910 50 $ 268,115 $ (977) $ 236,108 $ 16 December 31, 2017 Less than 12 months 12 months or greater Fair value Unrealized losses Fair value Unrealized losses (in thousands) Corporate Securities $ 79,290 $ (137) $ 43,980 $ (453) Government Securities 128,313 (461) 95,217 (748) $ 207,603 $ (598) $ 139,197 $ (1,201) The unrealized losses from the listed securities are due to a change in the interest rate environment and not a change in the credit quality of the securities.

Sale of Additional Common Stock

Sale of Additional Common Stock	12 Months Ended
	Dec. 31, 2018
Sale of Additional Common Stock
Sale of Additional Common Stock	4. Sale of Additional Common Stock In March 2015, we completed the sale of 8,625,000 shares of common stock which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option pursuant to a follow-on offering. We received net proceeds of $115.2 million, after underwriting discounts, commissions and estimated offering expenses. In December 2016, we completed the sale of 5,272,750 shares of common stock which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option pursuant to a follow-on financing. We received net proceeds of $119.3 million after underwriting discounts, commissions and offering expenses. In March 2018, we completed the sale of 8,395,000 shares of commons stock which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option pursuant to a follow-on financing. We received net proceeds of $245.5 million, after underwriters discounts and offering expenses. On September 19, 2016, we entered into an Equity Distribution Agreement (the Distribution Agreement) with Piper Jaffray & Co (Piper Jaffray) pursuant to which we may sell from time to time, at our option, up to an aggregate of $40 million of common stock through Piper Jaffray as sales agent. The issuance and sale of these shares by Xencor under the Distribution Agreement will be pursuant to our shelf registration statement on Form S-3 (File No.333-213700) declared effective by the SEC on October 5, 2016. We are not obligated sell any shares of common stock under the Distribution Agreement, and to date we have not sold any shares under the Distribution Agreement.

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property and Equipment
Property and Equipment	5. Property and Equipment Property and equipment consist of the following: December 31, 2018 2017 (In thousands) Computers, software and equipment $ 16,292 $ 10,874 Furniture and fixtures 173 152 Leasehold and tenant improvements 4,774 4,010 21,239 15,036 Less accumulated depreciation and amortization (9,426) (7,948) $ 11,813 $ 7,088 Depreciation and amortization expense related to property and equipment in 2018, 2017 and 2016 was $2.4 million, $1.2 million and $0.7 million, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Taxes
Income Taxes	6. Inc ome Taxes Our effective tax rate differs from the statutory federal income tax rate, primarily as a result of the changes in valuation allowance. There was no provision for income taxes for the year ended December 31, 2018. For the year ended December 31, 2017, the provision for income tax is a benefit of $0.5 million. Current tax expense of $1.0 million for the year ended December 31, 2016 represents federal and state alternative minimum tax. A reconciliation of the federal statutory income tax to our effective income tax is as follows (in thousands): Year Ended December 31, 2018 2017 2016 (Revised) (Revised) Federal statutory income tax $ (14,795) $ (13,243) $ 15,680 State and local income taxes (4,767) (1,806) 2,818 Research and development credit (6,170) (5,554) (2,544) Stock based compensation 444 2,709 733 Effect of the 2017 Tax Cut and Jobs Act — 19,596 — Other 414 720 1,008 Net change in valuation allowance 24,874 (2,885) (16,695) Income tax provision (benefit) $ — $ (463) $ 1,000 The tax effect of temporary differences that give rise to a significant portion of the deferred tax assets and liabilities at December 31, 2018 and 2017 is presented below (in thousands): December 31, 2018 2017 (Revised) Deferred income tax assets Net operating loss carryforwards $ 49,889 $ 25,565 Research credits 23,151 16,642 Depreciation 207 437 Unrealized loss on securities 269 504 Accrued compensation 1,097 748 Deferred revenue 11,222 16,820 State taxes — (2) Gross deferred income tax assets 85,835 60,714 Valuation allowance (82,537) (57,663) Net deferred income tax assets 3,298 3,051 Deferred income tax liabilities Patent costs (3,142) (2,873) Licensing costs (125) (142) Capitalized legal costs (31) (36) Gross deferred income tax liabilities (3,298) (3,051) Net deferred income tax asset $ — $ — The Tax Cuts and Jobs Act (TCJA) was enacted in December 2017 and made substantial changes in the U.S. tax system. One of the changes was elimination of the AMT tax system for corporations and allowance of an income tax refund for AMT tax credit carryforwards as of December 31, 2017. We have reported an income tax receivable of $1.6 million as of December 31, 2018 and 2017 to reflect the U.S. AMT credit carryforwards we have available. Due to the uncertainty surrounding the realization of the benefits of our deferred tax assets in future tax periods, we have placed a valuation allowance against our deferred tax assets at December 31, 2018 and 2017. The Company recognizes valuation allowances to reduce deferred tax assets to the amount that is more likely than not to be realized. The Company’s net deferred income tax asset is not more likely than not to be realized due to the lack of sufficient sources of future taxable income and cumulative losses that have resulted over the years. During the year ended December 31, 2018, the valuation allowance increased by $24.9 million. Upon analysis, there were changes in ownership under Section 382 of the Internal Revenue Code and related state provisions as a result of our sale of preferred stock and sale of common stock during 2013. Section 382 limits the amount of net operating losses and tax credit forwards that may be available after a change in ownership. The Company has adjusted its net operating loss and tax credit carryforwards to reflect the impact of the section 382 limitations. The Company’s tax returns remain open to potential inspection for the years 2013 and onwards for federal purposes and 2012 and onwards for state purposes. As of December 31, 2018, we had cumulative net operating loss carryforwards for federal and state income tax purposes of $191.1 million and $138.9 million respectively, and available tax credit carryforwards of approximately $14.9 million for federal income tax purposes and $8.2 million for state income tax purposes, which can be carried forward to offset future taxable income, if any. The federal net operating loss carryforwards consists of $102.6 million of losses incurred prior to January 1, 2018 and which can be used to offset 100% of future taxable income and, $88.6 million of losses incurred after January 1, 2018 which can be used to offset up to 80% of taxable income in subsequent years. Our federal net operating loss carryforwards expire starting in 2026, state net operating losses expire starting in 2032, and federal tax credit carryforwards expire starting in 2019. Utilization of the net operating losses and tax credits are subject to a substantial annual limitation due to ownership changes which occurred. As a result of these changes, provisions in the Internal Revenue Code of 1986 under Section 382 and similar state provisions may result in the expiration of certain of our net operating losses and tax credits before we can use them.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Stock-Based Compensation
Stock-Based Compensation	7. Stock‑Based Compensation Our Board of Directors and the requisite stockholders previously approved the 2010 Equity Incentive Plan (the 2010 Plan). In October 2013, our Board of Directors approved the 2013 Equity Incentive Plan (the 2013 Plan) and in November 2013 our stockholders approved the 2013 Plan. The 2013 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, performance cash awards, and other stock awards. The 2013 Plan became effective as of December 3, 2013, the date of the Company’s initial public offering. As of December 2, 2013, we suspended the 2010 Plan and no additional awards may be granted under the 2010 Plan. Any shares of common stock covered by awards granted under the 2010 Plan that terminate after December 2, 2013 by expiration, forfeiture, cancellation or other means without the issuance of such shares will be added to the 2013 Plan reserve. As of December 31, 2018, the total number of shares of common stock available for issuance under the 2013 Plan was 9,581,833. Unless otherwise determined by the Board, beginning January 1, 2014, and continuing until the expiration of the 2013 Plan, the total number of shares of common stock available for issuance under the 2013 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of December 31 of the immediate preceding year. On January 1, 2018, the total number of shares of common stock available for issuance under the 2013 Plan was automatically increased by 1,880,100 shares, which number is included in the number of shares available for issuance above. As of December 31, 2018 a total of 6,751,287 options have been issued under the 2013 Plan. During the year ended December 31, 2018, the Company awarded 33,933 Restricted Stock Units (RSUs) to certain employees pursuant to the 2013 Plan. Vesting of these awards will be in three equal annual installments and is contingent on continued employment terms. The fair value of these awards is determined based on the intrinsic value of the stock on the date of grant and will be recognized as stock-based compensation expense over the requisite service period. In November 2013, our Board of Directors and stockholders approved the 2013 Employee Stock Purchase Plan (ESPP), which became effective as of December 5, 2013. Under the ESPP our employees may elect to have between 1-15% of their compensation withheld to purchase shares of the Company’s common stock at a discount. The ESPP had an initial two-year term that includes four six-month purchase periods and employee withholding amounts may be used to purchase Company stock during each six-month purchase period. The initial two-year term ended in December 2015 and pursuant to the provisions of the ESPP, the second two-year term began automatically upon the end of the initial term. The total number of shares that can be purchased with the withholding amounts are based on the lower of 85% of the Company’s common stock price at the initial offering date or 85% of the Company’s stock price at each purchase date. We have reserved a total of 581,286 shares of common stock for issuance under the ESPP. Unless otherwise determined by our Board, beginning on January 1, 2014, and continuing until the expiration of the ESPP, the total number shares of common stock available for issuance under the ESPP will automatically increase annually on January 1 by the lesser of (i) 1% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year, or (ii) 621,814 shares of common stock. On January 1, 2014, the total number of shares of common stock available for issuance under the ESPP was automatically increased by 313,545 shares, which number is included in the number of shares reserved for issuance above. Pursuant to approval by our board, there were no increases in the number of authorized shares in the ESPP in years from 2015 to 2018. As of December 31, 2018, we have issued a total of 349,716 shares of common stock under the ESPP. Total employee, director and non‑employee stock‑based compensation expense recognized was as follows: Year Ended December 31, (In thousands) 2018 2017 2016 General and administrative $ 7,699 $ 5,617 $ 3,592 Research and development 12,849 8,034 4,256 $ 20,548 $ 13,651 $ 7,848 Year Ended December 31, (In thousands) 2018 2017 2016 Stock options $ 19,537 $ 13,153 $ 7,470 ESPP 744 498 378 Restricted stock units 267 — — $ 20,548 $ 13,651 $ 7,848 Information with respect to stock options outstanding is as follows: December 31, 2018 2017 2016 Exercisable options 3,058,659 2,558,941 1,743,765 Weighted average exercise price per share of exercisable options $ 15.12 $ 11.06 $ 8.87 Weighted average grant date fair value per share of options granted during the year $ 18.06 $ 16.92 $ 10.30 Options available for future grants 3,576,574 3,394,691 2,943,216 Weighted average remaining contractual life 7.51 7.62 7.82 The following table summarizes stock option activity for the years ended December 31, 2018 and 2017: Weighted- Weighted- Average Average Remaining Exercise Contractual Aggregate Number of Price Term Intrinsic Value Shares (Per Share) (1) (in years) (in thousands) (2) Balances at December 31, 2016 4,045,801 11.95 7.82 $ 58,131 Options granted 1,511,100 22.61 Options forfeited (96,856) 17.08 Options expired (3,000) 21.99 Options exercised(3) (363,603) 7.69 Balances at December 31, 2017 5,093,442 15.32 7.62 $ 35,495 Options granted 1,805,937 27.43 Options forfeited (107,720) 21.66 Options exercised(3) (824,731) 9.24 Balances at December 31, 2018 5,966,928 $ 19.71 7.51 $ 99,273 As of December 31, 2018 Options vested and expected to vest 5,966,928 $ 19.71 7.51 $ 99,273 Exercisable 3,058,659 $ 15.12 6.42 $ 64,417 (1) The weighted average exercise price per share is determined using exercise price per share for stock options. (2) The aggregate intrinsic value is calculated as the difference between the exercise price of the option and the fair value of our common stock for in‑ the‑money options at December 31, 2018 and 2017. (3) The total intrinsic value of stock options exercised was $23.6 million, $5.7 million and $11.2 million for the years ended December 31, 2018, 2017 and 2016 respectively. The stock options outstanding and exercisable by exercise price at December 31, 2018 are as follows: Stock Options Outstanding Stock Options Exercisable Weighted- Average Remaining Weighted- Weighted- Range of Contractual Average Average Exercise Number of Term Exercise Price Number of Exercise Price Prices Shares (in years) Per Share Shares Per Share $0.59 – $4.25 291,433 4.49 $ 3.63 291,433 $ 3.63 $9.78 – $14.75 1,560,249 6.39 $ 12.10 1,281,242 $ 11.97 $14.77 – $22.18 852,138 6.62 $ 17.00 706,562 $ 16.35 $22.20 – $33.78 2,929,571 8.42 $ 23.92 760,672 $ 23.08 $34.56 – $43.16 333,537 9.68 $ 39.26 18,750 $ 38.93 5,966,928 7.51 $ 19.71 3,058,659 $ 15.12 We estimated the fair value of employee and non‑employee awards using the Black‑Scholes valuation model. The fair value of employee stock options is being amortized on a straight‑line basis over the requisite service period of the awards. Management estimates the probability of non‑employee awards being vested based upon an evaluation of the non‑employee achieving their specific performance goals. Options granted after our initial public offering are issued at the fair market value of our stock on the date of grant. The fair value of employee stock options was estimated using the following weighted average assumptions for the years ended December 31, 2018, 2017 and 2016: Options 2018 2017 2016 Common stock fair value per share $ 21.80 - 43.16 $ 19.61 - 25.67 $ 11.50 - 26.76 Expected volatility 70.97% - 73.10% 77.42% - 96.73% 75.77% - 90.83% Risk-free interest rate 2.29% - 3.10% 0.96% - 2.37% 1.03% - 2.18% Expected dividend yield — — — Expected term (in years) 5.23 - 6.08 5.23 - 6.08 5.23 - 6.08 ESPP 2018 2017 2016 Expected term (years) 0.5 - 2.0 0.5 - 2.0 0.5 - 2.0 Expected volatility 57.0% - 71.4% 67.8% - 79.8% 67.8% - 79.8% Risk-free interest rate 1.47% - 2.70% 0.47% - 1.80% 0.47% - 0.93% Expected dividend yield — — — The expected term of stock options represents the average period the stock options are expected to remain outstanding. The expected stock price volatility for our stock options for the years ended December 31, 2018, 2017 and 2016 was determined by examining the historical volatilities for industry peers and adjusting for differences in our life cycle and financing leverage. Industry peers consist of several public companies in the biopharmaceutical industry. We determined the average expected life of stock options based on the simplified method because our common stock has not been publicly traded for an extended period and we do not have a track record of our stock being traded on the public markets for sufficient time to establish the volatility of our stock. The risk‑free interest rate assumption is based on the U.S. Treasury instruments whose term was consistent with the expected term of our stock options. The expected dividend assumption is based on our history and expectation of dividend payouts. The following table summarizes restricted stock units activity for the years ended December 31, 2018: Weighted- Average Grant Date Number of Fair Value Shares (Per Unit) Unvested at December 31, 2017 — — Granted 33,933 $ 27.64 Vested — — Forfeited — — Unvested at December 31, 2018 33,933 $ 27.64 As of December 31, 2018 and 2017, the unamortized compensation expense related to unvested stock options was $42.8 million and $30.7 million, respectively. The remaining unamortized compensation expense will be recognized over the next 2.71 years. At December 31, 2018 and 2017, the unamortized compensation expense was $0.8 million and $1.1 million respectively under our ESPP. The remaining unamortized expense will be recognized over the next 11.2 months. At December 31, 2018, the unamortized compensation expense related to unvested restricted stock units was $0.7 million, which will be recognized over the next 2.15 years.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Commitments and Contingencies	8. Commitments and Contingencies Operating leases The Company leases office and laboratory space in Monrovia, CA through June 2020. In July 2017, the Company entered into an amended lease agreement for additional space in the same building. The amended lease has a 64-month term with an option to renew for an additional five years. The lease terms for the original space were not amended. The Company also leases office space in San Diego, CA through June 2020. In June 2017, the Company entered into a new lease agreement for an additional office space. The new lease has a 61-month term beginning from the date of occupancy and includes an option to renew for an additional five years. At December 31, 2018 the future minimum lease payments under the operating leases were as follows: Operating Years ending December 31, Leases 2019 $ 2,752 2020 2,404 2021 1,980 2022 1,406 2023 — Thereafter — Rent expenses for the years ended December 31, 2018, 2017 and 2016 were $2.5 million, $1.7 million, and $0.6 million, respectively. Contingencies From time to time, the Company may be subject to various litigation and related matters arising in the ordinary course of business. The Company does not believe it is currently subject to any material matters where there is at least a reasonable possibility that a material loss may be incurred. On March 3, 2015, a verified class action complaint, captioned DePinto v. John S. Stafford, et al., C.A. No. 10742, was filed in the Court of Chancery of the State of Delaware against certain of the Company's current and former directors alleging cause of action for Breach of Fiduciary Duty and Invalidity of Director and Stockholder Consents and that the defendants breached their fiduciary duties in the course of approving a series of transactions. The complaint related to a financial recapitalization of the Company and certain related transactions that the Company completed in 2013. On September 27, 2016, the parties engaged in voluntary mediation and agreed to settle the complaint’s outstanding claims for a total payment of $2.375 million to the class certified by the Delaware Court of Chancery. The settlement was reached without any party admitting wrongdoing. Under the terms of the settlement, no payment shall be made to the plaintiffs by the Company or any of the defendants in the lawsuit other than payments covered by the Company’s insurance. On April 4, 2017, the Delaware Court of Chancery approved the settlement between the parties. On May 1, 2017, the Company’s insurance carriers fully funded the settlement account. We recognized legal costs related to the litigation as incurred and offset any insurance proceeds when approved and issued. For the year ended December 31, 2017 no amount of loss related to the settlement has been accrued. At December 31, 2016, we reported the outstanding settlement amount of $2.355 million as a payable and reflected a receivable of the same amount for the insurance coverage. This amount was paid by the insurance carrier on our behalf in May 2017. We are obligated to make future payments to third parties under in‑license agreements, including sublicense fees, royalties, and payments that become due and payable on the achievement of certain development and commercialization milestones. As the amount and timing of sublicense fees and the achievement and timing of these milestones are not probable and estimable, such commitments have not been included on our balance sheet. We have also entered into agreements with third party vendors which will require us to make future payments upon the delivery of goods and services in future periods. Guarantees In the normal course of business, we indemnify certain employees and other parties, such as collaboration partners and other parties that perform certain work on behalf of, or for the Company or take licenses to our technologies. We have agreed to hold these parties harmless against losses arising from our breach of representations or covenants, intellectual property infringement or other claims made against these parties in performance of their work with us. These agreements typically limit the time within which the party may seek indemnification by us and the amount of the claim. It is not possible to prospectively determine the maximum potential amount of liability under these indemnification agreements since we have not had any prior indemnification claims on which to base the calculation. Further, each potential claim would be based on the unique facts and circumstances of the claim and the particular provisions of each agreement. We are not aware of any potential claims and we did not record a liability as of December 31, 2018 and 2017.

Collaboration and Licensing Agr

Collaboration and Licensing Agreements	12 Months Ended
	Dec. 31, 2018
Collaboration and Licensing Agreements
Collaboration and Licensing Agreements	9. Collaboration and Licensing Agreements Following is a summary description of the material revenue arrangements, including arrangements that generated revenue in the period ended December 31, 2018, 2017, and 2016. The revenue reported for each agreement has been adjusted to reflect the adoption of ASC 606 for each period presented. Novartis In June 2016, the Company entered into a Collaboration and License Agreement (Novartis Agreement) with Novartis Institutes for BioMedical Research, Inc. (Novartis), to develop and commercialize bispecific and other Fc engineered antibody drug candidates using the Company’s proprietary XmAb® technologies and drug candidates. Pursuant to the Novartis Agreement: · The Company granted Novartis certain exclusive rights to research, develop and commercialize XmAb14045 and XmAb13676, two development stage products that incorporate the Company’s bispecific Fc technology; · The Company will apply its bispecific technology in up to four target pair antibodies identified by Novartis (each a Global Discovery Program); and · The Company will provide Novartis with a non-exclusive license to certain of its Fc technologies to apply against up to ten targets identified by Novartis. The Company received a non-refundable upfront payment under the Novartis Agreement of $150 million in July 2016 and is eligible to receive up to $2.1 billion in future development, regulatory and sales milestones in total for all programs that could be developed under the Novartis Agreement. In December 2018, Novartis notified the Company it was terminating its rights with respect to the XmAb13676 program, which will be effective June 2019. Under the Agreement, Novartis is responsible to fund its share of XmAb13676 development costs through June 2020. Under the Novartis Agreement, the Company granted Novartis a worldwide co-exclusive license with the Company to research, develop and manufacture XmAb14045. The Company also granted Novartis an exclusive license to commercialize XmAb14045 in all worldwide territories outside the U.S. The Company and Novartis will co-develop XmAb14045 worldwide and share development costs. The Company may elect to opt-out of the development of XmAb14045 by providing notice to Novartis. If the Company elects to opt-out, Novartis will receive the Company’s U.S. rights to XmAb14045 and the Company will receive low double-digit royalties on U.S. net sales in addition to the royalties on net sales outside the U.S. Pursuant to the Novartis Agreement, the Company will apply its bispecific technology to up to four target pair antibodies selected, if available for exclusive license to Novartis and not subject to a Company internal program. The Company will apply its bispecific technology to generate bispecific antibody candidates from starting target pair antibodies provided by Novartis for each of the four Global Discovery Programs and return the bispecific product candidate to Novartis for further testing, development and commercialization. Novartis has the right to substitute up to four of the original selected target pair antibodies during the research term provided that Novartis has not filed and received acceptance for an Investigational New Drug Application (IND) with the Company provided bispecific candidate. The research term is five years from the date of the Novartis Agreement. We completed delivery of a Global Discovery Program in 2017 and delivery of a second Global Discovery Program in 2018. Novartis will assume full responsibility for development and commercialization of each product candidate under each of the Global Discovery Programs. Under the Novartis Agreement, the Company has the right to participate in the development and commercialization of one of the Global Discovery Programs prior to filing an IND for the Global Discovery Program. If the Company elects to participate in development, it will assume responsibility for 25% of the worldwide development costs for the program and 50% of commercialization costs and will receive 50% of the U.S. profits on net sales of the product. Under the Novartis Agreement, the Company also granted Novartis a non-exclusive research license to use certain of the Company’s Fc technologies, specifically Cytotoxic, Xtend and Immune Inhibitor, to research, develop, commercialize and manufacture antibodies against up to ten targets selected by Novartis, if available for non-exclusive license and not subject to a Company internal program. Novartis will assume all research, development and commercialization costs for products that are developed from application of the Fc technologies. The Company evaluated the Novartis Agreement under the new revenue recognition standard ASC 606 and concluded that Novartis is a customer. The Company identified the following performance obligations that it deemed to be distinct at the inception of the contract: · License to certain rights to Xencor’s XmAb14045 and XmAb13676; · Develop four bispecific drug candidates against four targets identified by Novartis; and · License to Xencor’s Fc technologies for up to 10 targets identified and selected by Novartis. The Company considered the licenses as functional intellectual property as Novartis has the right to access its technology and such technology is functional to Novartis at the time that the Company provides access. Under the Novartis Agreement, Novartis has substitution rights under each discovery program provided it has not advanced to filing an IND. The Company’s obligation to provide services related to the discovery programs, and Novartis’ right to substitute programs is limited to the five-year period from the date of the Novartis agreement. The Company determined the transaction price at inception is the $150 million upfront payment to be allocated to the performance obligations. The Novartis Agreement includes variable consideration for potential future milestones and royalties that were contingent on future success factors for development programs. The Company used the “most likely” method to determine the variable consideration. None of the development, regulatory or sales milestones or royalties were included in the transaction price. The Company will re-evaluate the transaction price in each reporting period as uncertain events are resolved or other changes in circumstances occur. The Company determined the transaction price at inception of the Novartis Agreement and allocated it to the various performance obligations using the standalone selling price which is comparable to the relative selling price methodology used in the original accounting treatment for the transaction. The transaction price of $150 million was allocated to the performance obligations as follows: * $27.1 million to certain rights to the XmAb14045 Program; * $31.4 million to certain rights to the XmAb13676 Program; * $20.05 million to each of the four Global Discovery Programs; and * $11.3 million to the Fc licenses. Under ASC 606, revenue is recognized at the time that the Company’s performance obligation for each Global Discovery is completed upon delivery of each discovery program to Novartis. The Company delivered a discovery program to Novartis in 2017 and recognized $20.05 million of revenue in the period of delivery. In the third quarter of 2018, the Company delivered a second discovery program to Novartis and is recognizing an additional $20.04 million of revenue. Under ASC 606 the entire amount of revenue allocated to the Fc licenses is being recognized at inception of the Novartis Agreement, the second quarter of 2016. During the year ended December 31, 2018, 2017 and 2016, the Company recognized $20.0 million, $20.1 million and $69.9 million of revenue respectively. As of December 31, 2018 there is a receivable of $2.1 million and $40.1 million in deferred revenue related to the arrangement. Amgen Inc. In September 2015, the Company entered into a research and license agreement (the Amgen Agreement) with Amgen Inc. (Amgen) to develop and commercialize bispecific antibody product candidates using the Company’s proprietary XmAb® bispecific Fc technology. Under the Amgen Agreement, the Company granted an exclusive license to Amgen to develop and commercialize bispecific drug candidates from the Company’s preclinical program that bind the CD38 antigen and the cytotoxic T-cell binding domain CD3, (the CD38 Program). The Company also agreed to apply its bispecific technology to five previously identified Amgen provided targets (each a Discovery Program). The Company received a $45 million upfront payment from Amgen and is eligible to receive up to $600 million in future development, regulatory and sales milestones in total for programs in development and is eligible to receive royalties on any global net sales of products. Pursuant to the Amgen Agreement, the Company applied its bispecific technology to five Discovery Programs antibody molecules provided by Amgen that bind Discovery Program targets and returned the bispecific product candidates to Amgen for further testing, development and commercialization. The initial research term was three years from the date of the Amgen Agreement, but Amgen, at its option, may request an extension of one year. In May 2018, Amgen notified the Company that it was electing to extend the term of the research term for one year. Pursuant to the Amgen Agreement, Amgen and the Company will agree upon a detailed plan for services to be provided by the Company during the extended research term. The Company will receive research funding for the additional services provided during the extended research term. Amgen will assume full responsibility for development and commercialization of product candidates under each of the Discovery Programs. The Company evaluated the Amgen Agreement under ASC 606 and determined that it is a customer and that delivery of the CD38 Program and each of the five Discovery Programs represent the performance obligations under the contract. The Company determined the transaction price at inception is the $45 million upfront payment to be allocated to the performance obligations. The Amgen Agreement includes variable consideration for potential future milestones and royalties that were contingent on future success factors for development programs. The Company used the “most likely” method to determine the variable consideration. In the fourth quarter of 2017, the Company received a $10 million development milestone related to the CD38 program, now AMG424, and this payment was included in the transaction price as uncertainty associated with it has been resolved. In the fourth quarter of 2018, the Company received a $0.5 million preclinical milestone related to one of the discovery programs. No other development, regulatory or sales milestones or royalties were included in the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. In allocating the transaction price determined at inception, the Company determined that ASC 606 provides the use of a standalone selling price for the transaction. The transaction price of $55.5 million was allocated to the performance obligations as follows: * $23.75 million to the CD38 Program and * $6.25 million to each of the five Discovery Programs Under ASC 606, the amount of revenue recognized for the CD38 program is recognized at the inception of the contract when delivery of the CD38 Program and materials was transferred to Amgen. The $10 million milestone revenue was recognized in the period that the uncertainty regarding the event is resolved, i.e., when the milestone event occurred. The Company completed performance obligations for the five discovery programs in 2016 when all five of the Discovery Programs were delivered to Amgen. Pursuant to ASC 606 the Company recognized $31.25 million of revenue for delivery of the five discovery programs in 2016. In the fourth quarter of 2018, a $0.5 million milestone payment was received in connection with a preclinical development of a discovery program. In the third quarter of 2018, the Company and Amgen agreed upon additional scope of work to be performed by the Company; the work has been completed in 2018. During the years ended December 31, 2018, 2017 and 2016, the Company recognized $0.6 million, $10.0 million and $31.2 million in revenue, respectively, under this arrangement. As of December 31, 2018 there was no deferred revenue related to the arrangement. Novo Nordisk A/S In December 2014, the Company entered into a Collaboration and License Agreement with Novo Nordisk A/S (Novo). Under the terms of the agreement, the Company granted Novo a research license to use certain Company technologies during a two-year research term. The Company received an upfront payment of $2.5 million and research funding of $1.6 million per year over the research term. This agreement was terminated by Novo in 2016. There was no revenue recognized for the year ended December 31, 2018 and 2017. For the year ended in December 31, 2016, the total revenue recognized under this agreement was $2.7 million. As of December 31, 2018 the Company has no deferred revenue related to the agreement. MorphoSys Ag In June 2010, the Company entered into a Collaboration and License Agreement with MorphoSys AG (MorphoSys), which was subsequently amended in March 2012. The agreement provided us an upfront payment of $13 million in exchange for an exclusive worldwide license to the Company’s patents and know‑how to research, develop and commercialize our XmAb5574 product candidate (subsequently renamed MOR208) with the right to sublicense under certain conditions. Under the agreement, the Company agreed to collaborate with MorphoSys to develop and commercialize XmAb5574/MOR208. If certain developmental, regulatory and sales milestones are achieved, the Company is eligible to receive future milestone payments and royalties. In June 2017, MorphoSys initiated a Phase III clinical trial under the arrangement for which the Company received a milestone payment of $12.5 million. The Company recognized the payment as revenue in the period that the milestone event occurred. The Company recognized $12.5 million of revenue for the year end December 31, 2017. There were no revenues recognized under this arrangement for the years ended December 31, 2018 and 2016. As of December 31, 2018, the Company has no deferred revenue related to this agreement. Alexion Pharmaceuticals, Inc. In January 2013, the Company entered into an option and license agreement with Alexion Pharmaceuticals, Inc. (Alexion). Under the terms of the agreement, the Company granted to Alexion an exclusive research license, with limited sublicensing rights, to make and use our Xtend technology to evaluate and advance compounds against six different target programs during a five‑year research term under the agreement, up to completion of the first multi‑dose human clinical trial for each target compound. Under the agreement, the Company received an upfront payment of $3.0 million and also an annual maintenance fee of $0.5 million during the research term of the agreement. In addition, the Company is eligible to receive contractual milestones for certain development, regulatory and commercial achievements. If licensed products are successfully commercialized, the Company is also entitled to receive royalties based on a percentage of net sales of such products sold by Alexion, its affiliates or its sub licensees, which percentage is in the low single digits. Alexion’s royalty obligations continue on a product‑by‑product and country‑by‑ country basis until the expiration of the last‑to‑expire valid claim in a licensed patent covering the applicable product in such country. In December 2016, Alexion achieved a Phase 3 clinical development milestone for ALXN1210. In the third quarter of 2018, Alexion completed certain regulatory submission filings for ALXN1210 and the Company received $9 million in milestone payments. In the fourth quarter of 2018, Alexion completed certain regulatory submission filings for ALXN 1210 and also received FDA marketing approval for ALXN 1210, now Ultomiris, and the Company received $11 million in milestone payments. The Company determined Alexion to be a customer and the license of the Company’s Xtend intellectual property is functional intellectual property, distinct and is the only performance obligation under the agreement. The upfront fee, the net present value of the annual maintenance fees, the option exercise fee and milestone payments of $36.5 million already received represent the total transaction price at inception. The option exercise fee does not provide a discount on future services and does not grant a material right. Under ASC 606 the upfront payment and the present value of the annual licensing fees are recognized at inception of the agreement when Alexion was provided access to the technology. The total revenue recognized under this arrangement was $20.0 million and $5.0 million for the years ended December 31, 2018 and 2016, respectively. There was no revenue recognized for the year ended December 31, 2017. As of December 31, 2018 there is no deferred revenue related to this agreement. Boehringer Ingelheim International GmbH In 2007 the Company entered into a Research Licensee and Collaboration Agreement with Boehringer Ingelheim International GmbH (BI). Under the agreement, the Company provided BI with a three‑year research license to one of the Company’s technologies and commercial options. BI elected to exercise two commercial licenses from compounds identified during the research term and one compound is currently in clinical development. No revenue related to this arrangement was recognized in 2018, 2017 or 2016. There is no deferred revenue related to this agreement at December 31, 2018. CSL Limited In February 2009, the Company entered into a research license and commercialization agreement with CSL Limited (CSL). Under the agreement, the Company provided CSL with a research license to our Fc Cytotoxic technology and options to non-exclusive commercial licenses. CSL elected to exercise one commercial license for a compound, CSL362. In 2013 CSL sublicensed CSL362 (now called talacotuzumab) to Janssen Biotech Inc. (Janssen Biotech). In March 2017, CSL, through its sub-licensee, Janssen Biotech, initiated a Phase 3 clinical trial for CSL362 and the Company received a milestone payment of $3.5 million. There was no revenue recognized for the years ended December 31, 2018 and 2016. Total revenue recognized for the year ended December 31, 2017 was $3.5 million. As of December 31, 2018, there is no deferred revenue related to this agreement. Merck Sharp & Dohme Corp. In July 2013, the Company entered into a License Agreement with Merck Sharp & Dohme Corp (Merck). Under the terms of the agreement, the Company provided Merck with a non‑exclusive commercial license to certain patent rights to our Fc domains to apply to one of their compounds. The agreement provided for an upfront payment of $1.0 million and annual maintenance fees totaling $0.5 million. In February 2018, Merck provided notice that it is terminating the agreement. The Company did not recognize any revenue for each of the years ended December 31, 2018, 2017 and 2016. As of December 31, 2018, there is no deferred revenue related to this agreement. INmune Bio, Inc. In October 2017, the Company entered into a License Agreement with INmune Bio, Inc. (INmune). Under the terms of the agreement, the Company provided INmune with an exclusive license to certain rights to a proprietary protein, XPRO1595. Under the agreement the Company received an upfront payment of $100,000, a 19% fully-diluted equity interest in INmune and an option to acquire additional shares of INmune. The Company is eligible to receive a percentage of sublicensing revenue received for XPRO1595 and also royalties in the mid-single digit percent on the sale of approved products. The equity interest in INmune constituted 1,585,000 shares of common stock and the option is to purchase an additional 10% of the fully diluted interest in INmune for $10 million. In 2018, INmune filed a registration statement on a Form S-1 with the Securities and Exchange Commission (SEC) which was declared effective by the SEC as of December 19, 2018. Under ASC 606, the Company determined that the performance obligation under the agreement was the license to XPRO1595 and performance occurred at the effective date of the agreement. The total consideration under the agreement was determined to be $100,000 as the equity interest and the option had an insignificant fair value. The Company recognized $100,000 as revenue related to the agreement for the year ended December 31, 2017, and did not recognize any revenue related to the agreement for the year ended December 31, 2018. There is no deferred revenue as of December 31, 2018 related to this agreement. Revenue earned The $40.6 million, $46.2 million and $109.0 million of revenue recorded for the years ended December 31, 2018, 2017 and 2016, respectively, was earned principally from the following licensees (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) Amgen $ 0.6 $ 10.0 $ 31.2 Alexion 20.0 — 5.0 CSL — 3.5 — MorphoSys — 12.5 — Novo Nordisk — — 2.7 Novartis 20.0 20.1 69.9 Other — 0.1 0.2 Total $ 40.6 $ 46.2 $ 109.0 The below table summarizes the disaggregation of revenue recorded for the years ended December 31, 2018, 2017 and 2016 (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) Research collaboration $ 20.1 $ 20.1 $ 34.2 Milestone 20.5 26.0 5.0 Licensing — 0.1 69.8 Total $ 40.6 $ 46.2 $ 109.0 A portion of our revenue is earned from collaboration partners outside the United States. Non‑U.S. revenue is denominated in U.S. dollars. A breakdown of our revenue from U.S. and Non‑U.S. sources for the years ended December 31, 2018, 2017 and 2016 is as follows (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) U.S. Revenue $ 40.6 $ 30.2 $ 106.3 Non-U.S. Revenue — 16.0 2.7 Total $ 40.6 $ 46.2 $ 109.0 Remaining Performance Obligations and Deferred Revenue Our only remaining performance obligations are the Global Discovery Programs under the Novartis Agreement. As of December 31, 2018 and 2017, we have deferred revenue of $40.1 million and $60.1 million, respectively. We classified the deferred revenue as current liabilities as our obligations to perform services are due on demand when requested by Novartis under the Agreement.

401(k) Plan

401(k) Plan	12 Months Ended
	Dec. 31, 2018
401(k) Plan
401(k) Plan	10. 401(k) Plan We have a 401(k) plan covering all full‑time employees. Employees may make pre‑tax contributions up to the maximum allowable by the Internal Revenue Code. Effective January 1, 2018, the Company contributes 100% of the first 1% of participating employees’ contribution and 50% of the next 5% of participating employees’ contribution, for a maximum of 3.5% employer contribution. Participants are immediately vested in their employee contributions; employer contributions are vested over a three-year period with one-third for each year of a participating employee’s service. Employer contributions made for the year ended December 31, 2018 was $0.5 million. No employer contributions were made for the years ended December 31, 2017 and 2016.

Condensed Quarterly Financial D

Condensed Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2018
Condensed Quarterly Financial Data (unaudited)
Condensed Quarterly Financial Data (unaudited)	11. Condensed Quarterly Financial Data (unaudited) The following table contains selected unaudited financial data for each quarter of 2018 and 2017. The unaudited information should be read in conjunction with the Company’s financial statements and related notes included elsewhere in this report. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. Quarterly Financial Data (in thousands, except per share data): 2018 Quarter Ended March 31, June 30, September 30, December 31, Total revenue $ — $ — $ 29,039 $ 11,564 Income (loss) from operations (30,649) (28,290) 651 (21,082) Net income (loss) (29,493) (25,869) 3,150 (18,197) Basic net income (loss) per common share (0.62) (0.46) 0.06 (0.32) Diluted net income (loss) per common share (0.62) (0.46) 0.05 (0.32) 2017 Quarter Ended March 31, June 30, September 30, December 31, (As Revised) (As Revised) (As Revised) (As Revised) Total revenue $ 3,500 $ 12,500 $ — $ 30,150 Income (loss) from operations (16,359) (8,510) (23,580) 5,326 Net income (loss) (15,475) (7,725) (22,652) 7,366 Basic net income (loss) per common share (0.33) (0.17) (0.48) 0.16 Diluted net income (loss) per common share (0.33) (0.17) (0.48) 0.15

Prior-Period Financial Statemen

Prior-Period Financial Statements	12 Months Ended
	Dec. 31, 2018
Prior-Period Financial Statements
Prior-Period Financial Statements	12. Prior-Period Financial Statements The Company adopted ASC 606 on January 1, 2018 using the full retrospective method and as a result the Company has revised its comparative financial statements for the prior period as if ASC 606 had been in effect for that period. The most significant changes to revenue recognition under ASC 606 relate to the timing of revenue recognized for arrangements that include licensing of our technologies. Under ASC 606 revenue related to licensing of access to our technologies is recognized at inception of the agreement, generally the effective date of the agreement. For existing licensing arrangements, the effect of ASC 606 is to shift revenue to earlier periods. Approximately $11.3 million of licensing revenue that was being recognized over the five-year period 2016-2021 is being recognized in the second quarter of 2016. The other significant change under ASC 606 relates to the timing of collaboration revenue when the Company completes its performance obligations for delivery of a drug candidate to its collaboration partners after applying its technologies. For existing collaborations, the effect of ASC 606 is to accelerate revenue recognition to earlier periods. Approximately $6.25 million of collaboration revenue recognized in 2017 and 2018 under historical accounting guidance is being recognized in 2016 under ASC 606. An additional $20.5 million of collaboration revenue that would be recognized in 2018 is being recognized in 2017. The following tables summarize the effects of adopting ASC topic 606 on our financial statements. Balance Sheet As Reported Effect of Adoption of As Revised 2017 ASC 606 2017 Assets Current assets Cash and cash equivalents $ 16,528 $ — $ 16,528 Marketable securities 207,603 — 207,603 Accounts receivable 1,142 — 1,142 Prepaid expenses and other current assets 5,606 — 5,606 Total current assets 230,879 — 230,879 Property and equipment, net 7,088 — 7,088 Patents, licenses, and other intangible assets, net 11,148 — 11,148 Marketable securities - long term 139,198 — 139,198 Income tax receivable 1,524 — 1,524 Loan receivable — 86 86 Interest receivable — 14 14 Other assets 265 — 265 Total assets $ 390,102 $ 100 $ 390,202 Liabilities and stockholders’ equity Current liabilities Accounts payable $ 6,869 $ — $ 6,869 Accrued expenses 5,480 — 5,480 Current portion of deferred rent 26 — 26 Current portion of deferred revenue 88,813 (28,695) 60,118 Income taxes 157 — 157 Total current liabilities 101,345 (28,695) 72,650 Deferred rent, less current portion 1,088 — 1,088 Deferred revenue, less current portion 5,623 (5,623) — Total liabilities 108,056 (34,318) 73,738 Commitments and contingencies Stockholders’ equity Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at December 31, 2017 — — — Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017 470 — 470 Additional paid-in capital 570,670 — 570,670 Accumulated other comprehensive income loss (1,808) — (1,808) Accumulated deficit (287,286) 34,418 (252,868) Stockholders’ equity 282,046 34,418 316,464 Total liabilities and stockholders’ equity $ 390,102 $ 100 $ 390,202 Statement of Operation As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2017 ASC 606 2017 Revenue Collaborations, licenses and milestones $ 35,711 $ 10,439 $ 46,150 Operating expenses Research and development 71,772 — 71,772 General and administrative 17,501 — 17,501 Total operating expenses 89,273 — 89,273 Loss from operations (53,562) 10,439 (43,123) Other income (expenses) Interest income 4,194 — 4,194 Interest expense (13) — (13) Other income (7) — (7) Total other income, net 4,174 — 4,174 Loss before income tax benefit (49,388) 10,439 (38,949) Income tax benefit (463) — (463) Net loss (48,925) 10,439 (38,486) Other comprehensive loss Net unrealized loss on marketable securities (367) — (367) Comprehensive loss $ (49,292) $ 10,439 $ (38,853) Basic and diluted net loss per common share $ (1.05) $ 0.23 $ (0.82) As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2016 ASC 606 2016 Revenue Collaborations, licenses and milestones $ 87,520 $ 21,500 $ 109,020 Operating expenses Research and development 51,872 — 51,872 General and administrative 13,108 — 13,108 Total operating expenses 64,980 — 64,980 Income from operations 22,540 21,500 44,040 Other income (expenses) Interest income 2,091 — 2,091 Interest expense (21) — (21) Other income 6 — 6 Total other income, net 2,076 — 2,076 Income before income tax expense 24,616 21,500 46,116 Income tax expense 991 — 991 Net income 23,625 21,500 45,125 Other comprehensive loss Net unrealized loss on marketable securities (925) — (925) Comprehensive income $ 22,700 $ 21,500 $ 44,200 Basic net income per common share $ 0.57 $ 0.52 $ 1.09 Diluted net income per common share $ 0.56 $ 0.51 $ 1.07 Statement of Stockholders’ Equity Accumulated Additional Other Total Common Stock Paid Comprehensive Accumulated Stockholders’ Stockholders’ Equity Shares Amount in-Capital Loss Deficit Equity Balance, December 31, 2016 as originally reported 46,567,978 $ 466 $ 552,889 $ (1,441) $ (237,960) $ 313,954 Adoption of ASU 2016-09 — — 401 — (401) — Adoption of ASC 606 — — — — 23,979 23,979 Balance, December 31, 2016 as revised 46,567,978 466 553,290 (1,441) (214,382) 337,933 Issuance of common stock upon exercise of stock awards 363,603 4 2,793 — — 2,797 Issuance of common stock under the Employee Stock Purchase Plan 70,907 — 936 — — 936 Comprehensive loss — — — (367) (48,925) (49,292) Stock-based compensation — — 13,651 — — 13,651 Balance, December 31, 2017 47,002,488 $ 470 $ 570,670 $ (1,808) $ (263,307) $ 306,025 Adoption of ASC topic 606 — — — — 10,439 10,439 Balance, December 31, 2017 as revised 47,002,488 $ 470 $ 570,670 $ (1,808) $ (252,868) $ 316,464 Accumulated Additional Other Total Common Stock Paid Comprehensive Accumulated Stockholders’ Stockholders’ Equity Shares Amount in-Capital Loss Deficit Equity Balance, December 31, 2015 as originally reported 40,551,039 $ 405 $ 424,128 $ (516) $ (261,585) $ 162,432 Adoption of ASC Topic 606 — — — — 2,479 2,479 Balance, December 31, 2015 as revised 40,551,039 405 424,128 (516) (259,106) 164,911 Sale of common stock, net of issuance cost 5,272,750 53 119,216 — — 119,269 Issuance of common stock upon exercise of stock awards 699,066 7 1,153 — — 1,160 Issuance of common stock under the Employee Stock Purchase Plan 45,123 1 544 — — 545 Comprehensive income (loss) — — — (925) 23,625 22,700 Stock-based compensation — — 7,848 — — 7,848 Balance, December 31, 2016 46,567,978 466 552,889 (1,441) (235,481) 316,433 Adoption of ASC Topic 606 — — — — 21,500 21,500 Balance, December 31, 2016 as revised 46,567,978 $ 466 $ 552,889 $ (1,441) $ (213,981) $ 337,933 Statement of Cash Flows As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2017 ASC 606 2017 Cash flows from operating activities Net loss $ (48,925) $ 10,439 $ (38,486) Adjustments to reconcile net loss to net cash used in operating activities: Depreciation and amortization 2,030 — 2,030 Amortization of premium on marketable securities 2,845 — 2,845 Stock-based compensation 13,651 — 13,651 Abandonment of capitalized intangible assets 396 — 396 Loss on disposal of assets 83 — 83 Changes in operating assets and liabilities: Accounts receivable 7,474 — 7,474 Interest receivable (293) (14) (307) Prepaid expenses and other assets (2,705) — (2,705) Income tax receivable (1,524) — (1,524) Other assets (161) — (161) Accounts payable 2,989 — 2,989 Accrued expenses (1,212) — (1,212) Deferred rent 589 — 589 Income tax payable 91 — 91 Deferred revenue (9,011) (10,339) (19,350) Net cash used in operating activities (33,683) 86 (33,597) Cash flows from investing activities Proceeds from sale and maturities of marketable securities available-for-sale 115,757 — 115,757 Purchase of marketable securities (76,529) — (76,529) Purchase of intangible assets (1,967) — (1,967) Purchase of property and equipment (5,311) — (5,311) Issuance of loan — (86) (86) Net cash provided by (used in) investing activities 31,950 (86) 31,864 Cash flows from financing activities Proceeds from issuance of common stock upon exercise of stock awards 2,797 — 2,797 Proceeds from issuance of common stock from Employee Stock Purchase Plan 936 — 936 Net cash provided by financing activities 3,733 — 3,733 Net increase in cash and cash equivalents 2,000 — 2,000 Cash and cash equivalents, beginning of period 14,528 — 14,528 Cash and cash equivalents, end of period $ 16,528 $ — $ 16,528 As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2016 ASC 606 2016 Cash flows from operating activities Net income $ 23,625 $ 21,500 $ 45,125 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 1,466 — 1,466 Amortization of premium on marketable securities 2,037 — 2,037 Stock-based compensation 7,848 — 7,848 Abandonment of capitalized intangible assets 356 — 356 Gain on sale of marketable securities available for sale (5) — (5) Changes in operating assets and liabilities: Accounts receivable (8,572) — (8,572) Interest receivable (530) (79) (609) Prepaid expenses and other assets (1,700) — (1,700) Other assets (40) — (40) Accounts payable (2,520) — (2,520) Accrued expenses 3,058 — 3,058 Deferred rent (89) — (89) Deferred tax liability 65 — 65 Deferred revenue 69,618 (20,800) 48,818 Net cash provided by operating activities 94,617 621 95,238 Cash flows from investing activities Proceeds from sale and maturities of marketable securities available-for-sale 105,505 — 105,505 Purchase of marketable securities (316,149) — (316,149) Purchase of intangible assets (1,502) — (1,502) Purchase of property and equipment (1,507) — (1,507) Proceeds from repayment of (investment in) loan receivable — (621) (621) Net cash used in investing activities (213,653) (621) (214,274) Cash flows from financing activities Proceeds from issuance of common stock upon exercise of stock awards 1,160 — 1,160 Proceeds from issuance of common stock from Employee Stock Purchase Plan 545 — 545 Proceeds from issuance of common stock 126,546 — 126,546 Common stock issuance costs (7,277) — (7,277) Net cash provided by financing activities 120,974 — 120,974 Net increase in cash and cash equivalents 1,938 — 1,938 Cash and cash equivalents, beginning of period 12,590 — 12,590 Cash and cash equivalents, end of period $ 14,528 $ — $ 14,528

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2018
Subsequent Event
Subsequent Event	13. Subsequent Event Genentech Agreement In February 2019, the Company entered into a collaboration and license agreement (the Genentech Agreement) with Genentech, Inc. and F. Hoffmann La-Roche Ltd. (collectively, Genentech) for the development and commercialization of novel IL-15 Collaboration Products, including XmAb24306, an IL-15/IL15Rα cytokine complex engineered with the Company’s bispecific Fc and Xtend technologies. The Company’s IL-15 bispecific cytokine platform provides a more druggable version of IL-15 with potentially superior tolerability, slower receptor-mediated clearance and a prolonged half-life, and is intended for development with a wide-range of combination agents due to its proposed mechanism of activating tumor-killing immune cells. Under the terms of the Genentech Agreement, Genentech received an exclusive worldwide license to XmAb24306 and other Collaboration Products, including any new IL-15 programs identified during the joint research collaboration. The Company will receive a non-refundable upfront payment of $120 million after the Genentech Agreement becomes effective and is eligible to receive up to an aggregate of $160 million in clinical milestone payments for each Collaboration Product that advances to Phase 3 clinical trials. The Company is eligible to receive a 45% share of net profits for sales of XmAb24306 and other Collaboration Products, while also sharing in the net losses at the same percentage rate. The parties will also jointly share development and commercialization costs at the same percentage rate, while Genentech will bear launch costs entirely. The profit/cost share is subject to ratchet at the Company’s discretion and convertible to a royalty under certain circumstances. The Company and Genentech will also conduct joint research activities for a two-year period to discover additional IL-15 candidates developed from the Company’s cytokine and bispecific Fc technologies. The Company will receive a $20 million development milestone for each new Collaboration Product that is identified from the research efforts and advances into a Phase 1 clinical trial. The Genentech Agreement is subject to customary closing conditions, including clearance under the Hart-Scott-Rodino Antitrust Improvements Act, and closing is expected to occur in the first quarter of 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Polices)	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The Company’s financial statements as of December 31, 2018, 2017, and 2016 and for the years then ended have been prepared in accordance with accounting principles generally accepted in the United States (U.S.).
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates include useful lives of long-lived assets, the periods over which certain revenues and expenses will be recognized including collaboration revenue recognized from non-refundable upfront licensing payments, the amount of non-cash compensation costs related to share-based payments to employees and non-employees and the period over which these costs are expensed.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Pronouncements adopted in 2018 Effective January 1, 2018, the Company adopted Accounting Standards Codification Topic 606 (ASC 606), Revenue from Contracts with Customers, using the full retrospective transition method. Under this method, the Company is presenting its financial statements for the years ended December 31, 2017 and 2016 as if ASC 606 had been effective for those periods. Under ASC 606 an entity recognizes revenue when its customer obtains control of promised goods or services in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. A five-step model is used to achieve the core principle: (1) identify the customer contract, (2) identify the contract’s performance obligations, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations and (5) recognize revenue when or as a performance obligation is satisfied. The Company applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. The new guidance provides that revenue recognition for performance obligations related to delivery of certain goods or services occurs when control over the good or service is transferred to the customer. In addition, the timing of revenue recognition from licensing of our intellectual property that are functional and are distinct performance obligations changed from being recognized over the term of access to our license or technology to being recognized at a point in time. See Note 12 “Prior Period Financial Statements” for a complete discussion of the impact of adopting the new standard. Effective January 1, 2018, the Company adopted ASU 2016-01, Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Liabilities, which eliminates the available-for-sale classification for equity securities and requires equity securities to be measured at fair value with changes in the fair value recognized through net income. This ASU eliminates the available-for-sale classification for equity investments that recognized changes in the fair value as a component of other comprehensive income. The adoption had no effect on the Company’s financial statements. Effective January 1 2018, the Company adopted ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which addresses eight specific cash flow issues with the objective of reducing the existing diversity in practice. The standard clarifies when cash receipts and cash payments have aspects of more than one class of cash flows and cannot be separated. Classification will depend on the predominant source or use. The adoption did not have an effect on the Company’s statements of cash flow. Effective January 1, 2018, the Company adopted ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) . The standard applies when a company changes the terms of a stock compensation award previously granted to an employee where modification accounting applies. According to the standard, modification accounting is not required if (1) the fair value of the modified award (or the award’s calculated value or intrinsic value as appropriate) is the same as the value immediately prior to its modification, (2) the vesting conditions of the modified award are the same as the vesting conditions of the award immediately prior to its modification; and (3) the award’s classification as an equity or liability is the same after the modification as it was immediately prior to its modification. The Company did not have any modifications upon adopting the new standard; therefore, adoption had no effect on the Company’s financial statements. Pronouncements not yet effective In February 2016, the FASB issued ASU No. 2016-02 Leases (Topic 842) . The new guidance requires lessees to recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term for all leases not considered short term. The new standard will be effective for reporting periods beginning after December 15, 2018. In July 2018, the FASB issued ASU No. 2018-10 Codification Improvements to Topic 842, Leases and ASU No. 2018-11, Lease (Topic 842): Targeted Improvements , which provides narrow aspects of the guidance issued in ASU No. 2016-02 as well as an alternative transition method of adoption, permitting the recognition of cumulative-effect adjustment to retained earnings on the date of adoption. The new standard also provides a number of optional practical expedients in transition, including the “package of practical expedients,” which permits us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct cost. We will adopt the new standard on January 1, 2019 using the cumulative effect adjustment method, with the election of the package of practical expedients and hindsight practical expedient. We have completed a preliminary assessment of the new standard’s impact, which includes a total of five operating leases for facilities in Monrovia and San Diego. Under Topic 842, deferred rent liability under operating lease is no longer tracked separately; instead, it will integrate as part of the right-of-use (ROU) asset. As a result, we expect to adjust the cumulative effect to the beginning balance for deferred rent liability, and adopt the use of right-of-use asset and lease liability. We do not expect that this standard will have a material impact on our financial statements. Upon adoption, we estimate that we will have additional liabilities ranging from $10 million to $12 million with a corresponding ROU assets of a similar amount for lease agreements in effect as of December 31, 2018. This will result in an estimated increase to the beginning balance on both assets and liabilities after the adjustment of between $10 million and $12 million, with no impact on our retained earnings. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which amends the guidance on reporting credit losses for assets held at amortized cost basis and available-for-sale debt securities. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses rather than as a direct write-down to the security. Credit losses on available-for-sale securities will be required when the amortized cost is below the fair market value. The amendment is effective for fiscal years beginning after December 15, 2019 including interim periods within those fiscal years. In November 2018, the FASB issued ASU No. 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses , which clarifies that receivables arising from operating leases are not within the scope of Topic 326. We will apply the standard’s provision as a cumulative effect adjustment to retained earnings as of the beginning of the first effective reporting period. We do not expect the adoption to have a material impact on our results of operations or financial position. In March 2017, the FASB issued ASU No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities, which amends the guidance on the amortization period of premiums on certain purchased callable debt securities by shortening the amortization period of premiums to the earliest call date. The amendment affects all entities that hold investments in callable debt securities that have an amortized cost basis in excess of the amount that is repayable by the issuer at the earliest call date. The amendment is effective for fiscal years after December 31, 2018 with early adoption permitted. The Company will review the requirements of the standard but does not anticipate it will have a significant impact on our financial statements. In February 2018, the FASB issued ASU No. 2018-02. Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income , an amendment which permits companies to reclassify the income tax effects of the 2017 Tax Cuts and Jobs Act (TCJA) on items within accumulated other comprehensive income to retained earnings. The standard also requires new disclosures about these stranded tax effects and is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted and can be applied either in the period of adoption or retrospectively to each period (or periods) in which the effect of the change in the U.S. federal corporate income tax rate in the TCJA is recognized. The Company is currently evaluating the impact the guidance will have on its financial statements. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting , which expands the scope of Topic 718 to include share-based payments issued to nonemployees for goods and services. The standard is effective for fiscal years beginning after December 15, 2018 and interim periods within such fiscal year. The Company does not anticipate that the standard will have a significant impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement , which modifies the disclosures for transfers between Level 1 and Level 2 of the fair value hierarchy, modifies the level 3 disclosure requirements for non-public entities and requires additional disclosure for Level 3 fair value hierarchy. The amendment is effective for fiscal years beginning after December 15, 2019. The Company does not anticipate that the standard will have a significant impact on its financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract , which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendment. The amendment is effective for annual periods, including interim periods within those annual periods, beginning after December 15, 2019. The Company does not anticipate that the standard will have a significant impact on its financial statements. In October 2018, the FASB issued ASU No. 2018-17, Consolidation (Topic 810): Targeted Improvements to Related Party Guidance for Variable Interest Entities, which amends the guidance for determining whether a decision-making fee is a variable interest. The amendments require organizations to consider indirect interests held through related parties under common control on a proportional basis rather than as the equivalent of a direct interest in its entirety (as currently required). The Company does not anticipate that the standard will have a significant impact on tis financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606, which provides guidance on how to assess whether certain transactions between collaborative arrangement participants should be accounted for within the revenue recognition standard. The standard is effective for fiscal years beginning after December 15, 2019 and interim period within those years. The Company does not anticipate that the standard will have a significant impact on its financial statements.
Revenue Recognition	Revenue Recognition We have, to date, earned revenue from research and development collaborations, which may include research and development services, licenses of our internally-developed technologies, licenses of our internally-developed drug candidates, or combinations of these. The terms of our license and research and development and collaboration agreements generally include non-refundable upfront payments, research funding, co-development reimbursements, license fees and, milestone and other contingent payments to us for the achievement of defined collaboration objectives and certain clinical, regulatory and sales‑based events, as well as royalties on sales of any commercialized products. The terms of our licensing agreements include non‑refundable upfront fees, annual licensing fees, and contractual payment obligations for the achievement of pre‑defined preclinical, clinical, regulatory and sales‑based events by our partners. The licensing agreements also include royalties on sales of any commercialized products by our partners. We recognize revenue through the five-step process in accordance with ASC 606 Revenue Recognition when control of the promised goods or services is transferred to our customers in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services.
Deferred Revenue	Deferred Revenue Deferred revenue arises from payments received in advance of the culmination of the earnings process. We have classified deferred revenue for which we stand ready to perform within the next 12 months as a current liability. We recognize deferred revenue as revenue in future periods when the applicable revenue recognition criteria have been met. The total amounts reported as deferred revenue were $40.1 million and $60.1 million at December 31, 2018 and 2017, respectively.
Research and Development Expenses	Research and Development Expenses Research and development expenses include costs we incur for our own and for our collaborators’ research and development activities. Research and development costs are expensed as incurred. These costs consist primarily of salaries and benefits, including associated stock‑based compensation, laboratory supplies, facility costs, and applicable overhead expenses of personnel directly involved in the research and development of new technology and products, as well as fees paid to other entities that conduct certain research development activities on our behalf. We estimate preclinical study and clinical trial expenses based on the services performed pursuant to the contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on our behalf based on the actual time and expenses incurred by them. Further, we accrue expenses related to clinical trials based on the level of patient enrollment and activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. We capitalize acquired research and development technology licenses and third‑party contract rights and amortize the costs over the shorter of the license term or the expected useful life. We review the license arrangements and the amortization period on a regular basis and adjust the carrying value or the amortization period of the licensed rights if there is evidence of a change in the carrying value or useful life of the asset
Cash and Cash Equivalents	Cash and Cash Equivalents We consider cash equivalents to be only those investments which are highly liquid, readily convertible to cash and which mature within three months from the date of purchase.
Marketable Securities	Marketable Securities The Company has an investment policy that includes guidelines on acceptable investment securities, minimum credit quality, maturity parameters and concentration and diversification. The Company invests its excess cash primarily in marketable debt securities issued by investment grade institutions. The Company considers its marketable debt securities to be “available-for-sale”, as defined by authoritative guidance issued by the FASB. These assets are carried at fair value and the unrealized gains and losses are included in accumulated other comprehensive income (loss). Accrued interest on marketable debt securities is included in marketable securities. Accrued interest was $2.3 million and $1.7 million at December 31, 2018 and 2017, respectively. If a decline in the value of a marketable security in the Company’s investment portfolio is deemed to be other-than-temporary, the Company writes down the security to its current fair value and recognizes a loss as a charge against income. The Company reviews its portfolio of marketable debt securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost are other-than-temporary.
Concentrations of Risk	Concentrations of Risk Cash, cash equivalents and marketable debt securities are financial instruments that potentially subject the Company to concentrations of risk. We invest our cash in corporate debt securities and U.S. sponsored agencies with strong credit ratings. We have established guidelines relative to diversification and maturities that are designed to help ensure safety and liquidity. These guidelines are periodically reviewed to take advantage of trends in yields and interest rates. Cash and cash equivalents are maintained at financial institutions and, at times, balances may exceed federally insured limits. We have never experienced any losses related to these balances. Amounts on deposit in excess of federally insured limits at December 31, 2018 and 2017 approximated $26.0 million and $16.3 million, respectively. We have payables with four service providers that represent 49% of our total payables and two service providers that represented 40% of our total payables at December 31, 2018 and 2017, respectively. We rely on three critical suppliers for the manufacture of our drug product for use in our clinical trials. While we believe that there are alternative vendors available, a change in manufacturing vendors could cause a delay in the availability of drug product and result in a delay of conducting and completing our clinical trials. No other vendor accounted for more than 10% of total payables at December 31, 2018 or 2017.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Our financial instruments primarily consist of cash and cash equivalents, marketable debt securities, accounts receivable, accounts payable and accrued expenses. Marketable debt securities and cash equivalents are carried at fair value. The fair value of the other financial instruments closely approximate their fair value due to their short maturities. The Company accounts for recurring and non-recurring fair value measurements in accordance with FASB Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures (ASC 820). ASC 820 defines fair value, establishes a fair value hierarchy for assets and liabilities measured at fair value, and requires expanded disclosure about fair value measurements. The ASC 820 hierarchy ranks the quality of reliable inputs, or assumptions, used in the determination of fair value and requires assets and liabilities carried at fair value to be classified and disclosed in one of the following three categories: Level 1— Fair Value is determined by using unadjusted quoted prices that are available in active markets for identical assets or liabilities. Level 2— Fair Value is determined by using inputs other than Level 1 quoted prices that are directly or indirectly observable. Inputs can include quoted prices for similar assets or liabilities in active markets or quoted prices for identical assets or liabilities in markets that are not active. Related inputs can also include those used in valuation or other pricing models, such as interest rates and yield curves that can be corroborated by observable market data. Level 3— Fair value is determined by inputs that are unobservable and not corroborated by market data. Use of these inputs involves significant and subjective judgments to be made by the reporting entity –e.g. determining an appropriate discount factor for illiquidity associated with a given security. The Company measures the fair value of financial assets using the highest level of inputs that are reasonably available as of the measurement date. The assets recorded at fair value are classified within the hierarchy as follows for the periods reported (in thousands): December 31, 2018 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 18,270 $ 18,270 $ — Corporate Securities 104,967 — 104,967 Government Securities 399,256 — 399,256 $ 522,493 $ 18,270 $ 504,223 December 31, 2017 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 5,175 $ 5,175 $ — Corporate Securities 123,270 — 123,270 Government Securities 223,530 — 223,530 $ 351,975 $ 5,175 $ 346,800
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the assets. Expenditures for repairs and maintenance are charged to expense as incurred while renewals and improvements are capitalized. Useful lives by asset category are as follows: Computers, software and equipment 3 - 5 years Furniture and fixtures 5 - 7 years Leasehold improvements 5 - 7 years or remaining lease term, whichever is less
Patents, Licenses, and Other Intangible Assets	Patents, Licenses, and Other Intangible Assets The cost of acquiring licenses is capitalized and amortized on the straight‑line basis over the shorter of the term of the license or its estimated economic life, ranging from five to 25 years. Third‑party costs incurred for acquiring patents are capitalized. Capitalized costs are accumulated until the earlier of the period that a patent is issued or we abandon the patent claims. Cumulative capitalized patent costs are amortized on a straight‑line basis from the date of issuance over the shorter of the patent term or the estimated useful economic life of the patent, ranging from 13 to 20 years. Our senior management, with advice from outside patent counsel, assesses three primary criteria to determine if a patent will be capitalized initially: i) technical feasibility, ii) magnitude and scope of new technical function covered by the patent compared to the company’s existing technology and patent portfolio, particularly assessing the value added to our product candidates or licensing business, and iii) legal issues, primarily assessment of patentability and prosecution cost. We review our intellectual property on a regular basis to determine if there are changes in the estimated useful life of issued patents and if any capitalized costs for unissued patents should be abandoned. Capitalized patent costs related to abandoned patent filings are charged off in the period of the decision to abandon. During 2018, 2017 and 2016, we abandoned previously capitalized patent and licensing related charges of $0.2 million, $0.4 million and $0.4 million, respectively. The carrying amount and accumulated amortization of patents, licenses, and other intangibles is as follows (in thousands): December 31, 2018 2017 Patents, definite life $ 9,320 $ 8,915 Patents, pending issuance 5,644 4,360 Licenses and other amortizable intangible assets 2,011 2,011 Nonamortizable intangible assets (trademarks) 399 399 Total gross carrying amount 17,374 15,685 Accumulated amortization—patents (4,142) (3,413) Accumulated amortization—licenses and other (1,263) (1,124) Total intangible assets, net $ 11,969 $ 11,148 Amortization expense for patents, licenses, and other intangible assets was $0.9 million, $0.8 million, and $0.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. Future amortization expense for patent, licenses, and other intangible assets recorded as of December 31, 2018, and for which amortization has commenced, is as follows: Year ended December 31, (in thousands) 2019 $ 862 2020 860 2021 757 2022 730 2023 659 Thereafter 1,923 Total $ 5,791 The above amortization expense forecast is an estimate. Actual amounts of amortization expense may differ from estimated amounts due to additional intangible asset acquisitions, impairment of intangible assets, accelerated amortization of intangible assets, and other events. As of December 31, 2018, the Company has $5.6 million of intangible assets which are in‑process and have not been placed in service and, accordingly amortization on these assets has not commenced.
Long-Lived Assets	Long‑Lived Assets Management reviews long‑lived assets which include fixed assets and amortizable intangibles for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the fair value of the assets. We did not recognize a loss from impairment for the years ended December 31, 2018, 2017 or 2016.
Income Taxes	Income Taxes We account for income taxes in accordance with accounting guidance which requires an asset and liability approach to financial accounting and reporting for income taxes. Deferred income tax assets and liabilities are computed annually for differences between the financial statement and tax bases of assets and liabilities that will result in taxable or deductible amounts in the future based on enacted tax laws and rates applicable to the periods in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Income tax expense is the tax payable or refundable for the period plus or minus the change during the period in deferred tax assets and liabilities. We assess our income tax positions and record tax benefits for all years subject to examination based upon our evaluation of the facts, circumstances and information available at the reporting date. For those tax positions where there is greater than 50% likelihood that a tax benefit will be sustained, we have recorded the largest amount of tax benefit that may potentially be realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. For those income tax positions where there is a 50% or less likelihood that a tax benefit will be sustained, no tax benefit has been recognized in the financial statements. We did not have any material uncertain tax positions at December 31, 2018 or 2017. Our policy is to recognize interest and penalties on taxes, if any, as a component of income tax expense. The Tax Cuts and Jobs Act (tax reform) was enacted on December 22, 2017 and has several key provisions impacting accounting for and reporting of income taxes. The most significant provisions reduced the U.S. corporate statutory tax rate from 35% to 21%, eliminated the Alternative Minimum Tax (AMT) system, and made changes to the utilization and carryforward of net operating losses beginning on January 1, 2018. The tax reform provided for a refund of unused AMT carryforwards for years beginning after December 31, 2017. We recorded an income tax receivable as of December 31, 2018 of $1.6 million related to federal AMT carryforwards.
Stock-Based Compensation	Stock‑Based Compensation We recognize compensation expense using a fair‑value‑based method for costs related to all share‑based payments, including stock options and shares issued under our Employee Stock Purchase Plan (ESPP). Stock‑based compensation cost related to employees and directors is measured at the grant date, based on the fair‑value-based measurement of the award using the Black‑Scholes method, and is recognized as expense over the requisite service period on a straight‑line basis. We account for forfeitures when they occur. We recorded stock‑based compensation and expense for stock‑based awards to employees, directors and consultants of approximately $20.5 million, $13.7 million, and $7.8 million for the years ended December 31, 2018, 2017 and 2016 respectively. Included in the 2018, 2017, and 2016 balances for total compensation expense is $0.7 million, $0.5 million, and $0.4 million, respectively, relating to our ESPP. Options granted to individual service providers that are not employees or directors are accounted for at estimated fair value using the Black‑Scholes option‑pricing method and are subject to periodic re‑measurement over the period during which the services are rendered.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Basic net income (loss) per common share is computed by dividing the net income or loss by the weighted‑average number of common shares outstanding during the period. Potentially dilutive securities consisting of stock options for 2018 and 2017, and stock purchases under the Employee Stock Purchase Plan were not included in the diluted net loss per common shares calculation because the inclusion of such shares would have had an antidilutive effect as follows: Year Ended December 31, 2018 2017 2016 (in thousands) Options to purchase common stock 1,881 1,291 — Employee stock purchase plan shares 3 — — Total 1,884 1,291 — Year Ended December 31, 2018 2017 2016 (in thousands, except share and per share data) (As Revised) (As Revised) Basic Numerator: Net income (loss) attributable to common stockholders for basic net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted-average common shares outstanding 53,942,116 46,817,756 41,267,329 Basic net income (loss) per common share $ (1.31) $ (0.82) $ 1.09 Diluted Numerator: Net income (loss) attributable to common stockholders for diluted net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted average number of common shares outstanding used in computing basic net income (loss) per common share 53,942,116 46,817,756 41,267,329 Dilutive effect of employee stock options and ESPP — — 1,121,538 Weighted-average number of common shares outstanding used in computing diluted net income (loss) per common share 53,942,116 46,817,756 42,388,867 Diluted net income (loss) per common share $ (1.31) $ (0.82) $ 1.07
Segment Reporting	Segment Reporting The Company determines its segment reporting based upon the way the business is organized for making operating decisions and assessing performance. The Company has only one operating segment related to the development of pharmaceutical products.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Schedule of assets recorded at fair value	The Company measures the fair value of financial assets using the highest level of inputs that are reasonably available as of the measurement date. The assets recorded at fair value are classified within the hierarchy as follows for the periods reported (in thousands): December 31, 2018 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 18,270 $ 18,270 $ — Corporate Securities 104,967 — 104,967 Government Securities 399,256 — 399,256 $ 522,493 $ 18,270 $ 504,223 December 31, 2017 Total Fair Value Level 1 Level 2 Money Market Funds in Cash and Cash Equivalents $ 5,175 $ 5,175 $ — Corporate Securities 123,270 — 123,270 Government Securities 223,530 — 223,530 $ 351,975 $ 5,175 $ 346,800
Schedule of useful lives by asset category	Computers, software and equipment 3 - 5 years Furniture and fixtures 5 - 7 years Leasehold improvements 5 - 7 years or remaining lease term, whichever is less
Schedule of indefinite-lived intangible assets	The carrying amount and accumulated amortization of patents, licenses, and other intangibles is as follows (in thousands): December 31, 2018 2017 Patents, definite life $ 9,320 $ 8,915 Patents, pending issuance 5,644 4,360 Licenses and other amortizable intangible assets 2,011 2,011 Nonamortizable intangible assets (trademarks) 399 399 Total gross carrying amount 17,374 15,685 Accumulated amortization—patents (4,142) (3,413) Accumulated amortization—licenses and other (1,263) (1,124) Total intangible assets, net $ 11,969 $ 11,148
Future amortization expense for patents, licenses, and other intangible assets	Year ended December 31, (in thousands) 2019 $ 862 2020 860 2021 757 2022 730 2023 659 Thereafter 1,923 Total $ 5,791
Schedule of potentially dilutive securities	Year Ended December 31, 2018 2017 2016 (in thousands) Options to purchase common stock 1,881 1,291 — Employee stock purchase plan shares 3 — — Total 1,884 1,291 —
Schedule of basic and diluted net income (loss) per common share	Year Ended December 31, 2018 2017 2016 (in thousands, except share and per share data) (As Revised) (As Revised) Basic Numerator: Net income (loss) attributable to common stockholders for basic net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted-average common shares outstanding 53,942,116 46,817,756 41,267,329 Basic net income (loss) per common share $ (1.31) $ (0.82) $ 1.09 Diluted Numerator: Net income (loss) attributable to common stockholders for diluted net income (loss) per share $ (70,409) $ (38,486) $ 45,125 Denominator: Weighted average number of common shares outstanding used in computing basic net income (loss) per common share 53,942,116 46,817,756 41,267,329 Dilutive effect of employee stock options and ESPP — — 1,121,538 Weighted-average number of common shares outstanding used in computing diluted net income (loss) per common share 53,942,116 46,817,756 42,388,867 Diluted net income (loss) per common share $ (1.31) $ (0.82) $ 1.07

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Marketable Securities
Schedule of marketable securities	December 31, 2018 Gross Gross Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Money Market Funds $ 18,270 $ — $ — $ 18,270 Corporate Securities 105,311 1 (345) 104,967 Government Securities 399,873 187 (804) 399,256 $ 523,454 $ 188 $ (1,149) $ 522,493 Reported as Cash and cash equivalents $ 18,270 Marketable securities 504,223 Total investments $ 522,493 December 31, 2017 Gross Gross Amortized Unrealized Unrealized Cost Gains Losses Fair Value (in thousands) Money Market Funds $ 5,175 $ — $ — $ 5,175 Corporate Securities 123,860 — (590) 123,270 Government Securities 224,739 — (1,209) 223,530 $ 353,774 $ — $ (1,799) $ 351,975 Reported as Cash and cash equivalents $ 5,175 Marketable securities 346,800 Total investments $ 351,975
Schedule of maturities of marketable securities	Amortized Estimated Cost Fair Value (in thousands) Mature in one year or less $ 269,092 $ 268,115 Mature after one year through five years 236,092 236,108 $ 505,184 $ 504,223
Schedule of unrealized losses on available-for-sale investments	December 31, 2018 Less than 12 months 12 months or greater Fair value Unrealized losses Fair value Unrealized gain (losses) (in thousands) Corporate Securities $ 84,770 $ (310) $ 20,198 $ (34) Government Securities 183,345 (667) 215,910 50 $ 268,115 $ (977) $ 236,108 $ 16 December 31, 2017 Less than 12 months 12 months or greater Fair value Unrealized losses Fair value Unrealized losses (in thousands) Corporate Securities $ 79,290 $ (137) $ 43,980 $ (453) Government Securities 128,313 (461) 95,217 (748) $ 207,603 $ (598) $ 139,197 $ (1,201)

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property and Equipment
Schedule of property and equipment	December 31, 2018 2017 (In thousands) Computers, software and equipment $ 16,292 $ 10,874 Furniture and fixtures 173 152 Leasehold and tenant improvements 4,774 4,010 21,239 15,036 Less accumulated depreciation and amortization (9,426) (7,948) $ 11,813 $ 7,088

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Taxes
Reconciliation of the federal statutory income tax rate to our effective income tax rate	Year Ended December 31, 2018 2017 2016 (Revised) (Revised) Federal statutory income tax $ (14,795) $ (13,243) $ 15,680 State and local income taxes (4,767) (1,806) 2,818 Research and development credit (6,170) (5,554) (2,544) Stock based compensation 444 2,709 733 Effect of the 2017 Tax Cut and Jobs Act — 19,596 — Other 414 720 1,008 Net change in valuation allowance 24,874 (2,885) (16,695) Income tax provision (benefit) $ — $ (463) $ 1,000
Schedule of tax effect of temporary differences that give rise to a significant portion of the deferred tax assets and liabilities	December 31, 2018 2017 (Revised) Deferred income tax assets Net operating loss carryforwards $ 49,889 $ 25,565 Research credits 23,151 16,642 Depreciation 207 437 Unrealized loss on securities 269 504 Accrued compensation 1,097 748 Deferred revenue 11,222 16,820 State taxes — (2) Gross deferred income tax assets 85,835 60,714 Valuation allowance (82,537) (57,663) Net deferred income tax assets 3,298 3,051 Deferred income tax liabilities Patent costs (3,142) (2,873) Licensing costs (125) (142) Capitalized legal costs (31) (36) Gross deferred income tax liabilities (3,298) (3,051) Net deferred income tax asset $ — $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Stock-Based Compensation
Schedule of total employee, director and non-employee stock-based compensation expense recognized	Year Ended December 31, (In thousands) 2018 2017 2016 General and administrative $ 7,699 $ 5,617 $ 3,592 Research and development 12,849 8,034 4,256 $ 20,548 $ 13,651 $ 7,848 Year Ended December 31, (In thousands) 2018 2017 2016 Stock options $ 19,537 $ 13,153 $ 7,470 ESPP 744 498 378 Restricted stock units 267 — — $ 20,548 $ 13,651 $ 7,848
Schedule of stock options outstanding	December 31, 2018 2017 2016 Exercisable options 3,058,659 2,558,941 1,743,765 Weighted average exercise price per share of exercisable options $ 15.12 $ 11.06 $ 8.87 Weighted average grant date fair value per share of options granted during the year $ 18.06 $ 16.92 $ 10.30 Options available for future grants 3,576,574 3,394,691 2,943,216 Weighted average remaining contractual life 7.51 7.62 7.82
Summary of stock option activity	Weighted- Weighted- Average Average Remaining Exercise Contractual Aggregate Number of Price Term Intrinsic Value Shares (Per Share) (1) (in years) (in thousands) (2) Balances at December 31, 2016 4,045,801 11.95 7.82 $ 58,131 Options granted 1,511,100 22.61 Options forfeited (96,856) 17.08 Options expired (3,000) 21.99 Options exercised(3) (363,603) 7.69 Balances at December 31, 2017 5,093,442 15.32 7.62 $ 35,495 Options granted 1,805,937 27.43 Options forfeited (107,720) 21.66 Options exercised(3) (824,731) 9.24 Balances at December 31, 2018 5,966,928 $ 19.71 7.51 $ 99,273 As of December 31, 2018 Options vested and expected to vest 5,966,928 $ 19.71 7.51 $ 99,273 Exercisable 3,058,659 $ 15.12 6.42 $ 64,417
Schedule of stock options outstanding and exercisable by exercise price	Stock Options Outstanding Stock Options Exercisable Weighted- Average Remaining Weighted- Weighted- Range of Contractual Average Average Exercise Number of Term Exercise Price Number of Exercise Price Prices Shares (in years) Per Share Shares Per Share $0.59 – $4.25 291,433 4.49 $ 3.63 291,433 $ 3.63 $9.78 – $14.75 1,560,249 6.39 $ 12.10 1,281,242 $ 11.97 $14.77 – $22.18 852,138 6.62 $ 17.00 706,562 $ 16.35 $22.20 – $33.78 2,929,571 8.42 $ 23.92 760,672 $ 23.08 $34.56 – $43.16 333,537 9.68 $ 39.26 18,750 $ 38.93 5,966,928 7.51 $ 19.71 3,058,659 $ 15.12
Schedule of weighted average assumptions used for estimation of fair value of stock options	Options 2018 2017 2016 Common stock fair value per share $ 21.80 - 43.16 $ 19.61 - 25.67 $ 11.50 - 26.76 Expected volatility 70.97% - 73.10% 77.42% - 96.73% 75.77% - 90.83% Risk-free interest rate 2.29% - 3.10% 0.96% - 2.37% 1.03% - 2.18% Expected dividend yield — — — Expected term (in years) 5.23 - 6.08 5.23 - 6.08 5.23 - 6.08
Schedule of weighted average assumptions used for estimation of fair value of ESPP	ESPP 2018 2017 2016 Expected term (years) 0.5 - 2.0 0.5 - 2.0 0.5 - 2.0 Expected volatility 57.0% - 71.4% 67.8% - 79.8% 67.8% - 79.8% Risk-free interest rate 1.47% - 2.70% 0.47% - 1.80% 0.47% - 0.93% Expected dividend yield — — —
Summary of restricted stock unity activity	Weighted- Average Grant Date Number of Fair Value Shares (Per Unit) Unvested at December 31, 2017 — — Granted 33,933 $ 27.64 Vested — — Forfeited — — Unvested at December 31, 2018 33,933 $ 27.64

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Schedule of future minimum payments for non-cancellable operating leases	Operating Years ending December 31, Leases 2019 $ 2,752 2020 2,404 2021 1,980 2022 1,406 2023 — Thereafter —

Collaboration and Licensing A_2

Collaboration and Licensing Agreements (Tables)	12 Months Ended
	Dec. 31, 2018
Collaboration and Licensing Agreements
Schedule of revenue by licensees	The $40.6 million, $46.2 million and $109.0 million of revenue recorded for the years ended December 31, 2018, 2017 and 2016, respectively, was earned principally from the following licensees (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) Amgen $ 0.6 $ 10.0 $ 31.2 Alexion 20.0 — 5.0 CSL — 3.5 — MorphoSys — 12.5 — Novo Nordisk — — 2.7 Novartis 20.0 20.1 69.9 Other — 0.1 0.2 Total $ 40.6 $ 46.2 $ 109.0
Schedule of disaggregation of revenue	The below table summarizes the disaggregation of revenue recorded for the years ended December 31, 2018, 2017 and 2016 (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) Research collaboration $ 20.1 $ 20.1 $ 34.2 Milestone 20.5 26.0 5.0 Licensing — 0.1 69.8 Total $ 40.6 $ 46.2 $ 109.0
Schedule of revenue from U.S. and Non U.S. sources	A breakdown of our revenue from U.S. and Non‑U.S. sources for the years ended December 31, 2018, 2017 and 2016 is as follows (in millions): Year Ended December 31, 2018 2017 2016 (As Revised) (As Revised) U.S. Revenue $ 40.6 $ 30.2 $ 106.3 Non-U.S. Revenue — 16.0 2.7 Total $ 40.6 $ 46.2 $ 109.0

Condensed Quarterly Financial_2

Condensed Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Condensed Quarterly Financial Data (unaudited)
Schedule of Quarterly Financial Data	Quarterly Financial Data (in thousands, except per share data): 2018 Quarter Ended March 31, June 30, September 30, December 31, Total revenue $ — $ — $ 29,039 $ 11,564 Income (loss) from operations (30,649) (28,290) 651 (21,082) Net income (loss) (29,493) (25,869) 3,150 (18,197) Basic net income (loss) per common share (0.62) (0.46) 0.06 (0.32) Diluted net income (loss) per common share (0.62) (0.46) 0.05 (0.32) 2017 Quarter Ended March 31, June 30, September 30, December 31, (As Revised) (As Revised) (As Revised) (As Revised) Total revenue $ 3,500 $ 12,500 $ — $ 30,150 Income (loss) from operations (16,359) (8,510) (23,580) 5,326 Net income (loss) (15,475) (7,725) (22,652) 7,366 Basic net income (loss) per common share (0.33) (0.17) (0.48) 0.16 Diluted net income (loss) per common share (0.33) (0.17) (0.48) 0.15

Prior-Period Financial Statem_2

Prior-Period Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2018
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Summary of effect of adopting new accounting pronouncement	The following tables summarize the effects of adopting ASC topic 606 on our financial statements. Balance Sheet As Reported Effect of Adoption of As Revised 2017 ASC 606 2017 Assets Current assets Cash and cash equivalents $ 16,528 $ — $ 16,528 Marketable securities 207,603 — 207,603 Accounts receivable 1,142 — 1,142 Prepaid expenses and other current assets 5,606 — 5,606 Total current assets 230,879 — 230,879 Property and equipment, net 7,088 — 7,088 Patents, licenses, and other intangible assets, net 11,148 — 11,148 Marketable securities - long term 139,198 — 139,198 Income tax receivable 1,524 — 1,524 Loan receivable — 86 86 Interest receivable — 14 14 Other assets 265 — 265 Total assets $ 390,102 $ 100 $ 390,202 Liabilities and stockholders’ equity Current liabilities Accounts payable $ 6,869 $ — $ 6,869 Accrued expenses 5,480 — 5,480 Current portion of deferred rent 26 — 26 Current portion of deferred revenue 88,813 (28,695) 60,118 Income taxes 157 — 157 Total current liabilities 101,345 (28,695) 72,650 Deferred rent, less current portion 1,088 — 1,088 Deferred revenue, less current portion 5,623 (5,623) — Total liabilities 108,056 (34,318) 73,738 Commitments and contingencies Stockholders’ equity Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at December 31, 2017 — — — Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017 470 — 470 Additional paid-in capital 570,670 — 570,670 Accumulated other comprehensive income loss (1,808) — (1,808) Accumulated deficit (287,286) 34,418 (252,868) Stockholders’ equity 282,046 34,418 316,464 Total liabilities and stockholders’ equity $ 390,102 $ 100 $ 390,202 Statement of Operation As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2017 ASC 606 2017 Revenue Collaborations, licenses and milestones $ 35,711 $ 10,439 $ 46,150 Operating expenses Research and development 71,772 — 71,772 General and administrative 17,501 — 17,501 Total operating expenses 89,273 — 89,273 Loss from operations (53,562) 10,439 (43,123) Other income (expenses) Interest income 4,194 — 4,194 Interest expense (13) — (13) Other income (7) — (7) Total other income, net 4,174 — 4,174 Loss before income tax benefit (49,388) 10,439 (38,949) Income tax benefit (463) — (463) Net loss (48,925) 10,439 (38,486) Other comprehensive loss Net unrealized loss on marketable securities (367) — (367) Comprehensive loss $ (49,292) $ 10,439 $ (38,853) Basic and diluted net loss per common share $ (1.05) $ 0.23 $ (0.82) As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2016 ASC 606 2016 Revenue Collaborations, licenses and milestones $ 87,520 $ 21,500 $ 109,020 Operating expenses Research and development 51,872 — 51,872 General and administrative 13,108 — 13,108 Total operating expenses 64,980 — 64,980 Income from operations 22,540 21,500 44,040 Other income (expenses) Interest income 2,091 — 2,091 Interest expense (21) — (21) Other income 6 — 6 Total other income, net 2,076 — 2,076 Income before income tax expense 24,616 21,500 46,116 Income tax expense 991 — 991 Net income 23,625 21,500 45,125 Other comprehensive loss Net unrealized loss on marketable securities (925) — (925) Comprehensive income $ 22,700 $ 21,500 $ 44,200 Basic net income per common share $ 0.57 $ 0.52 $ 1.09 Diluted net income per common share $ 0.56 $ 0.51 $ 1.07 Statement of Stockholders’ Equity Accumulated Additional Other Total Common Stock Paid Comprehensive Accumulated Stockholders’ Stockholders’ Equity Shares Amount in-Capital Loss Deficit Equity Balance, December 31, 2016 as originally reported 46,567,978 $ 466 $ 552,889 $ (1,441) $ (237,960) $ 313,954 Adoption of ASU 2016-09 — — 401 — (401) — Adoption of ASC 606 — — — — 23,979 23,979 Balance, December 31, 2016 as revised 46,567,978 466 553,290 (1,441) (214,382) 337,933 Issuance of common stock upon exercise of stock awards 363,603 4 2,793 — — 2,797 Issuance of common stock under the Employee Stock Purchase Plan 70,907 — 936 — — 936 Comprehensive loss — — — (367) (48,925) (49,292) Stock-based compensation — — 13,651 — — 13,651 Balance, December 31, 2017 47,002,488 $ 470 $ 570,670 $ (1,808) $ (263,307) $ 306,025 Adoption of ASC topic 606 — — — — 10,439 10,439 Balance, December 31, 2017 as revised 47,002,488 $ 470 $ 570,670 $ (1,808) $ (252,868) $ 316,464 Accumulated Additional Other Total Common Stock Paid Comprehensive Accumulated Stockholders’ Stockholders’ Equity Shares Amount in-Capital Loss Deficit Equity Balance, December 31, 2015 as originally reported 40,551,039 $ 405 $ 424,128 $ (516) $ (261,585) $ 162,432 Adoption of ASC Topic 606 — — — — 2,479 2,479 Balance, December 31, 2015 as revised 40,551,039 405 424,128 (516) (259,106) 164,911 Sale of common stock, net of issuance cost 5,272,750 53 119,216 — — 119,269 Issuance of common stock upon exercise of stock awards 699,066 7 1,153 — — 1,160 Issuance of common stock under the Employee Stock Purchase Plan 45,123 1 544 — — 545 Comprehensive income (loss) — — — (925) 23,625 22,700 Stock-based compensation — — 7,848 — — 7,848 Balance, December 31, 2016 46,567,978 466 552,889 (1,441) (235,481) 316,433 Adoption of ASC Topic 606 — — — — 21,500 21,500 Balance, December 31, 2016 as revised 46,567,978 $ 466 $ 552,889 $ (1,441) $ (213,981) $ 337,933 Statement of Cash Flows As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2017 ASC 606 2017 Cash flows from operating activities Net loss $ (48,925) $ 10,439 $ (38,486) Adjustments to reconcile net loss to net cash used in operating activities: Depreciation and amortization 2,030 — 2,030 Amortization of premium on marketable securities 2,845 — 2,845 Stock-based compensation 13,651 — 13,651 Abandonment of capitalized intangible assets 396 — 396 Loss on disposal of assets 83 — 83 Changes in operating assets and liabilities: Accounts receivable 7,474 — 7,474 Interest receivable (293) (14) (307) Prepaid expenses and other assets (2,705) — (2,705) Income tax receivable (1,524) — (1,524) Other assets (161) — (161) Accounts payable 2,989 — 2,989 Accrued expenses (1,212) — (1,212) Deferred rent 589 — 589 Income tax payable 91 — 91 Deferred revenue (9,011) (10,339) (19,350) Net cash used in operating activities (33,683) 86 (33,597) Cash flows from investing activities Proceeds from sale and maturities of marketable securities available-for-sale 115,757 — 115,757 Purchase of marketable securities (76,529) — (76,529) Purchase of intangible assets (1,967) — (1,967) Purchase of property and equipment (5,311) — (5,311) Issuance of loan — (86) (86) Net cash provided by (used in) investing activities 31,950 (86) 31,864 Cash flows from financing activities Proceeds from issuance of common stock upon exercise of stock awards 2,797 — 2,797 Proceeds from issuance of common stock from Employee Stock Purchase Plan 936 — 936 Net cash provided by financing activities 3,733 — 3,733 Net increase in cash and cash equivalents 2,000 — 2,000 Cash and cash equivalents, beginning of period 14,528 — 14,528 Cash and cash equivalents, end of period $ 16,528 $ — $ 16,528 As Reported As Revised Year Ended Effect of Year Ended December 31, Adoption of December 31, 2016 ASC 606 2016 Cash flows from operating activities Net income $ 23,625 $ 21,500 $ 45,125 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 1,466 — 1,466 Amortization of premium on marketable securities 2,037 — 2,037 Stock-based compensation 7,848 — 7,848 Abandonment of capitalized intangible assets 356 — 356 Gain on sale of marketable securities available for sale (5) — (5) Changes in operating assets and liabilities: Accounts receivable (8,572) — (8,572) Interest receivable (530) (79) (609) Prepaid expenses and other assets (1,700) — (1,700) Other assets (40) — (40) Accounts payable (2,520) — (2,520) Accrued expenses 3,058 — 3,058 Deferred rent (89) — (89) Deferred tax liability 65 — 65 Deferred revenue 69,618 (20,800) 48,818 Net cash provided by operating activities 94,617 621 95,238 Cash flows from investing activities Proceeds from sale and maturities of marketable securities available-for-sale 105,505 — 105,505 Purchase of marketable securities (316,149) — (316,149) Purchase of intangible assets (1,502) — (1,502) Purchase of property and equipment (1,507) — (1,507) Proceeds from repayment of (investment in) loan receivable — (621) (621) Net cash used in investing activities (213,653) (621) (214,274) Cash flows from financing activities Proceeds from issuance of common stock upon exercise of stock awards 1,160 — 1,160 Proceeds from issuance of common stock from Employee Stock Purchase Plan 545 — 545 Proceeds from issuance of common stock 126,546 — 126,546 Common stock issuance costs (7,277) — (7,277) Net cash provided by financing activities 120,974 — 120,974 Net increase in cash and cash equivalents 1,938 — 1,938 Cash and cash equivalents, beginning of period 12,590 — 12,590 Cash and cash equivalents, end of period $ 14,528 $ — $ 14,528

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) $ in Thousands	Jan. 01, 2019USD ($)lease	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Collaborations, licenses and milestones		$ 11,564	$ 29,039	$ 30,150	$ 12,500	$ 3,500	$ 40,603	$ 46,150	$ 109,020
Increase in assets		576,732		390,202			576,732	390,202
Increase in liabilities		55,051		73,738			55,051	73,738
Current portion of deferred revenue		$ 40,079		$ 60,118			$ 40,079	$ 60,118
ASU 2016-02 | Forecast
Number of operating leases | lease	5
ASU 2016-02 | Maximum | Forecast
Right-of-Use Asset	$ 12,000
Lease liability	12,000
Increase in assets	12,000
Increase in liabilities	12,000
ASU 2016-02 | Minimum | Forecast
Right-of-Use Asset	10,000
Lease liability	10,000
Increase in assets	10,000
Increase in liabilities	$ 10,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Concentration Risk (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)customer	Dec. 31, 2017USD ($)customer
Concentrations of risk
Accrued interest | $	$ 2.3	$ 1.7
Amounts on deposit in excess of federally insured limits approximately | $	$ 26	$ 16.3
Payables | Service providers or vendors
Concentrations of risk
Number of service providers | customer	4	2
Concentration risk percentage	49.00%	40.00%
Number of critical suppliers | customer	3

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Instruments
Money Market Funds	$ 26,246	$ 16,528
Securities	504,223
Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Government Securities
Fair Value of Financial Instruments
Securities	399,256	223,530
Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	522,493	351,975
Fair Value, Measurements, Recurring | Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Fair Value, Measurements, Recurring | Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Fair Value, Measurements, Recurring | Government Securities
Fair Value of Financial Instruments
Securities	399,256	223,530
Level 1 | Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	18,270	5,175
Level 1 | Fair Value, Measurements, Recurring | Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	18,270	5,175
Level 2 | Fair Value, Measurements, Recurring
Fair Value of Financial Instruments
Total Fair Value	504,223	346,800
Level 2 | Fair Value, Measurements, Recurring | Corporate Securities
Fair Value of Financial Instruments
Securities	104,967	123,270
Level 2 | Fair Value, Measurements, Recurring | Government Securities
Fair Value of Financial Instruments
Securities	$ 399,256	$ 223,530

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Property, Plant, and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Computers, software and equipment | Minimum
Property and Equipment
Estimated useful lives of the assets	3 years
Computers, software and equipment | Maximum
Property and Equipment
Estimated useful lives of the assets	5 years
Furniture and fixtures | Minimum
Property and Equipment
Estimated useful lives of the assets	5 years
Furniture and fixtures | Maximum
Property and Equipment
Estimated useful lives of the assets	7 years
Leasehold improvements | Minimum
Property and Equipment
Estimated useful lives of the assets	5 years
Leasehold improvements | Maximum
Property and Equipment
Estimated useful lives of the assets	7 years

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Intangibles (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)item	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Patents, licenses, and other intangible assets
Number of primary criteria to determine capitalization of patent | item	3
Abandonment of capitalized intangible assets	$ 239	$ 396	$ 356
Total gross carrying amount	17,374	15,685
Patents, licenses, and other intangible assets, net	11,969	11,148
Amortization expense for patents, licenses, and other intangible assets	900	800	$ 800
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2,019	862
2,020	860
2,021	757
2,022	730
2,023	659
Thereafter	1,923
Total	5,791
Nonamortizable intangible assets (trademarks)
Patents, licenses, and other intangible assets
Nonamortizable intangible assets (trademarks)	$ 399	399
Acquired licenses | Minimum
Patents, licenses, and other intangible assets
Estimated economic life	5 years
Acquired licenses | Maximum
Patents, licenses, and other intangible assets
Estimated economic life	25 years
Patents
Patents, licenses, and other intangible assets
Accumulated amortization	$ (4,142)	(3,413)
Patents | Minimum
Patents, licenses, and other intangible assets
Estimated economic life	13 years
Patents | Maximum
Patents, licenses, and other intangible assets
Estimated economic life	20 years
Patents, definite life
Patents, licenses, and other intangible assets
Amortizable intangible assets	$ 9,320	8,915
Patents, pending issuance
Patents, licenses, and other intangible assets
Amortizable intangible assets	5,644	4,360
Licenses and other amortizable intangible assets
Patents, licenses, and other intangible assets
Amortizable intangible assets	2,011	2,011
Accumulated amortization	(1,263)	$ (1,124)
In-process intangible assets
Patents, licenses, and other intangible assets
Total gross carrying amount	$ 5,600

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Income Taxes [Line Items]
US corporate tax rate	35.00%	21.00%
Federal
Income Taxes [Line Items]
Income tax receivable	$ 1.6	$ 1.6

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Income (Loss) Per Share
Anti-dilutive securities (in shares)									1,884,000	1,291,000
Stock-Based Compensation
Stock-based compensation expense									$ 20,548	$ 13,651	$ 7,848
Basic numerator:
Net income (loss) attributable to common stockholders									$ (70,409)	$ (38,486)	$ 45,125
Denominator:
Weighted average common shares outstanding, basic									53,942,116	46,817,756	41,267,329
Net income (loss) per common share, basic (in dollars per share)	$ (0.32)	$ 0.06	$ (0.46)	$ (0.62)	$ 0.16	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.09
Diluted numerator:
Net income (loss) attributable to common stockholders for diluted net income (loss) per share									$ (70,409)	$ (38,486)	$ 45,125
Denominator
Weighted average common shares outstanding, basic									53,942,116	46,817,756	41,267,329
Dilutive effect of employee stock options and ESPP											1,121,538
Weighted average common shares outstanding, diluted									53,942,116	46,817,756	42,388,867
Net income (loss) per common share, diluted (in dollars per share)	$ (0.32)	$ 0.05	$ (0.46)	$ (0.62)	$ 0.15	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.07
Employee stock options
Net Income (Loss) Per Share
Anti-dilutive securities (in shares)									1,881,000	1,291,000
ESPP
Net Income (Loss) Per Share
Anti-dilutive securities (in shares)									3,000
Stock-Based Compensation
Stock-based compensation expense									$ 700	$ 500	$ 400

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Segments (Details)	12 Months Ended
	Dec. 31, 2018segment
Segment Reporting
Number of Operating Segments	1

Marketable Securities (Details)

Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 26,246	$ 16,528
Investments, amortized cost	523,454	353,774
Investments	522,493	351,975
Total amortized cost	505,184
Gross unrealized gains	188
Gross unrealized losses	(1,149)	(1,799)
Securities	504,223
Marketable securities
Schedule of Available-for-sale Securities
Securities	504,223	346,800
Money Market Funds
Schedule of Available-for-sale Securities
Cash and cash equivalents	18,270	5,175
Corporate Securities
Schedule of Available-for-sale Securities
Total amortized cost	105,311	123,860
Gross unrealized gains	1
Gross unrealized losses	(345)	(590)
Securities	104,967	123,270
Government Securities
Schedule of Available-for-sale Securities
Total amortized cost	399,873	224,739
Gross unrealized gains	187
Gross unrealized losses	(804)	(1,209)
Securities	399,256	223,530
Cash and Cash Equivalents
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 18,270	$ 5,175

Marketable Securities - Maturit

Marketable Securities - Maturities (Details) $ in Thousands	Dec. 31, 2018USD ($)
Amortized Cost
Maturing in one year or less	$ 269,092
Maturing after one year	236,092
Total amortized cost	505,184
Estimate Fair Value
Maturing in one year or less	268,115
Maturing after one year	236,108
Total estimated fair value	$ 504,223

Marketable Securities - Unreali

Marketable Securities - Unrealized losses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair value
Fair value, less than 12 months	$ 268,115	$ 207,603
Fair value, 12 months or greater	236,108	139,197
Unrealized losses
Unrealized losses, Less than 12 months	(977)	(598)
Unrealized gain (losses), 12 months or greater	16	(1,201)
Corporate Securities
Fair value
Fair value, less than 12 months	84,770	79,290
Fair value, 12 months or greater	20,198	43,980
Unrealized losses
Unrealized losses, Less than 12 months	(310)	(137)
Unrealized gain (losses), 12 months or greater	(34)	(453)
Government Securities
Fair value
Fair value, less than 12 months	183,345	128,313
Fair value, 12 months or greater	215,910	95,217
Unrealized losses
Unrealized losses, Less than 12 months	(667)	(461)
Unrealized gain (losses), 12 months or greater	$ 50	$ (748)

Sale of Additional Common Sto_2

Sale of Additional Common Stock (Details) - USD ($)	Sep. 19, 2016	Mar. 31, 2018	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2018	Dec. 31, 2016
Proceeds from sale of common stock					$ 245,504,000	$ 119,269,000
Common Stock
Sale of common stock		8,395,000	5,272,750	8,625,000	8,395,000	5,272,750
Proceeds from sale of common stock		$ 245,500,000	$ 119,300,000	$ 115,200,000	$ 84,000	$ 53,000
Piper Jaffray | Common Stock | Maximum
Net proceeds	$ 40,000,000

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property and equipment
Property and equipment, gross	$ 21,239	$ 15,036
Less accumulated depreciation and amortization	(9,426)	(7,948)
Property and equipment, net	11,813	7,088
Depreciation and amortization expense	2,400	1,200	$ 700
Computers, software and equipment
Property and equipment
Property and equipment, gross	16,292	10,874
Furniture and fixtures
Property and equipment
Property and equipment, gross	173	152
Leasehold and tenant improvements
Property and equipment
Property and equipment, gross	$ 4,774	$ 4,010

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of federal statutory income tax to effective income tax
Federal statutory income tax rate	$ (14,795)	$ (13,243)	$ 15,680
State and local income taxes	(4,767)	(1,806)	2,818
Research and development credit	(6,170)	(5,554)	(2,544)
Stock based compensation	444	2,709	733
Effect of the 2017 Tax Cuts and Jobs Act		19,596
Other	414	720	1,008
Net change in valuation allowance	$ 24,874	(2,885)	(16,695)
Income tax provision (benefit)		$ (463)	$ 1,000

Income Taxes - Deferred tax ass

Income Taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax assets
Net operating loss carryforwards	$ 49,889	$ 25,565
Research credits	23,151	16,642
Depreciation	207	437
Unrealized loss on securities	269	504
Accrued compensation	1,097	748
Deferred revenue	11,222	16,820
State taxes		(2)
Gross deferred income tax assets	85,835	60,714
Valuation allowance	(82,537)	(57,663)
Net deferred income tax assets	3,298	3,051
Deferred income tax liabilities
Patent costs	(3,142)	(2,873)
Licensing costs	(125)	(142)
Capitalized legal costs	(31)	(36)
Gross deferred income tax liabilities	$ (3,298)	$ (3,051)

Income Taxes - TCJA and Net ope

Income Taxes - TCJA and Net operating loss carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Federal
Income Taxes [Line Items]
Income tax receivable	$ 1.6	$ 1.6
Cumulative net operating loss carryforwards	191.1
Tax credit carryforwards	14.9
State
Income Taxes [Line Items]
Cumulative net operating loss carryforwards	138.9
Tax credit carryforwards	8.2
Years Prior to 2018 | Federal
Income Taxes [Line Items]
Cumulative net operating loss carryforwards	$ 102.6
Percentage of net operating losses which can offset future taxable income	100.00%
Tax Year 2018 | Federal
Income Taxes [Line Items]
Cumulative net operating loss carryforwards	$ 88.6
Percentage of net operating losses which can offset future taxable income	80.00%

Stock-Based Compensation - Plan

Stock-Based Compensation - Plan details (Details)	Jan. 01, 2018shares	Jan. 01, 2014shares	Dec. 31, 2018item$ / sharesshares	Dec. 31, 2017$ / sharesshares	Dec. 31, 2016$ / sharesshares	Dec. 31, 2015shares
Stock options
Stock-based compensation
Total number of shares of common stock available for issuance			3,576,574	3,394,691	2,943,216
Options granted (in shares)			1,805,937	1,511,100
Information with respect to stock options outstanding
Exercisable options (in shares)			3,058,659	2,558,941	1,743,765
Weighted-average exercise price per share of exercisable options (in dollars per share) | $ / shares			$ 15.12	$ 11.06	$ 8.87
Weighted average grant date fair value per share of options granted during the year (in dollars per share) | $ / shares			$ 18.06	$ 16.92	$ 10.30
Weighted-average remaining contractual life			7 years 6 months 4 days	7 years 7 months 13 days	7 years 9 months 26 days
ESPP
Stock-based compensation
Total number of shares of common stock available for issuance			581,286
Initial term of plan			2 years
Second term of plan			2 years
Number of six month purchase periods | item			4
Purchase period			6 months
Increase in shares of common stock available for issuance (in shares)		313,545	0	0	0	0
Awards issued under the plan (in shares)			349,716
ESPP | Minimum
Stock-based compensation
Percentage of compensation that employees may withhold to purchase stock at a discount			1.00%
ESPP | Maximum
Stock-based compensation
Percentage of compensation that employees may withhold to purchase stock at a discount			15.00%
Purchase price as percentage of stock price at the initial offering date			85.00%
Purchase price as percentage of stock price at the purchase date			85.00%
Annual percentage increase in shares of common stock available for issuance			1.00%
Annual increase in shares of common stock available for issuance (in shares)			621,814
Restricted stock units
Stock-based compensation
Restricted stock granted (in shares)			33,933
Annual installment vesting periods | item			3
The 2010 Plan
Stock-based compensation
Total number of shares of common stock available for issuance			0
The 2013 Plan
Stock-based compensation
Total number of shares of common stock available for issuance			9,581,833
Annual percentage increase in shares of common stock available for issuance			4.00%
Increase in shares of common stock available for issuance (in shares)	1,880,100
Awards issued under the plan (in shares)			6,751,287

Stock-Based Compensation - Empl

Stock-Based Compensation - Employee expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 20,548	$ 13,651	$ 7,848
Employee stock options
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	19,537	13,153	7,470
ESPP
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	744	498	378
Restricted stock units
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	267
General and administrative
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	7,699	5,617	3,592
Research and development
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 12,849	$ 8,034	$ 4,256

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock option activity, Number of Shares
Balance at the beginning of the period (in shares)	5,093,442	4,045,801
Options granted (in shares)	1,805,937	1,511,100
Options forfeited (in shares)	(107,720)	(96,856)
Options expired (in shares)		(3,000)
Options exercised (in shares)	(824,731)	(363,603)
Balance at the end of the period (in shares)	5,966,928	5,093,442	4,045,801
Options vested and expected to vest (in shares)	5,966,928
Exercisable (in shares)	3,058,659	2,558,941	1,743,765
Weighted Average Exercise Price (Per Share)
Balance at the beginning of the period (in dollars per share)	$ 15.32	$ 11.95
Options granted (in dollars per share)	27.43	22.61
Options forfeited (in dollars per share)	21.66	17.08
Options expired (in dollars per share)		21.99
Options exercised (in dollars per share)	9.24	7.69
Balance at the end of the period (in dollars per share)	19.71	15.32	$ 11.95
Options vested and expected to vest (in dollars per share)	19.71
Exercisable (in dollars per share)	$ 15.12	$ 11.06	$ 8.87
Additional information
Weighted-Average Remaining Contractual Term, Balance outstanding	7 years 6 months 4 days	7 years 7 months 13 days	7 years 9 months 26 days
Weighted-Average Remaining Contractual Term, Options vested and expected to vest	7 years 6 months 4 days
Weighted-Average Remaining Contractual Term, Exercisable	6 years 5 months 1 day
Aggregate Intrinsic Value, Balance outstanding	$ 99,273	$ 35,495	$ 58,131
Aggregate Intrinsic Value, Options vested and expected to vest	99,273
Aggregate Intrinsic Value, Exercisable	64,417
Intrinsic value of options exercised	$ 23,600	$ 5,700	$ 11,200

Stock-Based Compensation - Opti

Stock-Based Compensation - Options by exercise price (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Stock options outstanding and exercisable by exercise price
Stock Options Outstanding, Number of Shares | shares	5,966,928
Stock Options Outstanding, Remaining Contractual Term	7 years 6 months 4 days
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 19.71
Stock Options Exercisable, Number of Shares | shares	3,058,659
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 15.12
$0.59 - $4.25
Stock options outstanding and exercisable by exercise price
Range of Exercise Prices, lower limit (in dollars per share)	0.59
Range of Exercise Prices, upper limit (in dollars per share)	$ 4.25
Stock Options Outstanding, Number of Shares | shares	291,433
Stock Options Outstanding, Remaining Contractual Term	4 years 5 months 27 days
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 3.63
Stock Options Exercisable, Number of Shares | shares	291,433
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 3.63
$9.78 - $14.75
Stock options outstanding and exercisable by exercise price
Range of Exercise Prices, lower limit (in dollars per share)	9.78
Range of Exercise Prices, upper limit (in dollars per share)	$ 14.75
Stock Options Outstanding, Number of Shares | shares	1,560,249
Stock Options Outstanding, Remaining Contractual Term	6 years 4 months 21 days
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 12.10
Stock Options Exercisable, Number of Shares | shares	1,281,242
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 11.97
$14.77 – $22.18
Stock options outstanding and exercisable by exercise price
Range of Exercise Prices, lower limit (in dollars per share)	14.77
Range of Exercise Prices, upper limit (in dollars per share)	$ 22.18
Stock Options Outstanding, Number of Shares | shares	852,138
Stock Options Outstanding, Remaining Contractual Term	6 years 7 months 13 days
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 17
Stock Options Exercisable, Number of Shares | shares	706,562
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 16.35
$22.20 – $33.78
Stock options outstanding and exercisable by exercise price
Range of Exercise Prices, lower limit (in dollars per share)	22.20
Range of Exercise Prices, upper limit (in dollars per share)	$ 33.78
Stock Options Outstanding, Number of Shares | shares	2,929,571
Stock Options Outstanding, Remaining Contractual Term	8 years 5 months 1 day
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 23.92
Stock Options Exercisable, Number of Shares | shares	760,672
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 23.08
$34.56 – $43.16
Stock options outstanding and exercisable by exercise price
Range of Exercise Prices, lower limit (in dollars per share)	34.56
Range of Exercise Prices, upper limit (in dollars per share)	$ 43.16
Stock Options Outstanding, Number of Shares | shares	333,537
Stock Options Outstanding, Remaining Contractual Term	9 years 8 months 5 days
Stock Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 39.26
Stock Options Exercisable, Number of Shares | shares	18,750
Stock Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 38.93

Stock-Based Compensation - FV o

Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Common stock fair value per share minimum	21.80	19.61	11.50
Common stock fair value per share maximum	43.16	25.67	26.76
Expected volatility, low end of range (as a percent)	70.97%	77.42%	75.77%
Expected volatility, high end of range (as a percent)	73.10%	96.73%	90.83%
Risk-free interest rate, low end of range (as a percent)	2.29%	0.96%	1.03%
Risk-free interest rate, high end of range (as a percent)	3.10%	2.37%	2.18%
Compensation expense
Unamortized compensation expense related to unvested options	$ 42.8	$ 30.7
Period to recognize unamortized compensation expense	2 years 8 months 16 days
Employee stock options | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	5 years 2 months 23 days	5 years 2 months 23 days	5 years 2 months 23 days
Employee stock options | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 years 29 days	6 years 29 days	6 years 29 days
ESPP
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)	57.00%	67.80%	67.80%
Expected volatility, high end of range (as a percent)	71.40%	79.80%	79.80%
Risk-free interest rate, low end of range (as a percent)	1.47%	0.47%	0.47%
Risk-free interest rate, high end of range (as a percent)	2.70%	1.80%	0.93%
Compensation expense
Unamortized compensation expense related to unvested options	$ 0.8	$ 1.1
Period to recognize unamortized compensation expense	11 months 6 days
ESPP | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months	6 months
ESPP | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years	2 years

Stock Based Compensation - Rest

Stock Based Compensation - Restricted Stock Units (Details) - Restricted stock units $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Stock-based compensation
Granted | shares	33,933
Ending balance | shares	33,933
Weighted Average Grant Date Fair Value, Granted | $ / shares	$ 27.64
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 27.64
Compensation expense
Unamortized compensation expense related to unvested restricted stock units | $	$ 0.7
Period to recognize unamortized compensation expense	2 years 1 month 24 days

Commitments and Contingencies_2

Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2017	Jun. 30, 2017
Operating Leases
2,019	$ 2,752
2,020	2,404
2,021	1,980
2,022	1,406
Net rent expense	$ 2,500	$ 1,700	$ 600
Monrovia
Lease term				64 months
Renewal term				5 years
San Diego
Lease term					61 months
Renewal term					5 years

Commitments and Contingencies -

Commitments and Contingencies - Litigation (Details) - USD ($)	Sep. 27, 2016	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Loss contingency accrual		$ 0	$ 2,355,000
DePinto vs John S Stafford Litigation Case
Loss Contingencies [Line Items]
Paid legal award	$ 2,375,000
Amount paid not covered by insurance	$ 0

Collaboration and Licensing A_3

Collaboration and Licensing Agreements (Details)	1 Months Ended										3 Months Ended			12 Months Ended
	Oct. 31, 2017USD ($)shares	Mar. 31, 2017USD ($)	Jul. 31, 2016USD ($)	Jun. 30, 2016USD ($)item	Sep. 30, 2015USD ($)item	Dec. 31, 2014USD ($)	Jul. 31, 2013USD ($)item	Jan. 31, 2013USD ($)item	Jun. 30, 2010USD ($)	Feb. 28, 2009item	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)item	Dec. 31, 2016USD ($)	Dec. 31, 2007item
License Agreement | Merck Sharp & Dohme Corp.
Collaboration research and licensing agreements
Nonrefundable upfront payment							$ 1,000,000
Revenue recognized														$ 0	$ 0	$ 0
Deferred revenue											$ 0			0
Annual maintenance fees							$ 500,000
Number of compounds | item							1
License Agreement | INmune
Collaboration research and licensing agreements
Nonrefundable upfront payment	$ 100,000
Fully-diluted equity interests (as a percentage)	19.00%
Additional equity interests (as a percentage)	10.00%
Equity issued in shares | shares	1,585,000
Equity issues, value	$ 10,000,000
Revenue recognized														0	100,000
Deferred revenue											0			0
Option and license agreement | Alexion Pharmaceuticals, Inc.
Collaboration research and licensing agreements
Nonrefundable upfront payment								$ 3,000,000
Revenue recognized											11,000,000	$ 9,000,000		20,000,000	0	5,000,000
Deferred revenue											0			0
Number of different target programs | item								6
Research license term								5 years
Annual maintenance fees								$ 500,000
Milestone payment received														36,500,000
2009 Research License and Commercialization Agreement | CSL Limited
Collaboration research and licensing agreements
Revenue recognized		$ 3,500,000												0	3,500,000	0
Deferred revenue											0			0
Collaboration And License Agreement | Novartis
Collaboration research and licensing agreements
Nonrefundable upfront payment			$ 150,000,000
Revenue recognized														20,000,000	20,100,000	69,900,000
Deferred revenue											40,100,000		$ 60,100,000	40,100,000	60,100,000
Contract receivable											2,100,000			2,100,000
Collaboration And License Agreement | Novartis | Maximum
Collaboration research and licensing agreements
Potential milestone payment											2,100,000,000			$ 2,100,000,000
Collaboration And License Agreement | Novartis | Discovery Program
Collaboration research and licensing agreements
Right to substitute identified target period														5 years
Collaboration And License Agreement | Novartis | Bispecific FC Technologies
Collaboration research and licensing agreements
Number of development stage products | item				2
Number of previously identified products | item				4
Research license term				5 years
Collaboration And License Agreement | Novartis | Bispecific FC Technologies | XmAb14045
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 27,100,000
Collaboration And License Agreement | Novartis | Bispecific FC Technologies | XmAb13676
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 31,400,000
Collaboration And License Agreement | Novartis | Bispecific FC Technologies | Maximum
Collaboration research and licensing agreements
Number of antibody targets for which bispecific technology applied | item				4
Collaboration And License Agreement | Novartis | Global Discovery Program
Collaboration research and licensing agreements
Number of programs where Company has the right to participate | item				1
Revenue recognized												$ 20,040,000			20,050,000
Percentage of responsibility for development costs				25.00%
Percentage of responsibility for commercialization costs				50.00%
Percentage of profits on net sales of the product				50.00%
Allocation of consideration to deliverables				$ 20,050,000
Number of discovery programs | item				4
Collaboration And License Agreement | Novartis | Global Discovery Program | Maximum
Collaboration research and licensing agreements
Number of antibody targets for which bispecific technology applied | item				4
Collaboration And License Agreement | Novartis | FC Licenses
Collaboration research and licensing agreements
Allocation of consideration to deliverables				$ 11,300,000
Collaboration And License Agreement | Novartis | FC Licenses | Maximum
Collaboration research and licensing agreements
Number of targets against which non-exclusive license is provided | item				10
Collaboration And License Agreement | Novo Nordisk A/S
Collaboration research and licensing agreements
Nonrefundable upfront payment						$ 2,500,000
Revenue recognized														$ 0	0	2,700,000
Deferred revenue											0			0
Research license term						2 years
Milestone payment received						$ 1,600,000
Collaboration And License Agreement | MorphoSys
Collaboration research and licensing agreements
Nonrefundable upfront payment									$ 13,000,000
Revenue recognized														0	12,500,000	0
Deferred revenue											0			0
Research and License Agreement 2015 | Amgen, Inc.
Collaboration research and licensing agreements
Nonrefundable upfront payment					$ 45,000,000
Potential milestone payment					$ 600,000,000
Revenue recognized														600,000	10,000,000	31,200,000
Deferred revenue											0			0
Allocation of consideration to deliverables											55,500,000			55,500,000
Research and License Agreement 2015 | Amgen, Inc. | ASU 2014-09
Collaboration research and licensing agreements
Revenue recognized																31,250,000
Research and License Agreement 2015 | Amgen, Inc. | CD38 Program
Collaboration research and licensing agreements
Revenue recognized													$ 10,000,000
Allocation of consideration to deliverables											23,750,000			23,750,000
Research and License Agreement 2015 | Amgen, Inc. | Discovery Program
Collaboration research and licensing agreements
Revenue recognized											500,000
Research license term					3 years
Allocation of consideration to deliverables											6,250,000			6,250,000
Number of discovery programs | item					5
Additional research term					1 year
Previous targets which bispecific technology will be applied | item					5
Number of delivered discover programs | item					5
Research Licensee and Collaboration Agreement 2007 Agreement | CSL Limited
Collaboration research and licensing agreements
Number of commercial licenses | item										1
Research Licensee and Collaboration Agreement 2007 Agreement | Boehringer Ingelheim International GmbH
Collaboration research and licensing agreements
Revenue recognized														0	$ 0	$ 0
Deferred revenue											$ 0			$ 0
Research license term																	3 years
Number of commercial options | item																	1
Number of compounds | item															1
Number of commercial licenses | item															2

Collaborative and Licensing Agr

Collaborative and Licensing Agreements - Revenue Recognition (Details) - USD ($)	1 Months Ended	3 Months Ended					12 Months Ended
	Jun. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 11,564,000	$ 29,039,000	$ 30,150,000	$ 12,500,000	$ 3,500,000	$ 40,603,000	$ 46,150,000	$ 109,020,000
Research collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized							20,100,000	20,100,000	34,200,000
Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized							20,500,000	26,000,000	5,000,000
Licensing
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized								100,000	69,800,000
United States
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned							40,600,000	30,200,000	106,300,000
Non - United States
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned								16,000,000	2,700,000
Alexion Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned							20,000,000		5,000,000
Amgen, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned							600,000	10,000,000	31,200,000
CSL Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned								3,500,000
MorphoSys
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned								12,500,000
Novo Nordisk A/S
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned									2,700,000
Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned							20,000,000	20,100,000	69,900,000
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue earned								100,000	$ 200,000
2009 Research License and Commercialization Agreement | CSL Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue		0					0
Collaboration And License Agreement | MorphoSys
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment	$ 12,500,000
Deferred revenue		0					0
Collaboration And License Agreement | Novo Nordisk A/S
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue		0					0
Collaboration And License Agreement | Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue		$ 40,100,000		$ 60,100,000			$ 40,100,000	$ 60,100,000

401(k) Plan (Details)

401(k) Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan employer matching contribution percent	3.50%
Employer contributions	$ 500	$ 0	$ 0
Vesting period	3 years
Annual vesting percentage	33.00%
1% of participating employee contributions
Defined Benefit Plan Disclosure [Line Items]
Percentage of company matching to defined contribution plan	100.00%
Defined contribution plan employer matching contribution percent	1.00%
5% of participating employee contributions
Defined Benefit Plan Disclosure [Line Items]
Percentage of company matching to defined contribution plan	50.00%
Defined contribution plan employer matching contribution percent	5.00%

Condensed Quarterly Financial_3

Condensed Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Quarterly Financial Data (unaudited)
Total revenue	$ 11,564	$ 29,039			$ 30,150		$ 12,500	$ 3,500	$ 40,603	$ 46,150	$ 109,020
Income (loss) from operations	(21,082)	651	$ (28,290)	$ (30,649)	5,326	$ (23,580)	(8,510)	(16,359)	(79,370)	(43,123)	44,040
Net income (loss)	$ (18,197)	$ 3,150	$ (25,869)	$ (29,493)	$ 7,366	$ (22,652)	$ (7,725)	$ (15,475)	$ (70,409)	$ (38,486)	$ 45,125
Basic net income (loss) (in dollars per share)	$ (0.32)	$ 0.06	$ (0.46)	$ (0.62)	$ 0.16	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.09
Diluted net income (loss) (in dollars per share)	$ (0.32)	$ 0.05	$ (0.46)	$ (0.62)	$ 0.15	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.07

Prior-Period Financial Statem_3

Prior-Period Financial Statementss - (Details) - USD ($) $ in Thousands	3 Months Ended						12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaborations, licenses and milestones	$ 11,564	$ 29,039	$ 30,150	$ 12,500	$ 3,500		$ 40,603	$ 46,150	$ 109,020
Research collaboration
Revenue
Collaborations, licenses and milestones							$ 20,100	20,100	34,200
Licensing
Revenue
Collaborations, licenses and milestones								100	69,800
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Revenue
Collaborations, licenses and milestones								10,439	21,500
ASU 2014-09 | Research collaboration
Revenue
Collaborations, licenses and milestones								$ 20,500	$ 6,250
ASU 2014-09 | Licensing
Revenue
Prior revenue recognition period						5 years
Collaborations, licenses and milestones						$ 11,300

Prior-Period Financial Statem_4

Prior-Period Financial Statements - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 26,246	$ 16,528
Marketable securities	268,115	207,603
Accounts receivable	10,187	1,142
Prepaid expenses and other current assets	10,375	5,606
Total current assets	315,727	230,879
Property and equipment, net	11,813	7,088
Patents, licenses, and other intangible assets, net	11,969	11,148
Marketable securities - long term	236,108	139,198
Income tax receivable	804	1,524
Loan receivable		86
Interest receivable		14
Other assets	311	265
Total assets	576,732	390,202
Current liabilities
Accounts payable	3,797	6,869
Accrued expenses	9,662	5,480
Current portion of deferred rent	315	26
Current portion of deferred revenue	40,079	60,118
Income taxes payable		157
Total current liabilities	53,853	72,650
Deferred rent, less current portion	1,198	1,088
Total liabilities	55,051	73,738
Commitments and contingencies
Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at December 31, 2017
Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017	563	470
Additional paid-in capital	845,366	570,670
Accumulated other comprehensive loss	(971)	(1,808)
Accumulated deficit	(323,277)	(252,868)
Stockholders' equity	521,681	316,464	$ 337,933
Total liabilities and stockholders' equity	$ 576,732	390,202
As Reported Before ASC Topic 606
Current liabilities
Stockholders' equity			$ 316,433
ASU 2014-09 | As Reported Before ASC Topic 606
Current assets
Cash and cash equivalents		16,528
Marketable securities		207,603
Accounts receivable		1,142
Prepaid expenses and other current assets		5,606
Total current assets		230,879
Property and equipment, net		7,088
Patents, licenses, and other intangible assets, net		11,148
Marketable securities - long term		139,198
Income tax receivable		1,524
Other assets		265
Total assets		390,102
Current liabilities
Accounts payable		6,869
Accrued expenses		5,480
Current portion of deferred rent		26
Current portion of deferred revenue		88,813
Income taxes payable		157
Total current liabilities		101,345
Deferred rent, less current portion		1,088
Deferred revenue, less current portion		5,623
Total liabilities		108,056
Commitments and contingencies
Common stock, $0.01 par value: 200,000,000 authorized shares at December 31, 2017; 47,002,488 issued and outstanding at December 31, 2017		470
Additional paid-in capital		570,670
Accumulated other comprehensive loss		(1,808)
Accumulated deficit		(287,286)
Stockholders' equity		282,046
Total liabilities and stockholders' equity		390,102
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Current assets
Loan receivable		86
Interest receivable		14
Total assets		100
Current liabilities
Current portion of deferred revenue		(28,695)
Total current liabilities		(28,695)
Deferred revenue, less current portion		(5,623)
Total liabilities		(34,318)
Commitments and contingencies
Accumulated deficit		34,418
Stockholders' equity		34,418
Total liabilities and stockholders' equity		$ 100

Prior-Period Financial Statem_5

Prior-Period Financial Statements - Balance Sheet Stock Information (Details) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Condensed Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	56,279,542	47,002,488
Common stock, shares outstanding	56,279,542	47,002,488

Prior-Period Financial Statem_6

Prior-Period Financial Statements - Comprehensive Income (Loss) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaborations, licenses and milestones	$ 11,564	$ 29,039			$ 30,150		$ 12,500	$ 3,500	$ 40,603	$ 46,150	$ 109,020
Operating expenses
Research and development									97,501	71,772	51,872
General and administrative									22,472	17,501	13,108
Total operating expenses									119,973	89,273	64,980
Income (loss) from operations	(21,082)	651	$ (28,290)	$ (30,649)	5,326	$ (23,580)	(8,510)	(16,359)	(79,370)	(43,123)	44,040
Other income (expense)
Interest income									9,102	4,194	2,091
Interest expense									(16)	(13)	(21)
Other income (expense)									(125)	(7)	6
Total other income, net									8,961	4,174	2,076
Income (loss) before income tax									(70,409)	(38,949)	46,116
Income tax expense										(463)	991
Net income (loss)	$ (18,197)	$ 3,150	$ (25,869)	$ (29,493)	$ 7,366	$ (22,652)	$ (7,725)	$ (15,475)	(70,409)	(38,486)	45,125
Other comprehensive income
Net unrealized gain (loss) on marketable securities available-for-sale									837	(367)	(925)
Comprehensive income (loss)									$ (69,572)	$ (38,853)	$ 44,200
Basic and diluted net loss (in dollars per share)										$ (0.82)
Basic net income (loss) (in dollars per share)	$ (0.32)	$ 0.06	$ (0.46)	$ (0.62)	$ 0.16	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	(0.82)	$ 1.09
Diluted net income (loss) (in dollars per share)	$ (0.32)	$ 0.05	$ (0.46)	$ (0.62)	$ 0.15	$ (0.48)	$ (0.17)	$ (0.33)	$ (1.31)	$ (0.82)	$ 1.07
As Reported Before ASC Topic 606
Other comprehensive income
Comprehensive income (loss)										$ (49,292)	$ 22,700
ASU 2014-09 | As Reported Before ASC Topic 606
Revenue
Collaborations, licenses and milestones										35,711	87,520
Operating expenses
Research and development										71,772	51,872
General and administrative										17,501	13,108
Total operating expenses										89,273	64,980
Income (loss) from operations										(53,562)	22,540
Other income (expense)
Interest income										4,194	2,091
Interest expense										(13)	(21)
Other income (expense)										(7)	6
Total other income, net										4,174	2,076
Income (loss) before income tax										(49,388)	24,616
Income tax expense										(463)	991
Net income (loss)										(48,925)	23,625
Other comprehensive income
Net unrealized gain (loss) on marketable securities available-for-sale										(367)	(925)
Comprehensive income (loss)										$ (49,292)	$ 22,700
Basic and diluted net loss (in dollars per share)										$ (1.05)
Basic net income (loss) (in dollars per share)											$ 0.57
Diluted net income (loss) (in dollars per share)											$ 0.56
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Revenue
Collaborations, licenses and milestones										$ 10,439	$ 21,500
Operating expenses
Income (loss) from operations										10,439	21,500
Other income (expense)
Income (loss) before income tax										10,439	21,500
Net income (loss)										10,439	21,500
Other comprehensive income
Comprehensive income (loss)										$ 10,439	$ 21,500
Basic and diluted net loss (in dollars per share)										$ 0.23
Basic net income (loss) (in dollars per share)											$ 0.52
Diluted net income (loss) (in dollars per share)											$ 0.51

Prior-Period Financial Statem_7

Prior-Period Financial Statements - Stockholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity
Balance				$ 316,464	$ 337,933
Balance, revised		$ 164,911			316,464	$ 337,933
Sale of common stock, net of issuance cost				245,504		119,269
Issuance of common stock upon exercise of stock awards				7,617	2,797	1,160
Issuance of common stock under the Employee Stock Purchase Plan				1,120	936	545
Comprehensive income (loss)				(69,572)	(38,853)	44,200
Stock-based compensation expense				20,548	13,651	7,848
Balance				521,681	316,464	337,933
As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					316,433
Sale of common stock, net of issuance cost						119,269
Issuance of common stock upon exercise of stock awards					2,797	1,160
Issuance of common stock under the Employee Stock Purchase Plan					936	545
Comprehensive income (loss)					(49,292)	22,700
Stock-based compensation expense					13,651	7,848
Balance						316,433
ASU 2014-09 | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				282,046
Comprehensive income (loss)					(49,292)	22,700
Balance					282,046
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				34,418
Adoption of new accounting principle						21,500
Comprehensive income (loss)					10,439	$ 21,500
Balance					34,418
Common Stock
Increase (Decrease) in Stockholders' Equity
Balance				$ 470	$ 466
Balance (in shares)				47,002,488	46,567,978	40,551,039
Balance, revised		405			$ 470	$ 466
Sale of common stock, net of issuance cost	$ 245,500	$ 119,300	$ 115,200	$ 84		$ 53
Sale of common stock, net of issuance cost (in shares)	8,395,000	5,272,750	8,625,000	8,395,000		5,272,750
Issuance of common stock upon exercise of stock awards				$ 8	$ 4	$ 7
Issuance of common stock upon exercise of stock awards (in shares)				824,731	363,603	699,066
Issuance of common stock under the Employee Stock Purchase Plan				$ 1		$ 1
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				57,323	70,907	45,123
Balance				$ 563	$ 470	$ 466
Balance (in shares)		40,551,039		56,279,542	47,002,488	46,567,978
Common Stock | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					$ 466
Balance (in shares)					46,567,978
Sale of common stock, net of issuance cost						$ 53
Sale of common stock, net of issuance cost (in shares)						5,272,750
Issuance of common stock upon exercise of stock awards					$ 4	$ 7
Issuance of common stock upon exercise of stock awards (in shares)					363,603	699,066
Issuance of common stock under the Employee Stock Purchase Plan						$ 1
Issuance of common stock under the Employee Stock Purchase Plan (in shares)					70,907	45,123
Balance						$ 466
Balance (in shares)						46,567,978
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Balance				$ 570,670	$ 552,889
Balance, revised		$ 424,128			570,670	$ 553,290
Sale of common stock, net of issuance cost				245,420		119,216
Issuance of common stock upon exercise of stock awards				7,609	2,793	1,153
Issuance of common stock under the Employee Stock Purchase Plan				1,119	936	544
Stock-based compensation expense				20,548	13,651	7,848
Balance				845,366	570,670	552,889
Additional Paid-in Capital | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					552,889
Sale of common stock, net of issuance cost						119,216
Issuance of common stock upon exercise of stock awards					2,793	1,153
Issuance of common stock under the Employee Stock Purchase Plan					936	544
Stock-based compensation expense					13,651	7,848
Balance						552,889
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Balance				(1,808)	(1,441)
Balance, revised		(516)			(1,808)	(1,441)
Comprehensive income (loss)				837	(367)	(925)
Balance				(971)	(1,808)	(1,441)
Accumulated Other Comprehensive Loss | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					(1,441)
Comprehensive income (loss)					(367)	(925)
Balance						(1,441)
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Balance				(252,868)	(213,981)
Balance, revised		(259,106)			(252,868)	(214,382)
Comprehensive income (loss)				(70,409)	(38,486)	45,125
Balance				(323,277)	(252,868)	(213,981)
Accumulated Deficit | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance					(235,481)
Comprehensive income (loss)					(48,925)	23,625
Balance						(235,481)
Accumulated Deficit | ASU 2014-09 | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle						21,500
Scenario, Previously Reported
Increase (Decrease) in Stockholders' Equity
Balance					337,933	162,432
Balance		162,432				337,933
Scenario, Previously Reported | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				306,025	313,954	162,432
Balance		162,432			306,025	313,954
Scenario, Previously Reported | Common Stock
Increase (Decrease) in Stockholders' Equity
Balance					$ 466	$ 405
Balance (in shares)					46,567,978	40,551,039
Balance		$ 405				$ 466
Balance (in shares)		40,551,039				46,567,978
Scenario, Previously Reported | Common Stock | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				$ 470	$ 466	$ 405
Balance (in shares)				47,002,488	46,567,978	40,551,039
Balance		$ 405			$ 470	$ 466
Balance (in shares)		40,551,039			47,002,488	46,567,978
Scenario, Previously Reported | Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Balance					$ 552,889	$ 424,128
Balance		$ 424,128				552,889
Scenario, Previously Reported | Additional Paid-in Capital | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				$ 570,670	552,889	424,128
Balance		424,128			570,670	552,889
Scenario, Previously Reported | Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Balance					(1,441)	(516)
Balance		(516)				(1,441)
Scenario, Previously Reported | Accumulated Other Comprehensive Loss | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				(1,808)	(1,441)	(516)
Balance		(516)			(1,808)	(1,441)
Scenario, Previously Reported | Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Balance					(213,981)	(261,585)
Balance		(261,585)				(213,981)
Scenario, Previously Reported | Accumulated Deficit | As Reported Before ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Balance				$ (263,307)	(237,960)	(261,585)
Balance		(261,585)			(263,307)	(237,960)
Restatement Adjustment | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle		2,479
Restatement Adjustment | ASU 2014-09
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle		2,479
Restatement Adjustment | ASU 2014-09 | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle					10,439	23,979
Restatement Adjustment | Additional Paid-in Capital | ASU 2016-09
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle						401
Restatement Adjustment | Accumulated Deficit | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle		2,479
Restatement Adjustment | Accumulated Deficit | ASU 2016-09
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle						(401)
Restatement Adjustment | Accumulated Deficit | ASU 2014-09
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle		$ 2,479
Restatement Adjustment | Accumulated Deficit | ASU 2014-09 | Effect of Adoption of ASC Topic 606
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle					$ 10,439	$ 23,979

Prior-Period Financial Statem_8

Prior-Period Financial Statements - Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income (loss)	$ (70,409)	$ (38,486)	$ 45,125
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	3,251	2,030	1,466
Amortization of premium on marketable securities	(394)	2,845	2,037
Stock-based compensation	20,548	13,651	7,848
Abandonment of capitalized intangible assets	239	396	356
Loss on disposal of assets	102	83
Loss (gain) on sale of marketable securities available-for-sale	74		(5)
Changes in operating assets and liabilities:
Accounts receivable	(9,045)	7,474	(8,572)
Interest receivable	(535)	(307)	(609)
Prepaid expenses and other assets	(4,769)	(2,705)	(1,700)
Income tax receivable	(84)	(1,524)
Other assets	(46)	(161)	(40)
Accounts payable	(3,072)	2,989	(2,520)
Accrued expenses	4,182	(1,212)	3,058
Deferred rent	398	589	(89)
Income taxes payable	(157)	91	65
Deferred revenue	(20,039)	(19,350)	48,818
Net cash provided by (used in) operating activities	(79,756)	(33,597)	95,238
Cash flows from investing activities
Proceeds from sale and maturities of marketable securities available-for-sale	222,125	115,757	105,505
Purchase of marketable securities	(377,840)	(76,529)	(316,149)
Purchase of intangible assets	(1,935)	(1,967)	(1,502)
Purchase of property and equipment	(7,212)	(5,311)	(1,507)
Proceeds from sale of property and equipment	9
Repayment of loan	86	(86)	(621)
Net cash provided by (used in) investing activities	(164,767)	31,864	(214,274)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards	7,617	2,797	1,160
Proceeds from issuance of common stock under the Employee Stock Purchase Plan	1,120	936	545
Proceeds from issuance of common stock	260,245		126,546
Common stock issuance costs	(14,741)		(7,277)
Net cash provided by financing activities	254,241	3,733	120,974
Net increase in cash and cash equivalents	9,718	2,000	1,938	$ 1,938
Cash and cash equivalents, beginning of period	16,528	14,528	12,590
Cash and cash equivalents, end of period	26,246	16,528	14,528	12,590
ASU 2014-09 | As Reported Before ASC Topic 606
Cash flows from operating activities
Net income (loss)		(48,925)	23,625
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization		2,030	1,466
Amortization of premium on marketable securities		2,845	2,037
Stock-based compensation		13,651	7,848
Abandonment of capitalized intangible assets		396	356
Loss on disposal of assets		83
Loss (gain) on sale of marketable securities available-for-sale			(5)
Changes in operating assets and liabilities:
Accounts receivable		7,474	(8,572)
Interest receivable		(293)	(530)
Prepaid expenses and other assets		(2,705)	(1,700)
Income tax receivable		(1,524)
Other assets		(161)	(40)
Accounts payable		2,989	(2,520)
Accrued expenses		(1,212)	3,058
Deferred rent		589	(89)
Income taxes payable		91	65
Deferred revenue		(9,011)	69,618
Net cash provided by (used in) operating activities		(33,683)	94,617
Cash flows from investing activities
Proceeds from sale and maturities of marketable securities available-for-sale		115,757	105,505
Purchase of marketable securities		(76,529)	(316,149)
Purchase of intangible assets		(1,967)	(1,502)
Purchase of property and equipment		(5,311)	(1,507)
Net cash provided by (used in) investing activities		31,950	(213,653)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock awards		2,797	1,160
Proceeds from issuance of common stock under the Employee Stock Purchase Plan		936	545
Proceeds from issuance of common stock			126,546
Common stock issuance costs			(7,277)
Net cash provided by financing activities		3,733	120,974
Net increase in cash and cash equivalents		2,000		1,938
Cash and cash equivalents, beginning of period	$ 16,528	14,528	12,590
Cash and cash equivalents, end of period		16,528	14,528	$ 12,590
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Cash flows from operating activities
Net income (loss)		10,439	21,500
Changes in operating assets and liabilities:
Interest receivable		(14)	(79)
Deferred revenue		(10,339)	(20,800)
Net cash provided by (used in) operating activities		86	621
Cash flows from investing activities
Repayment of loan		(86)	(621)
Net cash provided by (used in) investing activities		$ (86)	$ (621)

Subsequent Event (Details)

Subsequent Event (Details) - Collaboration And License Agreement - Genentech - Subsequent Events $ in Millions	1 Months Ended
	Feb. 28, 2019USD ($)
Collaboration research and licensing agreements
Nonrefundable upfront payment	$ 120
Research license term	2 years
XmAb24306
Collaboration research and licensing agreements
Percentage of profits on net sales of the product	45.00%
Collaboration Product Which Advances To Phase 1 Clinical Trials
Collaboration research and licensing agreements
Potential milestone payment	$ 20
Collaboration Product Which Advances To Phase 3 Clinical Trials
Collaboration research and licensing agreements
Potential milestone payment	$ 160