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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Feb. 28, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	Merus N.V.
Trading Symbol	MRUS
Entity Central Index Key	0001651311
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Interactive Data Current	Yes
Entity Common Stock, Shares Outstanding		29,003,005
Entity Public Float			$ 342.7
Entity File Number	001-37773
Entity Incorporation, State or Country Code	P7
Entity Tax Identification Number	00-0000000
Entity Address, Address Line One	Yalelaan 62
Entity Address, City or Town	Utrecht
Entity Address, Postal Zip Code	3584 CM
Entity Address, Country	NL
City Area Code	+31 30
Local Phone Number	253 8800
Document Annual Report	true
Document Transition Report	false
Title of 12(b) Security	Common shares, nominal value €0.09 per share
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive proxy statement that the registrant intends to file with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the registrant’s 2020 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K to the extent stated herein.

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 197,612	$ 164,590
Marketable securities	42,153	70,761
Accounts receivable	941	369
Accounts receivable (related party)	1,711	2,982
Prepaid expenses and other current assets	4,951	4,733
Total current assets	247,368	243,435
Marketable securities	2,009
Property and equipment, net	3,715	2,706
Operating lease right-of-use assets	5,215	2,538
Intangible assets, net	2,876	2,864
Deferred tax assets	288	199
Other assets	1,905	1,231
Total assets	263,376	252,973
Current liabilities:
Accounts payable	3,029	3,239
Accrued expenses	13,536	10,386
Current portion of lease obligation	1,380	840
Current portion of deferred revenue	941	1,248
Current portion of deferred revenue (related party)	17,901	18,246
Total current liabilities	36,787	33,959
Lease obligation	3,872	1,849
Deferred revenue, net of current portion	780	683
Deferred revenue, net of current portion (related party)	90,637	110,625
Total liabilities	132,076	147,116
Commitments and contingencies (Note 10)
Stockholders’ equity:
Common shares	2,918	2,366
Additional paid-in capital	441,395	360,045
Accumulated other comprehensive income	1,586	2,894
Accumulated deficit	(314,599)	(259,448)
Total stockholders’ equity	131,300	105,857
Total liabilities and stockholders’ equity	$ 263,376	$ 252,973

CONSOLIDATED BALANCE SHEETS (Parenthetical) - € / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common shares, par value	€ 0.09	€ 0.09
Common shares, authorized	45,000,000	45,000,000
Common shares, issued	28,882,217	23,358,977
Common shares, outstanding	28,882,217	23,358,977

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Total revenue	$ 31,133	$ 38,378
Operating expenses:
Research and development	55,680	54,767
General and administrative	34,110	29,354
Total operating expenses	89,790	84,121
Operating loss	(58,657)	(45,743)
Other income, net:
Interest income, net	1,889	2,132
Foreign exchange gains	1,615	7,126
Miscellaneous income and gains	196	8,380
Other income, net	3,700	17,638
Loss before income taxes	(54,957)	(28,105)
Tax expense	194	205
Net loss	(55,151)	(28,310)
Other comprehensive loss:
Currency translation adjustment	(1,308)	(6,498)
Comprehensive loss	$ (56,459)	$ (34,808)
Loss per share allocable to common stockholders:
Basic and diluted	$ (2.28)	$ (1.27)
Weighted average shares outstanding:
Basic and diluted	24,218,083	22,286,720
Collaboration Revenue [Member]
Total revenue from contracts with customers	$ 5,517	$ 8,176
Collaboration Revenue (Related Party) [Member]
Total revenue from contracts with customers	25,831	29,969
Grant Revenue [Member]
Total revenue from contracts with customers	$ (215)	$ 233

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (55,151)	$ (28,310)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of property and equipment	902	645
Amortization of intangible assets	236	109
Foreign exchange gain	(1,068)	(6,139)
Stock-based compensation expense	7,834	9,754
Amortization of discount on investments	(531)	(559)
Deferred tax benefit	(89)	(212)
Changes in operating assets and liabilities:
Accounts receivable	633	(1,573)
Operating lease right-of-use assets and lease obligations	(112)	78
Prepaid expenses and other current assets	(1,029)	(2,830)
Accounts payable	22	(133)
Accrued expenses	3,317	(25)
Deferred revenue	(18,012)	(16,901)
Net cash used in operating activities	(63,048)	(46,096)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(60,413)	(89,735)
Proceeds from maturities of marketable securities	87,183	68,914
Purchases of intangible assets	(375)	(2,510)
Purchases of property and equipment	(2,223)	(1,834)
Net cash provided by (used in) investing activities	24,172	(25,165)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of issuance costs	74,184	62,008
Proceeds from stock options exercised	48	1,035
Net cash provided by financing activities	74,232	63,043
Foreign exchange impact on cash, cash equivalents and restricted cash	(2,273)	(6,557)
Net increase (decrease) in cash, cash equivalents and restricted cash	33,083	(14,775)
Cash, cash equivalents, and restricted cash, beginning of period	164,730	179,505
Cash, cash equivalents, and restricted cash, end of period	197,813	164,730
SUPPLEMENTAL DISCLOSURES:
Non-cash lease obligations acquired from operating lease right-of-use assets	3,875	1,663
Non-cash purchases of property, equipment and intangibles	187	553
Non-cash financing costs	164	0
Income taxes paid	(320)	(737)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and cash equivalents	197,612	164,590
Restricted cash included in other assets	201	140
Cash, cash equivalents, and restricted cash, end of period	$ 197,813	$ 164,730

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balance at Dec. 31, 2017	$ 67,868	$ 1,935	$ 287,679	$ (231,138)	$ 9,392
Beginning balance, shares at Dec. 31, 2017		19,429,848
Issuance of common stock, net	62,008	$ 407	61,601	0	0
Issuance of common stock, net, shares		3,699,997
Exercise of stock options and vesting of restricted stock units	1,035	$ 24	1,011	0	0
Exercise of stock options and vesting of restricted stock units, shares		229,132
Stock-based compensation	9,754	$ 0	9,754	0	0
Currency translation adjustment	(6,498)	0	0	0	(6,498)
Net loss	(28,310)	0	0	(28,310)	0
Ending balance at Dec. 31, 2018	105,857	$ 2,366	360,045	(259,448)	2,894
Ending balance, shares at Dec. 31, 2018		23,358,977
Issuance of common stock, net	74,020	$ 546	73,474	0	0
Issuance of common stock, net, shares		5,462,500
Exercise of stock options and vesting of restricted stock units	48	$ 6	42	0	0
Exercise of stock options and vesting of restricted stock units, shares		60,740
Stock-based compensation	7,834	$ 0	7,834	0	0
Currency translation adjustment	(1,308)	0	0	0	(1,308)
Net loss	(55,151)	0	0	(55,151)	0
Ending balance at Dec. 31, 2019	$ 131,300	$ 2,918	$ 441,395	$ (314,599)	$ 1,586
Ending balance, shares at Dec. 31, 2019		28,882,217

- Definition

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

+ References + Details

The Company

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

The Company

1. The Company

Merus N.V. is a clinical-stage immuno-oncology company developing innovative bispecific antibody therapeutics, headquartered in Utrecht, the Netherlands. Merus US, Inc. is a wholly-owned subsidiary of Merus N.V. located at 139 Main Street, Cambridge, Massachusetts, United States (collectively, the “Company”).

Since inception, the Company has generated an accumulated loss of $314.6 million as of December 31, 2019. The Company expects to continue to incur significant expenses and operating losses for the foreseeable future as its bispecific antibody candidates advance through discovery, pre-clinical development and clinical trials and as it seeks regulatory approval and pursues commercialization of any approved bispecific antibody candidate.

As a result, the Company may need additional financing to support its continuing operations. Until the Company can generate significant revenue from product sales, if ever, the Company expects to finance its operations through public equity offerings, debt financings, or other sources, which may include collaborations with third parties and business development opportunities. Adequate additional financing may not be available to the Company on acceptable terms, or at all. The Company’s inability to raise capital as and when needed would have a negative impact on its financial condition and ability to pursue its business strategy. The Company will need to generate significant revenues to achieve profitability and may never do so.

Based on the Company’s current operating plan, the Company expects that its existing cash and cash equivalents and marketable securities of $241.8 million as of December 31, 2019 will be sufficient to fund its operations into 2022.

Investments in Debt Securities

12 Months Ended

Dec. 31, 2019

Debt Securities [Abstract]

Investments in Debt Securities

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3. Investments in Debt Securities

Debt securities are classified in the consolidated balance sheet as follows:

		December 31,
		2019		2018
		Balance		Balance
	(in thousands)
Cash equivalents		$	34,053	$	23,457
Current marketable securities			42,153		70,761
Non-current marketable securities			2,009		—
Total		$	78,215	$	94,218

The following table summarizes debt securities by maturity at December 31, 2019 (in thousands):

Maturity	Amortized Cost
Within one year	$	76,206
After one year through five years		2,009
Total	$	78,215

The following table summarizes debt securities by credit-quality indicator:

	Credit Quality Indicator as of December 31, 2019
	AAA		AA- to AA+		A- to A+		Total
	(In thousands)
Money market funds	$	34,053	$	—	$	—	$	34,053
Corporate paper and notes		—		11,550		27,129		38,679
U.S. government agency securities		—		3,987		—		3,987
U.S. treasuries		—		1,496		—		1,496
Total	$	34,053	$	17,033	$	27,129	$	78,215

The credit quality indicator was derived from publicly available ratings published by Moody’s or a comparable credit rating agency, last updated as of December 31, 2019.

The following table summarizes the fair value of debt securities by major security type held at December 31, 2019 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	34,053	$	—	$	—		$	34,053
Corporate paper and notes		38,679		32		(2	)		38,709
U.S. government agency securities		3,987		7		—			3,994
U.S. treasuries		1,496		2		—			1,498
Total	$	78,215	$	41	$	(2	)	$	78,254

The following table summarizes the fair value of debt securities by major security type held at December 31, 2018 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	4,463	$	—	$	—		$	4,463
Term deposits		18,000		—		—			18,000
Corporate paper and notes		62,066		—		(86	)		61,980
U.S. government agency securities		1,980		—		—			1,980
U.S. treasuries		7,709		—		(1	)		7,708
Total	$	94,218	$	—	$	(87	)	$	94,131

The allowance for credit losses applicable to debt securities was immaterial in all periods presented.

Fair Value

The fair value of money market funds is determined based on publicly available market price for these funds (Level 1). The fair value of other debt securities is determined based on the publicly available inputs which includes a market price for the same or similar instruments adjusted for estimates in interest yield (Level 2).

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Preparation

The Company prepared its consolidated financial statements in compliance with generally accepted accounting principles in the U.S. ("U.S. GAAP"). Any reference in these notes to applicable guidance is meant to refer to authoritative U.S. GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB").

Principles of Consolidation

Subsidiaries are entities controlled by the Company, consisting of Merus N.V.’s wholly owned subsidiary Merus US, Inc. The Company controls an entity when it is exposed to, or has rights to, variable returns from its involvement with the entity and has the ability to affect those returns through its power over the entity. The financial statements of subsidiaries are included in the consolidated financial statements from the date on which control commences until the date on which control ceases. All significant intercompany balances and transactions have been eliminated in consolidation.

Functional and Presentation Currency

Items recorded in each of the Company’s entities are measured using the currency of the primary economic environment in which the entity operates (the "functional currency"). Merus US, Inc.’s functional currency is the U.S. dollar. The functional currency of Merus N.V. is the euro. After measuring foreign currency denominated transactions into an entity’s functional currency, to the extent that a subsidiary’s functional currency differs from its parent, a subsidiary’s financial position and results of operations are translated into its parent’s functional currency. The Company’s consolidated financial position and results of operations are translated into the U.S. dollar as the Company’s reporting currency.

Use of Estimates

The preparation of these consolidated financial statements in accordance with U.S. GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of collaboration revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer.

Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection.

Subsequent Events

The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission.

Fair Value Measurements

Fair value is defined as an exit price, representing the amount that would be received upon the sale of an asset or payment to transfer a liability in an orderly transaction between market participants. Fair value is a market-based measurement that is determined based on assumptions that market participants would use in pricing an asset or liability. A three-tier fair value hierarchy is used to prioritize the inputs in measuring fair value as follows:

•

Level 1 – Quoted market prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

•

Level 2 – Quoted market prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable, either directly or indirectly. Fair value determined through the use of models or other valuation methodologies.

•

Level 3 – Significant unobservable inputs for assets or liabilities that cannot be corroborated by market data. Fair value is determined by the reporting entity’s own assumptions utilizing the best information available and includes situations where there is little market activity for the asset or liability.

The asset’s or liability’s fair value measurement within the fair value hierarchy is based upon the lowest level of any input that is significant to the fair value measurement. Our policy is to recognize transfers between levels of the fair value hierarchy in the period the event or change in circumstances that caused the transfer. There were no transfers into or out of Level 1, 2, or 3 during the periods presented.

The Company considers its cash, cash equivalents, accounts receivable, marketable securities due with maturities 12 months or less, and accounts payable financial instruments to reflect their fair value given their short maturity and risk profile of the counterparty.

Going Concern

At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern.

The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of December 31, 2019, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued.

Cash and Cash Equivalents

The Company considers all highly liquid debt securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Instruments subject to restrictions are not included in cash and cash equivalents.

Marketable Securities

The Company classifies marketable securities that are debt securities with a remaining maturity when purchased of greater than three months as held-to-maturity as the Company has the positive intent and ability to hold such debt securities through maturity.

Debt securities that are classified as held-to-maturity are initially recognized and measured at fair value. Subsequent to initial recognition, they are measured at amortized cost using the effective interest rate method. Interest income from these debt securities is included in interest income. Marketable securities are classified as current if their expected maturity is within one year or less of the balance sheet date and non-current if their maturity is beyond one year of the balance sheet date.

Accounts Receivable

Accounts receivable are amounts due from collaboration partners as a result of research and development services provided or milestones achieved but not yet paid.

Allowance for Credit Losses

The Company evaluates its cash equivalents, accounts receivable and held-to-maturity marketable securities financial assets for expected credit losses. Expected credit losses represent the portion of the amortized cost basis of a financial asset that an entity does not expect to collect. An allowance for expected credit losses is meant to reflect a risk of loss even if remote, irrespective of the expectation of collection from a particular issuer or debt security. The Company has not historically experienced any credit losses on any of its financial assets.

With respect to cash equivalents and accounts receivable, given consideration of their short maturity, historical losses and the current environment, the Company concluded there is generally no expected credit losses for these financial assets. With respect to held-to-maturity marketable securities which are comprised of debt securities, the Company evaluates expected credit losses on a pooled basis based on issuer-type which have similar credit risk characteristics. The allowance for credit losses is immaterial for all periods presented.

Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation or amortization, from the related accounts, and the resulting gain or loss is reflected in the results of operations.

Intangible Assets

Intangible assets are identifiable non-monetary assets without physical substance. An asset is a resource that is controlled by the enterprise as a result of past events (for example, purchase or self-creation) and from which future economic benefits (inflows of cash or other assets) are expected. The useful lives of intangible assets are assessed to be definite-lived and amortized over the useful economic life. The Company’s intangible assets are comprised of purchased licenses to intellectual property and software licenses.

Impairment of Long-Lived Assets

The Company reviews long-lived assets to be held and used, including property and equipment, operating lease right-of-use assets and definite-lived intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable.

Evaluation of recoverability is first based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded during 2019 or 2018.

Leases

The Company determines if an arrangement is or contains a lease at inception. For leases with a term of 12 months or less, the Company does not recognize a right-of-use asset or lease liability. The Company does not have any finance leases.

Right-of-use assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease, and excludes non-lease payments. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As the Company’s leases typically do not provide an implicit rate, the Company uses an estimate of its incremental borrowing rate based on the information available at the lease commencement date in determining the present value of lease payments.

Operating lease right-of-use assets also include the effect of any lease payments made and excludes lease incentives. The lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Operating lease expense is recognized on a straight-line basis over the lease term.

The Company has real estate operating lease agreements with lease and non-lease components, which are generally accounted for separately as operating lease costs and variable lease costs. Non-lease components in real estate leases refer to services provided by the lessor related to the premises. Fixed and variable lease payments are both allocated to lease and non-lease components. The allocation is determined on a relative fair value basis of the services provided relative to the operating lease of premises. With respect to equipment leases, the Company has elected not to allocate payments amongst lease and non-lease components as a practical expedient as afforded under ASC 842, Leases.

Taxes

Deferred Taxes

The Company records deferred taxes to recognize the future effects of temporary differences between the tax basis and financial statement carrying amount of assets and liabilities. The Company measures the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available, reversing temporary differences and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made.

Unrecognized Tax Benefits

The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the technical merits, facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to an underpayment of income taxes, if applicable, as a component of income tax expense.

Revenue Recognition

The Company 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. To determine revenue recognition for an arrangement, the Company performs the following five step analysis:

identify the contract(s) with a customer;

ii.

identify the performance obligations in the contract;

iii.

determine the transaction price;

iv.

allocate the transaction price to the performance obligations in the contract; and

recognize revenue when (or as) the entity satisfies a performance obligation.

The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, Revenue from Contracts with Customers, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to product candidates or future product candidates directed to specific targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed targets. The Company also derives revenue from government grants.

As part of the accounting for these arrangements, the Company must use judgment to determine:

the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract;

the transaction price under step (iii) above; and

the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above.

The Company uses judgment to determine whether milestones or other variable consideration, except for sales-based royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In validating its estimated stand-alone selling price, the Company evaluates whether changes in the key assumptions used to determine its estimated stand-alone selling price will have a significant effect on the allocation of arrangement consideration between performance obligations.

Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as unbilled receivables.

Exclusive Licenses

If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license.

In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the rights and obligations set out in the contract, the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises.

For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue.

The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement.

The Company’s arrangements may provide the collaboration partner with the right to select a target for licensing either at the inception of the arrangement or in the future. Under these arrangements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires.

For arrangements that include sales-based milestones and royalties, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any sales-based milestones or royalty revenue resulting from any of its arrangements.

Research and Development Services

The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under Exclusive Licenses above.

Reimbursements from the partner are evaluated as to whether the Company acts as a principal or an agent in such relationships. The Company evaluates whether control over the underlying goods or services were obtained prior to transferring these goods or services to the collaboration partner. Where the Company does not control the goods or services prior to transferring these goods or services to the collaboration partner, such reimbursements are presented net of costs.

At the inception of each arrangement that includes development milestone payments in respect of development efforts, the Company evaluates whether the development milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated development milestone value is included in the transaction price. Development milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular development milestone in making this assessment. There is judgment involved in determining whether it is probable that a significant revenue reversal would not occur.

At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of all development milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur.

Government Grants

The Company receives certain government and regional grants, which support its research efforts in defined projects, and include contributions towards the R&D cost. When there is reasonable assurance that the Company will comply with the conditions attached to a received grant, and when there is reasonable assurance that the grant will be received, government grants are recognized as revenue on a gross basis in the consolidated statement of profit or loss and comprehensive loss on a systematic basis over the periods in which the Company recognizes expenses for the related costs for which the grants are intended to compensate. In the case of grants related to assets, the received grant will be deducted from the carrying amount of the asset. Government grant revenue may be subject to review by a government authority in periods subsequent to their recognition and may result in the reversal of grant revenue previously recognized. Reversals of grant revenue are presented as contra revenue in the consolidated statement of operations.

Research and Development Expenses

Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.

When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history in conducting similar activities and the expected duration of the third-party service contract, among other considerations.

The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of research and development expenses.

The WBSO (afdrachtvermindering speur- en ontwikkelingswerk) is a Dutch fiscal facility that provides subsidies to companies, knowledge centers and self-employed people who perform research and development activities (as defined in the WBSO Act). Under this act, a contribution is paid towards the labor costs of employees directly involved in research and development. For the year ended December 31, 2019 and 2018, the Company recognized $4.5 million and $5.0 million as a reduction of research and development expenses, respectively.

Share-Based Payments

The Company measures employee share-based compensation based on the grant date fair value of the share-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable market prices.

For share-based payments subject time-based vesting, the Company recognizes employee stock-based compensation expense on a straight-line basis over the requisite service period of the awards, generally from the date of grant through each vesting date. The Company recognizes forfeitures at the time they occur. The actual expense recognized over the vesting period will only represent those options that vest; the effect of forfeitures in the recognition of periodic compensation expense are not estimated prior to their occurrence.

Earnings (Loss) per Share

The Company computes basic earnings (loss) per share by dividing income (loss) allocable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted earnings (loss) per share after giving consideration to the dilutive effect of stock options and restricted stock units (“RSU”) that are outstanding during the period, except where such non-participating securities would be anti-dilutive.

Segment Information

The Company operates in one reportable segment, which comprises the discovery and development of innovative bispecific therapeutics.

Pending Accounting Pronouncements

In August 2018, the FASB issued ASU 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. The new guidance aligns the requirements for capitalizing implementation costs incurred in a cloud-based 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). This ASU is effective for the Company at the beginning of fiscal 2021, including interim periods within that reporting period, although early adoption is permitted. The Company does not expect the impact of adoption to be significant.

In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808), which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The ASU will be effective for the Company in the first quarter of fiscal 2021, with early adoption permitted. As of December 31, 2019, none of the Company’s arrangements fall within the scope of ASC 808. However, as the Company may engage in future collaborative arrangements in the future, this ASU may apply to those new arrangements.

Recently Adopted Accounting Pronouncements

In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments along with subsequent related updates which replaces the existing incurred loss impairment methodology with an expected credit loss methodology and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates and amends disclosure requirements. As permitted under this ASU, the Company has early-adopted this ASU, applied retrospectively to all periods presented.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. Among other changes described in the ASU, Part I changes the classification analysis of certain equity-linked financial instruments (or embedded features) with down round features. When determining whether certain financial instruments should be classified as liabilities or equity instruments, a down round feature no longer precludes equity classification when assessing whether the instrument is indexed to an entity’s own stock. As a result, a freestanding equity-linked financial instrument (or embedded conversion option) no longer would be accounted for as a derivative liability at fair value as a result of the existence of a down round feature. Prior to the earliest period presented in these consolidated financial statements and prior to the initial public offering of securities in which all such instruments were converted or exchanged for common shares, the Company issued instruments that involved down round features. As permitted under this ASU, the Company has early-adopted this ASU, applied retrospectively to all periods presented.

In June 2018, the FASB issued ASU 2018-07, Improvements to Non-employee Share-Based Payment Accounting (Topic 718). The ASU expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from non-employees. The standard aligns measurement and classification guidance for share-based payments to non-employees with the guidance applicable to employees. The ASU was early-adopted by the Company and is reflected in these consolidated financial statements, applied retrospectively to all periods presented.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820); Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement. This ASU modified disclosure requirements to remove, modify and add certain disclosures with an aim to improve disclosure effectiveness. The ASU was early-adopted by the Company and is reflected in these consolidated financial statements, applied retrospectively to all periods presented.

In November 2019, the FASB issued ASU 2019-08, Compensation—Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606), Codification Improvements—Share-Based Consideration Payable to a Customer. This ASU requires that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. The amount recorded as a reduction of the transaction price is required to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718. The ASU was early-adopted by the Company and is reflected in these consolidated financial statements, applied retrospectively to all periods presented.

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740); Simplifying the Accounting for Income Taxes. Among the amendments in this ASU were amendments to simplify the accounting for income taxes by (1) removing the exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year; and (2) requiring that an entity reflect the effect of an enacted change in tax laws or rates in the annual effective tax rate computation in the interim period that includes the enactment date. Other aspects of this ASU are not expected to have an impact on the Company’s financial statements in the current or future periods. The Company early-adopted this ASU and is reflected in these consolidated financial statements.

Prepaid Expenses and Other Assets

12 Months Ended

Dec. 31, 2019

Prepaid Expense And Other Assets Current [Abstract]

Prepaid Expenses and Other Assets

4. Prepaid Expenses and Other Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2019		2018
	(In thousands)
Prepaid clinical and manufacturing costs	$	2,779	$	2,106
Interest receivable		259		244
Other		1,913		2,383
Total	$	4,951	$	4,733

Restricted cash included in other assets totaled $0.2 million and $0.1 million as of December 31, 2019 and 2018, respectively. The nature of the restriction relates to amounts held as collateral for a credit card borrowing arrangement.

Property and Equipment, net

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, net

5. Property and Equipment, net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Laboratory equipment	$	4,538		$	3,159
Office equipment and furniture		1,186			713
Leasehold improvements		79			40
Property and equipment		5,803			3,912
Less: accumulated depreciation and amortization		(2,088	)		(1,206	)
Property and equipment, net	$	3,715		$	2,706

Depreciation and amortization expense was $0.9 million, and $0.6 million for the years ended December 31, 2019, and 2018, respectively. Property and equipment are predominantly located in the Netherlands.

Intangible Assets, Net

12 Months Ended

Dec. 31, 2019

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible Assets, Net

6. Intangible assets, net

Intangible assets, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Licenses of intellectual property	$	3,568		$	3,532
Software licenses		264			65
Intangible assets		3,832			3,597
Less: accumulated amortization		(956	)		(733	)
Intangible assets, net	$	2,876		$	2,864

Amortization expense was $0.2 million and $0.1 million for the years ended December 31, 2019, and 2018, respectively. Intangible assets are predominantly located in the Netherlands.

Amortization expense over the next five years are expected to be as follows (in thousands):

Year	Expected amortization
2020	$	269
2021		269
2022		269
2023		206
2024		200
Thereafter		1,663
Total remaining value	$	2,876

Accrued Expenses

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Accrued Expenses

7. Accrued Expenses

Accrued expenses consisted of the following:

	December 31,
	2019		2018
	(In thousands)
Accrued research and development expenses	$	6,618	$	5,048
Accrued general and administrative expenses		2,402		2,891
Accrued personnel costs		4,495		2,386
Other		21		61
Accrued expenses	$	13,536	$	10,386

Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Taxes

8. Taxes

The components of loss from operations before income taxes are as follows:

	Year ended December 31,
	2019			2018
	(In thousands)
United States	$	(1,363	)	$	(2,731	)
Netherlands		(53,594	)		(25,374	)
Total loss before income taxes	$	(54,957	)	$	(28,105	)

The components of the income tax expense (benefit) from continuing operations are as follows:

	December 31,
	2019			2018
	(In thousands)
U.S. federal	$	243		$	251
U.S. state		40			166
Total current tax expense	$	283		$	417

U.S. federal	$	(63	)	$	(150	)
U.S state		(26	)		(62	)
Total deferred tax benefit	$	(89	)	$	(212	)

Total tax expense	$	194		$	205

The parent company is subject to income tax in the Netherlands where a greater proportion of economic activity is attributed. A reconciliation of the Netherlands statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2019			2018
Netherlands statutory income tax rate		25.0	%		25.0	%
U.S. tax rate differential		—			(0.1	)
Changes in tax rates		1.5			(35.9	)
Non-deductible expenses		(3.7	)		(8.7	)
Other		0.1			—
Change in valuation allowance		(23.3	)		19.0
Effective income tax rate		(0.4	)%		(0.7	)%

In 2018, Dutch tax authorities enacted new tax rates. The effect of the change in the valuation allowance each year reflects the increase or decrease in the valuation allowance against deferred tax assets attributable to the Netherlands.

The components of the Company’s deferred tax assets (liabilities) consist of the following:

	December 31,
	2019			2018
	(In thousands)
Deferred tax assets:
Net operating loss carryforwards	$	33,917		$	17,798
Deferred revenue		23,926			26,814
Lease obligation		1,333			551
Accrued expenses and other		319			206
Total deferred tax assets		59,495			45,369
Deferred tax asset valuation allowance		(57,876	)		(44,643	)
Total deferred tax assets, net of valuation allowance		1,619			726

Deferred tax liabilities:
Operating lease right-of-use assets	$	1,331		$	520
Other		—			7
Total deferred tax liabilities		1,331			527

Net deferred tax asset	$	288		$	199

After consideration of all positive and negative evidence, we believe that it is more-likely-than-not that our Netherlands deferred tax assets will not be realized that are not supported by reversing temporary differences. As a result, we established a valuation allowance of $57.9 million and $44.6 million as of December 31, 2019 and 2018, respectively. The increase in the valuation allowance of $13.2 million and $5.3 million is primarily attributable to the increase in net operating loss carryforward deferred tax assets for which a full valuation allowance applies and change in tax rates in 2018, respectively. As of December 31, 2019, the portion of the valuation allowance for deferred tax assets for which subsequently recognized tax benefits would be credited directly to contributed capital totaled $1.4 million.

As of December 31, 2019, the Company did not have any net operating losses for US federal or state income tax purposes. The Company had net operating loss carryforwards for Dutch income tax purposes, the amount and expiry are as follows (in thousands):

Expiry Year	Netherlands Tax Loss Carryforward
2024	$	21,174
2025		100,864
2026		—
2027		34,263
Total	$	156,301

As of December 31, 2019, the Company had no unrecognized tax benefits. As of December 31, 2019, the Company had no accrued interest or penalties related to underpayments of income taxes and no amounts have been recognized in the consolidated statements of operations. The Company will recognize interest and penalties related to an underpayment of income taxes in income tax expense.

The Company files income tax returns in the U.S. federal and Massachusetts jurisdictions as well as in the Netherlands. The statute of limitations for assessment by the Internal Revenue Service (IRS), and Massachusetts tax authorities is closed for tax years prior to 2016. The statute of limitations for assessment by the Netherlands tax authorities is closed for tax years prior to 2014. The Company is not currently under examination by the IRS or any other jurisdictions for any tax years.

Operating Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Operating Leases

9. Operating Leases

Merus N.V. leases its corporate headquarters under an agreement term of five years, which expires in the fourth quarter of 2021. On May 1, 2018, Merus N.V. leased additional space to expand its corporate headquarters under a separate agreement. Under the terms of the new agreement, the term began on May 1, 2018 and expires in the fourth quarter of 2021. If the leases are not terminated by Merus N.V., they will be automatically renewed for a period of two years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term. Fixed lease payments increase annually and include an increase based on an inflationary measure. Variable payments include amounts due to the lessor for additional services and cost reimbursements.

In March 2019, Merus US, Inc. entered into a lease agreement for office space in Cambridge, Massachusetts. The lease commenced in the second quarter of 2019 and has a term of seven years, and may be extended for another five years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term. Fixed lease payments increase annually and include an increase on an inflationary measure. Variable payments include amounts due to the lessor for additional services and cost reimbursements.

The Company’s operating leases relate to its real estate leases that are not classified as finance leases.

In July 2019, the Company entered into a lease with Kadans Science Partner XII B.V. (“Kadans”), pursuant to which the Company agreed to lease approximately 5,070 square meters of office and laboratory space in a new multi-tenant office building that is to be constructed in Utrecht, the Netherlands. The initial term of the lease is ten years from the date that the premises are completed in accordance with certain specifications provided in a development agreement (described below), which is expected to occur in mid-2022. The lease will renew for two 5-year terms following the initial term, unless earlier terminated by the Company or Kadans, except that the earliest Kadans may terminate the lease is 20 years from the completion date. The lease provides for an estimated initial rent of approximately €1.3 million per annum. The rent amount is subject to adjustment based on the consumer price index (the “CPI”) beginning on January 1, 2019 through the completion date and then annually thereafter, subject to certain limitations if the CPI is greater than 3.0%. The final initial rent amount is contingent upon, among other things, the parameters of the final constructed premises, the final floor area, and the CPI adjustment described above, and will be determined upon the completion date and recorded in a first rider, signed by the Company and Kadans, to the lease. The Company is also responsible for certain fit-out costs and service fees related to the premises.

In July 2019, the Company also entered into a development agreement with Kadans and another party, Genmab B.V., which provides for the design, development and construction of the new multi-tenant office building of which the premises is a part.

The components of lease cost recorded in the Company’s consolidated statement of operations and statement of cash flows were as follows:

	For the Year Ended December 31,
	2019		2018
	(In thousands)
Operating lease cost	$	1,386	$	834
Variable lease cost		297		177
Total lease cost included in operating expenses	$	1,683	$	1,011

Cash paid to lessors included in operating cash outflows	$	2,273	$	1,199

The Company’s non-lease cost and other costs paid to the lessor are primarily related to services provided by the lessor in operating the premises that includes fees, operating costs, taxes and insurance related to the leased premises.

Maturities of the Company’s operating lease obligations as of December 31, 2019 were as follows (in thousands):

Year	Operating Leases
2020	$	1,602
2021		1,495
2022		614
2023		630
2024		645
Thereafter		884
Total lease payments		5,870
Less: amount representing interest		(618	)
Total lease obligations	$	5,252

The weighted-average remaining lease terms and discount rates related to the Company’s leases were as follows:

	As of December 31,
	2019			2018
Weighted-average remaining operating lease term (in years)		4.8			2.8
Weighted-average discount rate for operating leases		5.0	%		6.0	%

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Indemnities

The Company indemnifies its officers and directors for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ and officers’ insurance coverage that is intended to limit its exposure and enable it to recover a portion of any future amounts paid.

The Company enters into certain agreements with other parties in the ordinary course of business that contain indemnification provisions. These typically include agreements with directors and officers, business partners, contractors, landlords, clinical sites and customers. Under these provisions, the Company may indemnify and hold harmless the indemnified party for losses suffered or incurred by the indemnified party as a result of the Company’s activities, such as gross negligence, willful misconduct or at times, other activities. These indemnification provisions may survive termination of the underlying agreements. The maximum potential amount of future payments the Company could be required to make under these indemnification provisions may be unlimited. However, to date the Company has not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. As a result, the estimated fair value of these obligations is minimal. Accordingly, the Company did not have any liabilities recorded for these obligations as of December 31, 2019.

Litigation

On March 11, 2014, Regeneron Pharmaceuticals, Inc. (“Regeneron”) filed a complaint in the U.S. District Court for the Southern District of New York, alleging that the Company was infringing one or more claims in Regeneron’s U.S. Patent No. 8,502,018, entitled “Methods of Modifying Eukaryotic Cells.” On December 20, 2018, the Company signed a global settlement and cross-license agreement with Regeneron, where the parties have agreed to end all pending litigation and opposition proceedings pertaining to the Company’s and Regeneron’s respective antibody generation technologies. Regeneron also purchased 600,000 of the Company’s common shares at a price of $25 per share for total aggregate proceeds of $15.0 million. The cross-license and stock purchase were made in conjunction with the agreement to withdraw Regeneron’s appeal of the fee award, and agreement to dismissal of all claims to approximately $10.5 million for the reimbursement of attorneys’ fees and other expenses, plus interest, awarded to Merus by the trial court. Under the terms of the settlement, Regeneron has withdrawn its appeal of the decision awarding attorneys’ fees to the Company as a result of the U.S. District Court litigation described above. In addition, Regeneron has dismissed its stayed case in the Netherlands asserting the EP 1 360 287 B1 patent, and both parties have withdrawn all pending oppositions as of December 20, 2018. As a result of the settlement, the Company recognized a gain of $8.1 million included in other income in the consolidated statement of operations and comprehensive loss for the year ended December 31, 2018.

On April 5, 2018, an unnamed third party and Regeneron Pharmaceuticals Inc. or Regeneron filed notices of opposition against the Company’s EP 2604625 patent, entitled “Generation of Binding Molecules,” in the European Opposition Division of the European Patent Office (the “EPO”). The notices asserted, as applicable, added subject matter, lack of novelty, lack of inventive step, and insufficiency. Regeneron withdrew its opposition pursuant to a global December 20, 2018 settlement with Merus. On August 20, 2018, the Company timely responded to these submissions with respect to the unnamed third party. An opposition hearing was held in June 2019, wherein the EPO revoked the EP 2604625 patent in its entirety under Art. 123(2) EPC. We timely appealed that decision in December 2019 before the Technical Board of Appeals for the EPO seeking reinstatement of the patent and proposing auxiliary requests for certain amended claims, with further proceedings to be scheduled in the future. As this opposition proceeding continues, the Company cannot be certain that the Company will ultimately prevail.

From time to time, the Company may be involved in various other claims and legal proceedings relating to claims arising out of the Company’s operations. The Company is not currently a party to any other material legal proceedings.

Stockholders' Equity

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Stockholders' Equity

11. Stockholders’ Equity

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors.

Private Placement of Common Shares

On February 13, 2018, the Company entered into the Purchase Agreement. Pursuant to the Purchase Agreement, the Company agreed to sell an aggregate of 3,099,997 of its common shares to the Investors at a purchase price equal to $18.00 per share. The Purchase Agreement contains customary representations and warranties from the Company and the Investors and customary closing conditions. On February 15, 2018, the Company completed the sale under the Private Placement and received net proceeds of approximately $55.1 million.

Share Subscription Agreement with Regeneron

On December 20, 2018, the Company entered into a share subscription agreement (the “Regeneron Subscription Agreement”) with Regeneron Pharmaceuticals, Inc. (“Regeneron”). Pursuant to the Regeneron Subscription Agreement, the Company agreed to sell an aggregate of 600,000 of its common shares to Regeneron at a purchase price equal to $25.00 per share. On December 21, 2018, the Company completed the sale under the Regeneron Subscription Agreement and received gross proceeds of $15.0 million. Accordingly, the Company recorded the common shares issued at the fair value of the underlying securities on the date of issuance. The difference between the total proceeds received of $15.0 million, and the aggregate value of common shares issued of $6.9 million, was recorded as other income of $8.1 million relating to the litigation settlement during the year ended December 31, 2018.

Public Share Offerings

On November 7, 2019, the Company completed an underwritten public offering in which the Company sold 5,462,500 common shares, including 715,500 common shares pursuant to the underwriters’ option to purchase additional shares, at a price to the public of $14.50 for aggregate net proceeds of $74.0 million.

Equity Compensation Plan

As of January 1, 2020, a total of 1,901,162 shares of common stock were reserved for additional grants of stock awards under the Company’s 2016 Incentive Award Plan. Stock-based compensation expense related to the equity compensation plan is more fully described in Note 13, Employee Benefit Plans.

Collaborations

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaborations

12. Collaborations

Incyte

In December 2016, pending regulatory clearance, Incyte agreed to pay the Company a $120.0 million, non-refundable upfront payment, and purchased 3.2 million common shares at a stated price per share of $25.00, for an aggregate purchase price of $80.0 million. In exchange, the Company granted Incyte with a license to certain of its intellectual property and committed to collaborate with Incyte to research, discover and develop monospecific or bispecific antibodies utilizing the Company’s proprietary bispecific technology platform. The collaboration is managed by a joint steering committee in which both parties are represented and is tasked with overseeing the activities which significantly contributes to the collaboration. The collaboration may encompass up to 11 product candidates that result from the Company’s application of its proprietary Biclonics^® technology platform. During the course of the initial research term, Merus proposes product candidates to Incyte, which evaluates whether to designate proposed product candidates from the Company to make a selection for further research. Proposed product candidates begin at a pre-clinical stage of development. Incyte has certain rights to replace product candidates, including the right to substitute a product candidate after initial selection. The Company would be entitled to future consideration in the form of cost reimbursements for research services, development milestones, commercialization milestones and royalties related to the programs under the arrangement.

At inception of the collaboration, two potential bispecific product candidates were under preliminary evaluation. After further research, a lead candidate was ultimately selected for the first product candidate, designated MCLA-145, and the other potential product candidate was not pursued. For the designated product candidate (MCLA-145), the Company retains the exclusive right to develop and commercialize products and product candidates in the United States, while Incyte has the exclusive right to develop and commercialize products and product candidates arising from such program outside the United States. For MCLA-145, the parties will conduct and share equally the costs of mutually agreed global development activities and will be solely responsible for independent development activities in each party’s respective territories. For all other programs under the arrangement to be selected by Incyte, Incyte will be responsible for all research, development and commercialization costs. The Company may elect to co-fund the development of certain of the other programs in the future, in which case costs and benefits would be shared. The Company has not elected to co-fund any programs to date.

At inception of the arrangement, the Company identified a performance obligation comprised of a combined delivery of a license and related activities, including the activities of the joint steering committee, to which to allocate consideration. The arrangement also allowed for optional future research services to advance selected product candidates through discovery and research. The transaction price was comprised of fixed consideration of an upfront payment of $120.0 million and proceeds from the sale of shares of $80.0 million. All other consideration under the arrangement was determined to be variable consideration and fully constrained at inception. $152.6 million of the transaction price was allocated to the license and related activities performance obligation after accounting for the purchase of common shares by Incyte.

On January 23, 2017, the Company completed the sale of shares and exchange of the license. The Company initially deferred the transaction price allocated to the license and related activities performance obligation as deferred revenue, to be recognized as revenue over time as the primary benefit of the license to Incyte is access to the platform for the generation of potential product candidates. Development milestones, commercialization milestones and royalties are variable consideration, fully constrained, to be recognized in future periods in accordance with the Company’s revenue recognition policy. Cost reimbursements for research services are recognized as they are performed over time as these are considered a separate performance obligation.

At December 31, 2019, the Company is currently engaged in research and development activities for MCLA-145 and developing candidates for the other programs. No development or commercialization milestones have been achieved to date.

ONO

On March 14, 2018, the Company granted ONO an exclusive, worldwide, royalty-bearing license, with the right to sublicense, research, test, make, use and market bispecific antibody candidates based on the Company’s Biclonics^® technology platform against two undisclosed targets directed to a particular undisclosed target combination. ONO is responsible for identifying lead candidates and conducting further non-clinical and clinical development activities for such licensed bispecific antibodies and pharmaceutical products containing such antibodies, including manufacture and process development. Additionally, ONO controls and has exclusive rights over the worldwide commercialization of any approved products, including worldwide supply, and is solely responsible for all costs and expenses related to commercialization. ONO has also agreed to fund the Company’s research and development activities and be responsible for the payment of all costs and expenses for its own research and development activities, which are set out in a mutually agreed upon research plan. The Company retains all rights to use and commercialize any antibodies that are generated under the collaborative research program, excluding the up to five lead and/or selected antibodies against the targets ONO is pursuing, provided that the use and commercialization is not with respect to the particular target combination.

ONO agreed to pay the Company an upfront, non-refundable payment of €0.7 million. In addition, the Company was entitled to €0.3 million intended to compensate the Company for research services already completed upon entering into the agreement, and €0.2 million to be paid to the Company over time for full time equivalent funding. The Company is entitled to research and development milestones in addition to royalties on future sales.

The Company identified performance obligations for: (1) provision of a license for the target combination, and (2) research and development services. The Company concluded that Ono would be able to develop and benefit from the license, independent of the research and development services. The research and development services are capable of being performed by third parties with an appropriate sub-license, and are recognized over time as these services are delivered. Milestone payments are fully constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

On March 3, 2016, the Company was engaged to perform manufacturing services for Ono in support of their clinical trials that was the subject of a prior arrangement with Ono initiated in 2014. The Company was entitled to compensation to oversee and support a manufacturing process with the use of an outside third-party and to deliver quantities of manufactured product. The Company identified one performance obligation to provide manufacturing supervision services involving the transfer of know-how and the development of a manufacturing process with the third-party and three performance obligations related to the services provided in connection with the delivery of product each having separate and distinct requirements. The Company acts as an agent for Ono with respect to the third-party manufacturing costs incurred for the underlying product and presents the recovery of such costs net in research and development expense in the consolidated statement of operations.

The Company received €3.7 million (approximately $4.1 million) for the year ended December 31, 2019 for development milestones received from Ono based on their progress. The amounts are recognized as revenue in the consolidated statement of operations. Research and development services were completed in 2018. The Company recognized €4.0 million (approximately $4.7 million) of the transaction price as revenue in the year ended December 31, 2018 mainly related to the three performance obligations for the services provided in connection with the delivery of product completed in the period.

Simcere

In January 2018, the Company granted Simcere an exclusive license to develop and commercialize three bispecific antibodies to be produced by Merus utilizing the Company’s Biclonics^® technology platform in the area of immuno-oncology in China. The Company will retain all rights outside of China. The Company has agreed to lead research and discovery activities, while Simcere has agreed to be responsible for the Investigational New Drug (“IND”) enabling studies, clinical development, regulatory filings and commercialization of these product candidates in China. The Company received an upfront, non-refundable payment of $2.75 million, relating to three separate research programs. The Company may be entitled to future development milestone payments.

At inception of the arrangement, the Company identified three performance obligations comprised of the combined delivery of a license and performance of research and development activities with respect to each program. The Company performs research and development activities to achieve candidate nomination. The Company concluded that these activities were not distinct from the underlying license for each program as Simcere would not be able to benefit from the license apart from research and development activities at this phase of development.

The transaction price under the arrangement comprised fixed consideration of $2.75 million. The transaction price was allocated to each separate performance obligation on a relative standalone fair value basis. The Company deferred the portion of the upfront payment allocated to the three performance obligations as deferred revenue, to be recognized over time. Compensation for research and development services prior to candidate nomination are allocated to each program performance obligation and also recognized over time. Development milestone payments allocated to each of the program performance obligations are constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

At December 31, 2019, research and development for two of the three programs is on-going.

Betta

On December 10, 2018, the Company granted Betta an exclusive license to develop and commercialize in China MCLA-129, a proprietary Biclonics^® produced by its Biclonics^® technology platform. The Company retains all rights outside of China. Betta has agreed to retain a contract manufacturing organization with experience in filing IND applications with U.S. regulatory authorities and CTAs with European regulatory authorities in order to produce clinical trial materials for the Chinese market and rest of the world. As a key strategic component of the collaboration, Betta will be responsible for IND enabling studies and manufacturing of clinical trial materials in China, which the Company intends to use to assist regulatory filing and early stage clinical development in the rest of the world.

In addition to a non-refundable upfront payment of $1.0 million, Betta and the Company will share equally the cost of the transfer of the manufacturing technology to a contract manufacturing organization. The Company is also eligible to receive an aggregate of $12.0 million in milestone payments contingent upon Betta achieving certain specified development and commercial goals as well as tiered royalty payments of net sales of any products resulting from the collaboration in China.

The Company identified a single combined performance obligation, being the delivery of the MCLA-129 license including activities necessary to complete the technology transfer. The Company had no other commitments. The transaction price is comprised of fixed consideration of $1.0 million and fully allocated to the single performance obligation which would be fulfilled at a point in time. The technology transfer to deliver the license was completed in 2018 and Company recognized the revenue related to this performance obligation of $1.0 million as revenue for the year ended December 31, 2018. Development milestone payments allocated to the performance obligation are constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

Contract Assets, Liabilities, Revenues and Expenses

The following tables provide amounts by year indicated and by line item included in the Company's accompanying consolidated financial statements attributable to transactions arising from its collaboration arrangements. The dollar amounts in the tables below are in thousands.

	Related Party			Third Party
	Incyte			Ono			Other			Total
Contract assets
Accounts receivable
Balance at January 1, 2019	$	—		$	70		$	29		$	99
Billings		8,480			4,504			949			5,453
Cash receipts		(8,511	)		(3,788	)		(978	)		(4,766	)
Foreign exchange		31			—			—			—
Balance at December 31, 2019		—			786			—			786

Unbilled receivables
Balance at January 1, 2019	$	2,982		$	270		$	—		$	270
Accrued receivables		7,270			4,239			1,104			5,343
Billings		(8,480	)		(4,504	)		(949	)		(5,453	)
Foreign exchange		(61	)		(5	)		—			(5	)
Balance at December 31, 2019		1,711			—			155			155

Contract liabilities
Deferred revenue
Balance at January 1, 2019	$	128,871		$	543		$	1,388		$	1,931
Cash receipts		—			—			949			949
Revenue recognized in the period		(17,839	)		(196	)		(926	)		(1,122	)
Foreign exchange		(2,494	)		(11	)		(26	)		(37	)
Balance at December 31, 2019		108,538			336			1,385			1,721
Less: current portion		(17,901	)		(197	)		(744	)		(941	)
Non-current balance at December 31, 2019		90,637			139			641			780

The balance of unbilled receivables predominantly represents reimbursement revenue under the Company’s collaboration arrangements earned in the period to be billed and collected in the next period, generally quarterly. Incyte is a related party as a shareholder, as more fully described in Note 15.

	For the Year Ended December 31, 2019
	Related Party		Third Party
	Incyte		Ono		Other		Total
Upfront payments	$	17,839	$	196	$	643	$	839
Reimbursement revenue		7,992		99		153		252
Milestones		—		4,142		284		4,426
Total collaboration revenue	$	25,831	$	4,437	$	1,080	$	5,517

Operating expenses:
Research and development expense	$	680	$	—	$	—	$	—
General and administrative expense		—		—		—		—
Total operating expenses from collaborations	$	680	$	—	$	—	$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	17,839	$	196	$	945	$	1,141

	For the Year Ended December 31, 2018
	Related Party		Third Party
	Incyte		Ono		Other		Total
Upfront payments	$	18,819	$	341	$	1,660	$	2,001
Reimbursement revenue		11,150		1,036		—		1,036
Milestones		—		4,724		415		5,139
Total collaboration revenue	$	29,969	$	6,101	$	2,075	$	8,176

Operating expenses:
Research and development expense	$	678	$	—	$	—	$	—
General and administrative expense		44		—		—		—
Total operating expenses from collaborations	$	722	$	—	$	—	$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,819	$	73	$	—	$	73

Employee Benefit Plans

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

13. Employee Benefit Plans

Share-Based Payments

2010 Plan

In 2010, the Company established the Merus B.V. 2010 Employee Option Plan (the “2010 Plan”) that entitled key management personnel, staff and consultants providing similar services to purchase shares in the Company. Under the 2010 Plan, holders of vested options were entitled to purchase depositary receipts for common shares at the exercise price determined at the date of grant. Upon exercise of the option, common shares were issued to a foundation established to facilitate administration of share-based compensation awards and pool the voting interests of the underlying shares, and depositary receipts were issued by the foundation to the individual holders. In connection with the IPO, the 2010 Plan was amended to cancel the depositary receipts and allow individual holders to directly hold the common shares obtained upon exercise of their options.

Options granted under the 2010 Plan generally vest in installments over a four-year period from the grant date: 25% percent on the first anniversary of the vesting commencement date, and the remaining 75% of the options vest in 36 monthly installments for each full month of continuous service provided thereafter. Options expire after 8 years from the date of grant. The last grant of options pursuant to the 2010 Plan occurred in 2016, with no further grants expected.

2016 Plan

In 2016, the Company established the 2016 Incentive Award Plan (the “2016 Plan”). All incentive award grants since 2016 are being made under the 2016 Plan.

Options granted to employees under the 2016 Plan generally vest in installments over a four-year period from the grant date: 25% percent on the first anniversary of the vesting commencement date, and the remaining 75% of the options vest in 36 monthly installments for each full month of continuous service provided thereafter. Options expire after 10 years from the date of grant.

Options granted to non-executive directors consist of initial option grant as well as subsequent annual awards. The initial award of options granted vest in installments over a three-year period: 33% of the options vest on the first anniversary of the vesting commencement date, and 67% of the options vest in 24 monthly installments thereafter. Each subsequent award vest over a one-year period in 12 monthly installments thereafter. The Company measures the fair value of an option through the application of an option pricing model, as more fully described below.

The RSUs granted to employees under the 2016 Plan vest in installments over a four-year period from the grant date. Each RSU represents the right to receive one common share. The fair value of an RSU is determined by reference to the price of the underlying common share.

The number of common shares authorized for issuance for future grants under the plan as of January 1, 2020 totaled 1,901,162.

Share-Based Compensation Expense

Share-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:

	Year Ended December 31,
	2019		2018
	(In thousands)
Research and development	$	3,186	$	3,333
General and administrative		4,648		6,421
Total	$	7,834	$	9,754

As of December 31, 2019, there was $4.6 million in unrecognized stock-based compensation that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.4 years.

Stock Option Valuation

The Company uses the Hull & White option-pricing model to measure the fair value of stock option awards. Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows:

	Year Ended December 31,
	2019			2018
Risk-free interest rate		2.5	%		2.9	%
Expected life of options (years)		10.0			10.0
Expected volatility of underlying stock		87.0	%		93.7	%
Expected dividend yield		0.0	%		0.0	%

The risk-free interest rate is based upon the U.S. Treasury yield curve in effect at the time of grant, with a term that approximates the expected life of the option. The Company determines the expected volatility using a blended approach encompassing its historical experience and the historical volatility of a peer group of comparable publicly traded companies with product candidates in similar stages of development to the Company’s product candidates. The Company has applied an expected dividend yield of 0.0% as the Company has not historically declared a dividend and does not anticipate declaring a dividend during the expected life of the options.

Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2019:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(years)		(In thousands)
Outstanding at January 1, 2019		2,633,039		$	17.20
Granted		1,145,221			12.18
Exercised		(12,080	)		4.01
Forfeited or expired		(451,309	)		15.80
Outstanding at December 31, 2019		3,314,871		$	14.67		7.3	$	7,757
Exercisable at December 31, 2019		1,795,089		$	14.61		6.0	$	5,572

	Year Ended December 31,
	2019		2018
Weighted-average fair value of options granted	$	7.54	$	11.69

RSU Activity

The following is a summary of RSU activity for the year ended December 31, 2019:

	Number of RSUs			Weighted Average Grant-date Fair Value
Non-vested at January 1, 2019		101,302		$	21.11
Granted		30,000			17.17
Vested		(48,660	)		21.11
Non-vested at December 31, 2019		82,642		$	19.68

Intrinsic Value of Stock Options Exercised and Vested RSUs

	Year Ended December 31,
	(In thousands)
	2019		2018
Total fair value of RSUs vested	$	726	$	1,775
Aggregate intrinsic value of options exercised		112		1,642

Post-Employment Benefit Plan

The Company has established a post-employment benefit plan for employees of the Netherlands that entitles executive officers and other staff members to retire at the age of 67 and receive annual payments based upon the average salary earned during the service period. The Company has insured the benefit liabilities through purchased non-participating annuities from an insurance company and has no other obligation other than to pay the annual insurance premiums to the insurance company. After purchasing the insurance, the Company has no further obligation (legal or constructive) to pay further amounts if the insurance fund has insufficient assets to pay all employee benefits relating to current and prior service. Contributions to purchase non-participating annuities are expensed as incurred as service costs. Company contributions to the post-employment benefit plan totaled $1.3 million and $0.8 million in the years ended December 31, 2019 and 2018, respectively.

401(k) Savings Plan

The Company has a defined contribution 401(k) savings plan (the “401(k) Plan”). The 401(k) Plan covers substantially all U.S. employees, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company matches contributions to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation. Company contributions to the 401(k) Plan totaled $0.1 million and $0.1 million in the years ended December 31, 2019 and 2018, respectively.

Executive Settlement

In December 2019, in connection with the departure of the Chief Executive Officer of the Company, the Company awarded benefits, including the following: cash compensation of $0.9 million, a grant of 30,000 RSUs, extended vesting of his equity incentive awards through June 30, 2021 and extended exercisability of his equity incentive awards through December 31, 2021. The cash compensation is to be paid by the Company January 31, 2020. There were no substantive service conditions associated with the benefits awarded other than the passage of time. The Company incrementally recognized $1.8 million in general and administrative expense associated with these benefits in the consolidated statement of operations for the year ended December 31, 2019.

Loss per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Loss per Share

14. Loss per Share

The two-class method was not applied for the years ended December 31, 2019 and 2018 due to the net loss recognized in each of those periods.

Basic and diluted loss per share allocable to common stockholders are computed as follows:

	Year Ended December 31,
	2019			2018
	(In thousands except per share data)
Net loss	$	(55,151	)	$	(28,310	)
Weighted average shares outstanding		24,218,083			22,286,720
Basic and diluted loss per share allocable to common stockholders	$	(2.28	)	$	(1.27	)

Related Party Transactions

12 Months Ended

Dec. 31, 2019

Related Party Transactions

15. Related Party Transactions

The Company has entered into the Incyte collaboration and license agreement and the Incyte share subscription agreement in which the terms and transactional amounts incurred between Incyte and the Company are more fully described in Note 12. Incyte is a shareholder with holdings representing approximately 11.1% of the outstanding shares of the Company as of December 31, 2019, and 13.7% as of December 31, 2018.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Basis of Preparation

Principles of Consolidation

Functional and Presentation Currency

Use of Estimates

Concentration of Credit Risk

Subsequent Events

Fair Value Measurements

•

Level 1 – Quoted market prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

•

Going Concern

Cash and Cash Equivalents

Marketable Securities

Accounts Receivable

Accounts receivable are amounts due from collaboration partners as a result of research and development services provided or milestones achieved but not yet paid.

Allowance for Credit Losses

Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

Intangible Assets

Impairment of Long-Lived Assets

Leases

Taxes

Deferred Taxes

Unrecognized Tax Benefits

Revenue Recognition

identify the contract(s) with a customer;

ii.

identify the performance obligations in the contract;

iii.

determine the transaction price;

iv.

allocate the transaction price to the performance obligations in the contract; and

recognize revenue when (or as) the entity satisfies a performance obligation.

As part of the accounting for these arrangements, the Company must use judgment to determine:

the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract;

the transaction price under step (iii) above; and

the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above.

Exclusive Licenses

Research and Development Services

Government Grants

Research and Development Expenses

Share-Based Payments

Earnings (Loss) per Share

Segment Information

The Company operates in one reportable segment, which comprises the discovery and development of innovative bispecific therapeutics.

Pending Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Investments in Debt Securities (Tables)

12 Months Ended

Dec. 31, 2019

Debt Securities [Abstract]

Summary of Classification of Debt Securities in Consolidated Balance Sheet

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Debt securities are classified in the consolidated balance sheet as follows:

		December 31,
		2019		2018
		Balance		Balance
	(in thousands)
Cash equivalents		$	34,053	$	23,457
Current marketable securities			42,153		70,761
Non-current marketable securities			2,009		—
Total		$	78,215	$	94,218

Summary of Debt Securities by Maturity

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The following table summarizes debt securities by maturity at December 31, 2019 (in thousands):

Maturity	Amortized Cost
Within one year	$	76,206
After one year through five years		2,009
Total	$	78,215

Summary of Debt Securities by Credit Quality Indicator

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The following table summarizes debt securities by credit-quality indicator:

	Credit Quality Indicator as of December 31, 2019
	AAA		AA- to AA+		A- to A+		Total
	(In thousands)
Money market funds	$	34,053	$	—	$	—	$	34,053
Corporate paper and notes		—		11,550		27,129		38,679
U.S. government agency securities		—		3,987		—		3,987
U.S. treasuries		—		1,496		—		1,496
Total	$	34,053	$	17,033	$	27,129	$	78,215

Summary of Fair Value of Debt Securities by Major Security Type

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The following table summarizes the fair value of debt securities by major security type held at December 31, 2019 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	34,053	$	—	$	—		$	34,053
Corporate paper and notes		38,679		32		(2	)		38,709
U.S. government agency securities		3,987		7		—			3,994
U.S. treasuries		1,496		2		—			1,498
Total	$	78,215	$	41	$	(2	)	$	78,254

The following table summarizes the fair value of debt securities by major security type held at December 31, 2018 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	4,463	$	—	$	—		$	4,463
Term deposits		18,000		—		—			18,000
Corporate paper and notes		62,066		—		(86	)		61,980
U.S. government agency securities		1,980		—		—			1,980
U.S. treasuries		7,709		—		(1	)		7,708
Total	$	94,218	$	—	$	(87	)	$	94,131

Schedule of Allowance for Credit Losses Applicable to Debt Securities by Major Security Type

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Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Estimated Useful Lives of Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

Prepaid Expenses and Other Assets (Tables)

12 Months Ended

Dec. 31, 2019

Prepaid Expense And Other Assets Current [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2019		2018
	(In thousands)
Prepaid clinical and manufacturing costs	$	2,779	$	2,106
Interest receivable		259		244
Other		1,913		2,383
Total	$	4,951	$	4,733

Property and Equipment, net (Tables)

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Laboratory equipment	$	4,538		$	3,159
Office equipment and furniture		1,186			713
Leasehold improvements		79			40
Property and equipment		5,803			3,912
Less: accumulated depreciation and amortization		(2,088	)		(1,206	)
Property and equipment, net	$	3,715		$	2,706

Intangible Assets, Net (Tables)

12 Months Ended

Dec. 31, 2019

Intangible Assets Gross Excluding Goodwill [Abstract]

Summary of Intangible Assets, Net

Intangible assets, net consists of the following:

	December 31,
	2019			2018
	(In thousands)
Licenses of intellectual property	$	3,568		$	3,532
Software licenses		264			65
Intangible assets		3,832			3,597
Less: accumulated amortization		(956	)		(733	)
Intangible assets, net	$	2,876		$	2,864

Schedule of Expected Amortization Expense

Amortization expense over the next five years are expected to be as follows (in thousands):

Year	Expected amortization
2020	$	269
2021		269
2022		269
2023		206
2024		200
Thereafter		1,663
Total remaining value	$	2,876

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Summary of Accrued Expenses

Accrued expenses consisted of the following:

	December 31,
	2019		2018
	(In thousands)
Accrued research and development expenses	$	6,618	$	5,048
Accrued general and administrative expenses		2,402		2,891
Accrued personnel costs		4,495		2,386
Other		21		61
Accrued expenses	$	13,536	$	10,386

Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Summary of Components of Loss from Operations Before Income Taxes

The components of loss from operations before income taxes are as follows:

	Year ended December 31,
	2019			2018
	(In thousands)
United States	$	(1,363	)	$	(2,731	)
Netherlands		(53,594	)		(25,374	)
Total loss before income taxes	$	(54,957	)	$	(28,105	)

Summary of Components of Expense for (Benefit) from Income Taxes

The components of the income tax expense (benefit) from continuing operations are as follows:

	December 31,
	2019			2018
	(In thousands)
U.S. federal	$	243		$	251
U.S. state		40			166
Total current tax expense	$	283		$	417

U.S. federal	$	(63	)	$	(150	)
U.S state		(26	)		(62	)
Total deferred tax benefit	$	(89	)	$	(212	)

Total tax expense	$	194		$	205

Reconciliation of Netherlands Statutory Income Tax Rate and Effective Income Tax Rate

	Year Ended December 31,
	2019			2018
Netherlands statutory income tax rate		25.0	%		25.0	%
U.S. tax rate differential		—			(0.1	)
Changes in tax rates		1.5			(35.9	)
Non-deductible expenses		(3.7	)		(8.7	)
Other		0.1			—
Change in valuation allowance		(23.3	)		19.0
Effective income tax rate		(0.4	)%		(0.7	)%

Summary of Components of Deferred Tax Assets (Liabilities)

The components of the Company’s deferred tax assets (liabilities) consist of the following:

	December 31,
	2019			2018
	(In thousands)
Deferred tax assets:
Net operating loss carryforwards	$	33,917		$	17,798
Deferred revenue		23,926			26,814
Lease obligation		1,333			551
Accrued expenses and other		319			206
Total deferred tax assets		59,495			45,369
Deferred tax asset valuation allowance		(57,876	)		(44,643	)
Total deferred tax assets, net of valuation allowance		1,619			726

Deferred tax liabilities:
Operating lease right-of-use assets	$	1,331		$	520
Other		—			7
Total deferred tax liabilities		1,331			527

Net deferred tax asset	$	288		$	199

Summary of Net Operating Loss Carryforwards for Dutch Income Tax Purposes

The Company had net operating loss carryforwards for Dutch income tax purposes, the amount and expiry are as follows (in thousands):

Expiry Year	Netherlands Tax Loss Carryforward
2024	$	21,174
2025		100,864
2026		—
2027		34,263
Total	$	156,301

Operating Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Summary of Components of Lease Cost

The components of lease cost recorded in the Company’s consolidated statement of operations and statement of cash flows were as follows:

	For the Year Ended December 31,
	2019		2018
	(In thousands)
Operating lease cost	$	1,386	$	834
Variable lease cost		297		177
Total lease cost included in operating expenses	$	1,683	$	1,011

Cash paid to lessors included in operating cash outflows	$	2,273	$	1,199

Schedule of Maturities of Operating Lease Obligations

Maturities of the Company’s operating lease obligations as of December 31, 2019 were as follows (in thousands):

Year	Operating Leases
2020	$	1,602
2021		1,495
2022		614
2023		630
2024		645
Thereafter		884
Total lease payments		5,870
Less: amount representing interest		(618	)
Total lease obligations	$	5,252

Schedule of Weighted Average Remaining Lease Terms and Discount Rates

The weighted-average remaining lease terms and discount rates related to the Company’s leases were as follows:

	As of December 31,
	2019			2018
Weighted-average remaining operating lease term (in years)		4.8			2.8
Weighted-average discount rate for operating leases		5.0	%		6.0	%

Collaborations (Tables)

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Summary of Company's Accompanying Consolidated Financial Statements Attributable to Transactions Arising From Collaboration Arrangements

	Related Party			Third Party
	Incyte			Ono			Other			Total
Contract assets
Accounts receivable
Balance at January 1, 2019	$	—		$	70		$	29		$	99
Billings		8,480			4,504			949			5,453
Cash receipts		(8,511	)		(3,788	)		(978	)		(4,766	)
Foreign exchange		31			—			—			—
Balance at December 31, 2019		—			786			—			786

Unbilled receivables
Balance at January 1, 2019	$	2,982		$	270		$	—		$	270
Accrued receivables		7,270			4,239			1,104			5,343
Billings		(8,480	)		(4,504	)		(949	)		(5,453	)
Foreign exchange		(61	)		(5	)		—			(5	)
Balance at December 31, 2019		1,711			—			155			155

Contract liabilities
Deferred revenue
Balance at January 1, 2019	$	128,871		$	543		$	1,388		$	1,931
Cash receipts		—			—			949			949
Revenue recognized in the period		(17,839	)		(196	)		(926	)		(1,122	)
Foreign exchange		(2,494	)		(11	)		(26	)		(37	)
Balance at December 31, 2019		108,538			336			1,385			1,721
Less: current portion		(17,901	)		(197	)		(744	)		(941	)
Non-current balance at December 31, 2019		90,637			139			641			780

Summary of Company's Collaboration Arrangements Earned in Period to Be Billed and Collected in Next Period

	For the Year Ended December 31, 2019
	Related Party		Third Party
	Incyte		Ono		Other		Total
Upfront payments	$	17,839	$	196	$	643	$	839
Reimbursement revenue		7,992		99		153		252
Milestones		—		4,142		284		4,426
Total collaboration revenue	$	25,831	$	4,437	$	1,080	$	5,517

Operating expenses:
Research and development expense	$	680	$	—	$	—	$	—
General and administrative expense		—		—		—		—
Total operating expenses from collaborations	$	680	$	—	$	—	$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	17,839	$	196	$	945	$	1,141

	For the Year Ended December 31, 2018
	Related Party		Third Party
	Incyte		Ono		Other		Total
Upfront payments	$	18,819	$	341	$	1,660	$	2,001
Reimbursement revenue		11,150		1,036		—		1,036
Milestones		—		4,724		415		5,139
Total collaboration revenue	$	29,969	$	6,101	$	2,075	$	8,176

Operating expenses:
Research and development expense	$	678	$	—	$	—	$	—
General and administrative expense		44		—		—		—
Total operating expenses from collaborations	$	722	$	—	$	—	$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,819	$	73	$	—	$	73

Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss

Share-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:

	Year Ended December 31,
	2019		2018
	(In thousands)
Research and development	$	3,186	$	3,333
General and administrative		4,648		6,421
Total	$	7,834	$	9,754

Summary of Key Weighted Average Assumptions Used in Pricing Model on Date of Grant for Options Granted to Employees

	Year Ended December 31,
	2019			2018
Risk-free interest rate		2.5	%		2.9	%
Expected life of options (years)		10.0			10.0
Expected volatility of underlying stock		87.0	%		93.7	%
Expected dividend yield		0.0	%		0.0	%

Summary of Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2019:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(years)		(In thousands)
Outstanding at January 1, 2019		2,633,039		$	17.20
Granted		1,145,221			12.18
Exercised		(12,080	)		4.01
Forfeited or expired		(451,309	)		15.80
Outstanding at December 31, 2019		3,314,871		$	14.67		7.3	$	7,757
Exercisable at December 31, 2019		1,795,089		$	14.61		6.0	$	5,572

	Year Ended December 31,
	2019		2018
Weighted-average fair value of options granted	$	7.54	$	11.69

Summary of RSU Activity

The following is a summary of RSU activity for the year ended December 31, 2019:

	Number of RSUs			Weighted Average Grant-date Fair Value
Non-vested at January 1, 2019		101,302		$	21.11
Granted		30,000			17.17
Vested		(48,660	)		21.11
Non-vested at December 31, 2019		82,642		$	19.68

Intrinsic Value of Stock Options Exercised and Vested RSUs

	Year Ended December 31,
	(In thousands)
	2019		2018
Total fair value of RSUs vested	$	726	$	1,775
Aggregate intrinsic value of options exercised		112		1,642

Loss per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Summary of Basic and Diluted Loss per Share

Basic and diluted loss per share allocable to common stockholders are computed as follows:

	Year Ended December 31,
	2019			2018
	(In thousands except per share data)
Net loss	$	(55,151	)	$	(28,310	)
Weighted average shares outstanding		24,218,083			22,286,720
Basic and diluted loss per share allocable to common stockholders	$	(2.28	)	$	(1.27	)

The Company - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated loss	$ (314,599)	$ (259,448)
Cash and cash equivalents and marketable securities	$ 241,800

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Cash and cash equivalents, maturities	three months or less
Impairment charges of long-lived assets	$ 0	$ 0
Reduction in research and development expenses	$ 4,500,000	$ 5,000,000
Number of reportable segment \| Segment	1

Summary of Significant Accounting Policies - Summary of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Office Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	Shorter of useful life or term of lease

Investments in Debt Securities - Summary of Classification of Debt Securities in Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	$ 78,215	$ 94,218
Cash equivalents [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	34,053	23,457
Current marketable securities [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	42,153	70,761
Non-current marketable securities [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	$ 2,009	$ 0

Investments in Debt Securities - Summary of Debt Securities by Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Held To Maturity Securities Debt Maturities Single Maturity Date Amortized Cost Basis [Abstract]
Within one year	$ 76,206
After one year through five years	2,009
Total	$ 78,215	$ 94,218

Investments in Debt Securities - Summary of Debt Securities by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 78,215	$ 94,218
AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	34,053
AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	17,033
A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	27,129
Money market funds [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	34,053	4,463
Money market funds [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	34,053
Money market funds [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Money market funds [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Corporate paper and notes [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	38,679	62,066
Corporate paper and notes [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Corporate paper and notes [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	11,550
Corporate paper and notes [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	27,129
U.S. government agency securities [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	3,987	1,980
U.S. government agency securities [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. government agency securities [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	3,987
U.S. government agency securities [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. treasuries [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	1,496	$ 7,709
U.S. treasuries [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. treasuries [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	1,496
U.S. treasuries [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 0

Investments in Debt Securities - Summary of Fair Value of Debt Securities by Major Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Total	$ 78,215	$ 94,218
Unrealized Gains	41	0
Unrealized Losses	(2)	(87)
Fair Value	78,254	94,131
Money market funds [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	34,053	4,463
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	34,053	4,463
Corporate paper and notes [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	38,679	62,066
Unrealized Gains	32	0
Unrealized Losses	(2)	(86)
Fair Value	38,709	61,980
U.S. government agency securities [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	3,987	1,980
Unrealized Gains	7	0
Unrealized Losses	0	0
Fair Value	3,994	1,980
U.S. treasuries [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	1,496	7,709
Unrealized Gains	2	0
Unrealized Losses	0	(1)
Fair Value	$ 1,498	7,708
Term Deposits [member]
Schedule Of Held To Maturity Securities [Line Items]
Total		18,000
Unrealized Gains		0
Unrealized Losses		0
Fair Value		$ 18,000

Prepaid Expenses and Other Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical and manufacturing costs	$ 2,779	$ 2,106
Interest receivable	259	244
Other	1,913	2,383
Total	$ 4,951	$ 4,733

Prepaid Expenses and Other Assets - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Other Assets
Restricted cash held on account as collateral	$ 0.2	$ 0.1

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 5,803	$ 3,912
Less: accumulated depreciation and amortization	(2,088)	(1,206)
Property and equipment, net	3,715	2,706
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and Equipment, gross	4,538	3,159
Office Equipment and Furniture
Property Plant And Equipment [Line Items]
Property and Equipment, gross	1,186	713
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 79	$ 40

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Netherlands
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 0.9	$ 0.6

Intangible Assets, Net - Summary of Intangible Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 3,832	$ 3,597
Less: accumulated amortization	(956)	(733)
Intangible assets, net	2,876	2,864
Licenses of Intellectual Property [Member]
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	3,568	3,532
Software Licenses [Member]
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 264	$ 65

Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Intangible Assets Net Excluding Goodwill [Abstract]
Amortization of intangible assets	$ 236	$ 109

Intangible Assets, Net - Schedule of Expected Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Finite Lived Intangible Assets Future Amortization Expense Current And Five Succeeding Fiscal Years [Abstract]
2020	$ 269
2021	269
2022	269
2023	206
2024	200
Thereafter	1,663
Intangible assets, net	$ 2,876	$ 2,864

Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued research and development expenses	$ 6,618	$ 5,048
Accrued general and administrative expenses	2,402	2,891
Accrued personnel costs	4,495	2,386
Other	21	61
Accrued expenses	$ 13,536	$ 10,386

Taxes - Summary of Components of Loss from Operations Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (1,363)	$ (2,731)
Netherlands	(53,594)	(25,374)
Loss before income taxes	$ (54,957)	$ (28,105)

Taxes - Summary of Components of Expense for (Benefit) from Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal	$ 243	$ 251
U.S. state	40	166
Total current tax expense	283	417
U.S. federal	(63)	(150)
U.S state	(26)	(62)
Total deferred tax benefit	(89)	(212)
Total tax expense	$ 194	$ 205

Taxes - Reconciliation of Netherlands Statutory Income Tax Rate and Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Netherlands statutory income tax rate	25.00%	25.00%
U.S. tax rate differential	0.00%	(0.10%)
Changes in tax rates	1.50%	(35.90%)
Non-deductible expenses	(3.70%)	(8.70%)
Other	0.10%	0.00%
Change in valuation allowance	(23.30%)	19.00%
Effective income tax rate	(0.40%)	(0.70%)

Taxes - Summary of Components of Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 33,917	$ 17,798
Deferred revenue	23,926	26,814
Lease obligation	1,333	551
Accrued expenses and other	319	206
Total deferred tax assets	59,495	45,369
Deferred tax asset valuation allowance	(57,876)	(44,643)
Total deferred tax assets, net of valuation allowance	1,619	726
Deferred tax liabilities:
Operating lease right-of-use assets	1,331	520
Other	0	7
Total deferred tax liabilities	1,331	527
Net deferred tax asset	$ 288	$ 199

Taxes - Additional Information (Details) - USD ($)	12 Months Ended
Taxes - Additional Information (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Deferred tax asset valuation allowance	$ 57,876,000	$ 44,643,000
Unrecognized tax benefits	0
Accrued interest or penalties related to uncertain tax positions	0
Unrecognized tax benefits, tax related penalties and interest expense	0
Netherlands
Income Tax Disclosure [Line Items]
Increase in valuation allowance, attributable to increase in net operating loss carryforward deferred tax assets	13,200,000	$ 5,300,000
Deferred tax asset valuation allowance	$ 1,400

- Definition

Amount of valuation allowance for tax benefit to be credited to shareholders' equity if tax benefit subsequently becomes realizable. Includes, but is not limited to, adjustment to opening balance of retained earnings for change in accounting principle or correction of error, gain (loss) in comprehensive income (loss) but excluded from net income (loss), deductible expenditure for income tax purposes reported as reduction of shareholders' equity, and deductible temporary difference existing at date of quasi-reorganization.

+ References + Details

Taxes - Summary of Net Operating Loss Carryforwards for Dutch Income Tax Purposes (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Operating Loss Carryforwards [Line Items]
Netherlands Tax Loss Carryforward	$ 156,301
Tax Year 2024
Operating Loss Carryforwards [Line Items]
Expiry Year	2024
Netherlands Tax Loss Carryforward	$ 21,174
Tax Year 2025
Operating Loss Carryforwards [Line Items]
Expiry Year	2025
Netherlands Tax Loss Carryforward	$ 100,864
Tax Year 2026
Operating Loss Carryforwards [Line Items]
Expiry Year	2026
Tax Year 2027
Operating Loss Carryforwards [Line Items]
Expiry Year	2027
Netherlands Tax Loss Carryforward	$ 34,263

Operating Leases - Additional Information (Details) € in Millions	1 Months Ended		12 Months Ended
	Jul. 31, 2019 EUR (€) m² RenewalTerm	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2019
Lessee Lease Description [Line Items]
Operating lease, term of agreement			5 years
Operating lease, description			Merus N.V. leases its corporate headquarters under an agreement term of five years, which expires in the fourth quarter of 2021. On May 1, 2018, Merus N.V. leased additional space to expand its corporate headquarters under a separate agreement. Under the terms of the new agreement, the term began on May 1, 2018 and expires in the fourth quarter of 2021. If the leases are not terminated by Merus N.V., they will be automatically renewed for a period of two years.
Operating lease, renewal term			2 years
Operating lease, expiration period			fourth quarter of 2021.
Scenario Plan [Member] \| Kadans Science Partner XII BV \| Office Building
Lessee Lease Description [Line Items]
Operating lease, term of agreement	10 years
Operating lease, renewal term	5 years
Area of property available for operating lease \| m²	5,070
Operating lease, number of renewal term \| RenewalTerm	2
Operating lease, termination term	20 years
Operating lease, estimated rent expense \| €	€ 1.3
Operating lease, rent adjustment description	The rent amount is subject to adjustment based on the consumer price index (the “CPI”) beginning on January 1, 2019 through the completion date and then annually thereafter, subject to certain limitations if the CPI is greater than 3.0%.
Scenario Plan [Member] \| Kadans Science Partner XII BV \| Minimum \| Office Building
Lessee Lease Description [Line Items]
Consumer price Index	3.00%
Merus US Inc
Lessee Lease Description [Line Items]
Operating lease, term of agreement				7 years
Operating lease, description		Merus US, Inc. entered into a lease agreement for office space in Cambridge, Massachusetts. The lease commenced in the second quarter of 2019 and has a term of seven years, and may be extended for another five years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term.
Operating lease, extended term of agreement		5 years
Operating lease, option to extend		true
Operating lease, options to extend description		may be extended for another five years.

Operating Leases - Summary of Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Lease Cost [Abstract]
Operating lease cost	$ 1,386	$ 834
Variable lease cost	297	177
Total lease cost included in operating expenses	1,683	1,011
Cash paid to lessors included in operating cash outflows	$ 2,273	$ 1,199

Operating Leases - Schedule of Maturities of Operating Lease Obligations (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Lease Cost [Abstract]
2020	$ 1,602
2021	1,495
2022	614
2023	630
2024	645
Thereafter	884
Total lease payments	5,870
Less: amount representing interest	(618)
Total lease obligations	$ 5,252

Operating Leases - Schedule of Weighted Average Remaining Lease Terms and Discount Rates (Details)	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Weighted-average remaining operating lease term (in years)	4 years 9 months 18 days	2 years 9 months 18 days
Weighted-average discount rate for operating leases	5.00%	6.00%

Commitments and Contingencies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 20, 2018	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Proceeds from issuance of common stock, net of issuance costs		$ 74,184	$ 62,008
Common Stock [Member]
Loss Contingencies [Line Items]
Issuance of common stock, net, shares		5,462,500	3,699,997
Regeneron Pharmaceuticals, Inc
Loss Contingencies [Line Items]
Proceeds from issuance of common stock, net of issuance costs	$ 15,000
Litigation claims dismissal amount	$ 10,500
Regeneron Pharmaceuticals, Inc \| Common Stock [Member]
Loss Contingencies [Line Items]
Issuance of common stock, net, shares	600,000
Issue price per share	$ 25
Other Income
Loss Contingencies [Line Items]
Gain on litigation settlement			$ 8,100

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands						12 Months Ended
	Nov. 07, 2019	Dec. 21, 2018	Dec. 20, 2018	Feb. 15, 2018	Feb. 13, 2018	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Class Of Stock [Line Items]
Common stock voting rights						one vote
Net proceeds from sale under private placements				$ 55,100
Issuance of common stock, net						$ 74,020	$ 62,008
Proceeds from issuance of common stock, net of issuance costs						$ 74,184	62,008
2016 Incentive Award Plan \| Subsequent Event
Class Of Stock [Line Items]
Common stock reserved for additional grants								1,901,162
Regeneron Subscription Agreement
Class Of Stock [Line Items]
Issuance of common stock, net, shares			600,000
Issue price per share			$ 25.00
Issuance of common stock, net		$ 15,000					15,000
Proceeds from issuance of common stock, net of issuance costs							6,900
Gain on litigation settlement							$ 8,100
Private Placement
Class Of Stock [Line Items]
Issuance of common stock, net, shares					3,099,997
Issue price per share					$ 18.00
Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock, net, shares	5,462,500
Issue price per share	$ 14.50
Net proceeds from public offering	$ 74,000
Number of common shares pursuant to underwriters' option to purchase additional shares	715,500

Collaborations - Additional Information (Details) $ / shares in Units, $ in Thousands, € in Millions, shares in Millions		1 Months Ended		12 Months Ended
	Mar. 14, 2018 EUR (€) Antibody	Jan. 31, 2018 USD ($) Antibody PerformanceObligation	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 10, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestones received				$ 4,100	€ 3.7
Transaction price as revenue				$ 4,700	€ 4.0
Performance obligations related to transaction price as revenue				three	three
Incyte Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable			$ 120,000
Issuance of common stock, net, shares \| shares			3.2
Purchase price per share \| $ / shares			$ 25.00
Collaborations, transaction price as revenue			$ 80,000
Incyte Collaboration and License Agreement \| License and Related Activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborations, transaction price as revenue			$ 152,600
Second ONO Research and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable \| €	€ 0.7
Payment to compensate research services \| €	0.3
Over time payment for full time equivalent funding \| €	€ 0.2
Second ONO Research and License Agreement \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of bispecific antibodies under collaboration and license agreement \| Antibody	5
Simcere Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborations, transaction price as revenue		$ 2,750
Number of bispecific antibodies under collaboration and license agreement \| Antibody		3
Non-refundable upfront payment received		$ 2,750
Description of transaction price under fixed consideration				research and development for two of the three programs is on-going	research and development for two of the three programs is on-going
Simcere Collaboration and License Agreement \| License and Performance \| Research and Development
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of performance obligation \| PerformanceObligation		3
Betta Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable							$ 1,000
Contingent milestone payments receivable							$ 12,000
Revenue related performance obligation as revenue						$ 1,000

Collaborations - Summary of Company's Accompanying Consolidated Financial Statements Attributable to Transactions Arising From Collaboration Arrangements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accrued expenses	$ 13,536	$ 10,386
Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	17,839	18,819
Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	196	73
Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	945	0
Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	1,141	73
Accounts Receivable \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	0
Billings	8,480
Cash receipts	(8,511)
Foreign exchange	31
Balance at December 31, 2019	0	0
Accounts Receivable \| Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	70
Billings	4,504
Cash receipts	(3,788)
Foreign exchange	0
Balance at December 31, 2019	786	70
Accounts Receivable \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	29
Billings	949
Cash receipts	(978)
Foreign exchange	0
Balance at December 31, 2019	0	29
Accounts Receivable \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	99
Billings	5,453
Cash receipts	(4,766)
Foreign exchange	0
Balance at December 31, 2019	786	99
Unbilled Receivables \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	2,982
Billings	(8,480)
Foreign exchange	(61)
Balance at December 31, 2019	1,711	2,982
Accrued expenses	7,270
Unbilled Receivables \| Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	270
Billings	(4,504)
Foreign exchange	(5)
Balance at December 31, 2019	0	270
Accrued expenses	4,239
Unbilled Receivables \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	0
Billings	(949)
Foreign exchange	0
Balance at December 31, 2019	155	0
Accrued expenses	1,104
Unbilled Receivables \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2019	270
Billings	(5,453)
Foreign exchange	(5)
Balance at December 31, 2019	155	270
Accrued expenses	5,343
Deferred Revenue \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash receipts	0
Foreign exchange	(2,494)
Balance at January 1, 2019	128,871
Revenue recognized in the period	(17,839)
Balance at December 31, 2019	108,538	128,871
Less: current portion	(17,901)
Non-current balance at December 31, 2019	90,637
Deferred Revenue \| Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash receipts	0
Foreign exchange	(11)
Balance at January 1, 2019	543
Revenue recognized in the period	(196)
Balance at December 31, 2019	336	543
Less: current portion	(197)
Non-current balance at December 31, 2019	139
Deferred Revenue \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash receipts	(949)
Foreign exchange	(26)
Balance at January 1, 2019	1,388
Revenue recognized in the period	(926)
Balance at December 31, 2019	1,385	1,388
Less: current portion	(744)
Non-current balance at December 31, 2019	641
Deferred Revenue \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash receipts	(949)
Foreign exchange	(37)
Balance at January 1, 2019	1,931
Revenue recognized in the period	(1,122)
Balance at December 31, 2019	1,721	$ 1,931
Less: current portion	(941)
Non-current balance at December 31, 2019	$ 780

Collaborations - Summary of Company's Collaboration Arrangements Earned in Period to Be Billed and Collected in Next Period (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 55,680	$ 54,767
General and administrative	34,110	29,354
Total operating expenses	89,790	84,121
Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	17,839	18,819
Reimbursement revenue	7,992	11,150
Milestones	0	0
Total collaboration revenue	25,831	29,969
Operating expenses:
Research and development	680	678
General and administrative	0	44
Total operating expenses	680	722
Revenue recognized in the period	17,839	18,819
Ono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	196	341
Reimbursement revenue	99	1,036
Milestones	4,142	4,724
Total collaboration revenue	4,437	6,101
Operating expenses:
Research and development	0	0
General and administrative	0	0
Total operating expenses	0	0
Revenue recognized in the period	196	73
Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	643	1,660
Reimbursement revenue	153	0
Milestones	284	415
Total collaboration revenue	1,080	2,075
Operating expenses:
Research and development	0	0
General and administrative	0	0
Total operating expenses	0	0
Revenue recognized in the period	945	0
Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	839	2,001
Reimbursement revenue	252	1,036
Milestones	4,426	5,139
Total collaboration revenue	5,517	8,176
Operating expenses:
Research and development	0	0
General and administrative	0	0
Total operating expenses	0	0
Revenue recognized in the period	$ 1,141	$ 73

Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
401 (K) Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Defined contribution plan , description	The 401(k) Plan covers substantially all U.S. employees, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company matches contributions to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation.
Percentage of employer matching contribution	50.00%
Defined contribution plan, cost	$ 0.1	$ 0.1
401 (K) Plan \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employer matching contribution, percent of employees compensation	3.00%
Postemployment Retirement Benefits
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Contributions to post-employment benefit plan	$ 1.3	$ 0.8
Executive settlement, cash compensation to be paid	0.9
Executive settlement expense	1.8
Stock Option Activity
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation	$ 4.6
Weighted average remaining vesting period	1 year 4 months 24 days
Expected dividend yield	0.00%	0.00%
Share-Based Payments
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Share-Based Payments \| 2010 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments	4 years
Options expiration period	8 years
Share-Based Payments \| 2010 Plan \| First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	25.00%
Share-Based Payments \| 2010 Plan \| Thereafter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	75.00%
Award vesting rights, required service period	36 months	36 months
Share-Based Payments \| 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments	4 years	4 years
Options expiration period	10 years	10 years
Share-Based Payments \| 2016 Plan \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares authorized for issuance for future grants			1,901,162
Share-Based Payments \| 2016 Plan \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments	3 years	3 years
Share-Based Payments \| 2016 Plan \| Non-Executive Directors \| Subsequent Award
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments	1 year	1 year
Award vesting rights, required service period	12 months	12 months
Share-Based Payments \| 2016 Plan \| First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	25.00%	25.00%
Share-Based Payments \| 2016 Plan \| First Anniversary \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	33.00%	33.00%
Share-Based Payments \| 2016 Plan \| Thereafter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	75.00%	75.00%
Award vesting rights, required service period	36 months	36 months
Share-Based Payments \| 2016 Plan \| Thereafter \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage	67.00%	67.00%
Award vesting rights, required service period	24 months	24 months
RSUs \| Postemployment Retirement Benefits
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant of RSUs	30,000
RSUs \| 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Description of RSUs granted to employees	Each RSU represents the right to receive one common share.
Vesting period of RSUs granted share-based payments	4 years	4 years

Employee Benefit Plans - Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	$ 7,834	$ 9,754
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	3,186	3,333
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	$ 4,648	$ 6,421

Employee Benefit Plans - Summary of Key Weighted Average Assumptions used in Pricing Model on date of Grant for Options Granted to Employees (Details) - Stock Option Activity	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.50%	2.90%
Expected life of options (years)	10 years	10 years
Expected volatility of underlying stock	87.00%	93.70%
Expected dividend yield	0.00%	0.00%

Employee Benefit Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning balance	2,633,039
Number of Options, Granted	1,145,221
Number of Options, Exercised	(12,080)
Number of Options, Forfeited or expired	(451,309)
Number of Options, Outstanding, Ending balance	3,314,871	2,633,039
Number of Options, Exercisable	1,795,089
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 17.20
Weighted Average Exercise Price per Share, Granted	12.18
Weighted Average Exercise Price per Share, Exercised	4.01
Weighted Average Exercise Price per Share, Forfeited or expired	15.80
Weighted Average Exercise Price per Share, Outstanding, Ending balance	14.67	$ 17.20
Weighted Average Exercise Price per Share, Exercisable	$ 14.61
Weighted Average Remaining Contractual Term, Outstanding	7 years 3 months 18 days
Weighted Average Remaining Contractual Term, Exercisable	6 years
Aggregate Intrinsic Value, Outstanding	$ 7,757
Aggregate Intrinsic Value, Exercisable	$ 5,572
Weighted-average fair value of options granted	$ 7.54	$ 11.69

Employee Benefit Plans - Summary of RSU Activity (Details) - RSUs	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSUs, Non-vested, Beginning balance \| shares	101,302
Number of RSUs, Granted \| shares	30,000
Number of RSUs, Vested \| shares	(48,660)
Number of RSUs, Non-vested, Ending balance \| shares	82,642
Weighted Average Grant-date Fair Value, Non-vested, Beginning balance \| $ / shares	$ 21.11
Weighted Average Grant-date Fair Value, Granted \| $ / shares	17.17
Weighted Average Grant-date Fair Value, Vested \| $ / shares	21.11
Weighted Average Grant-date Fair Value, Non-vested, Ending balance \| $ / shares	$ 19.68

Employee Benefit Plans - Intrinsic Value of Stock Options Exercised and Vested RSUs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of options exercised	$ 112	$ 1,642
RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total fair value of RSUs vested	$ 726	$ 1,775

Loss per Share - Summary of Basic and Diluted Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss	$ (55,151)	$ (28,310)
Weighted average shares outstanding	24,218,083	22,286,720
Basic and diluted	$ (2.28)	$ (1.27)

- Definition

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

+ References + Details

Related Party Transactions - Additional Information (Details)	Dec. 31, 2019	Dec. 31, 2018
Incyte \| Merus N V [Member]
Related Party Transaction [Line Items]
Ownership percentage	11.10%	13.70%