Arena Pharmaceuticals (ARNA) 10-Q 7 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 02, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	ARENA PHARMACEUTICALS INC
Trading Symbol	ARNA
Entity Central Index Key	1,080,709
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		49,348,189

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 501,217	$ 158,837
Short-term investments, available-for-sale	91,190	88,240
Accounts receivable	2,334	2,357
Insurance recovery receivable		12,025
Prepaid expenses and other current assets	6,539	2,681
Assets of disposal group held for sale		17,140
Total current assets	601,280	281,280
Investments, available-for-sale		24,242
Land, property and equipment, net	28,756	30,131
Other non-current assets	7,524	3,622
Total assets	637,560	339,275
Current liabilities:
Accounts payable and other accrued liabilities	6,855	7,916
Accrued clinical and preclinical study fees	9,668	7,706
Accrued litigation settlement		24,000
Current portion of deferred revenues	800	1,110
Current portion of lease financing obligations	3,594	4,000
Liabilities of disposal group held for sale		27,595
Total current liabilities	20,917	72,327
Other long-term liabilities	1,011	989
Deferred revenues, less current portion	615	1,067
Lease financing obligations, less current portion	56,179	57,748
Commitments and contingencies
Stockholders' equity:
Common stock	5	4
Additional paid-in capital	2,093,958	1,698,543
Accumulated other comprehensive loss	(177)	(1,216)
Accumulated deficit	(1,534,948)	(1,490,187)
Total stockholders' equity	558,838	207,144
Total liabilities and stockholders' equity	$ 637,560	$ 339,275

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 3,994	$ 1,898	$ 5,749	$ 3,558
Operating Costs and Expenses:
Research and development	26,755	17,700	48,328	33,044
General and administrative	10,405	7,150	21,556	14,670
Total operating costs and expenses	37,160	24,850	69,884	47,714
Loss from operations	(33,166)	(22,952)	(64,135)	(44,156)
Interest and Other Income (Expense):
Interest income	2,502	16	3,276	50
Interest expense	(1,448)	(1,538)	(2,920)	(3,108)
Other income	279	711	813	900
Total interest and other income (expense), net	1,333	(811)	1,169	(2,158)
Loss from continuing operations	(31,833)	(23,763)	(62,966)	(46,314)
Income (loss) from discontinued operations		147	(830)	201
Net loss	(31,833)	(23,616)	(63,796)	(46,113)
Less net loss attributable to noncontrolling interest in consolidated variable interest entity		299		743
Net loss attributable to stockholders of Arena	(31,833)	(23,317)	(63,796)	(45,370)
Amounts attributable to stockholders of Arena:
Loss from continuing operations	$ (31,833)	(23,464)	(62,966)	(45,571)
Income (loss) from discontinued operations		$ 147	$ (830)	$ 201
Net income (loss) attributable to stockholders of Arena per share, basic and diluted:
Continuing operations	$ (0.65)	$ (0.77)	$ (1.41)	$ (1.66)
Discontinued operations			(0.02)
Net income (loss) attributable to stockholders of Arena per share, basic and diluted	$ (0.65)	$ (0.77)	$ (1.43)	$ (1.66)
Shares used in calculating net income (loss) attributable to stockholders of Arena per share, basic and diluted:	49,263	30,299	44,655	27,371
Comprehensive Loss:
Net loss	$ (31,833)	$ (23,616)	$ (63,796)	$ (46,113)
Foreign currency translation gain (loss)	(50)	1,910	(33)	2,714
Unrealized gain (loss) on available-for-sale investments	113		(31)
Comprehensive loss	(31,770)	(21,706)	(63,860)	(43,399)
Less comprehensive loss attributable to noncontrolling interest in consolidated variable interest entity		299		743
Comprehensive loss attributable to stockholders of Arena	(31,770)	(21,407)	(63,860)	(42,656)
Collaboration and Other Revenue
Revenues:
Total revenues	3,133	$ 1,898	4,161	$ 3,558
Royalty Revenue
Revenues:
Total revenues	$ 861		$ 1,588

Condensed Consolidated Stateme4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities:
Net loss	$ (63,796)	$ (46,113)
Adjustments to reconcile net loss to net cash used in operating activities:
(Income) loss from discontinued operations	830	(201)
Depreciation and amortization	1,902	2,190
Non-cash collaboration consideration	(1,500)
Share-based compensation	8,563	3,881
Amortization of prepaid financing costs	55	68
Amortization of original issue discounts, net of premiums, on available-for-sale investments	(162)
Gain on disposal of property and equipment		(393)
Changes in operating assets and liabilities:
Accounts receivable	165	11,974
Prepaid expenses and other assets	(635)	(921)
Payables and accrued liabilities	349	(5,623)
Accrued litigation settlement	(11,975)
Deferred revenues	(646)	(2,190)
Other long-term liabilities	(551)	(28)
Net cash used in operating activities - continuing operations	(67,401)	(37,356)
Net cash used in operating activities - discontinued operations	(102)	(4,829)
Net cash used in operating activities	(67,503)	(42,185)
Investing Activities:
Purchases of available-for-sale investments	(25,477)
Proceeds from sale and maturity of available-for-sale investments	46,900
Purchases of property and equipment	(528)	(25)
Other non-current assets	2	(6)
Net cash used in investing activities - continuing operations	20,897	(31)
Net cash provided by investing activities - discontinued operations	3,405	31
Net cash provided by investing activities	24,302
Financing Activities:
Principal payments on lease financing obligations	(1,976)	(1,684)
Proceeds from issuance of common stock, net	387,009	81,496
Net cash provided by financing activities	385,033	79,812
Effect of exchange rate changes on cash	548	2,424
Net increase in cash, cash equivalents and restricted cash	342,380	40,051
Cash, cash equivalents and restricted cash at beginning of period	159,700	91,575
Cash, cash equivalents and restricted cash at end of period	$ 502,080	$ 131,626

Basis of Presentation

Basis of Presentation	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	1. Basis of Presentation The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2017, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2017. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. Beacon Discovery, Inc., or Beacon, is a variable interest entity in which we had a controlling financial interest until December 2017. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statement of operations and comprehensive loss for the three and six months ended June 30, 2017 . We Liquidity. As of June 30, 2018, we had cash, cash equivalents and available-for-sale investments of approximately $592.4 million. We believe our cash and cash equivalents and available-for-sale investments will be sufficient to fund our operations for at least the next 12 months. We will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, as this process typically takes many years and potentially hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates, programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions. Changes in Accounting Policies - Revenue Recognition. Effective January 1, 2018, we adopted Accounting Standard Codification 606, Revenue from Contracts with Customers , or ASC 606, issued by the Accounting Standards Board, or FASB. As a result, we have changed our accounting policy for revenue recognition as detailed below. We implemented ASC 606 using the modified retrospective method by recognizing the cumulative effect of initially applying ASC 606 as an adjustment to the opening balance of our accumulated deficit at January 1, 2018. Therefore, the comparative period information has not been adjusted. We applied ASC 606 using a practical expedient for contracts that were modified before the implementation date, which allowed us to determine an aggregate effect of all modifications that occurred before January 1, 2018, when determining the satisfied and unsatisfied performance obligations, the transaction price, and allocating that transaction price to the performance obligations instead of retrospectively restating the contracts for such contract modifications. The cumulative impact to our accumulated deficit balance at January 1, 2018, as a result of the adoption of ASC 606 was a decrease of $19.0 million. The decrease arose primarily from a reduction of deferred revenue balances related to upfront payments received from customers and recognition of contract assets due to a combination of (i) the effects of applying the practical expedient for contract modifications and our conclusions related to satisfied and unsatisfied performance obligations, which resulted in a relatively higher portion of the total transaction price recognized as revenue in periods prior to our adoption of ASC 606, (ii) the effect of the bill-and-hold accounting guidance for inventory in ASC 606 and (iii) the inclusion of estimated future royalty payments related to our intellectual property in the total transaction price to the extent such intellectual property was legally sold to our customer rather than licensed. The cumulative effect adjustment is net of an impairment loss of $13.1 million which was a direct effect of the adoption of ASC 606 on the asset group of the Manufacturing Operations, which was classified as assets of disposal group held for sale since December 2017. The following table summarizes the impacts of adopting ASC 606 on our condensed consolidated financial statements, in thousands. Impact of Changes in Accounting Policies Three months ended June 30, 2018 As reported Adjustments Balances without adoption of ASC 606 Collaboration and other revenue $ 3,133 $ (15 ) $ 3,118 Royalty revenue 861 426 1,287 Total revenue 3,994 411 4,405 Loss from operations (33,166 ) 411 (32,755 ) Loss from continuing operations (31,833 ) 411 (31,422 ) Net loss (31,833 ) 411 (31,422 ) Net loss attributable to stockholders of Arena (31,833 ) 411 (31,422 ) Six months ended June 30, 2018 Collaboration and other revenue $ 4,161 $ 44 $ 4,205 Royalty revenue 1,588 688 2,276 Total revenue 5,749 732 6,481 Loss from operations (64,135 ) 732 (63,403 ) Loss from continuing operations (62,966 ) 1,880 (61,086 ) Income (loss) from discontinued operations (830 ) 13,660 12,830 Net loss (63,796 ) 15,540 (48,256 ) Net loss attributable to stockholders of Arena (63,796 ) 15,540 (48,256 ) As of June 30, 2018 Prepaid expenses and other current assets $ 6,539 $ (1,059 ) $ 5,480 Total current assets 601,280 (1,059 ) 600,221 Other non-current assets 7,524 (2,361 ) 5,163 Total assets 637,560 (3,420 ) 634,140 Current portion of deferred revenues 800 20 820 Total current liabilities 20,917 20 20,937 Deferred revenues, less current portion 615 52 667 Accumulated deficit (1,534,948 ) (3,492 ) (1,538,440 ) Total stockholders' equity 558,838 (3,492 ) 555,346 Total liabilities and stockholders' equity 637,560 (3,420 ) 634,140 Our revenues to date have been generated primarily through collaboration agreements. Our collaboration agreements frequently contain multiple elements including (i) intellectual property licenses, (ii) product research, development and regulatory services and (iii) product manufacturing. Consideration we receive under these arrangements may include upfront payments, research and development funding, cost reimbursements, milestone payments, payments for product sales and royalty payments. Our customers include Everest Medicines Limited, or Everest, Outpost Medicine, LLC, or Outpost, Eisai Inc. and Eisai Co., Ltd., or collectively, Eisai, Axovant Sciences GmbH, or Axovant, Boehringer Ingelheim International GmbH, or Boehringer Ingelheim, and Siegfried. Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services. The amount of revenue recognized reflects the consideration that we expect to be entitled to receive in exchange for these services and excludes sales incentives and amounts collected i) Identify the contract with a customer . We consider the terms and conditions of our collaboration agreements to identify contracts within the scope of ASC 606. We consider that we have a contract with a customer when the contract is approved, we can identify each party's rights regarding the goods and services to be transferred, we can identify the payment terms for the goods and services, we have determined the customer has the ability and intent to pay and the contract has commercial substance. We use judgment in determining the customer's ability and intent to pay, which is based upon factors including the customer's historical payment experience or, for new customers, credit and financial information pertaining to the customers. ii) Identify the performance obligations in the contract. Performance obligations in our collaboration agreements are identified based on the goods and services that will be transferred to the customer that are both capable of being distinct, whereby the customer can benefit from the service either on its own or together with other resources that are readily available from third parties or from us, and are distinct in the context of the contract, whereby the transfer of the services is separately identifiable from other promises in the contract. Our performance obligations generally consist of intellectual property licenses, research, development and/or regulatory services and manufacturing and supply commitments. iii) Determine the transaction price. We determine the transaction price based on the consideration to which we expect to be entitled in exchange for transferring goods and services to the customer. In determining the transaction price, any variable consideration would be considered, to the extent applicable, if, in our judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. In accordance with the royalty exception under ASC 606 for licenses of intellectual property, the transaction price excludes future royalty payments to be received from our customers. None of our collaboration agreements contain consideration payable to our customer or a significant financing component. iv) Allocate the transaction price to performance obligations in the contract. v) Recognize revenue when or as we satisfy a performance obligation. Revenue is recognized at the time the related performance obligation is satisfied by transferring the promised goods or services to a customer. We recognize revenue when we transfer control of the goods or services to our customers for an amount that reflects the consideration that we expect to receive in exchange for those services. Performance Obligations. The following is a description of principal goods and services from which we generate revenue. Intellectual property licenses We generate revenue from licensing our intellectual property including know-how and development and commercialization rights. These licenses provide customers with a term-based license to further research, develop and commercialize our internally-discovered drug candidates. The consideration we receive in the form of nonrefundable upfront consideration related to the functional intellectual property licenses is recognized when we transfer such license to the customer unless the license is combined with other goods or services into one performance obligation, in which case the revenue is recognized over a period of time based on our estimated pattern in which we satisfy the combined performance obligation. Intellectual property sales We generate royalty revenue from sales of our intellectual property. We estimate the future royalty payments and recognize revenue with a corresponding contract asset at a point in time when we transfer the intellectual property to the customer. Research, development and regulatory services We generate revenue from research, development and regulatory services we provide to our customers in connection with the licensed intellectual property. The services we provide to our customers primarily include scientific research activities, preparation for and management of clinical trials, and assistance during the regulatory approval application process. Revenue associated with these services is recognized based on our estimate of total consideration to be received for such services and the pattern in which we perform the services. The pattern of performance is generally determined to be the amount of incurred expenses reimbursed by the customer as a percentage of total expected reimbursable expenses associated with the contract. Product manufacturing We generate revenue from manufacturing and clinical supply promises to our customers in connection with securing a supply of drug products for development and clinical trial purposes. The drug products are generally manufactured by our contract manufacturing organizations. We used our product manufacturing facility in Zofingen, Switzerland for a portion of the product manufacturing requirements until we sold the Manufacturing Operations on March 31, 2018 (see Note 2). Revenue associated with product manufacturing obligations is recognized at a point in time as control of the related product is transferred to the customer. Contracts with Multiple Performance Obligations. Most of our collaboration agreements with customers contain multiple promised goods or services. Based on the characteristics of the promised goods and services we analyze whether they are separate or combined performance obligations. The transaction price is allocated to the separate performance obligations on a relative standalone selling price basis. We determine standalone selling price based on our overall pricing and discounting objectives, taking into consideration the type of services, estimates of hourly market rates, and stage of the development and clinical trials. Variable Consideration. Our contracts with customers primarily include two types of variable consideration: (i) development and regulatory milestone payments, which are due to us upon achievement of specific development and regulatory milestones and (ii) one-time sales-based payments and sales-based royalties associated with sold or licensed intellectual property. Due to uncertainty associated with achievement of the development and Product sales-based royalties under licensed intellectual property and one-time payments are accounted for under the royalty exception. We recognize revenue for sales-based royalties under licensed intellectual property and one-time payments at the later of when the sales occur or the performance obligation is satisfied or partially satisfied. Disaggregation of Revenue. We operate in one reportable business segment. We provide goods and services to our customers in collaboration agreements pursuant to various geographical markets. In the following table, revenue is disaggregated by major customers, timing of revenue recognition and revenue classification, in thousands. Customers Three months ended June 30, 2018 Six months ended June 30, 2018 Outpost $ 2,750 $ 2,750 Eisai 861 3,089 Siegfried — 942 Axovant 200 768 Boehringer Ingelheim 163 623 Other 20 84 Total $ 3,994 $ 8,256 Timing of revenue recognition Goods and services transferred at a point in time $ 2,770 $ 5,277 Goods and services transferred over time 1,224 2,979 Total $ 3,994 $ 8,256 Classification Revenue from continuing operations $ 3,994 $ 5,749 Revenue reported under discontinued operations — 2,507 Total $ 3,994 $ 8,256 Contract Assets and Contract Liabilities. We receive payments from customers based on contractual terms. Accounts receivable are recorded when the right to consideration becomes unconditional. For research and development services, we generally bill our customers monthly or quarterly as the services are performed. Product sales are generally billed as completed. Payment terms on invoiced amounts are typically 30 days. Contract assets include amounts related to our contractual right to consideration for both completed and partially completed performance obligations that have not been invoiced. The current portion of contract assets is included in prepaid expenses and other current assets in the condensed consolidated balance sheet. The non-current portion of contract assets is included in other non-current assets in the condensed consolidated balance sheet. As of January 1, 2018, we recorded a contract asset of $4.1 million, of which $1.4 million is classified as current and $2.7 million is classified as non-current, related to future royalties associated with intellectual property patents previously sold to a customer which do not qualify for the royalty exception in ASC 606. We estimated the amount of the contract asset by applying the expected value method to our estimate of future royalty payments we will receive from this customer. Any future changes to this estimate will be recorded as an adjustment to revenue in the period in which the change in estimate is made. Contract liabilities consist of deferred revenue and include payments received in advance of performance under the contract. The following table provides detail of changes in our contract assets and deferred revenues, in thousands. The deferred revenue balances as of December 31, 2017, presented in the following table include balances classified as liabilities of disposal group held for sale: Contract Assets - Current Contract Assets - Non-Current Current Portion of Deferred Revenues Deferred Revenues, Less Current Portion Balances at December 31, 2017 $ — $ — $ 26,560 $ 1,067 ASC 606 implementation adjustments 7,527 2,694 (25,526 ) (40 ) Reductions of contract assets (2,070 ) (333 ) — — Impact of the disposal of the Manufacturing Operations (see Note 2) (4,543 ) — — — Foreign currency translation adjustment 145 — — — Recognized as revenue during the period — — (234 ) (412 ) Balances at June 30, 2018 $ 1,059 $ 2,361 $ 800 $ 615 Cost to Obtain and Fulfill a Contract. We generally do not incur costs to obtain new contracts. Costs to fulfill contracts are expensed as incurred. Remaining Performance Obligations. The following table provides detail of estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) pursuant to our existing collaboration agreements as of June 30, 2018, in thousands: As of June 30, 2018 Rest of 2018 $ 1,332 2019 2,069 2020 672 2021 and thereafter — Total $ 4,073 Under the royalty exception in ASC 606 for licensed intellectual property we do not recognize any revenue for the variable amounts related to sales-based royalties until the later of when the sales occur or the performance obligation is satisfied or partially satisfied. Accordingly, the revenue related to future royalties and sales-based milestones is not included in the table above. Recent Accounting Pronouncements . In February 2016, the FASB issued ASU No. 2016-02, Leases FASB has subsequently issued additional ASUs to clarify certain elements of the new lease accounting guidance. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash The following table provides a reconciliation of the components of cash, cash equivalents, and restricted cash reported in our condensed consolidated balance sheets to the total of the amount presented in the condensed consolidated statements of cash flows, in thousands: June 30, December 31, 2018 2017 Cash and cash equivalents $ 501,217 $ 158,837 Restricted cash included in other non-current assets 863 863 Total cash, cash equivalents and restricted cash presented in the condensed consolidated statement of cash flows $ 502,080 $ 159,700 The restricted cash relates to our property leases. The restriction will lapse when the related leases expire. Use of Estimates. The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions. Contingencies. We disclose information regarding each material claim where the likelihood of a loss contingency is probable or reasonably possible. If a loss contingency is probable and the amount of the loss can be reasonably estimated, we record an accrual for the loss. In such cases, there may be an exposure to potential loss in excess of the amount accrued. The insurance recoveries are recorded in the period when the insurance reimbursement is deemed probable. The ability to predict the ultimate outcome of such matters involves judgments, estimates and inherent uncertainties. The actual outcome of such matters could differ materially from management’s estimates.

Sale of Manufacturing Operation

Sale of Manufacturing Operations	6 Months Ended
	Jun. 30, 2018
Discontinued Operations And Disposal Groups [Abstract]
Sale of Manufacturing Operations	2. Sale of Manufacturing Operations In order to further focus our efforts and resources on our strategic objectives of developing our pipeline drug candidates, on March 9, 2018, we entered into an Asset Purchase Agreement, or Sale Agreement, with Siegfried Pharma AG and Siegfried AG, (collectively and individually, Siegfried). Under the Sale Agreement, we agreed to sell and assign to Siegfried, and Siegfried agreed to purchase and assume from Arena GmbH, certain drug product finishing facility assets and know-how, including fixtures, equipment, other personal property and real estate assets located in Zofingen, Switzerland and related contracts and certain related liabilities, or collectively, the Manufacturing Operations. We refer to this transaction as the Siegfried Transaction. The Siegfried Transaction was completed on March 31, 2018. In connection with the Siegfried Transaction, all of Arena GmbH’s approximately 50 employees transferred to Siegfried. We have excluded from our continuing operations for all periods presented in this report revenues and expenses associated with the disposed Manufacturing Operations, which are reported as discontinued operations. The total sales price for the Manufacturing Operations was approximately CHF 4 million of which approximately CHF 3 million was received in cash in March 2018 with the remaining portion to be received in March 2019, net of any qualifying claims by Siegfried. In the event Siegfried agrees to sell or transfer some or all of the transferred assets to certain third parties on or prior to December 31, 2018, for a consideration in excess of a specified amount, Arena GmbH will be entitled to percentage of such excess amount. We have retrospectively revised the condensed consolidated statements of operations for the three and six months ended June 30, 2017, and cash flows for the six months ended June 30, 2017, to reflect the operations and cash flows of the disposed Manufacturing Operations as discontinued operations. The following table summarizes the results of discontinued operations for the periods presented in the condensed consolidated statements of operations, in thousands: Three months ended June 30, Six months ended June 30, Revenues 2018 2017 2018 2017 Net product sales $ — $ 2,059 $ 1,129 $ 4,770 Other collaboration revenue — 1,781 372 3,316 Toll manufacturing — 754 1,006 1,472 Total revenues — 4,594 2,507 9,558 Costs and expenses Cost of product sales — 1,497 1,858 4,029 Cost of toll manufacturing — 1,074 1,411 1,993 Research and development — 222 — 389 General and administrative — 86 329 730 Other expenses, net — 1,568 464 2,216 Total costs and expenses — 4,447 4,062 9,357 Income (loss) from operations of discontinued operations — 147 (1,555 ) 201 Gain on sale of discontinued operations — — 725 — Income (loss) from discontinued operations $ — $ 147 $ (830 ) $ 201

Fair Value Disclosures

Fair Value Disclosures	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Disclosures	3. Fair Value Disclosures We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities: Level 1 - Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2 - Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly. Level 3 - Significant unobservable inputs based on our assumptions. The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis, in thousands: Fair Value Measurements at June 30, 2018 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds(1) $ 419,361 $ 419,361 $ — $ — US government and government agency notes(2) 28,243 28,243 — — Corporate debt instruments(2) 87,149 — 87,149 — Fair Value Measurements at December 31, 2017 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds(1) $ 48,750 $ 48,750 $ — $ — US government and government agency notes(2) 50,335 50,335 — — Corporate debt instruments(2) 94,639 — 94,639 — (1) Included in cash and cash equivalents in the accompanying (2) Included in either cash and cash equivalents or available-for-sale investments in the accompanying condensed consolidated balance sheets.

Investments, Available-for-Sale

Investments, Available-for-Sale	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Investments, Available-for-Sale	4. Investments, Available-for-Sale I nvestments, available-for-sale, consisted of the following June 30, 2018 Maturity in years Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value US government and government agency notes < 1 $ 28,251 $ — $ (8 ) $ 28,243 Corporate debt securities < 1 63,103 — (156 ) 62,947 Short-term investments, available-for-sale $ 91,354 $ — $ (164 ) $ 91,190 December 31, 2017 Maturity in years Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value US government and government agency notes < 1 $ 34,873 $ — $ (8 ) $ 34,865 Corporate debt securities < 1 53,438 — (63 ) 53,375 Short-term investments, available-for-sale $ 88,311 $ — $ (71 ) $ 88,240 Corporate debt securities 1 – 5 24,304 — (62 ) 24,242 Investments, available-for-sale $ 24,304 $ — $ (62 ) $ 24,242

Land, Property and Equipment

Land, Property and Equipment	6 Months Ended
	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Land, Property and Equipment	5. Land, Property and Equipment Land, property and equipment consisted of the following, in thousands: June 30, December 31, 2018 2017 Cost $ 88,772 $ 88,244 Less accumulated depreciation and amortization (60,016 ) (58,113 ) Land, property and equipment, net $ 28,756 $ 30,131

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	6. Accounts Payable and Other Accrued Liabilities Accounts payable and other accrued liabilities consisted of the following, in thousands: June 30, December 31, 2018 2017 Accounts payable $ 1,129 $ 1,599 Accrued compensation 3,913 5,255 Other accrued liabilities 1,813 1,062 Total accounts payable and other accrued liabilities $ 6,855 $ 7,916

Collaborations

Collaborations	6 Months Ended
	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	7. Collaborations Refer to our Annual Report on Form 10-K for the year ended December 31, 2017, for more information on our significant collaboration agreements. Outpost Medicine LLC. In the second quarter of 2018, we and Outpost entered into a license agreement, or Outpost Agreement, under which Outpost has an exclusive right to advance an undisclosed, preclinical compound with potential utility in treating genitourinary disorders Under the Outpost Agreement, we received an upfront payment of $3.0 million, of which $1.5 million was in the form of an equity interest in Outpost. milestone payments success of The promised goods and services under the Outpost Outpost A Eisai. In July 2010, we granted Eisai In December 2016, we and Eisai amended and restated the terms of the marketing and supply agreement for lorcaserin with Eisai by entering into a Transaction Agreement and a Supply Agreement (collectively, the Eisai Agreement) with Eisai. Under the Transaction Agreement, Eisai acquired an exclusive royalty-bearing license or transfer of intellectual property to global commercialization and manufacturing rights to lorcaserin, including in the territories retained by us under the prior agreement, with control over global development and commercialization decisions. Eisai is responsible for all lorcaserin development expenses going forward. We also assigned to Eisai our rights under the commercial lorcaserin distribution agreements with Ildong Pharmaceutical Co., Ltd., or Ildong, for South Korea; CY Biotech Company Limited, or CYB, for Taiwan; and Teva Pharmaceuticals Ltd.’s Israeli subsidiary, Abic Marketing Limited, or Teva, for Israel. Under the Supply Agreement, Eisai paid us $10.0 million in December 2016 to acquire our entire on-hand inventory of bulk lorcaserin and the precursor material for manufacturing lorcaserin. Eisai also paid us for finished drug product plus monthly manufacturing support payments through June 2018 totaling CHF 8.7 million. Until March 31, 2018, when we sold the Manufacturing Operations, including the assignment of the Supply Agreement, to Siegfried (see Note 2), we manufactured lorcaserin at our manufacturing facility in Zofingen, Switzerland. Revenues earned for (i) lorcaserin sold by us to Eisai under the manufacturing and supply commitment within the Supply Agreement and (ii) the manufacturing support payments are classified within discontinued operations as part of the Manufacturing Operations in the condensed consolidated statements of operations (see Note 2). All other revenues earned under the Transaction Agreement, such as royalties, are classified within continuing operations in the condensed consolidated statements of operations. Royalty payments. Pursuant to the Transaction Agreement, we are eligible to receive royalty payments from Eisai based on the global net sales of lorcaserin. The royalty rates are as follows: • 9.5% on annual net sales less than or equal to $175.0 million • 13.5% on annual net sales greater than $175.0 million but less than or equal to $500.0 million • 18.5% of annual net sales greater than $500.0 million Upfront payments. Prior to the Transaction Agreement, we received from Eisai total upfront payments of $115.0 million under prior lorcaserin collaboration agreements and $7.5 million from the prior commercial lorcaserin distribution agreements with Ildong, CYB, and Teva. Revenues from these upfront payments were previously deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, these payments were recognized ratably as revenue over the periods in which we expected the services to be rendered. Milestone payments. Prior to the Transaction Agreement, we received a total of $102.1 million in milestone payments from Eisai, Ildong, CYB, and Teva. These payments were recognized as revenue upon the achievement of the milestones. We are eligible to receive an additional sales-based milestone of $25.0 million upon the achievement of global net sales of lorcaserin for a calendar year first exceeding $250.0 million. Accounting for Eisai Agreement under ASC 606. Upon implementation of ASC 606 on January 1, 2018, we applied a practical expedient for contract modifications applicable to contracts that were modified before the implementation date. The promised goods and services under the Eisai Agreement were assessed in combination with promised goods and services under our previous agreements with Eisai and commercial lorcaserin distribution agreements with Ildong, CYB, and Teva. The total estimated transaction price of these contracts at the implementation date was $344.4 million, which included previously received upfront payments, milestone payments, proceeds from net products sales, reimbursement of development expenses, reimbursement of patent expenses, manufacturing support payments received and expected to be received under the Supply Agreement, proceeds from the sale of on-hand inventory of bulk lorcaserin and the precursor material, royalty payments received through December 31, 2017, and estimated future royalty payments related to intellectual property sold to Eisai. The estimated total transaction price In connection with the sale of the Manufacturing Operations on March 31, 2018, we derecognized the remaining portion of the contract asset associated with the Supply Agreement. Based on the bill-and-hold accounting guidance in ASC 606, effective January 1, 2018, we derecognized $3.6 million of inventory of bulk lorcaserin and the precursor material previously sold to Eisai for which the revenue recognition criteria were met on the implementation date under ASC 606. For the three and six months ended June 30, 2018, we recorded royalty revenue of $0.9 million and $1.6 million related to the Transaction Agreement compared to no royalty revenue recorded for the three and six months ended June 30, 2017. For the six months ended June 30, 2018, we recognized as revenue $1.5 million related to the Supply Agreement (classified under discontinued operations), all of which was recorded during the first quarter of 2018 and primarily consisting of net product sales and other collaboration revenue. For the three and six months ended June 30, 2017, we recognized as revenue $3.8 million and $8.1 million (classified under discontinued operations), respectively, related to the Supply Agreement and primarily consisting of net product sales and other collaboration revenue. In the condensed consolidated balance sheet at December 31, 2017, the deferred revenues of $25.5 million relating to the Eisai Agreement (primarily comprised of the deferred portion of the previously received upfront payments and the $10.0 million payment received from Eisai in December 2016) were classified as liabilities of disposal group held for sale. Axovant Sciences GmbH. We and Axovant have a development, marketing and supply agreement, or Axovant Agreement, under which Axovant has exclusive worldwide rights to develop and commercialize nelotanserin, subject to regulatory approval. Under the Axovant Agreement, we received an upfront payment of $4.0 million in May 2015. We will receive payments from potential sales of nelotanserin under the Axovant Agreement and are eligible to receive purchase price adjustment payments based on Axovant’s annual net product sales. We are eligible to receive up to an aggregate of $41.5 million in success milestone payments in case of full development and regulatory success of nelotanserin. The promised goods and services under the Axovant Agreement are accounted for as two separate performance obligations: (i) a combined performance obligation consisting of commercialization rights and development and regulatory services and (ii) a manufacturing and supply commitment. As of June 30, 2018, the estimated total transaction price was $9.9 million, of which $3.7 million is associated with performance obligations to be satisfied in the future. The future potential purchase price adjustment payments and milestone payments were excluded from the estimated total transaction price as they are considered constrained. We recognize revenue for the combined performance obligation consisting of commercialization rights and development and regulatory services based on the amount of incurred development expenses reimbursed by the customer as a percentage of total expected reimbursable expenses associated with the contract. For the three and six months ended June 30, 2018, we recorded revenue of $0.2 million and $0.8 million, respectively, related to the Axovant Agreement. For the three and six months ended June 30, 2017, we recorded revenue of $0.5 million and $1.0 million, respectively, related to the Axovant Agreement. Boehringer Ingelheim International GmbH. We and Boehringer Ingelheim have a collaboration and license agreement, or Boehringer Ingelheim Agreement, to conduct joint research to identify drug candidates targeting an undisclosed G protein-coupled receptor, or GPCR, that belongs to the group of orphan central nervous system, or CNS, receptors. The Boehringer Ingelheim Agreement was in effect through January 2018. We and Boehringer Ingelheim agreed to extend the original term of the Boehringer Ingelheim Agreement by twelve months through January 2019. In partial consideration of the exclusive rights to our intellectual property necessary or useful to conduct the joint research under the Boehringer Ingelheim Agreement, we received from Boehringer Ingelheim an upfront payment of $7.5 million in 2016. Under the Boehringer Ingelheim Agreement, we are eligible to receive up to an aggregate of $251.0 million in success milestone payments in case of full commercial success multiple drug products. The promised goods and services under the Boehringer Ingelheim Boehringer Ingelheim For the three and six months ended June 30, 2018, we recorded revenue of $0.2 million and $0.6 million, respectively, related to the Boehringer Ingelheim Agreement. For the three and six months ended June 30, 2017, we recorded revenue of $1.3 million and $2.5 million, respectively, related to the Boehringer Ingelheim Agreement.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 2018
Stockholders Equity Note [Abstract]
Stockholders' Equity	8. Stockholders’ Equity In March 2018, we completed the sale of an aggregate of 9,775,000 shares of our common stock under an underwritten public offering. Net proceeds from the offering were approximately $383.1 million after deducting underwriting discounts and commissions and offering expenses payable by us.

Share-based Compensation

Share-based Compensation	6 Months Ended
	Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	9. Share-based Compensation We recognized share-based compensation expense as follows, in thousands: Three months ended Six months ended June 30, June 30, 2018 2017 2018 2017 Research and development $ 1,917 $ 576 $ 3,567 $ 955 General and administrative 2,613 1,534 4,996 2,926 Discontinued operations — 20 11 87 Total share-based compensation expense $ 4,530 $ 2,130 $ 8,574 $ 3,968 The following table summarizes our stock option activity during the six months ended June 30, 2018, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2018 3,755 $ 20.00 Granted 2,485 36.91 Exercised (219 ) 17.66 Forfeited/cancelled/expired (212 ) 34.62 Outstanding at June 30, 2018 5,809 $ 26.79 During the six months ended June 30, 2018, 32,322 shares were issued to the holders of the remaining PRSUs, granted in March 2015 based on the TSR, of our common stock relative to the TSR of the Nasdaq Biotechnology Index over the three-year performance period that began on March 1, 2015

Concentrations of Credit Risk

Concentrations of Credit Risk	6 Months Ended
	Jun. 30, 2018
Risks And Uncertainties [Abstract]
Concentrations of Credit Risk	10. Concentrations of Credit Risk Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash, cash equivalents and available-for-sale investments. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency or government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors.

Income (Loss) Per Share

Income (Loss) Per Share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Income (Loss) Per Share	11. Income (Loss) Per Share We calculate basic and diluted loss from continuing operations, income (loss) from discontinued operations and net loss per share using the weighted-average number of shares of common stock outstanding during the period. Since we have a loss from continuing operations for the three and six months ended June 30, 2018, and 2017, in addition to excluding potentially dilutive out-of-the money securities, we have excluded from our calculation of diluted income (loss) per share all potentially dilutive in-the-money (i) stock options, (ii) restricted stock unit awards, or RSUs, (iii) PRSUs and (iv) unvested restricted stock in our deferred compensation plan, and our diluted net income (loss) per share is the same as our basic net income (loss) per share. The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net income (loss) per share, in thousands: Three months ended Six months ended June 30, June 30, 2018 2017 2018 2017 Stock options 5,722 4,002 5,542 3,567 RSUs and unvested restricted stock 5 3 15 3 Total 5,727 4,005 5,557 3,570 Because the market condition for the outstanding PRSUs was not satisfied at June 30, 2017, such securities are excluded from the table above. There are no outstanding PRSUs at June 30, 2018.

Legal Proceedings

Legal Proceedings	6 Months Ended
	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Legal Proceedings	12. Legal Proceedings Beginning on September 20, 2010, a number of complaints were filed in the US District Court for the Southern District of California, or District Court, against us and certain of our current and former employees and directors on behalf of certain purchasers of our common stock. The complaints were brought as purported stockholder class actions, and, in general, include allegations that we and certain of our current and former employees and directors violated federal securities laws by making materially false and misleading statements regarding our BELVIQ program, thereby artificially inflating the price of our common stock. The plaintiffs sought unspecified monetary damages and other relief. On August 8, 2011, the District Court consolidated the actions and appointed a lead plaintiff and lead counsel. On November 1, 2011, the lead plaintiff filed a consolidated amended complaint. On March 28, 2013, the District Court dismissed the consolidated amended complaint without prejudice. On May 13, 2013, the lead plaintiff filed a second consolidated amended complaint. On November 5, 2013, the District Court dismissed the second consolidated amended complaint without prejudice as to all parties except for Robert E. Hoffman, who was dismissed from the action with prejudice. On November 27, 2013, the lead plaintiff filed a motion for leave to amend the second consolidated amended complaint. On March 20, 2014, the District Court denied plaintiff’s motion and dismissed the second consolidated amended complaint with prejudice. On April 18, 2014, the lead plaintiff filed a notice of appeal, and on August 27, 2014, the lead plaintiff filed his appellate brief in the US Court of Appeals for the Ninth Circuit, or Ninth Circuit. On October 24, 2014, we filed our answering brief in response to the lead plaintiff’s appeal. On December 5, 2014, the lead plaintiff filed his reply brief. A panel of the Ninth Circuit heard oral argument on the appeal on May 4, 2016. On October 26, 2016, the Ninth Circuit panel reversed the District Court’s dismissal of the second consolidated amended complaint and remanded the case back to the District Court for further proceedings. On January 25, 2017, the District Court permitted us to submit a renewed motion to dismiss the second consolidated amended complaint. On February 2, 2017, we filed the renewed motion to dismiss. On February 23, 2017, the lead plaintiff filed his opposition, and on March 2, 2017, we filed our reply. On April 28, 2017, the District Court denied our renewed motion to dismiss. On November 3, 2017, we and the lead plaintiff signed a stipulation and agreement of settlement, or Stipulation, to resolve the consolidated class action. Under the terms of the Stipulation, and in exchange for a release of all claims by class members and a dismissal of the consolidated class action with prejudice, we have agreed that (i) our insurers would pay class members and their attorneys a total of approximately $12.025 million and (ii) Arena would pay class members and their attorneys approximately $11.975 million in either shares of our common stock or cash at our election. Accordingly, in the third quarter of 2017, we recognized $11.975 million of net expense for the portion of the settlement that we agreed to pay in either common stock or cash. On November 30, 2017, the District Court preliminary approved the settlement and the form of notice to potential class members of the proposed settlement and the procedure by which they can become class members. On April 12, 2018, the District Court entered its final approval order approving the settlement and the plan of allocation and request for attorneys’ fees and expense. In the second quarter of 2018, we and our insurer made settlement payments in cash to the class members and their attorneys to settle our liability under the Stipulation. On September 30, 2016, we and Eisai Inc. filed a patent infringement lawsuit against Lupin Limited and Lupin Pharmaceuticals, Inc. (collectively, Lupin) in the U.S. District Court for the District of Delaware. The lawsuit relates to a “Paragraph IV certification” notification that we and Eisai Inc. received regarding an abbreviated new drug application, or ANDA, submitted to the FDA by Lupin requesting approval to engage in the commercial manufacture, use, importation, offer for sale or sale of a generic version of BELVIQ (lorcaserin hydrochloride tablets, 10 mg). In its notification, Lupin alleged that no valid, enforceable claim of any of the patents that are listed in the FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations, or Orange Book, for BELVIQ will be infringed by Lupin’s manufacture, importation, use, sale or offer for sale of the product described in its ANDA for 10 mg lorcaserin hydrochloride tablets. Lupin is accused of infringing U.S. Patent Nos. 6,953,787; 7,514,422; 7,977,329; 8,207,158 and 8,273,734. In accordance with the Hatch-Waxman Act, as a result of filing a patent infringement lawsuit within 45 days of receipt of Lupin’s notification, the FDA cannot approve Lupin’s ANDA any earlier than 7.5 years from NDA approval unless a District Court finds that all of the asserted claims of the patents-in-suit are invalid, unenforceable or not infringed. On January 11, 2017, Lupin filed an answer, defenses and counterclaims to the September 30, 2016 complaint. We and Eisai Inc. filed an answer to Lupin’s counterclaims on February 1, 2017. We and Eisai Inc. are seeking a determination from the court that, among other things, Lupin has infringed our patents, Lupin’s ANDA for 10 mg lorcaserin hydrochloride tablets should not be approved until the expiration date of our patents, and Lupin should be enjoined from commercializing a product that infringes our patents. Trial is currently scheduled for April 15, 2019. The parties are currently in the fact discovery phase of the case. On March 6, 2017, we and Eisai Inc. filed a patent infringement lawsuit against Teva Pharmaceuticals USA, Inc. and Teva Pharmaceutical Industries Ltd. (collectively, Teva) in the U.S. District Court for the District of Delaware. The lawsuit also relates to a “Paragraph IV certification” notification that we and Eisai Inc. received regarding an ANDA submitted to the FDA by Teva requesting approval to engage in the commercial manufacture, use, importation, offer for sale or sale of a generic version of BELVIQ XR (lorcaserin hydrochloride extended-release tablets, 20 mg). In its notification, Teva alleged that no valid, enforceable claim of any of the patents that are listed in the Orange Book for BELVIQ XR will be infringed by Teva’s manufacture, importation, use, sale or offer for sale of the product described in its ANDA. Teva is accused of infringing U.S. Patent Nos. 6,953,787; 7,514,422; 7,977,329; 8,207,158 and 8,273,734. In accordance with the Hatch-Waxman Act, as a result of filing a patent infringement lawsuit within 45 days of receipt of Teva’s notification, the FDA cannot approve Teva’s ANDA any earlier than 7.5 years from NDA approval unless a District Court finds that all of the asserted claims of the patents-in-suit are invalid, unenforceable or not infringed. On April 18, 2017, Teva filed an amended answer, defenses and counterclaims to the March 6, 2017, complaint. We and Eisai Inc. are seeking a determination from the court that, among other things, Teva has infringed our patents, Teva’s ANDA should not be approved until the expiration date of our patents, and Teva should be enjoined from commercializing a product that infringes our patents. On May 1, 2017, the Teva and Lupin actions were consolidated for all purposes and will follow the case schedule that was previously entered in the Lupin action. We and Eisai Inc. filed an answer to Teva’s amended counterclaims on May 3, 2017. On or about October 16, 2017, we and Eisai Inc. received a “Paragraph IV certification” notification from Teva alleging that no valid, enforceable claim of U.S. Patent No. 9,770,455, which was listed in the Orange Book for BELVIQ and BELVIQ XR We and Eisai Inc. also received a “Paragraph IV certification” notification from Lupin alleging that no valid, enforceable claim of any of the patents that are listed in the Orange Book for BELVIQ and BELVIQ XR will be infringed by Lupin’s manufacture, importation, use, sale or offer for sale of the product described in its ANDA for 20 mg lorcaserin hydrochloride extended-release tablets. Because Lupin is not the first applicant to submit a substantially complete application containing a Paragraph IV certification for approval of a generic equivalent of BELVIQ XR, absent extenuating circumstances, Lupin would not be able to launch its 20 mg lorcaserin hydrochloride extended-release tablets before Teva was able to launch its respective product. On March 23, 2018, we and Eisai Inc. filed a second amended complaint against Lupin and Teva, adding infringement of U.S. Patent Nos. 6,953,787, 7,514,422, 7,977,329, 8,207,158, 8,273,734, and 9,770,455 by Lupin’s generic equivalent of BELVIQ XR. This consolidated action against Lupin and Teva is currently in fact discovery. We cannot predict the ultimate outcome of any proceeding.

Beacon Discovery, Inc.

Beacon Discovery, Inc.	6 Months Ended
	Jun. 30, 2018
Variable Interest Entity Disclosure [Abstract]
Beacon Discovery, Inc.	13. Beacon Discovery, Inc. On September 1, 2016, we entered into a As Beacon’s equity investment at risk is not sufficient to permit Beacon to finance its activities without subordinated financial support, Beacon is considered a variable interest entity in which we hold a significant variable interest pursuant to the agreements with Beacon. We do not own any equity interest in Beacon; however, as the agreements provided us the controlling financial interest in Beacon until December 2017, we consolidated Beacon’s balances and activity within our consolidated financial statements until December 2017 as we were determined to be the primary beneficiary of Beacon. Pursuant to a contract Beacon entered into with a third party in December 2017 which provided Beacon with a certain amount of upfront funding, we determined we no longer held the controlling financial interest as of that date and, therefore, deconsolidated Beacon from our consolidated financial statements as we were no longer deemed to be the primary beneficiary. Our condensed consolidated financial statements for the three and six months ended June 30, 2017, include Beacon’s results of operations and cash flows. As of June 30, 2018, Beacon’s total assets of $7.0 million, total liabilities of $7.0 million and total stockholders’ deficit of less than $0.1 million are excluded from our consolidated balance sheet. For the three and six months ended June 30, 2018, Beacon recognized revenues of $2.6 million and $4.1 million, respectively, of which $0.3 million and $0.7 million, respectively was earned by Beacon from agreements with us. For the three and six months ended June 30, 2018, Beacon reported net income of $0.8 million and $0.7 million, respectively. For the three and six months ended June 30, 2017, Beacon recognized revenues of $0.7 million and $1.3 million, substantially all of which was earned by Beacon from agreements with us. For the three and six months ended June 30, 2017, Beacon incurred a net loss of $0.3 million and $0.7 million which is fully presented as net loss attributable to noncontrolling interest in consolidated variable interest entity in our condensed consolidated statements of operations and comprehensive loss. As of June 30, 2018, a note receivable from Beacon with a balance of $0.5 million is included in prepaid expenses and other current assets in our condensed consolidated balance sheet. We believe that our maximum exposure to loss as a result of our involvement with Beacon is limited to the note receivable due to us from Beacon.

Basis of Presentation (Policies

Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2018
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2017, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2017. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. Beacon Discovery, Inc., or Beacon, is a variable interest entity in which we had a controlling financial interest until December 2017. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statement of operations and comprehensive loss for the three and six months ended June 30, 2017 . We
Liquidity	Liquidity. As of June 30, 2018, we had cash, cash equivalents and available-for-sale investments of approximately $592.4 million. We believe our cash and cash equivalents and available-for-sale investments will be sufficient to fund our operations for at least the next 12 months. We will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, as this process typically takes many years and potentially hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates, programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions.
Recent Accounting Pronouncements	Recent Accounting Pronouncements . In February 2016, the FASB issued ASU No. 2016-02, Leases FASB has subsequently issued additional ASUs to clarify certain elements of the new lease accounting guidance. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash The following table provides a reconciliation of the components of cash, cash equivalents, and restricted cash reported in our condensed consolidated balance sheets to the total of the amount presented in the condensed consolidated statements of cash flows, in thousands: June 30, December 31, 2018 2017 Cash and cash equivalents $ 501,217 $ 158,837 Restricted cash included in other non-current assets 863 863 Total cash, cash equivalents and restricted cash presented in the condensed consolidated statement of cash flows $ 502,080 $ 159,700 The restricted cash relates to our property leases. The restriction will lapse when the related leases expire.
Use of Estimates	Use of Estimates. The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.
Contingencies	Contingencies. We disclose information regarding each material claim where the likelihood of a loss contingency is probable or reasonably possible. If a loss contingency is probable and the amount of the loss can be reasonably estimated, we record an accrual for the loss. In such cases, there may be an exposure to potential loss in excess of the amount accrued. The insurance recoveries are recorded in the period when the insurance reimbursement is deemed probable. The ability to predict the ultimate outcome of such matters involves judgments, estimates and inherent uncertainties. The actual outcome of such matters could differ materially from management’s estimates.
Fair Value Disclosures	We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities: Level 1 - Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2 - Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly. Level 3 - Significant unobservable inputs based on our assumptions.
Concentrations of Credit Risk	Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash, cash equivalents and available-for-sale investments. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency or government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors.
Income (Loss) Per Share	We calculate basic and diluted loss from continuing operations, income (loss) from discontinued operations and net loss per share using the weighted-average number of shares of common stock outstanding during the period. Since we have a loss from continuing operations for the three and six months ended June 30, 2018, and 2017, in addition to excluding potentially dilutive out-of-the money securities, we have excluded from our calculation of diluted income (loss) per share all potentially dilutive in-the-money (i) stock options, (ii) restricted stock unit awards, or RSUs, (iii) PRSUs and (iv) unvested restricted stock in our deferred compensation plan, and our diluted net income (loss) per share is the same as our basic net income (loss) per share.
ASC 606
Changes in Accounting Policies - Revenue Recognition	Changes in Accounting Policies - Revenue Recognition. Effective January 1, 2018, we adopted Accounting Standard Codification 606, Revenue from Contracts with Customers , or ASC 606, issued by the Accounting Standards Board, or FASB. As a result, we have changed our accounting policy for revenue recognition as detailed below. We implemented ASC 606 using the modified retrospective method by recognizing the cumulative effect of initially applying ASC 606 as an adjustment to the opening balance of our accumulated deficit at January 1, 2018. Therefore, the comparative period information has not been adjusted. We applied ASC 606 using a practical expedient for contracts that were modified before the implementation date, which allowed us to determine an aggregate effect of all modifications that occurred before January 1, 2018, when determining the satisfied and unsatisfied performance obligations, the transaction price, and allocating that transaction price to the performance obligations instead of retrospectively restating the contracts for such contract modifications. The cumulative impact to our accumulated deficit balance at January 1, 2018, as a result of the adoption of ASC 606 was a decrease of $19.0 million. The decrease arose primarily from a reduction of deferred revenue balances related to upfront payments received from customers and recognition of contract assets due to a combination of (i) the effects of applying the practical expedient for contract modifications and our conclusions related to satisfied and unsatisfied performance obligations, which resulted in a relatively higher portion of the total transaction price recognized as revenue in periods prior to our adoption of ASC 606, (ii) the effect of the bill-and-hold accounting guidance for inventory in ASC 606 and (iii) the inclusion of estimated future royalty payments related to our intellectual property in the total transaction price to the extent such intellectual property was legally sold to our customer rather than licensed. The cumulative effect adjustment is net of an impairment loss of $13.1 million which was a direct effect of the adoption of ASC 606 on the asset group of the Manufacturing Operations, which was classified as assets of disposal group held for sale since December 2017. The following table summarizes the impacts of adopting ASC 606 on our condensed consolidated financial statements, in thousands. Impact of Changes in Accounting Policies Three months ended June 30, 2018 As reported Adjustments Balances without adoption of ASC 606 Collaboration and other revenue $ 3,133 $ (15 ) $ 3,118 Royalty revenue 861 426 1,287 Total revenue 3,994 411 4,405 Loss from operations (33,166 ) 411 (32,755 ) Loss from continuing operations (31,833 ) 411 (31,422 ) Net loss (31,833 ) 411 (31,422 ) Net loss attributable to stockholders of Arena (31,833 ) 411 (31,422 ) Six months ended June 30, 2018 Collaboration and other revenue $ 4,161 $ 44 $ 4,205 Royalty revenue 1,588 688 2,276 Total revenue 5,749 732 6,481 Loss from operations (64,135 ) 732 (63,403 ) Loss from continuing operations (62,966 ) 1,880 (61,086 ) Income (loss) from discontinued operations (830 ) 13,660 12,830 Net loss (63,796 ) 15,540 (48,256 ) Net loss attributable to stockholders of Arena (63,796 ) 15,540 (48,256 ) As of June 30, 2018 Prepaid expenses and other current assets $ 6,539 $ (1,059 ) $ 5,480 Total current assets 601,280 (1,059 ) 600,221 Other non-current assets 7,524 (2,361 ) 5,163 Total assets 637,560 (3,420 ) 634,140 Current portion of deferred revenues 800 20 820 Total current liabilities 20,917 20 20,937 Deferred revenues, less current portion 615 52 667 Accumulated deficit (1,534,948 ) (3,492 ) (1,538,440 ) Total stockholders' equity 558,838 (3,492 ) 555,346 Total liabilities and stockholders' equity 637,560 (3,420 ) 634,140 Our revenues to date have been generated primarily through collaboration agreements. Our collaboration agreements frequently contain multiple elements including (i) intellectual property licenses, (ii) product research, development and regulatory services and (iii) product manufacturing. Consideration we receive under these arrangements may include upfront payments, research and development funding, cost reimbursements, milestone payments, payments for product sales and royalty payments. Our customers include Everest Medicines Limited, or Everest, Outpost Medicine, LLC, or Outpost, Eisai Inc. and Eisai Co., Ltd., or collectively, Eisai, Axovant Sciences GmbH, or Axovant, Boehringer Ingelheim International GmbH, or Boehringer Ingelheim, and Siegfried. Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services. The amount of revenue recognized reflects the consideration that we expect to be entitled to receive in exchange for these services and excludes sales incentives and amounts collected i) Identify the contract with a customer . We consider the terms and conditions of our collaboration agreements to identify contracts within the scope of ASC 606. We consider that we have a contract with a customer when the contract is approved, we can identify each party's rights regarding the goods and services to be transferred, we can identify the payment terms for the goods and services, we have determined the customer has the ability and intent to pay and the contract has commercial substance. We use judgment in determining the customer's ability and intent to pay, which is based upon factors including the customer's historical payment experience or, for new customers, credit and financial information pertaining to the customers. ii) Identify the performance obligations in the contract. Performance obligations in our collaboration agreements are identified based on the goods and services that will be transferred to the customer that are both capable of being distinct, whereby the customer can benefit from the service either on its own or together with other resources that are readily available from third parties or from us, and are distinct in the context of the contract, whereby the transfer of the services is separately identifiable from other promises in the contract. Our performance obligations generally consist of intellectual property licenses, research, development and/or regulatory services and manufacturing and supply commitments. iii) Determine the transaction price. We determine the transaction price based on the consideration to which we expect to be entitled in exchange for transferring goods and services to the customer. In determining the transaction price, any variable consideration would be considered, to the extent applicable, if, in our judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. In accordance with the royalty exception under ASC 606 for licenses of intellectual property, the transaction price excludes future royalty payments to be received from our customers. None of our collaboration agreements contain consideration payable to our customer or a significant financing component. iv) Allocate the transaction price to performance obligations in the contract. v) Recognize revenue when or as we satisfy a performance obligation. Revenue is recognized at the time the related performance obligation is satisfied by transferring the promised goods or services to a customer. We recognize revenue when we transfer control of the goods or services to our customers for an amount that reflects the consideration that we expect to receive in exchange for those services. Performance Obligations. The following is a description of principal goods and services from which we generate revenue. Intellectual property licenses We generate revenue from licensing our intellectual property including know-how and development and commercialization rights. These licenses provide customers with a term-based license to further research, develop and commercialize our internally-discovered drug candidates. The consideration we receive in the form of nonrefundable upfront consideration related to the functional intellectual property licenses is recognized when we transfer such license to the customer unless the license is combined with other goods or services into one performance obligation, in which case the revenue is recognized over a period of time based on our estimated pattern in which we satisfy the combined performance obligation. Intellectual property sales We generate royalty revenue from sales of our intellectual property. We estimate the future royalty payments and recognize revenue with a corresponding contract asset at a point in time when we transfer the intellectual property to the customer. Research, development and regulatory services We generate revenue from research, development and regulatory services we provide to our customers in connection with the licensed intellectual property. The services we provide to our customers primarily include scientific research activities, preparation for and management of clinical trials, and assistance during the regulatory approval application process. Revenue associated with these services is recognized based on our estimate of total consideration to be received for such services and the pattern in which we perform the services. The pattern of performance is generally determined to be the amount of incurred expenses reimbursed by the customer as a percentage of total expected reimbursable expenses associated with the contract. Product manufacturing We generate revenue from manufacturing and clinical supply promises to our customers in connection with securing a supply of drug products for development and clinical trial purposes. The drug products are generally manufactured by our contract manufacturing organizations. We used our product manufacturing facility in Zofingen, Switzerland for a portion of the product manufacturing requirements until we sold the Manufacturing Operations on March 31, 2018 (see Note 2). Revenue associated with product manufacturing obligations is recognized at a point in time as control of the related product is transferred to the customer. Contracts with Multiple Performance Obligations. Most of our collaboration agreements with customers contain multiple promised goods or services. Based on the characteristics of the promised goods and services we analyze whether they are separate or combined performance obligations. The transaction price is allocated to the separate performance obligations on a relative standalone selling price basis. We determine standalone selling price based on our overall pricing and discounting objectives, taking into consideration the type of services, estimates of hourly market rates, and stage of the development and clinical trials. Variable Consideration. Our contracts with customers primarily include two types of variable consideration: (i) development and regulatory milestone payments, which are due to us upon achievement of specific development and regulatory milestones and (ii) one-time sales-based payments and sales-based royalties associated with sold or licensed intellectual property. Due to uncertainty associated with achievement of the development and Product sales-based royalties under licensed intellectual property and one-time payments are accounted for under the royalty exception. We recognize revenue for sales-based royalties under licensed intellectual property and one-time payments at the later of when the sales occur or the performance obligation is satisfied or partially satisfied. Disaggregation of Revenue. We operate in one reportable business segment. We provide goods and services to our customers in collaboration agreements pursuant to various geographical markets. In the following table, revenue is disaggregated by major customers, timing of revenue recognition and revenue classification, in thousands. Customers Three months ended June 30, 2018 Six months ended June 30, 2018 Outpost $ 2,750 $ 2,750 Eisai 861 3,089 Siegfried — 942 Axovant 200 768 Boehringer Ingelheim 163 623 Other 20 84 Total $ 3,994 $ 8,256 Timing of revenue recognition Goods and services transferred at a point in time $ 2,770 $ 5,277 Goods and services transferred over time 1,224 2,979 Total $ 3,994 $ 8,256 Classification Revenue from continuing operations $ 3,994 $ 5,749 Revenue reported under discontinued operations — 2,507 Total $ 3,994 $ 8,256 Contract Assets and Contract Liabilities. We receive payments from customers based on contractual terms. Accounts receivable are recorded when the right to consideration becomes unconditional. For research and development services, we generally bill our customers monthly or quarterly as the services are performed. Product sales are generally billed as completed. Payment terms on invoiced amounts are typically 30 days. Contract assets include amounts related to our contractual right to consideration for both completed and partially completed performance obligations that have not been invoiced. The current portion of contract assets is included in prepaid expenses and other current assets in the condensed consolidated balance sheet. The non-current portion of contract assets is included in other non-current assets in the condensed consolidated balance sheet. As of January 1, 2018, we recorded a contract asset of $4.1 million, of which $1.4 million is classified as current and $2.7 million is classified as non-current, related to future royalties associated with intellectual property patents previously sold to a customer which do not qualify for the royalty exception in ASC 606. We estimated the amount of the contract asset by applying the expected value method to our estimate of future royalty payments we will receive from this customer. Any future changes to this estimate will be recorded as an adjustment to revenue in the period in which the change in estimate is made. Contract liabilities consist of deferred revenue and include payments received in advance of performance under the contract. The following table provides detail of changes in our contract assets and deferred revenues, in thousands. The deferred revenue balances as of December 31, 2017, presented in the following table include balances classified as liabilities of disposal group held for sale: Contract Assets - Current Contract Assets - Non-Current Current Portion of Deferred Revenues Deferred Revenues, Less Current Portion Balances at December 31, 2017 $ — $ — $ 26,560 $ 1,067 ASC 606 implementation adjustments 7,527 2,694 (25,526 ) (40 ) Reductions of contract assets (2,070 ) (333 ) — — Impact of the disposal of the Manufacturing Operations (see Note 2) (4,543 ) — — — Foreign currency translation adjustment 145 — — — Recognized as revenue during the period — — (234 ) (412 ) Balances at June 30, 2018 $ 1,059 $ 2,361 $ 800 $ 615 Cost to Obtain and Fulfill a Contract. We generally do not incur costs to obtain new contracts. Costs to fulfill contracts are expensed as incurred. Remaining Performance Obligations. The following table provides detail of estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) pursuant to our existing collaboration agreements as of June 30, 2018, in thousands: As of June 30, 2018 Rest of 2018 $ 1,332 2019 2,069 2020 672 2021 and thereafter — Total $ 4,073 Under the royalty exception in ASC 606 for licensed intellectual property we do not recognize any revenue for the variable amounts related to sales-based royalties until the later of when the sales occur or the performance obligation is satisfied or partially satisfied. Accordingly, the revenue related to future royalties and sales-based milestones is not included in the table above.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	6 Months Ended
	Jun. 30, 2018
Reconciliation of Components of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of the components of cash, cash equivalents, and restricted cash reported in our condensed consolidated balance sheets to the total of the amount presented in the condensed consolidated statements of cash flows, in thousands: June 30, December 31, 2018 2017 Cash and cash equivalents $ 501,217 $ 158,837 Restricted cash included in other non-current assets 863 863 Total cash, cash equivalents and restricted cash presented in the condensed consolidated statement of cash flows $ 502,080 $ 159,700
ASC 606
Summary of Impacts of Adopting ASC 606 on Condensed Consolidated Statements of Financial Statements	The following table summarizes the impacts of adopting ASC 606 on our condensed consolidated financial statements, in thousands. Impact of Changes in Accounting Policies Three months ended June 30, 2018 As reported Adjustments Balances without adoption of ASC 606 Collaboration and other revenue $ 3,133 $ (15 ) $ 3,118 Royalty revenue 861 426 1,287 Total revenue 3,994 411 4,405 Loss from operations (33,166 ) 411 (32,755 ) Loss from continuing operations (31,833 ) 411 (31,422 ) Net loss (31,833 ) 411 (31,422 ) Net loss attributable to stockholders of Arena (31,833 ) 411 (31,422 ) Six months ended June 30, 2018 Collaboration and other revenue $ 4,161 $ 44 $ 4,205 Royalty revenue 1,588 688 2,276 Total revenue 5,749 732 6,481 Loss from operations (64,135 ) 732 (63,403 ) Loss from continuing operations (62,966 ) 1,880 (61,086 ) Income (loss) from discontinued operations (830 ) 13,660 12,830 Net loss (63,796 ) 15,540 (48,256 ) Net loss attributable to stockholders of Arena (63,796 ) 15,540 (48,256 ) As of June 30, 2018 Prepaid expenses and other current assets $ 6,539 $ (1,059 ) $ 5,480 Total current assets 601,280 (1,059 ) 600,221 Other non-current assets 7,524 (2,361 ) 5,163 Total assets 637,560 (3,420 ) 634,140 Current portion of deferred revenues 800 20 820 Total current liabilities 20,917 20 20,937 Deferred revenues, less current portion 615 52 667 Accumulated deficit (1,534,948 ) (3,492 ) (1,538,440 ) Total stockholders' equity 558,838 (3,492 ) 555,346 Total liabilities and stockholders' equity 637,560 (3,420 ) 634,140
Summary of Revenue Disaggregated by Major Customers, Timing of Revenue Recognition and Revenue Classification	In the following table, revenue is disaggregated by major customers, timing of revenue recognition and revenue classification, in thousands. Customers Three months ended June 30, 2018 Six months ended June 30, 2018 Outpost $ 2,750 $ 2,750 Eisai 861 3,089 Siegfried — 942 Axovant 200 768 Boehringer Ingelheim 163 623 Other 20 84 Total $ 3,994 $ 8,256 Timing of revenue recognition Goods and services transferred at a point in time $ 2,770 $ 5,277 Goods and services transferred over time 1,224 2,979 Total $ 3,994 $ 8,256 Classification Revenue from continuing operations $ 3,994 $ 5,749 Revenue reported under discontinued operations — 2,507 Total $ 3,994 $ 8,256
Summary of Changes in Contract Assets and Deferred Revenues	The following table provides detail of changes in our contract assets and deferred revenues, in thousands. The deferred revenue balances as of December 31, 2017, presented in the following table include balances classified as liabilities of disposal group held for sale: Contract Assets - Current Contract Assets - Non-Current Current Portion of Deferred Revenues Deferred Revenues, Less Current Portion Balances at December 31, 2017 $ — $ — $ 26,560 $ 1,067 ASC 606 implementation adjustments 7,527 2,694 (25,526 ) (40 ) Reductions of contract assets (2,070 ) (333 ) — — Impact of the disposal of the Manufacturing Operations (see Note 2) (4,543 ) — — — Foreign currency translation adjustment 145 — — — Recognized as revenue during the period — — (234 ) (412 ) Balances at June 30, 2018 $ 1,059 $ 2,361 $ 800 $ 615
Summary of Estimated Revenue Expected to be Recognized in Future Related to Performance Obligations	The following table provides detail of estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) pursuant to our existing collaboration agreements as of June 30, 2018, in thousands: As of June 30, 2018 Rest of 2018 $ 1,332 2019 2,069 2020 672 2021 and thereafter — Total $ 4,073

Sale of Manufacturing Operati20

Sale of Manufacturing Operations (Tables)	6 Months Ended
	Jun. 30, 2018
Discontinued Operations And Disposal Groups [Abstract]
Summary of Discontinued Operations for Periods Presented in the Condensed Consolidated Statements of Operations	The following table summarizes the results of discontinued operations for the periods presented in the condensed consolidated statements of operations, in thousands: Three months ended June 30, Six months ended June 30, Revenues 2018 2017 2018 2017 Net product sales $ — $ 2,059 $ 1,129 $ 4,770 Other collaboration revenue — 1,781 372 3,316 Toll manufacturing — 754 1,006 1,472 Total revenues — 4,594 2,507 9,558 Costs and expenses Cost of product sales — 1,497 1,858 4,029 Cost of toll manufacturing — 1,074 1,411 1,993 Research and development — 222 — 389 General and administrative — 86 329 730 Other expenses, net — 1,568 464 2,216 Total costs and expenses — 4,447 4,062 9,357 Income (loss) from operations of discontinued operations — 147 (1,555 ) 201 Gain on sale of discontinued operations — — 725 — Income (loss) from discontinued operations $ — $ 147 $ (830 ) $ 201

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present our valuation hierarchy for our financial assets and liabilities that are measured at fair value on a recurring basis, in thousands: Fair Value Measurements at June 30, 2018 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds(1) $ 419,361 $ 419,361 $ — $ — US government and government agency notes(2) 28,243 28,243 — — Corporate debt instruments(2) 87,149 — 87,149 — Fair Value Measurements at December 31, 2017 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds(1) $ 48,750 $ 48,750 $ — $ — US government and government agency notes(2) 50,335 50,335 — — Corporate debt instruments(2) 94,639 — 94,639 — (1) Included in cash and cash equivalents in the accompanying (2) Included in either cash and cash equivalents or available-for-sale investments in the accompanying condensed consolidated balance sheets.

Investments, Available-for-Sa22

Investments, Available-for-Sale (Tables)	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Schedule of Investments, Available-for-Sale	I nvestments, available-for-sale, consisted of the following June 30, 2018 Maturity in years Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value US government and government agency notes < 1 $ 28,251 $ — $ (8 ) $ 28,243 Corporate debt securities < 1 63,103 — (156 ) 62,947 Short-term investments, available-for-sale $ 91,354 $ — $ (164 ) $ 91,190 December 31, 2017 Maturity in years Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value US government and government agency notes < 1 $ 34,873 $ — $ (8 ) $ 34,865 Corporate debt securities < 1 53,438 — (63 ) 53,375 Short-term investments, available-for-sale $ 88,311 $ — $ (71 ) $ 88,240 Corporate debt securities 1 – 5 24,304 — (62 ) 24,242 Investments, available-for-sale $ 24,304 $ — $ (62 ) $ 24,242

Land, Property and Equipment (T

Land, Property and Equipment (Tables)	6 Months Ended
	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Land, Property and Equipment	Land, property and equipment consisted of the following, in thousands: June 30, December 31, 2018 2017 Cost $ 88,772 $ 88,244 Less accumulated depreciation and amortization (60,016 ) (58,113 ) Land, property and equipment, net $ 28,756 $ 30,131

Accounts Payable and Other Ac24

Accounts Payable and Other Accrued Liabilities (Tables)	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following, in thousands: June 30, December 31, 2018 2017 Accounts payable $ 1,129 $ 1,599 Accrued compensation 3,913 5,255 Other accrued liabilities 1,813 1,062 Total accounts payable and other accrued liabilities $ 6,855 $ 7,916

Share-based Compensation (Table

Share-based Compensation (Tables)	6 Months Ended
	Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share Based Compensation Expense	We recognized share-based compensation expense as follows, in thousands: Three months ended Six months ended June 30, June 30, 2018 2017 2018 2017 Research and development $ 1,917 $ 576 $ 3,567 $ 955 General and administrative 2,613 1,534 4,996 2,926 Discontinued operations — 20 11 87 Total share-based compensation expense $ 4,530 $ 2,130 $ 8,574 $ 3,968
Summary of Stock Option Activity	The following table summarizes our stock option activity during the six months ended June 30, 2018, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2018 3,755 $ 20.00 Granted 2,485 36.91 Exercised (219 ) 17.66 Forfeited/cancelled/expired (212 ) 34.62 Outstanding at June 30, 2018 5,809 $ 26.79

Income (Loss) Per Share (Tables

Income (Loss) Per Share (Tables)	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Potentially Dilutive Securities Excluded from Calculation of Diluted Income (Loss) Per Share	The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net income (loss) per share, in thousands: Three months ended Six months ended June 30, June 30, 2018 2017 2018 2017 Stock options 5,722 4,002 5,542 3,567 RSUs and unvested restricted stock 5 3 15 3 Total 5,727 4,005 5,557 3,570

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Jan. 01, 2018USD ($)
Subsidiary Sale Of Stock [Line Items]
Cash and cash equivalents and available-for-sale investments	$ 592,400,000
Number of reportable business segment | Segment	1
Customers billing method, description	We receive payments from customers based on contractual terms. Accounts receivable are recorded when the right to consideration becomes unconditional. For research and development services, we generally bill our customers monthly or quarterly as the services are performed.
Payment terms on invoiced amounts	30 days
Contract asstes		$ 4,100,000
Current contract assets with related to future royalties	$ 1,059,000	1,400,000
Non-current contract assets with related to future royalties	2,361,000	2,700,000
Revenue remaining performance obligations	4,073,000
ASC 606
Subsidiary Sale Of Stock [Line Items]
Cumulative impact to accumulated deficit, decrease in amount			$ 19,000,000
Impairment loss		$ 13,100,000
Revenue remaining performance obligations	$ 0

Basis of Presentation - Summary

Basis of Presentation - Summary of Impacts of Adopting ASC 606 on Condensed Consolidated Statements of Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	$ 3,994	$ 1,898	$ 5,749	$ 3,558
Loss from operations	(33,166)	(22,952)	(64,135)	(44,156)
Loss from continuing operations	(31,833)	(23,763)	(62,966)	(46,314)
Income (loss) from discontinued operations		147	(830)	201
Net loss	(31,833)	(23,616)	(63,796)	(46,113)
Net loss attributable to stockholders of Arena	(31,833)	(23,317)	(63,796)	(45,370)
Prepaid expenses and other current assets	6,539		6,539		$ 2,681
Total current assets	601,280		601,280		281,280
Other non-current assets	7,524		7,524		3,622
Total assets	637,560		637,560		339,275
Current portion of deferred revenues	800		800		1,110
Total current liabilities	20,917		20,917		72,327
Deferred revenues, less current portion	615		615		1,067
Accumulated deficit	(1,534,948)		(1,534,948)		(1,490,187)
Total stockholders' equity	558,838		558,838		207,144
Total liabilities and stockholders' equity	637,560		637,560		$ 339,275
Collaboration and Other Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	3,133	$ 1,898	4,161	$ 3,558
Royalty Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	861		1,588
ASC 606 | Adjustments
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	411		732
Loss from operations	411		732
Loss from continuing operations	411		1,880
Income (loss) from discontinued operations			13,660
Net loss	411		15,540
Net loss attributable to stockholders of Arena	411		15,540
Prepaid expenses and other current assets	(1,059)		(1,059)
Total current assets	(1,059)		(1,059)
Other non-current assets	(2,361)		(2,361)
Total assets	(3,420)		(3,420)
Current portion of deferred revenues	20		20
Total current liabilities	20		20
Deferred revenues, less current portion	52		52
Accumulated deficit	(3,492)		(3,492)
Total stockholders' equity	(3,492)		(3,492)
Total liabilities and stockholders' equity	(3,420)		(3,420)
ASC 606 | Adjustments | Collaboration and Other Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	(15)		44
ASC 606 | Adjustments | Royalty Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	426		688
ASC 606 | Balances without adoption of ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	4,405		6,481
Loss from operations	(32,755)		(63,403)
Loss from continuing operations	(31,422)		(61,086)
Income (loss) from discontinued operations			12,830
Net loss	(31,422)		(48,256)
Net loss attributable to stockholders of Arena	(31,422)		(48,256)
Prepaid expenses and other current assets	5,480		5,480
Total current assets	600,221		600,221
Other non-current assets	5,163		5,163
Total assets	634,140		634,140
Current portion of deferred revenues	820		820
Total current liabilities	20,937		20,937
Deferred revenues, less current portion	667		667
Accumulated deficit	(1,538,440)		(1,538,440)
Total stockholders' equity	555,346		555,346
Total liabilities and stockholders' equity	634,140		634,140
ASC 606 | Balances without adoption of ASC 606 | Collaboration and Other Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	3,118		4,205
ASC 606 | Balances without adoption of ASC 606 | Royalty Revenue
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total revenues	$ 1,287		$ 2,276

Basis of Presentation - Summa29

Basis of Presentation - Summary of Revenue Disaggregated by Major Customers, Timing of Revenue Recognition and Revenue Classification (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 3,994		$ 8,256
Revenue from continuing operations	3,994	$ 1,898	5,749	$ 3,558
Revenue reported under discontinued operations		$ 4,594	2,507	$ 9,558
Goods and Services Transferred Over Time
Disaggregation Of Revenue [Line Items]
Revenue	1,224		2,979
Goods and Services Transferred At a Point in Time
Disaggregation Of Revenue [Line Items]
Revenue	2,770		5,277
Outpost
Disaggregation Of Revenue [Line Items]
Revenue	2,750		2,750
Eisai
Disaggregation Of Revenue [Line Items]
Revenue	861		3,089
Siegfried
Disaggregation Of Revenue [Line Items]
Revenue			942
Axovant
Disaggregation Of Revenue [Line Items]
Revenue	200		768
Boehringer Ingelheim
Disaggregation Of Revenue [Line Items]
Revenue	163		623
Other
Disaggregation Of Revenue [Line Items]
Revenue	$ 20		$ 84

Basis of Presentation - Summa30

Basis of Presentation - Summary of Changes in Contract Assets and Deferred Revenues (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Change In Contract With Customer Asset Liability And Deferred Revenues [Line Items]
Contract Assets - Current, Beginning balance	$ 1,400
Contract Assets - Current, Reductions of contract assets	(2,070)
Contract Assets - Current, Impact of the disposal of the Manufacturing Operations (see Note 2)	(4,543)
Contract Asset - Current, Foreign currency translation adjustment	145
Contract Assets - Current, Ending balance	1,059
Contract Assets - Non-Current, Beginning balance	2,700
Contract Assets - Non-Current, Reductions of contract assets	(333)
Contract Assets - Non-Current, Ending balance	2,361
Current Portion of Deferred Revenues, Beginning balance	26,560
Current Portion of Deferred Revenues, Recognized as revenue during the period	(234)
Current Portion of Deferred Revenues, Ending balance	800
Deferred Revenues less Current Portion, Beginning balance	1,067
Deferred Revenues less Current Portion, Recognized as revenue during the period	(412)
Deferred Revenues less Current Portion, Ending balance	615
ASC 606 | Difference between Revenue Guidance in Effect before and after Topic 606
Change In Contract With Customer Asset Liability And Deferred Revenues [Line Items]
Contract Assets - Current, Implementation adjustments	7,527
Contract Assets - Non-Current, Implementation adjustments	2,694
Current Portion of Deferred Revenues, Implementation adjustments	(25,526)
Deferred Revenues less Current Portion, Implementation adjustments	(40)
Deferred Revenues less Current Portion, Ending balance	$ 52

Basis of Presentation - Summa31

Basis of Presentation - Summary of Estimated Revenue Expected to be Recognized in Future Related to Performance Obligations (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 4,073
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-07-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 1,332
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 2,069
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 672
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year

Basis of Presentation - Summa32

Basis of Presentation - Summary of Estimated Revenue Expected to be Recognized in Future Related to Performance Obligations (Details 1) $ in Thousands	Jun. 30, 2018USD ($)
Revenue Performance Obligation [Abstract]
Revenue remaining performance obligations	$ 4,073

Basis of Presentation - Reconci

Basis of Presentation - Reconciliation of Components of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 501,217	$ 158,837
Restricted cash included in other non-current assets	$ 863	$ 863
Restricted Cash, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents and restricted cash presented in the condensed consolidated statement of cash flows	$ 502,080	$ 159,700

Sale of Manufacturing Operati34

Sale of Manufacturing Operations - Additional Information (Detail) SFr in Millions	1 Months Ended
	Mar. 31, 2018CHF (SFr)	Jun. 30, 2018employee
Siegfried
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of employees transferred | employee		50
Arena Gmb H
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Sales price of business	SFr 4
Cash received from sale of business	SFr 3

Sale of Manufacturing Operati35

Sale of Manufacturing Operations - Summary of Discontinued Operations for Periods Presented in the Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net product sales	$ 2,059	$ 1,129	$ 4,770
Other collaboration revenue	1,781	372	3,316
Toll manufacturing	754	1,006	1,472
Total revenues	4,594	2,507	9,558
Costs and expenses
Cost of product sales	1,497	1,858	4,029
Cost of toll manufacturing	1,074	1,411	1,993
Research and development	222		389
General and administrative	86	329	730
Other expenses, net	1,568	464	2,216
Total costs and expenses	4,447	4,062	9,357
Income (loss) from operations of discontinued operations	147	(1,555)	201
Gain on sale of discontinued operations		725
Income (loss) from discontinued operations	$ 147	$ (830)	$ 201

Fair Value Disclosures - Financ

Fair Value Disclosures - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Money market funds	$ 419,361	$ 48,750
US government and government agency notes	28,243	50,335
Corporate debt instruments	87,149	94,639
Quoted Prices in Active Markets (Level 1)
Assets:
Money market funds	419,361	48,750
US government and government agency notes	28,243	50,335
Significant Other Observable Inputs (Level 2)
Assets:
Corporate debt instruments	$ 87,149	$ 94,639

Investments, Available-for-Sa37

Investments, Available-for-Sale - Schedule of Investments, Available-for-Sale (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term investments, available-for-sale
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 91,354	$ 88,311
Gross Unrealized Losses	(164)	(71)
Estimated Fair Value	91,190	88,240
Short-term investments, available-for-sale | US government and government agency notes
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	28,251	34,873
Gross Unrealized Losses	(8)	(8)
Estimated Fair Value	$ 28,243	$ 34,865
Short-term investments, available-for-sale | US government and government agency notes | Minimum
Schedule Of Available For Sale Securities [Line Items]
Maturity in years	1 year	1 year
Short-term investments, available-for-sale | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 63,103	$ 53,438
Gross Unrealized Losses	(156)	(63)
Estimated Fair Value	$ 62,947	$ 53,375
Short-term investments, available-for-sale | Corporate debt securities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Maturity in years	1 year	1 year
Investments, available-for-sale
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 24,304
Gross Unrealized Losses		(62)
Estimated Fair Value		24,242
Investments, available-for-sale | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		24,304
Gross Unrealized Losses		(62)
Estimated Fair Value		$ 24,242
Investments, available-for-sale | Corporate debt securities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Maturity in years		1 year
Investments, available-for-sale | Corporate debt securities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Maturity in years		5 years

Land, Property and Equipment -

Land, Property and Equipment - Land, Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Cost	$ 88,772	$ 88,244
Less accumulated depreciation and amortization	(60,016)	(58,113)
Land, property and equipment, net	$ 28,756	$ 30,131

Accounts Payable and Other Ac39

Accounts Payable and Other Accrued Liabilities - Accounts Payable and Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 1,129	$ 1,599
Accrued compensation	3,913	5,255
Other accrued liabilities	1,813	1,062
Total accounts payable and other accrued liabilities	$ 6,855	$ 7,916

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	1 Months Ended		3 Months Ended		6 Months Ended			12 Months Ended
	Jan. 31, 2018USD ($)	May 31, 2015USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2018CHF (SFr)	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract asset associated with agreement									$ 4,100,000
Revenues			$ 3,994,000	$ 1,898,000	$ 5,749,000		$ 3,558,000
Payment received from Eisai								$ 10,000,000
Estimated total transaction price associated with performance obligations			4,073,000		4,073,000
Royalty Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues			861,000		1,588,000
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			25,000,000		$ 25,000,000
Eisai | 9.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					9.50%	9.50%
Eisai | 13.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					13.50%	13.50%
Eisai | 18.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales					18.50%	18.50%
Lorcaserin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales					$ 250,000,000
Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments		$ 4,000,000
Additional milestone payments on achievement			41,500,000		41,500,000
Estimated total transaction price			9,900,000		9,900,000
Estimated total transaction price associated with performance obligations			3,700,000		3,700,000
Recognized revenues			200,000	500,000	800,000		1,000,000
Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments								$ 7,500,000
Additional milestone payments on achievement			251,000,000		251,000,000
Revenues			200,000	1,300,000	600,000		2,500,000
Estimated total transaction price			700,000		700,000
Estimated total transaction price associated with performance obligations			400,000		$ 400,000
Collaboration and license agreement extension description					We and Boehringer Ingelheim agreed to extend the original term of the Boehringer Ingelheim Agreement by twelve months through January 2019	We and Boehringer Ingelheim agreed to extend the original term of the Boehringer Ingelheim Agreement by twelve months through January 2019
Collaboration and license agreement extension period					12 months	12 months
Estimated total transaction price under original term recognized as revenue	$ 10,500,000
Eisai First Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date					2012-05	2012-05
Eisai Second Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement initiation date					2013-11	2013-11
Eisai Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from sale of inventory					$ 10,000,000
Additional manufacturing support payments received | SFr						SFr 8,700,000
Estimated transaction price					344,400,000
Reduction in deferred revenue associated with agreement					25,500,000
Contract asset associated with agreement			6,100,000		6,100,000
Estimated future royalty payments					4,100,000
Reversal balance of hand on inventory					3,600,000
Deferred revenues									$ 25,500,000
Eisai Agreement | Royalty Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues			900,000	0	1,600,000		0
Prior to Transaction Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments					115,000,000
Prior to Transaction Agreement | Ildong
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			102,100,000		102,100,000
Prior to Commercial Distribution Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments					7,500,000
Supply Agreement | Product Sales
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues				$ 3,800,000	1,500,000		$ 8,100,000
Maximum | Eisai | 9.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					175,000,000
Maximum | Eisai | 13.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					500,000,000
Minimum | Eisai | 13.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					175,000,000
Minimum | Eisai | 18.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount					500,000,000
Outpost Medicine, LLC
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments			3,000,000
Collaborative agreement upfront payments in the form of equity interest			1,500,000
Outpost Medicine, LLC | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			$ 96,500,000		$ 96,500,000

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017
Stockholders Equity [Line Items]
Aggregate gross proceeds from sale of common stock		$ 387,009	$ 81,496
Underwritten Public Offering | Common Stock
Stockholders Equity [Line Items]
Sale of common stock shares	9,775,000
Aggregate gross proceeds from sale of common stock	$ 383,100

Share-based Compensation - Shar

Share-based Compensation - Share Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 4,530	$ 2,130	$ 8,574	$ 3,968
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,917	576	3,567	955
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 2,613	1,534	4,996	2,926
Discontinued operations
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 20	$ 11	$ 87

Share-based Compensation - Summ

Share-based Compensation - Summary of Stock Option Activity (Detail) - Stock options shares in Thousands	6 Months Ended
	Jun. 30, 2018$ / sharesshares
Number of stock options
Outstanding at January 1, 2018 | shares	3,755
Granted | shares	2,485
Exercised | shares	(219)
Forfeited/cancelled/expired | shares	(212)
Outstanding at June 30, 2018 | shares	5,809
Weighted- Average Exercise Price
Outstanding at January 1, 2018 | $ / shares	$ 20
Granted | $ / shares	36.91
Exercised | $ / shares	17.66
Forfeited/cancelled/expired | $ / shares	34.62
Outstanding at June 30, 2018 | $ / shares	$ 26.79

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Detail) - Performance restricted stock units - shares	1 Months Ended	6 Months Ended
	Mar. 31, 2015	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Shares issued		32,322

Income (Loss) Per Share - Poten

Income (Loss) Per Share - Potentially Dilutive Securities Excluded from Calculation of Diluted Income (Loss) Per Share (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share (shares)	5,727	4,005	5,557	3,570
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share (shares)	5,722	4,002	5,542	3,567
RSUs and unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share (shares)	5	3	15	3

Income (Loss) Per Share - Addit

Income (Loss) Per Share - Additional Information (Detail)	Jun. 30, 2018shares
Performance restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of shares outstanding PRSUs	0

Legal Proceedings - Additional

Legal Proceedings - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Nov. 03, 2017
Loss Contingencies [Line Items]
Accrued litigation settlement		$ 24,000
Litigation settlement expense, net	$ 11,975
Litigation Amount Settlement By Insurer
Loss Contingencies [Line Items]
Accrued litigation settlement			$ 12,025
Litigation Amount Settlement By Parent
Loss Contingencies [Line Items]
Accrued litigation settlement			$ 11,975

Beacon Discovery, Inc. - Additi

Beacon Discovery, Inc. - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Variable Interest Entity [Line Items]
Revenues	$ 3,994,000	$ 1,898,000	$ 5,749,000	$ 3,558,000
Net income	(31,833,000)	(23,317,000)	(63,796,000)	(45,370,000)
Net loss attributable to noncontrolling interest in consolidated variable interest entity		(299,000)		(743,000)
Prepaid Expenses and Other Current Assets
Variable Interest Entity [Line Items]
Note receivable from Beacon, balance	500,000		500,000
Variable Interest Entity, Primary Beneficiary | Beacon Discovery, Inc.
Variable Interest Entity [Line Items]
Total assets Beacon's excluded from consolidated balance sheet	7,000,000		7,000,000
Total liabilities Beacon's excluded from consolidated balance sheet	7,000,000		7,000,000
Revenues	2,600,000	700,000	4,100,000	1,300,000
Net income	800,000		700,000
Revenues from third parties	300,000		700,000
Net loss attributable to noncontrolling interest in consolidated variable interest entity		$ (300,000)		$ (700,000)
Variable Interest Entity, Primary Beneficiary | Beacon Discovery, Inc. | Maximum
Variable Interest Entity [Line Items]
Total stockholders' deficit Beacon's excluded from consolidated balance sheet	$ 100,000		$ 100,000