Arena Pharmaceuticals (ARNA) 10-Q 9 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 04, 2016
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	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		243,313,807

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 101,629	$ 156,184
Accounts receivable	18,304	4,934
Inventory	10,166	9,502
Prepaid expenses and other current assets	4,768	4,218
Total current assets	134,867	174,838
Land, property and equipment, net	64,980	71,828
Intangibles, net	7,518	7,775
Other non-current assets	2,883	2,351
Total assets	210,248	256,792
Current liabilities:
Accounts payable and other accrued liabilities	14,767	8,334
Accrued clinical and preclinical study fees	6,532	3,286
Payable to Eisai	12,065	12,080
Accrued restructuring charges	1,244	1,793
Current portion of deferred revenues	22,522	21,425
Current portion of lease financing obligations	3,378	2,978
Total current liabilities	60,508	49,896
Other long-term liabilities	853	470
Deferred revenues, less current portion	82,132	87,617
Lease financing obligations, less current portion	62,678	65,267
Commitments and contingencies
Equity:
Common stock	24	24
Additional paid-in capital	1,439,745	1,430,917
Accumulated other comprehensive income (loss)	867	(1,179)
Accumulated deficit	(1,437,308)	(1,376,220)
Total equity attributable to stockholders of Arena	3,328	53,542
Equity attributable to noncontrolling interest in consolidated variable interest entity	749	0
Total equity	4,077	53,542
Total liabilities and equity	$ 210,248	$ 256,792

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues:
Net product sales	$ 3,323	$ 4,884	$ 11,104	$ 15,787
Toll manufacturing	1,228	1,463	3,276	3,199
Total revenues	19,242	9,138	38,601	30,575
Operating Costs and Expenses:
Cost of product sales	882	1,635	4,161	6,129
Cost of toll manufacturing	1,930	1,584	4,876	3,798
Research and development	17,466	22,072	54,514	68,241
General and administrative	8,590	9,028	23,979	26,311
Restructuring charges	231	0	6,346	0
Total operating costs and expenses	29,099	34,319	93,876	104,479
Loss from operations	(9,857)	(25,181)	(55,275)	(73,904)
Interest and Other Income (Expense):
Interest income	54	37	247	105
Interest expense	(1,609)	(1,683)	(4,907)	(5,133)
Gain from valuation of derivative liabilities	0	852	0	474
Other	(1,067)	(443)	(1,275)	938
Total interest and other expense, net	(2,622)	(1,237)	(5,935)	(3,616)
Net loss	(12,479)	(26,418)	(61,210)	(77,520)
Less net loss attributable to noncontrolling interest in consolidated variable interest entity	122	0	122	0
Net loss attributable to stockholders of Arena	$ (12,357)	$ (26,418)	$ (61,088)	$ (77,520)
Net loss attributable to stockholders of Arena per share:
Basic	$ (0.05)	$ (0.11)	$ (0.25)	$ (0.32)
Diluted	$ (0.05)	$ (0.11)	$ (0.25)	$ (0.32)
Shares used in calculating net loss attributable to stockholders of Arena per share:
Basic	243,254	242,257	243,069	240,033
Diluted	243,254	242,257	243,069	240,033
Comprehensive Loss:
Net loss	$ (12,479)	$ (26,418)	$ (61,210)	$ (77,520)
Foreign currency translation gain (loss)	694	(2,881)	2,046	(2,499)
Comprehensive loss	(11,785)	(29,299)	(59,164)	(80,019)
Less comprehensive loss attributable to noncontrolling interest in consolidated variable interest entity	122	0	122	0
Comprehensive loss attributable to stockholders of Arena	(11,663)	(29,299)	(59,042)	(80,019)
Eisai
Revenues:
Net product sales	1,868	3,274	7,179	11,603
Other collaboration revenue	12,954	2,065	18,155	7,414
Total revenues	14,822	5,339	25,334	19,017
Other
Revenues:
Other collaboration revenue	$ 1,737	$ 726	$ 6,066	$ 4,175

Condensed Consolidated Stateme4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating Activities
Net loss	$ (61,210)	$ (77,520)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	7,005	7,401
Amortization of intangibles	104	147
Share-based compensation	9,261	11,795
Gain from valuation of derivative liabilities	0	(474)
Amortization of prepaid financing costs	102	102
Loss on disposal of property and equipment	798	1,007
Changes in assets and liabilities:
Accounts receivable	(13,122)	116
Inventory	157	1,240
Prepaid expenses and other assets	(514)	(366)
Payables and accrued liabilities	8,820	(21,271)
Deferred revenues	(4,929)	4,506
Other long-term liabilities	62	78
Net cash used in operating activities	(53,466)	(73,239)
Investing Activities
Purchases of property and equipment	(644)	(10,800)
Proceeds from sale of property and equipment	786	2,232
Other non-current assets	(659)	(55)
Net cash used in investing activities	(517)	(8,623)
Financing Activities
Principal payments on lease financing obligations	(2,189)	(1,829)
Proceeds from issuance of common stock	288	102,934
Net cash provided by (used in) financing activities	(1,901)	101,105
Effect of exchange rate changes on cash	1,329	(1,172)
Net increase (decrease) in cash and cash equivalents	(54,555)	18,071
Cash and cash equivalents at beginning of period	156,184	163,209
Cash and cash equivalents at end of period	$ 101,629	$ 181,280

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2016
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, 2015, as filed with the Securities and Exchange Commission, or SEC, from which we derived our balance sheet as of December 31, 2015. The accompanying 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 consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying 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. The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity for which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheet. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon is 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. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU No. 2014-09 outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU No. 2014-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017. ASU No. 2014-09 allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying ASU No. 2014-09 is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We have not yet selected an adoption method as we are currently evaluating the impact of ASU No. 2014-09 on our consolidated financial statements. In August 2014, the FASB issued ASU No. 2014-15, “Presentation of Financial Statements – Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern.” ASU No. 2014-15 requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and provides guidance on determining when and how to disclose going concern uncertainties in the financial statements. Certain disclosures will be required if conditions give rise to substantial doubt about an entity’s ability to continue as a going concern. ASU No. 2014-15 applies to all entities and is effective for annual and interim periods ending after December 15, 2016, with early adoption permitted. We do not expect the adoption of ASU No. 2014-15 to have a material impact on our consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities.” ASU No. 2016-01 supersedes and amends the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and require equity securities to be measured at fair value with changes in the fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU No. 2016-01 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017, and calls for prospective application, with early application permitted. We do not expect the adoption of ASU No. 2016-01 to have a material impact on our consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, “Leases.” ASU No. 2016-02 amends the accounting guidance for leases. The amendments contain principles that will require lessees to recognize most leases on the balance sheet by recording a right-of-use asset and a lease liability, unless the lease is a short-term lease that has an accounting lease term of 12 months or less. The amendments also contain other changes to the current lease guidance that may result in changes to how entities determine which contractual arrangements qualify as a lease, the accounting for executory costs (such as property taxes and insurance), as well as which lease origination costs will be capitalizable. The new standard also requires expanded quantitative and qualitative disclosures. ASU No. 2016-02 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2018, with early adoption permitted. ASU No. 2016-02 requires the use of the modified retrospective transition method, whereby the new guidance will be applied at the beginning of the earliest period presented in the financial statements of the period of adoption. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements. In March 2016, the FASB issued ASU No. 2016-09, “Improvements to Employee Share-Based Payment Accounting.” ASU No. 2016-09 is designed to simplify several aspects of accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and forfeiture rate calculations. ASU No. 2016-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2016, with early adoption permitted. We are currently evaluating the impact of ASU No. 2016-09 on our consolidated financial statements. 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.

Fair Value Disclosures

Fair Value Disclosures	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair Value Disclosures	2. 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 September 30, 2016 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 56,327 $ 56,327 $ 0 $ 0 Fair Value Measurements at December 31, 2015 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 113,080 $ 113,080 $ 0 $ 0 (1) Included in cash and cash equivalents on our condensed consolidated balance sheets.

Inventory

Inventory	9 Months Ended
	Sep. 30, 2016
Inventory Disclosure [Abstract]
Inventory	3. Inventory Inventory consisted of the following, in thousands: September 30, December 31, 2016 2015 Raw materials $ 2,692 $ 2,487 Work in process 2,969 2,781 Finished goods at Arena GmbH 727 165 Finished goods at Eisai 2,492 3,309 Finished goods at Ildong 1,286 760 Total inventory $ 10,166 $ 9,502

Land, Property and Equipment

Land, Property and Equipment	9 Months Ended
	Sep. 30, 2016
Property Plant And Equipment [Abstract]
Land, Property and Equipment	4. Land, Property and Equipment Land, property and equipment consisted of the following, in thousands: September 30, December 31, 2016 2015 Cost $ 166,420 $ 172,729 Less accumulated depreciation and amortization (101,440 ) (100,901 ) Land, property and equipment, net $ 64,980 $ 71,828

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	9 Months Ended
	Sep. 30, 2016
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	5. Accounts Payable and Other Accrued Liabilities Accounts payable and other accrued liabilities consisted of the following, in thousands: September 30, December 31, 2016 2015 Accounts payable $ 7,645 $ 2,078 Accrued compensation 5,663 5,118 Other accrued liabilities 1,459 1,138 Total accounts payable and other accrued liabilities $ 14,767 $ 8,334

Collaborations

Collaborations	9 Months Ended
	Sep. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	6. Collaborations Please refer to our Annual Report on Form 10-K for the year ended December 31, 2015, for additional information regarding the collaborations described below. Eisai. We have a collaboration agreement with Eisai Inc. and Eisai Co., Ltd. (collectively with Eisai Inc., Eisai). Under this agreement, or Eisai Agreement, Eisai is the exclusive distributor of BELVIQ in the United States and most other countries in the world except for South Korea, Taiwan, Australia, New Zealand and Israel. In July 2016, the US Food and Drug Administration approved the New Drug Application for our once-daily formulation of lorcaserin for chronic weight management under the brand name BELVIQ XR. Eisai will pay us a $10.0 million substantive milestone payment earned from this achievement. Eisai has the exclusive rights to distribute BELVIQ XR in the United States. In October 2016, Eisai announced the commercial launch of BELVIQ XR in the United States. In July 2016, the Federal Commission for the Protection Against Sanitary Risk approved the Marketing Authorization Application in Mexico for our twice-daily formulation of lorcaserin for chronic weight management. The product will be sold under the brand name VENESPRI. Eisai will pay us a $1.0 million substantive milestone payment earned from this achievement. Eisai has the exclusive rights to distribute VENESPRI in Mexico. Under the Eisai Agreement, in addition to the $11.0 million in milestones mentioned above and the other $86.5 million in milestones previously achieved since we entered the agreement in 2010, we are eligible to receive up to 16 substantive regulatory milestones totaling $165.0 million. These payments are based on 16 milestone events of which eight milestones with an aggregate value of $105.0 million are related to a second or third product approval. The following table summarizes the revenues we recognized for the periods presented under the Eisai Agreement, in thousands: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Net product sales $ 1,868 $ 3,274 $ 7,179 $ 11,603 Milestone payments 11,000 0 11,000 0 Amortization of upfront payments 1,886 1,886 5,656 5,656 Reimbursement of development expenses 11 107 1,242 1,454 Reimbursement of patent and trademark expenses 57 72 257 304 Subtotal other Eisai collaborative revenue 12,954 2,065 18,155 7,414 Total $ 14,822 $ 5,339 $ 25,334 $ 19,017 The following table summarizes the deferred revenues under the Eisai Agreement, in thousands: September 30, December 31, 2016 2015 Upfront payments $ 81,278 $ 86,933 Net product sales 7,331 10,754 Total deferred revenues attributable to Eisai 88,609 97,687 Less current portion (14,872 ) (18,295 ) Deferred revenues attributable to Eisai, less current portion $ 73,737 $ 79,392 Ildong Pharmaceutical Co., Ltd. We and Ildong Pharmaceutical Co., Ltd., or Ildong, have an exclusive agreement, or Ildong Agreement, under which Ildong has exclusive rights to commercialize BELVIQ in South Korea for weight loss or weight management in obese and overweight patients. We also provide certain services and manufacture and sell BELVIQ to Ildong. There are no milestones we are eligible to receive in the future under the Ildong Agreement. CY Biotech Company Limited We have an agreement with CY Biotech Company Limited, or CYB. Under this agreement, we granted CYB exclusive rights to commercialize BELVIQ in Taiwan for weight loss or weight management in obese and overweight patients, subject to regulatory approval of BELVIQ by the Taiwan Food and Drug Administration. We also provide certain services and will manufacture and sell BELVIQ to CYB. There are no material milestones we are eligible to receive in the future under this agreement. Abic Marketing Limited (Teva) We granted Teva exclusive rights to commercialize BELVIQ in Israel for weight loss or weight management in obese and overweight patients, subject to regulatory approval of BELVIQ by the Israeli Ministry of Health, or MOH. We also provide certain services and will manufacture and sell BELVIQ to Teva. There are no material milestones we are eligible to receive in the future under this agreement. Axovant Sciences Ltd. We and Axovant Sciences, Ltd., or Axovant, have an exclusive agreement, or Axovant Agreement, under which Axovant has exclusive worldwide rights to develop and commercialize nelotanserin, subject to regulatory approval. We also provide certain services and will manufacture and sell nelotanserin to Axovant. Under the Axovant Agreement, we are eligible to receive up to an aggregate of $41.5 million in success milestones in case of full development and regulatory success of nelotanserin. Of these payments, two development milestones totaling $4.0 million are substantive and four regulatory milestones totaling $37.5 million are substantive. Boehringer Ingelheim International GmbH. We and Boehringer Ingelheim GmbH, or Boehringer Ingelheim, have an exclusive 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. Under the Boehringer Ingelheim Agreement, we are eligible to receive up to an aggregate of $251.0 million in success milestones in case of full commercial success of multiple drug products. Of these payments, three development milestones totaling $7.0 million are substantive, three development milestones totaling $30.0 million are non-substantive, nine regulatory milestones totaling $84.0 million are non-substantive and four commercial milestones totaling $130.0 million are non-substantive.

Share-based Activity

Share-based Activity	9 Months Ended
	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Activity	7. Share-based Activity Share-based Compensation. We recognized share-based compensation expense as follows, in thousands: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Cost of product sales $ 0 $ 0 $ 20 $ 0 Research and development 1,200 2,097 4,940 6,338 General and administrative 981 1,751 3,269 5,457 Restructuring charges 0 0 1,032 0 Total share-based compensation expense $ 2,181 $ 3,848 $ 9,261 $ 11,795 Total share-based compensation expense capitalized into inventory $ 64 $ 41 $ 149 $ 146 Share-based Award Activity. The following table summarizes our stock option activity during the nine months ended September 30, 2016, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2016 16,407 $ 5.01 Granted 16,502 1.61 Exercised (63 ) 1.57 Forfeited/cancelled/expired (7,281 ) 3.70 Outstanding at September 30, 2016 25,565 $ 3.20 The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the nine months ended September 30, 2016, in thousands (except per share data): RSUs Weighted- Average Grant-Date Fair Value Unvested at January 1, 2016 273 $ 4.67 Granted 0 Vested (193 ) 4,51 Forfeited/cancelled (9 ) 4.31 Unvested at September 30, 2016 71 $ 5.16 During the nine months ended September 30, 2016, the remaining Total Stockholder Return, or TSR, performance restricted stock unit, or PRSU, awards that we granted to our executive officers in March 2013 were forfeited without any earnout 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, 2013. In the aggregate, the target number of shares of common stock that could have been earned under the PRSUs granted in March 2013 was 780,000. Except for those cancelled due to employment separation from Arena, the PRSU awards granted in March 2014 and March 2015 are still outstanding at September 30, 2016.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Major Customers	9 Months Ended
	Sep. 30, 2016
Risks And Uncertainties [Abstract]
Concentrations of Credit Risk and Major Customers	8. Concentrations of Credit Risk and Major Customers Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. 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 and 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. Eisai and Ildong are the exclusive distributors of BELVIQ in the United States and South Korea, respectively. Eisai is also the exclusive distributor of BELVIQ XR in the United States. We also produce drug products for Siegfried AG, or Siegfried, and, to a lesser extent, another third party under toll manufacturing agreements. Percentages of our total revenues are as follows: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Eisai Agreement (See Note 6) 77.0 % 58.4 % 65.6 % 62.2 % Ildong Agreement 8.2 % 18.6 % 11.0 % 24.4 % Toll manufacturing agreements 6.4 % 16.0 % 8.5 % 10.5 % Boehringer Ingelheim Agreement (See Note 6) 5.6 % 0.0 % 10.0 % 0.0 % Axovant Agreement (See Note 6) 2.4 % 6.1 % 4.3 % 2.2 % Other collaborative agreements 0.4 % 0.9 % 0.6 % 0.7 % Total percentage of revenues 100.0 % 100.0 % 100.0 % 100.0 %

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Net Loss Per Share	9. Net Loss Per Share We calculate basic and diluted net loss attributable to stockholders of Arena per share using the weighted-average number of shares of common stock outstanding during the period. Since we are in a net loss position, in addition to excluding potentially dilutive out-of-the money securities, we exclude from our calculation of diluted net loss attributable to stockholders of Arena per share all potentially dilutive in-the-money (i) stock options, (ii) RSUs, (iii) PRSUs, (iv) unvested restricted stock in our deferred compensation plan and (v) our previously outstanding warrant, and our diluted net loss per share is the same as our basic net loss per share. The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss attributable to stockholders of Arena per share, in thousands: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Stock options 28,559 17,462 24,791 17,033 RSUs and unvested restricted stock 175 690 242 576 Warrant 0 0 0 26 Total 28,734 18,152 25,033 17,635 Because the market conditions for the PRSUs were not satisfied at September 30, 2016, or September 30, 2015, such securities are excluded from the table above.

Legal Proceedings

Legal Proceedings	9 Months Ended
	Sep. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Legal Proceedings	10. Legal Proceedings Beginning on September 20, 2010, a number of complaints were filed in the US District Court for the Southern District of California 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 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 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 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 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. 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. Due to the stage of these proceedings, we are not able to predict or reasonably estimate the ultimate outcome or possible losses relating to these claims. On September 30, 2016, we and Eisai filed a patent infringement lawsuit against Lupin Limited and Lupin Pharmaceuticals, Inc. (collectively with Lupin Limited, Lupin) in the U.S. District Court for the District of Delaware. The lawsuit relates to a “Paragraph IV certification” notification we and Eisai 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 which are listed in the FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations (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. The lawsuit claims infringement of U.S. Patent Nos. 6,953,787; 7,514,422; 7,977,329; 8,207,158; 8,273,734; 8,546,379; 8,575,149; 8,999,970 and 9,169,213. 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 the 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. We and Eisai are seeking a determination from the court that, among other things, Lupin has infringed our patents, Lupin’s ANDA should not be approved until the expiration date of our patents, and Lupin should be enjoined from commercializing a product that infringes our patents. We cannot predict the ultimate outcome of any proceeding.

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2016
Restructuring And Related Activities [Abstract]
Restructuring Charges	11. Restructuring Charges In the fourth quarter of 2015, we committed to a reduction in our US workforce of approximately 35%, or approximately 80 employees, which we substantially completed by the end of 2015. In the fourth quarter of 2015, we committed to a reduction in our Swiss workforce of approximately 17%, or approximately 14 employees, which we substantially completed by the end of the second quarter of 2016. As a result of these workforce reductions, we recorded a restructuring charge in the fourth quarter of 2015 for termination benefits, including severance and other benefits, of $4.0 million, and at September 30, 2016, all of this charge has been paid. In the second quarter of 2016, we committed to a reduction in our US workforce of approximately 73%, or approximately 100 employees, which we substantially completed in the third quarter of 2016. As a result of this workforce reduction, we recorded a restructuring charge in the second quarter of 2016 of $6.1 million for termination benefits, including severance and other benefits. Included within this amount is non-cash, share-based compensation expense of $1.0 million related to the accelerated vesting of stock options and the extension of the exercise period of vested options for employees impacted by the workforce reduction. At September 30, 2016, $4.0 million of the cash portion of the restructuring charge has been paid, resulting in a remaining accrual of $1.1 million. In the third quarter of 2016, we committed to a reduction of our manufacturing workforce in Zofingen, Switzerland of approximately 23%, or approximately 15 employees, which we plan to substantially complete by the end of the first quarter of 2017. As a result of this workforce reduction, we recorded a restructuring charge in the third quarter of 2016 of $0.2 million for cash termination benefits. At September 30, 2016, $0.1 million of this charge has been paid, resulting in a remaining accrual of $0.1 million.

Management Changes

Management Changes	9 Months Ended
	Sep. 30, 2016
Compensation And Retirement Disclosure [Abstract]
Management Changes	12. Management Changes Appointment of President and Chief Executive Officer. In May 2016, our Board of Directors appointed Amit Munshi as our President and Chief Executive Officer, and he joined our Board of Directors in June 2016 following our 2016 Annual Stockholders’ Meeting. Harry F. Hixson, Jr., Ph.D., who served as our interim Chief Executive Officer from October 2015 to May 2016, continues to serve on our Board of Directors. In connection with Mr. Munshi’s appointment as an officer, our Board of Directors’ Compensation Committee approved an inducement stock option grant to Mr. Munshi to purchase 3,800,000 shares of our common stock under our 2013 Long-Term Incentive Plan, as amended in May 2016, to reserve an additional 3,800,000 shares of common stock for inducement awards. The nonstatutory stock options have a seven-year term and will vest over four years, with 25% of the shares subject to vesting one year after grant and the remainder of the shares vesting quarterly over the following three years in equal installments, subject to his continued service through the applicable vesting dates and possible acceleration in specified circumstances. Termination of Chief Medical Officer. In June 2016, our Board of Directors terminated without cause our former Senior Vice President and Chief Medical Officer, William R. Shanahan, Jr., M.D., J.D. Under our Amended and Restated Severance Benefit Plan, as amended, or Severance Benefit Plan, Dr. Shanahan is entitled to receive the following termination benefits: (1) a cash severance payment of approximately $0.5 million (subject to applicable withholdings); (2) continuation of health insurance coverage for a period of 12 months; (3) acceleration of the stock options and RSUs (other than PRSUs) held by Dr. Shanahan that would otherwise have vested through the 12-month period following the date of his termination, provided that, for purposes of calculating such vesting acceleration, any unvested portion of such equity awards that were scheduled to vest in annual installments are treated as if the original grant provided for vesting in equal monthly installments rather than annually; and (4) continued stock option exercisability until the later of (i) the end of the original post-termination exercise period provided in the applicable stock option agreement or (ii) 12 months (but not beyond the original contractual life of the option). In addition, with respect to outstanding PRSUs, when our Board of Directors’ Compensation Committee determines our relative performance for an applicable performance period, a pro-rata portion of the relevant PRSUs held by Dr. Shanahan is eligible to vest (based on the percentage of the performance period that Dr. Shanahan provided service prior to his termination). The pro-rata vesting may be accelerated if we undergo a change in control before the scheduled end of the performance period. We recorded a charge of $1.0 million in the second quarter of 2016 related to these benefits, including non-cash, share-based compensation expense of $0.4 million, which is included in research and development expenses in our condensed consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2016. As of September 30, 2016, there are remaining accruals for these benefits of $0.6 million included in accounts payable and other accrued expenses, the majority of which we expect to pay in the fourth quarter of 2016. In July 2016, we and Dr. Shanahan entered into a one-year services agreement whereby Dr. Shanahan performs services for us relating to our research and development programs. As compensation, Dr. Shanahan receives a fixed monthly fee along with reimbursement of certain pre-approved expenses and continued stock option exercisability until 24 months from the July 2016 effective date of this agreement (but not beyond the original contractual life of the option). We recorded a charge of $0.1 million in the third quarter of 2016 related to this compensation, including non-cash, share-based compensation expense, which is included in research and development expenses in our condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016. Appointment of Chief Financial Officer. In June 2016, our Board of Directors appointed Kevin R. Lind as our Executive Vice President and Chief Financial Officer. In connection with such appointment, our Board’s Compensation Committee approved an inducement stock option grant to Mr. Lind to purchase 800,000 shares of our common stock under our 2013 Long-Term Incentive Plan, as amended in May 2016 and June 2016, to reserve an additional 800,000 shares of common stock for inducement awards in addition to the 3,800,000 shares it previously reserved for such awards. The nonstatutory stock options have a seven-year term and will vest over 4 years, with 25% of the shares subject to vesting one year after grant and the remainder of the shares vesting monthly over the following three years in equal installments, subject to his continued service through the applicable vesting dates and possible acceleration in specified circumstances. Appointment of Chief Business Officer. In August 2016, our Board of Directors appointed Vincent Aurentz as our Executive Vice President and Chief Business Officer. In connection with such appointment, our Board’s Compensation Committee approved an inducement stock option grant to Mr. Aurentz to purchase 800,000 shares of our common stock under our 2013 Long-Term Incentive Plan, as amended in May 2016, June 2016 and August 2016, to reserve an additional 800,000 shares of common stock for inducement awards in addition to the 4,600,000 shares it previously reserved for such awards. The nonstatutory stock options have a seven-year term and will vest over 4 years, with 25% of the shares subject to vesting one year after grant and the remainder of the shares vesting monthly over the following three years in equal installments, subject to his continued service through the applicable vesting dates and possible acceleration in specified circumstances. Resignation of Chief Scientific Officer. In September 2016, Dominic P. Behan, Ph.D., D.Sc., our former Executive Vice President and Chief Scientific Officer resigned from the Company, including from our Board of Directors, and became the Chief Executive Officer of Beacon (see Note 13). Dr. Behan's resignation follows our strategic shift in priorities to emphasize our proprietary clinical stage pipeline, which was announced on June 30, 2016. Dr. Behan’s resignation was for good reason under our Severance Benefit Plan resulting from Dr. Behan’s materially diminished duties and responsibilities following this strategic shift. Following his resignation from the Company, Dr. Behan acts as the Chair of our Scientific Advisory Board and provides consulting services to us regarding our research and development program under a five-year consulting agreement, or Consulting Agreement, which may be terminated earlier by either party on 30 days advanced written notice. Dr. Behan receives a market rate hourly consulting fee, along with reimbursement of certain pre-approved expenses. In addition, Dr. Behan’s consulting services constitute continuous service with us, and as a result, the outstanding equity awards we previously granted to Dr. Behan continue to vest and/or be exercisable, as those services are provided in accordance with the applicable plan(s) and written grant instrument(s) for such awards. Under the Severance Benefit Plan, Dr. Behan is entitled to receive the following termination benefits: (1) a cash severance payment of approximately $0.9 million (subject to applicable withholdings); (2) continuation of health insurance coverage for a period of 18 months; (3) acceleration of the stock options and RSUs (other than PRSUs) held by Dr. Behan that would otherwise have vested through the 18-month period following his Arena employee-status termination date, provided that, for purposes of calculating such vesting acceleration, any unvested portion of such equity awards that were scheduled to vest in annual installments are treated as if the original grant provided for vesting in equal monthly installments rather than annually; (4) for options that were vested as of his Arena employee-status termination date, including those for which vesting was accelerated upon his termination, continued stock option exercisability until the later of (i) the end of the original post-termination exercise period provided in the applicable stock option agreement measured from the date of cessation of services under the Consulting Agreement or (ii) 18 months following his Arena employee status termination date (but not beyond the original contractual life of the option) and (5) for options that were not vested as of his Arena employee-status termination date, continued stock option exercisability, to the extent vested as of the date of cessation of services under the Consulting Agreement, until the end of the original post-termination exercise period provided in the applicable stock option agreement measured from the date of cessation of services under the Consulting Agreement (but not beyond the original contractual life of the option). We recorded a charge of $2.6 million in the third quarter of 2016 related to these benefits, including non-cash, share-based compensation expense of $1.6 million, which is included in research and development expenses in our condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016. As of September 30, 2016, there are remaining accruals for these benefits of $0.9 million included in accounts payable and other accrued expenses, the majority of which we expect to pay in the first quarter of 2017.

Beacon Discovery, Inc.

Beacon Discovery, Inc.	9 Months Ended
	Sep. 30, 2016
Variable Interest Entity Disclosure [Abstract]
Beacon Discovery, Inc.	13. Beacon Discovery, Inc. In the third quarter of 2016, Beacon was formed. Beacon is a privately held drug discovery incubator which focuses on identifying and advancing molecules targeting GCPRs. Dr. Behan is the Chief Executive Officer of Beacon (see Note 12). On September 1, 2016, we entered into various agreements with Beacon described in further detail below. We entered into a license and collaboration agreement with Beacon, pursuant to which we transferred certain equipment to Beacon and granted Beacon a non-exclusive, non-assignable and non-sublicensable license to certain database information relating to compounds, receptors and pharmacology, and transferred certain equipment to Beacon. Beacon will seek to engage global partners to facilitate discovery and development. Beacon has agreed to assign to us any intellectual property relating to our existing research and development programs developed in the course of performing research for us, and grant us a non-exclusive license to any intellectual property developed outside the course of performing work for us that is reasonably necessary or useful for developing or commercializing the products under our research and development programs. We are also entitled to rights of negotiation and rights of first refusal to potentially obtain licenses to compounds discovered and developed by Beacon. In addition, we are entitled to receive (i) a percentage of any revenue received by Beacon on or after the second anniversary of the effective date of the agreement from any third party pursuant to a third party license, including upfront payments, milestone payments and royalties; (ii) single-digit royalties on the aggregate net sales of any related products sold by Beacon and its affiliates; and (iii) in the event that Beacon is sold, a percentage of the consideration for such sale transaction. We entered a master services agreement with Beacon, pursuant to which Beacon performs certain research services for us. We also entered into a separate services agreement with Beacon, pursuant to which Beacon now performs our research obligations under the Boehringer Ingelheim Agreement. In consideration for performing these research obligations, Beacon is entitled to receive the applicable FTE payments that are paid to us by Boehringer Ingelheim for the research services and certain milestone payments. We also entered into a sublease agreement, or Sublease, with Beacon, pursuant to which we sublease approximately 15,000 square feet of laboratory, office and meeting room space to Beacon for a period of five years. Beacon can defer payments due to us under the Sublease by increasing the outstanding principal amount under a secured promissory note, or Note, we issued to Beacon. The outstanding principal amount and all accrued or unpaid interest thereon (calculated at a simple interest rate of seven percent per annum) shall be due and payable on the earlier of (i) August 31, 2022 or (ii) Beacon receiving cumulative cash proceeds of $10,000,000 from the sale of equity, issuance of debt or third party license revenue. As Beacon would not be able to finance its activities without the financial support we are providing pursuant to these agreements, Beacon is a variable interest entity. Arena does not own any equity in Beacon; however, as these agreements provide us the controlling financial interest in Beacon, we consolidate Beacon’s balances and activity within our condensed consolidated financial statements. The noncontrolling interest attributable to Beacon presented on our condensed consolidated financial statements is comprised of Beacon’s equity ownership interests as we do not own any voting interest in Beacon. The following table presents a reconciliation of the equity attributable to the stockholders of Arena and the equity attributable to Beacon, in thousands: Equity Attributable to Stockholders of Arena Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity Total Equity Balance at December 31, 2015 $ 53,542 $ 0 $ 53,542 Net loss (61,088 ) (122 ) (61,210 ) Translation gain 2,046 0 2,046 Contribution of equipment and other assets from Arena to Beacon (871 ) 871 0 Other 9,699 0 9,699 Balance at September 30, 2016 $ 3,328 $ 749 $ 4,077 The following table presents the assets and liabilities of Beacon which are included in our condensed consolidated balance sheet at September 30, 2016, in thousands. The assets include only those assets that can be used to settle obligations of Beacon. The liabilities include third party liabilities of Beacon. As of September 30, 2016, Beacon had no creditors with recourse to the general credit of Arena. The assets and liabilities exclude intercompany balances that eliminate in consolidation: Assets of Beacon that can only be used to settle obligations of Beacon Cash and cash equivalents $ 288 Prepaid expense and other current assets 42 Land, property and equipment, net 793 Total assets of Beacon that can only be used to settle obligation of Beacon $ 1,123 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 8 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 8

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2016
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events In October 2016, Eisai announced the commercial launch of BELVIQ XR in the United States (see Note 6). See Note 10 for the update to our legal proceedings which occurred subsequent to September 30, 2016.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
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, 2015, as filed with the Securities and Exchange Commission, or SEC, from which we derived our balance sheet as of December 31, 2015. The accompanying 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 consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying 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. The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity for which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheet. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon is 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.
New Accounting Guidance	In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU No. 2014-09 outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU No. 2014-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017. ASU No. 2014-09 allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying ASU No. 2014-09 is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We have not yet selected an adoption method as we are currently evaluating the impact of ASU No. 2014-09 on our consolidated financial statements. In August 2014, the FASB issued ASU No. 2014-15, “Presentation of Financial Statements – Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern.” ASU No. 2014-15 requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued and provides guidance on determining when and how to disclose going concern uncertainties in the financial statements. Certain disclosures will be required if conditions give rise to substantial doubt about an entity’s ability to continue as a going concern. ASU No. 2014-15 applies to all entities and is effective for annual and interim periods ending after December 15, 2016, with early adoption permitted. We do not expect the adoption of ASU No. 2014-15 to have a material impact on our consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities.” ASU No. 2016-01 supersedes and amends the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and require equity securities to be measured at fair value with changes in the fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU No. 2016-01 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017, and calls for prospective application, with early application permitted. We do not expect the adoption of ASU No. 2016-01 to have a material impact on our consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, “Leases.” ASU No. 2016-02 amends the accounting guidance for leases. The amendments contain principles that will require lessees to recognize most leases on the balance sheet by recording a right-of-use asset and a lease liability, unless the lease is a short-term lease that has an accounting lease term of 12 months or less. The amendments also contain other changes to the current lease guidance that may result in changes to how entities determine which contractual arrangements qualify as a lease, the accounting for executory costs (such as property taxes and insurance), as well as which lease origination costs will be capitalizable. The new standard also requires expanded quantitative and qualitative disclosures. ASU No. 2016-02 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2018, with early adoption permitted. ASU No. 2016-02 requires the use of the modified retrospective transition method, whereby the new guidance will be applied at the beginning of the earliest period presented in the financial statements of the period of adoption. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements. In March 2016, the FASB issued ASU No. 2016-09, “Improvements to Employee Share-Based Payment Accounting.” ASU No. 2016-09 is designed to simplify several aspects of accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and forfeiture rate calculations. ASU No. 2016-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2016, with early adoption permitted. We are currently evaluating the impact of ASU No. 2016-09 on our consolidated financial statements.
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.
Fair Value Measurements	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.
Concentration of Credit Risk and Major Customers	Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. 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 and 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. Eisai and Ildong are the exclusive distributors of BELVIQ in the United States and South Korea, respectively. Eisai is also the exclusive distributor of BELVIQ XR in the United States. We also produce drug products for Siegfried AG, or Siegfried, and, to a lesser extent, another third party under toll manufacturing agreements. Percentages of our total revenues are as follows: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Eisai Agreement (See Note 6) 77.0 % 58.4 % 65.6 % 62.2 % Ildong Agreement 8.2 % 18.6 % 11.0 % 24.4 % Toll manufacturing agreements 6.4 % 16.0 % 8.5 % 10.5 % Boehringer Ingelheim Agreement (See Note 6) 5.6 % 0.0 % 10.0 % 0.0 % Axovant Agreement (See Note 6) 2.4 % 6.1 % 4.3 % 2.2 % Other collaborative agreements 0.4 % 0.9 % 0.6 % 0.7 % Total percentage of revenues 100.0 % 100.0 % 100.0 % 100.0 %
Net Loss Per Share	We calculate basic and diluted net loss attributable to stockholders of Arena per share using the weighted-average number of shares of common stock outstanding during the period. Since we are in a net loss position, in addition to excluding potentially dilutive out-of-the money securities, we exclude from our calculation of diluted net loss attributable to stockholders of Arena per share all potentially dilutive in-the-money (i) stock options, (ii) RSUs, (iii) PRSUs, (iv) unvested restricted stock in our deferred compensation plan and (v) our previously outstanding warrant, and our diluted net loss per share is the same as our basic net loss per share.

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	9 Months Ended
	Sep. 30, 2016
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 September 30, 2016 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 56,327 $ 56,327 $ 0 $ 0 Fair Value Measurements at December 31, 2015 Balance Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Money market funds 1 $ 113,080 $ 113,080 $ 0 $ 0 (1) Included in cash and cash equivalents on our condensed consolidated balance sheets.

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2016
Inventory Disclosure [Abstract]
Components of Inventory	Inventory consisted of the following, in thousands: September 30, December 31, 2016 2015 Raw materials $ 2,692 $ 2,487 Work in process 2,969 2,781 Finished goods at Arena GmbH 727 165 Finished goods at Eisai 2,492 3,309 Finished goods at Ildong 1,286 760 Total inventory $ 10,166 $ 9,502

Land, Property and Equipment (T

Land, Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2016
Property Plant And Equipment [Abstract]
Land, Property and Equipment	Land, property and equipment consisted of the following, in thousands: September 30, December 31, 2016 2015 Cost $ 166,420 $ 172,729 Less accumulated depreciation and amortization (101,440 ) (100,901 ) Land, property and equipment, net $ 64,980 $ 71,828

Accounts Payable and Other Ac23

Accounts Payable and Other Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2016
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following, in thousands: September 30, December 31, 2016 2015 Accounts payable $ 7,645 $ 2,078 Accrued compensation 5,663 5,118 Other accrued liabilities 1,459 1,138 Total accounts payable and other accrued liabilities $ 14,767 $ 8,334

Collaborations (Tables)

Collaborations (Tables) - Eisai	9 Months Ended
	Sep. 30, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues Recognized under Eisai Agreement	The following table summarizes the revenues we recognized for the periods presented under the Eisai Agreement, in thousands: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Net product sales $ 1,868 $ 3,274 $ 7,179 $ 11,603 Milestone payments 11,000 0 11,000 0 Amortization of upfront payments 1,886 1,886 5,656 5,656 Reimbursement of development expenses 11 107 1,242 1,454 Reimbursement of patent and trademark expenses 57 72 257 304 Subtotal other Eisai collaborative revenue 12,954 2,065 18,155 7,414 Total $ 14,822 $ 5,339 $ 25,334 $ 19,017
Deferred Revenues under Eisai Agreement	The following table summarizes the deferred revenues under the Eisai Agreement, in thousands: September 30, December 31, 2016 2015 Upfront payments $ 81,278 $ 86,933 Net product sales 7,331 10,754 Total deferred revenues attributable to Eisai 88,609 97,687 Less current portion (14,872 ) (18,295 ) Deferred revenues attributable to Eisai, less current portion $ 73,737 $ 79,392

Share-based Activity (Tables)

Share-based Activity (Tables)	9 Months Ended
	Sep. 30, 2016
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 Nine months ended September 30, September 30, 2016 2015 2016 2015 Cost of product sales $ 0 $ 0 $ 20 $ 0 Research and development 1,200 2,097 4,940 6,338 General and administrative 981 1,751 3,269 5,457 Restructuring charges 0 0 1,032 0 Total share-based compensation expense $ 2,181 $ 3,848 $ 9,261 $ 11,795 Total share-based compensation expense capitalized into inventory $ 64 $ 41 $ 149 $ 146
Summary of Stock Option Activity	The following table summarizes our stock option activity during the nine months ended September 30, 2016, in thousands (except per share data): Options Weighted- Average Exercise Price Outstanding at January 1, 2016 16,407 $ 5.01 Granted 16,502 1.61 Exercised (63 ) 1.57 Forfeited/cancelled/expired (7,281 ) 3.70 Outstanding at September 30, 2016 25,565 $ 3.20
Summary of Restricted Stock Unit Awards Activity	The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the nine months ended September 30, 2016, in thousands (except per share data): RSUs Weighted- Average Grant-Date Fair Value Unvested at January 1, 2016 273 $ 4.67 Granted 0 Vested (193 ) 4,51 Forfeited/cancelled (9 ) 4.31 Unvested at September 30, 2016 71 $ 5.16

Concentrations of Credit Risk26

Concentrations of Credit Risk and Major Customers (Tables)	9 Months Ended
	Sep. 30, 2016
Sales Revenue, Net
Concentration Risk [Line Items]
Percentages of Total Revenues	Percentages of our total revenues are as follows: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Eisai Agreement (See Note 6) 77.0 % 58.4 % 65.6 % 62.2 % Ildong Agreement 8.2 % 18.6 % 11.0 % 24.4 % Toll manufacturing agreements 6.4 % 16.0 % 8.5 % 10.5 % Boehringer Ingelheim Agreement (See Note 6) 5.6 % 0.0 % 10.0 % 0.0 % Axovant Agreement (See Note 6) 2.4 % 6.1 % 4.3 % 2.2 % Other collaborative agreements 0.4 % 0.9 % 0.6 % 0.7 % Total percentage of revenues 100.0 % 100.0 % 100.0 % 100.0 %

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Attributable to Stockholders of Arena Per Share	The following table presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss attributable to stockholders of Arena per share, in thousands: Three months ended Nine months ended September 30, September 30, 2016 2015 2016 2015 Stock options 28,559 17,462 24,791 17,033 RSUs and unvested restricted stock 175 690 242 576 Warrant 0 0 0 26 Total 28,734 18,152 25,033 17,635

Beacon Discovery, Inc. (Tables)

Beacon Discovery, Inc. (Tables)	9 Months Ended
	Sep. 30, 2016
Variable Interest Entity [Line Items]
Reconciliation of Equity Attributable to Stockholders of Arena and Equity Attributable to Beacon	The following table presents a reconciliation of the equity attributable to the stockholders of Arena and the equity attributable to Beacon, in thousands: Equity Attributable to Stockholders of Arena Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity Total Equity Balance at December 31, 2015 $ 53,542 $ 0 $ 53,542 Net loss (61,088 ) (122 ) (61,210 ) Translation gain 2,046 0 2,046 Contribution of equipment and other assets from Arena to Beacon (871 ) 871 0 Other 9,699 0 9,699 Balance at September 30, 2016 $ 3,328 $ 749 $ 4,077
Beacon Discovery, Inc.
Variable Interest Entity [Line Items]
Summary of Assets and Liabilities Included in Condensed Consolidated Balance Sheet	The following table presents the assets and liabilities of Beacon which are included in our condensed consolidated balance sheet at September 30, 2016, in thousands. The assets include only those assets that can be used to settle obligations of Beacon. The liabilities include third party liabilities of Beacon. As of September 30, 2016, Beacon had no creditors with recourse to the general credit of Arena. The assets and liabilities exclude intercompany balances that eliminate in consolidation: Assets of Beacon that can only be used to settle obligations of Beacon Cash and cash equivalents $ 288 Prepaid expense and other current assets 42 Land, property and equipment, net 793 Total assets of Beacon that can only be used to settle obligation of Beacon $ 1,123 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 8 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 8

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	Sep. 30, 2016	Dec. 31, 2015
Assets:
Money market funds	$ 56,327	$ 113,080
Quoted Prices in Active Markets (Level 1)
Assets:
Money market funds	56,327	113,080
Significant Other Observable Inputs (Level 2)
Assets:
Money market funds	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Money market funds	$ 0	$ 0

Inventory - Components of Inven

Inventory - Components of Inventory (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Inventory [Line Items]
Raw materials	$ 2,692	$ 2,487
Work in process	2,969	2,781
Total inventory	10,166	9,502
Arena GmbH
Inventory [Line Items]
Finished goods	727	165
Eisai
Inventory [Line Items]
Finished goods	2,492	3,309
Ildong
Inventory [Line Items]
Finished goods	$ 1,286	$ 760

Land, Property and Equipment (D

Land, Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Cost	$ 166,420	$ 172,729
Less accumulated depreciation and amortization	(101,440)	(100,901)
Land, property and equipment, net	$ 64,980	$ 71,828

Accounts Payable and Other Ac32

Accounts Payable and Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Accounts payable	$ 7,645	$ 2,078
Accrued compensation	5,663	5,118
Other accrued liabilities	1,459	1,138
Total accounts payable and other accrued liabilities	$ 14,767	$ 8,334

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	1 Months Ended	3 Months Ended		9 Months Ended		81 Months Ended
	Jul. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)Milestone	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue	$ 11,000,000	$ 11,000,000	$ 0	$ 11,000,000	$ 0	$ 86,500,000
Eisai | Substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of regulatory milestone | Milestone				16
Additional milestone payments on achievement		165,000,000		$ 165,000,000		165,000,000
Eisai | Second or Third Product Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of regulatory milestone | Milestone				8
Additional milestone payments on achievement		105,000,000		$ 105,000,000		105,000,000
Ildong
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		0		0		0
CY Biotech
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		0		0		0
Abic Marketing
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		0		0		0
Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		41,500,000		$ 41,500,000		41,500,000
Axovant | Substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of regulatory milestone | Milestone				4
Number of development milestone | Milestone				2
Axovant | Substantive | Development Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		4,000,000		$ 4,000,000		4,000,000
Axovant | Substantive | Regulatory Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		37,500,000		37,500,000		37,500,000
Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		251,000,000		$ 251,000,000		251,000,000
Boehringer Ingelheim | Substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of development milestone | Milestone				3
Boehringer Ingelheim | Substantive | Development Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		7,000,000		$ 7,000,000		7,000,000
Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of regulatory milestone | Milestone				9
Number of development milestone | Milestone				3
Number of commercial milestone | Milestone				4
Boehringer Ingelheim | Non-substantive | Development Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		30,000,000		$ 30,000,000		30,000,000
Boehringer Ingelheim | Non-substantive | Regulatory Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		84,000,000		84,000,000		84,000,000
Boehringer Ingelheim | Non-substantive | Commercial Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement		$ 130,000,000		$ 130,000,000		$ 130,000,000
US Food and Drug Administration New Drug Application Approval | Eisai | Substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue	10,000,000
Federal Commission for the Protection Against Sanitary Risk Mexico Approval | Eisai | Substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue	$ 1,000,000

Collaborations - Revenues Recog

Collaborations - Revenues Recognized under Eisai Agreement (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		81 Months Ended
	Jul. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales		$ 3,323	$ 4,884	$ 11,104	$ 15,787
Total revenues		19,242	9,138	38,601	30,575
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales		1,868	3,274	7,179	11,603
Milestone payments	$ 11,000	11,000	0	11,000	0	$ 86,500
Amortization of upfront payments		1,886	1,886	5,656	5,656
Subtotal other Eisai collaborative revenue		12,954	2,065	18,155	7,414
Total revenues		14,822	5,339	25,334	19,017
Eisai | Development expenses
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses		11	107	1,242	1,454
Eisai | Patent and trademark expenses
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses		$ 57	$ 72	$ 257	$ 304

Collaborations - Deferred Reven

Collaborations - Deferred Revenues under Eisai Agreement (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]
Less current portion	$ (22,522)	$ (21,425)
Deferred revenues, less current portion	82,132	87,617
Eisai
Deferred Revenue Arrangement [Line Items]
Deferred revenues attributable to Eisai	88,609	97,687
Less current portion	(14,872)	(18,295)
Deferred revenues, less current portion	73,737	79,392
Eisai | Upfront payments
Deferred Revenue Arrangement [Line Items]
Deferred revenues attributable to Eisai	81,278	86,933
Eisai | Net product sales
Deferred Revenue Arrangement [Line Items]
Deferred revenues attributable to Eisai	$ 7,331	$ 10,754

Share-based Activity - Share Ba

Share-based Activity - Share Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 2,181		$ 3,848	$ 9,261	$ 11,795
Total share-based compensation expense capitalized into inventory	64		41	149	146
Cost of product sales
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	0		0	20	0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,200		2,097	4,940	6,338
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	981		1,751	3,269	5,457
Restructuring charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 0	$ 1,000	$ 0	$ 1,032	$ 0

Share-based Activity - Summary

Share-based Activity - Summary of Stock Option Activity (Detail) - Stock options shares in Thousands	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Number of stock options
Outstanding at January 1, 2016 | shares	16,407
Granted | shares	16,502
Exercised | shares	(63)
Forfeited/cancelled/expired | shares	(7,281)
Outstanding at September 30, 2016 | shares	25,565
Weighted- Average Exercise Price
Outstanding at January 1, 2016 | $ / shares	$ 5.01
Granted | $ / shares	1.61
Exercised | $ / shares	1.57
Forfeited/cancelled/expired | $ / shares	3.70
Outstanding at September 30, 2016 | $ / shares	$ 3.20

Share-based Activity - Summar38

Share-based Activity - Summary of Restricted Stock Unit Awards Activity (Detail) - Restricted Stock Units (RSU) shares in Thousands	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Number of Restricted Stock Units
Unvested at January 1, 2016	273
Granted	0
Vested	(193)
Forfeited/cancelled	(9)
Unvested at September 30, 2016	71
Weighted- Average Grant-Date Fair Value
Unvested at January 1, 2016 | $ / shares	$ 4.67
Vested | $ / shares	451
Forfeited/cancelled | $ / shares	4.31
Unvested at September 30, 2016 | $ / shares	$ 5.16

Share-based Activity - Addition

Share-based Activity - Additional Information (Detail) - Performance restricted stock units	1 Months Ended
	Mar. 31, 2013shares
Stockholders Equity [Line Items]
Performance period	3 years
Units granted (shares)	780,000

Concentrations of Credit Risk40

Concentrations of Credit Risk and Major Customers - Percentages of Total Revenues (Detail) - Sales Revenue, Net	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Concentration Risk [Line Items]
Percentage of revenues	100.00%	100.00%	100.00%	100.00%
Eisai Agreement
Concentration Risk [Line Items]
Percentage of revenues	77.00%	58.40%	65.60%	62.20%
Boehringer Ingelheim Agreement
Concentration Risk [Line Items]
Percentage of revenues	5.60%	0.00%	10.00%	0.00%
Ildong Agreement
Concentration Risk [Line Items]
Percentage of revenues	8.20%	18.60%	11.00%	24.40%
Toll manufacturing agreements
Concentration Risk [Line Items]
Percentage of revenues	6.40%	16.00%	8.50%	10.50%
Axovant Agreement
Concentration Risk [Line Items]
Percentage of revenues	2.40%	6.10%	4.30%	2.20%
Other collaborative agreements
Concentration Risk [Line Items]
Percentage of revenues	0.40%	0.90%	0.60%	0.70%

Net Loss Per Share - Potentiall

Net Loss Per Share - Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss Attributable to Stockholders of Arena Per Share (Detail) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	28,734	18,152	25,033	17,635
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	28,559	17,462	24,791	17,033
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 attributable to stockholders of Arena per share (shares)	175	690	242	576
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	0	0	0	26

Restructuring Charges (Details)

Restructuring Charges (Details) $ in Thousands	Sep. 30, 2016USD ($)	Sep. 30, 2016USD ($)employee	Jun. 30, 2016USD ($)employee	Dec. 31, 2015USD ($)employee	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charge		$ 231		$ 4,000	$ 0	$ 6,346	$ 0
Non-cash share-based compensation expense		2,181			3,848	9,261	11,795
Accrued restructuring charges	$ 1,244	1,244		$ 1,793		1,244
Restructuring charges
Restructuring Cost and Reserve [Line Items]
Non-cash share-based compensation expense		0	$ 1,000		$ 0	1,032	$ 0
UNITED STATES
Restructuring Cost and Reserve [Line Items]
Positions to be eliminated			73.00%	35.00%
Number of positions to be eliminated | employee			100	80
Restructuring charge			$ 6,100
Restructuring payments	4,000
Accrued restructuring charges	1,100	$ 1,100				1,100
SWITZERLAND
Restructuring Cost and Reserve [Line Items]
Positions to be eliminated		23.00%		17.00%
Number of positions to be eliminated | employee		15		14
Restructuring charge		$ 200
Restructuring payments	100
Accrued restructuring charges	$ 100	$ 100				$ 100

Management Changes (Details)

Management Changes (Details) - USD ($) $ in Thousands	May 31, 2016	Sep. 30, 2016	Aug. 31, 2016	Jul. 31, 2016	Jun. 30, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Management Changes [Line Items]
Share-based compensation								$ 9,261	$ 11,795
Chief Medical Officer
Management Changes [Line Items]
Cash severance payment					$ 500
Duration					12 months
Expense							$ 1,000
Share-based compensation								400
Benefits		$ 600				$ 600		600
Service agreement term				1 year
Service charge						100
Chief Scientific Officer
Management Changes [Line Items]
Cash severance payment		$ 900
Duration		18 months
Expense						2,600
Share-based compensation						1,600		1,600
Benefits		$ 900				$ 900		$ 900
Consulting agreement term		5 years
2013 Long-Term Incentive Plan as Amended | Chief Executive Officer
Management Changes [Line Items]
Options granted (shares)	3,800,000
Additional shares authorized (shares)	3,800,000
Term	7 years
Vesting period	4 years
Vesting percentage	25.00%
2013 Long-Term Incentive Plan as Amended | Chief Financial Officer
Management Changes [Line Items]
Options granted (shares)					800,000
Additional shares authorized (shares)	3,800,000				800,000
Term					7 years
Vesting period					4 years
Vesting percentage					25.00%
2013 Long-Term Incentive Plan as Amended | Chief Business Officer
Management Changes [Line Items]
Options granted (shares)			800,000
Additional shares authorized (shares)	4,600,000		800,000		800,000
Term			7 years
Vesting period			4 years
Vesting percentage			25.00%
2013 Long-Term Incentive Plan as Amended | Cliff Vesting After One Year | Chief Executive Officer
Management Changes [Line Items]
Vesting period	1 year
2013 Long-Term Incentive Plan as Amended | Cliff Vesting After One Year | Chief Financial Officer
Management Changes [Line Items]
Vesting period					1 year
2013 Long-Term Incentive Plan as Amended | Cliff Vesting After One Year | Chief Business Officer
Management Changes [Line Items]
Vesting period			1 year
2013 Long-Term Incentive Plan as Amended | Vesting Years Two Through Four | Chief Executive Officer
Management Changes [Line Items]
Vesting period	3 years
2013 Long-Term Incentive Plan as Amended | Vesting Years Two Through Four | Chief Financial Officer
Management Changes [Line Items]
Vesting period					3 years
2013 Long-Term Incentive Plan as Amended | Vesting Years Two Through Four | Chief Business Officer
Management Changes [Line Items]
Vesting period			3 years

Beacon Discovery, Inc. - Additi

Beacon Discovery, Inc. - Additional Information (Detail) - Beacon Discovery, Inc. - Variable Interest Entity, Primary Beneficiary	1 Months Ended	9 Months Ended
	Sep. 30, 2016USD ($)ft²	Sep. 30, 2016USD ($)ft²
Variable Interest Entity [Line Items]
Promissory note interest rate	7.00%	7.00%
Promissory note repayment due date		Aug. 31, 2022
Sale of equity, issuance of debt or third party license revenue threshold amount for repayment of promissory note | $	$ 10,000,000	$ 10,000,000
Sublease Agreement
Variable Interest Entity [Line Items]
Area of space subleased | ft²	15,000	15,000
Period of sublease	5 years

Beacon Discovery, Inc. - Reconc

Beacon Discovery, Inc. - Reconciliation of Equity Attributable to Stockholders of Arena and Equity Attributable to Beacon (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reconciliation Of Stockholders Equity [Line Items]
Balance at December 31, 2015			$ 53,542
Net loss	$ (12,479)	$ (26,418)	(61,210)	$ (77,520)
Translation gain	694	$ (2,881)	2,046	$ (2,499)
Contribution of equipment and other assets from Arena to Beacon			0
Other			9,699
Balance at September 30, 2016	4,077		4,077
Equity Attributable to Stockholders of Arena
Reconciliation Of Stockholders Equity [Line Items]
Balance at December 31, 2015			53,542
Net loss			(61,088)
Translation gain			2,046
Contribution of equipment and other assets from Arena to Beacon			(871)
Other			9,699
Balance at September 30, 2016	3,328		3,328
Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity | Variable Interest Entity, Primary Beneficiary
Reconciliation Of Stockholders Equity [Line Items]
Balance at December 31, 2015			0
Net loss			(122)
Translation gain			0
Contribution of equipment and other assets from Arena to Beacon			871
Other			0
Balance at September 30, 2016	$ 749		$ 749

Beacon Discovery, Inc. - Summar

Beacon Discovery, Inc. - Summary of Assets and Liabilities Included in Condensed Consolidated Balance Sheet (Detail) - Variable Interest Entity, Primary Beneficiary - Beacon Discovery, Inc. $ in Thousands	Sep. 30, 2016USD ($)
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	$ 1,123
Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena	8
Cash and Cash Equivalents
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	288
Prepaid Expense and Other Current Assets
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	42
Land, Property and Equipment, Net
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	793
Accounts Payable and Other Accrued Liabilities
Variable Interest Entity [Line Items]
Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena	$ 8