Arena Pharmaceuticals (ARNA) 10-K 15 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Mar. 10, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Entity Registrant Name	ARENA PHARMACEUTICALS INC
Entity Central Index Key	1,080,709
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Amendment Flag	false
Trading Symbol	ARNA
Entity Common Stock, Shares Outstanding		245,469,142
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 416

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 90,712	$ 156,184
Accounts receivable	20,162	4,934
Inventory	6,708	9,502
Prepaid expenses and other current assets	2,307	4,218
Total current assets	119,889	174,838
Land, property and equipment, net	43,828	71,828
Intangibles, net	2,357	7,775
Other non-current assets	2,936	2,351
Total assets	169,010	256,792
Current liabilities:
Accounts payable and other accrued liabilities	12,116	10,127
Payable to Eisai	9,074	12,080
Accrued clinical and preclinical study fees	3,883	3,286
Current portion of deferred revenues	35,288	21,425
Current portion of lease financing obligations	3,518	2,978
Total current liabilities	63,879	49,896
Other long-term liabilities	821	470
Deferred revenues, less current portion	2,167	87,617
Lease financing obligations, less current portion	61,748	65,267
Commitments and contingencies
Equity:
Preferred stock, $0.0001 par value, 7,500,000 shares authorized, no shares issued and outstanding at December 31, 2016, and 2015
Common stock, $0.0001 par value, 367,500,000 shares authorized at December 31, 2016, and 2015; 243,400,800 shares issued and outstanding at December 31, 2016; 242,871,179 shares issued and outstanding at December 31, 2015	24	24
Additional paid-in capital	1,441,715	1,430,917
Accumulated other comprehensive loss	(3,099)	(1,179)
Accumulated deficit	(1,398,736)	(1,376,220)
Total equity attributable to stockholders of Arena	39,904	53,542
Equity attributable to noncontrolling interest in consolidated variable interest entity	491
Total equity	40,395	53,542
Total liabilities and equity	$ 169,010	$ 256,792

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	7,500,000	7,500,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	367,500,000	367,500,000
Common stock, shares issued	243,400,800	242,871,179
Common stock, shares outstanding	243,400,800	242,871,179

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Net product sales	$ 26,349	$ 19,726	$ 15,983
Toll manufacturing	4,129	4,250	1,497
Total revenues	123,975	38,326	36,970
Operating costs and expenses
Cost of product sales	9,297	8,590	6,369
Cost of toll manufacturing	6,044	4,585	1,390
Research and development	66,425	88,411	100,347
General and administrative	31,243	35,966	34,137
Restructuring charges	6,346	3,972
Impairment of long-lived assets	21,766
Total operating costs and expenses	141,121	141,524	142,243
Loss from operations	(17,146)	(103,198)	(105,273)
Interest and other income (expense)
Interest income	290	158	83
Interest expense	(6,512)	(6,828)	(6,915)
Gain from valuation of derivative liabilities		474	4,418
Gain on sale of available-for-sale securities			49,553
Other income (expense)	472	1,415	(2,374)
Total interest and other income (expense), net	(5,750)	(4,781)	44,765
Net loss	(22,896)	(107,979)	(60,508)
Less net loss attributable to noncontrolling interest in consolidated variable interest entity	380
Net loss attributable to stockholders of Arena	$ (22,516)	$ (107,979)	$ (60,508)
Net loss attributable to stockholders of Arena per share:
Basic	$ (0.09)	$ (0.45)	$ (0.28)
Diluted	$ (0.09)	$ (0.45)	$ (0.28)
Shares used in calculating net loss attributable to stockholders of Arena per share:
Basic	243,133	240,671	219,734
Diluted	243,133	240,671	219,734
Comprehensive Loss:
Net loss	$ (22,896)	$ (107,979)	$ (60,508)
Foreign currency translation adjustment	(1,920)	(4,087)	(2,820)
Comprehensive loss	(24,816)	(112,066)	(63,328)
Less comprehensive loss attributable to noncontrolling interest in consolidated variable interest entity	380
Comprehensive loss attributable to stockholders of Arena	(24,436)	(112,066)	(63,328)
Eisai
Revenues
Net product sales	19,196	14,236	15,983
Other collaboration revenue	79,701	9,505	18,611
Total revenues	98,897	23,741	34,594
Operating costs and expenses
Impairment of long-lived assets	32,900
Other
Revenues
Other collaboration revenue	$ 13,796	$ 4,845	$ 879

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total Equity Attributable to Stockholders of Arena	Equity Attributable to Noncontrolling Interest in Consolidated Variable Interest Entity
Beginning Balance (in shares) at Dec. 31, 2013		218,816,242
Beginning Balance at Dec. 31, 2013	$ 91,857	$ 22	$ 1,293,840	$ 5,728	$ (1,207,733)	$ 91,857
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		1,115,068
Issuance of common stock upon exercise of options	$ 4,078		4,078			4,078
Issuance of common stock under employee stock purchase plan (in shares)	304,085	304,085
Issuance of common stock under employee stock purchase plan	$ 1,148		1,148			1,148
Issuance of common stock upon vesting of restricted stock unit awards (in shares)		86,250
Share-based compensation expense | Expensed	13,509		13,509			13,509
Share-based compensation expense | Capitalized Cost	81		81			81
Translation loss	(2,820)			(2,820)		(2,820)
Net loss	(60,508)				(60,508)	(60,508)
Ending Balance (in shares) at Dec. 31, 2014		220,321,645
Ending Balance at Dec. 31, 2014	47,345	$ 22	1,312,656	2,908	(1,268,241)	47,345
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock to underwriters (in shares)		21,000,000
Issuance of common stock to underwriters	100,658	$ 2	100,656			100,658
Issuance of common stock upon exercise of options (in shares)		1,154,084
Issuance of common stock upon exercise of options	$ 2,211		2,211			2,211
Issuance of common stock under employee stock purchase plan (in shares)	327,950	327,950
Issuance of common stock under employee stock purchase plan	$ 758		758			758
Issuance of common stock upon vesting of restricted stock unit awards (in shares)		67,500
Share-based compensation expense | Expensed	14,463		14,463			14,463
Share-based compensation expense | Capitalized Cost	173		173			173
Translation loss	(4,087)			(4,087)		(4,087)
Net loss	$ (107,979)				(107,979)	(107,979)
Ending Balance (in shares) at Dec. 31, 2015	242,871,179	242,871,179
Ending Balance at Dec. 31, 2015	$ 53,542	$ 24	1,430,917	(1,179)	(1,376,220)	53,542
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		115,564
Issuance of common stock upon exercise of options	$ 179		179			179
Issuance of common stock under employee stock purchase plan (in shares)	141,397	141,397
Issuance of common stock under employee stock purchase plan	$ 203		203			203
Issuance of common stock upon vesting of restricted stock unit awards (in shares)		272,660
Share-based compensation expense | Expensed	11,117		11,117			11,117
Share-based compensation expense | Capitalized Cost	170		170			170
Contribution to variable interest entity			(871)			(871)	$ 871
Translation loss	(1,920)			(1,920)		(1,920)
Net loss	$ (22,896)				(22,516)	(22,516)	(380)
Ending Balance (in shares) at Dec. 31, 2016	243,400,800	243,400,800
Ending Balance at Dec. 31, 2016	$ 40,395	$ 24	$ 1,441,715	$ (3,099)	$ (1,398,736)	$ 39,904	$ 491

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities:
Net loss	$ (22,896)	$ (107,979)	$ (60,508)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	9,144	9,804	8,655
Amortization of intangibles	211	238	506
Impairment of long-lived assets	21,766
Share-based compensation	11,117	14,463	13,509
Gain from valuation of derivative liabilities		(474)	(4,418)
Gain on sale of available-for-sale securities			(49,553)
Amortization of prepaid financing costs	136	136	136
Loss on disposal or sale of equipment	1,270	1,007	172
Changes in operating assets and liabilities:
Accounts receivable	(15,903)	(1,425)	6,407
Inventory	3,193	1,858	870
Prepaid expenses and other assets	1,962	575	(772)
Payables and accrued liabilities	(103)	(16,970)	13,240
Deferred revenues	(72,066)	553	(29,764)
Deferred rent	30	101	122
Net cash used in operating activities	(62,139)	(98,113)	(101,398)
Investing activities:
Proceeds from sale of available-for-sale securities			49,553
Purchases of land, property and equipment	(950)	(10,992)	(8,905)
Proceeds from sale of equipment	954	2,232	47
Other non-current assets	(754)	609	209
Net cash provided by (used in) investing activities	(750)	(8,151)	40,904
Financing activities:
Principal payments on lease financing obligations	(2,979)	(2,492)	(2,057)
Proceeds from issuance of common stock	370	103,628	5,225
Other financing activities	320
Net cash provided by (used in) financing activities	(2,289)	101,136	3,168
Effect of exchange rate changes on cash	(294)	(1,897)	(1,343)
Net decrease in cash and cash equivalents	(65,472)	(7,025)	(58,669)
Cash and cash equivalents at beginning of year	156,184	163,209	221,878
Cash and cash equivalents at end of year	90,712	156,184	163,209
Supplemental disclosure of cash flow information:
Interest paid	$ 6,303	$ 6,562	6,778
Siegfried
Supplemental disclosure of non-cash investing and financing information:
Payable to Siegfried for acquisition of land and building			$ 8,217

The Company and Summary of Sign

The Company and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
The Company and Summary of Significant Accounting Policies	1. The Company and Summary of Significant Accounting Policies The Company Arena Pharmaceuticals, Inc., or Arena, was incorporated on April 14, 1997, and commenced operations in July 1997. We are a biopharmaceutical company focused on developing novel, small molecule drugs with optimized receptor pharmacology designed to deliver broad clinical utility across multiple therapeutic areas. Our proprietary pipeline includes potentially first or best in class programs for which we own global commercial rights. Our three most advanced investigational clinical programs are etrasimod (formerly APD334) in Phase 2 evaluation for multiple inflammatory indications, ralinepag (formerly APD811) in Phase 2 evaluation for pulmonary arterial hypertension (PAH), and APD371 entering Phase 2 evaluation for the treatment of pain associated with Crohn's disease. Additionally, we have collaborations with the following pharmaceutical companies: Eisai Inc. and Eisai Co., Ltd. (collectively, Eisai) (commercial stage), Axovant Sciences Ltd., or Axovant, (Phase 2 candidate), and Boehringer Ingelheim International GmbH, or Boehringer Ingelheim, (preclinical candidate). We operate in one business segment. Our US operations are located in San Diego, California. Our primary clinical operations are located in Zug, Switzerland, and our commercial manufacturing facility is located in Zofingen, Switzerland. We internally discovered the drug lorcaserin, which has been commercially sold in a twice-daily formulation under the brand name BELVIQ® in the United States since June 2013 and in South Korea since February 2015. The commercial launch of lorcaserin in a once-daily formulation under the brand name BELVIQ XR® in the United States was announced in October 2016. On December 28, 2016, we amended and restated the terms of marketing and supply agreement for lorcaserin with Eisai by entering into a new Transaction Agreement and a new Supply Agreement (collectively with the Transaction Agreement, the Eisai Agreement) with Eisai. Under the Eisai Agreement, Eisai acquired 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. Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with the U.S. generally accepted accounting principles, or GAAP, and reflect all of our activities, including those of our wholly owned subsidiaries. All material intercompany accounts and transactions have been eliminated in consolidation. The accompanying consolidated financial statements include the balances and activity of Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 16). 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 consolidated balance sheets. 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 consolidated statements of operations and comprehensive loss. Liquidity It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several 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 and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions. Our prospects are subject to the risks and uncertainties frequently encountered by companies in the early stages of development and commercialization, especially those companies in rapidly evolving and technologically advanced industries such as the biotechnology field. Our future viability approvals novel, small molecule drugs We incurred net losses of $22.9 million, $108.0 million and $60.5 million for the years ended December 31, 2016, 2015, and 2014, respectively. Additionally, we have used net cash of $62.1 million, $98.1 million and $101.4 million to fund our operating activities for years ended December 31, 2016, 2015, and 2014, respectively. We believe our plans to raise additional cash from outside sources and, if necessary, our cost containment efforts are sufficient to allow us to continue operations for the next twelve months. Our plans include pursuing additional cash through strategic corporate partnerships and possibly engaging in future sales of equity or debt. There is no guarantee that adequate funds will be available when needed from equity financing or additional debt, development and commercialization partnerships, increased results of operations, or from other sources, or on terms acceptable to us. If our efforts to obtain sufficient additional funds are not successful, we would be required to delay, scale back, or eliminate some or all of our research or development, manufacturing operations, administrative operations, and clinical or regulatory activities, which could negatively affect our ability to achieve certain corporate goals. Recent Accounting Pronouncements Revenue recognition. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers The new guidance 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 the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018, and not restate prior periods. The Company has continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard. Other. 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. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases modified In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash Use of Estimates The preparation of financial statements in conformity 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. Cash and Cash Equivalents Cash and cash equivalents consist of cash and highly liquid investments with remaining maturities of three months or less when purchased. Inventory Inventory is stated at the lower of cost or market. We determine cost, which includes amounts related to materials, labor and overhead, using a first-in, first-out basis. We evaluate our inventory each period to identify potential obsolete, excess or otherwise non-saleable items. If non-saleable items are observed and there are no alternate uses for the inventory, we will record a write-down to net realizable value in the period that the decline in value is first recognized. Concentrations of Risk 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 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. BELVIQ has been exclusively sold in the United States and South Korea by Eisai and Ildong, respectively, which are the only jurisdictions for which BELVIQ has been commercially sold. 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: Years ended December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 79.8 % 61.9 % 93.6 % Ildong Agreement (see Note 9) 9.2 % 23.2 % 1.0 % Toll manufacturing agreements 3.3 % 11.1 % 4.0 % Other collaboration agreements 7.7 % 3.8 % 1.4 % Total percentage of revenues 100.0 % 100.0 % 100.0 % Percentages of our total accounts receivable are as follows: December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 93.1 % 77.5 % 93.1 % Toll manufacturing agreements 2.1 % 9.6 % 0.0 % Ildong Agreement (see Note 9) 2.0 % 1.3 % 0.4 % Other collaboration agreements 2.8 % 11.6 % 6.5 % Total percentage of accounts receivable 100.0 % 100.0 % 100.0 % We purchase raw materials, starting materials, intermediates, API, excipients and other materials from commercial sources. To decrease the risk of an interruption to our supply, when we believe it is reasonable for us to do so, we source these materials from multiple suppliers so that, in general, the loss of any one source of supply would not have a material adverse effect on commercial production. However, currently we have only one or a limited number of suppliers for some of these materials. The loss of a primary source of supply would potentially delay our production. Our facility in Zofingen, Switzerland is currently the only manufacturer of finished drug product for BELVIQ. Eisai maintains a safety stock of BELVIQ to help mitigate risks related to having only one manufacturer of finished drug product. Property and Equipment Property and equipment are stated at cost and depreciated over the estimated useful lives of the assets (generally 3 to 15 years) using the straight-line method. Buildings are stated at cost and depreciated over an estimated useful life of approximately 20 years using the straight-line method. Leasehold improvements are stated at cost and amortized over the shorter of the estimated useful lives of the assets or the lease term using the straight-line method. Capital improvements are stated at cost and amortized over the estimated useful lives of the underlying assets using the straight-line method. On December 31, 2016, for the property and equipment located at our Zofingen, Switzerland facility, we recorded an impairment charge of $17.2 million, changed our estimate of the useful life for these assets to be two remaining years and expect to depreciate the remaining carrying value of these assets using the straight-line method over this period pursuant to the Eisai Agreement (see Note 9). Intangibles Intangible assets consist of our manufacturing facility production licenses we acquired from Siegfried in January 2008. Through December 2016, we amortized these assets using the straight-line method over their estimated useful life of 20 years. On December 31, 2016, we recorded an impairment charge of $4.6 million for these assets, changed our estimate of the useful life for these assets to be two remaining years and expect to amortize the remaining carrying value of these assets using the straight-line method over this period pursuant to the Eisai Agreement (see Note 9). Long-lived Assets If indicators of impairment exist, we assess the recoverability of the affected long-lived assets by determining whether the carrying value of such assets can be recovered through undiscounted cash flows. If impairment is indicated, we measure the impairment loss by comparing the fair value to the carrying value of the asset. Deferred Rent For financial reporting purposes, rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under lease agreements is recorded as deferred rent in the liability section of our consolidated balance sheets. Derivative Liabilities We account for warrants and other derivative financial instruments as either equity or liabilities based upon the characteristics and provisions of each instrument. Warrants classified as equity are recorded as additional paid-in capital on our consolidated balance sheets and no further adjustments to their valuation are made. Warrants classified as derivative liabilities and other derivative financial instruments that require separate accounting as liabilities are recorded on our consolidated balance sheets at their fair value on the date of issuance and are revalued on each balance sheet date until such instruments are exercised or expire, with changes in the fair value between reporting periods recorded as other income or expense. There were no warrants classified as derivative liabilities as of December 31, 2016, and 2015. Foreign Currency The functional currency of our wholly owned subsidiaries in Switzerland, Arena GmbH and Arena Pharmaceuticals Development GmbH is the Swiss franc. Accordingly, all assets and liabilities of these subsidiaries are translated to US dollars based on the applicable exchange rate on the balance sheet date. Revenue and expense components are translated to US dollars at weighted-average exchange rates in effect during the period. Gains and losses resulting from foreign currency translation are reported as a separate component of accumulated other comprehensive income or loss in the equity section of our consolidated balance sheets. Foreign currency transaction gains and losses, which are primarily the result of remeasuring US dollar-denominated receivables and payables at Arena GmbH, are recorded in the interest and other income (expense) section of our consolidated statements of operations and comprehensive loss. For the year ended December 31, 2016, we recognized foreign currency transaction gains, net of $0.9 million. For the year ended December 31, 2015, we recognized foreign currency transaction gains, net of $2.0 million. For the year ended December 31, 2014, we recognized foreign currency transaction losses, net of $2.2 million. Share-based Compensation Our share-based awards are measured at fair value and recognized over the requisite service or performance period. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option pricing model, based on the market price of the underlying common stock, expected life, expected stock price volatility and expected risk-free interest rate. Expected volatility is computed using a combination of historical volatility for a period equal to the expected term and implied volatilities from traded options to buy our common stock, with historical volatility being weighted at 75%. The expected life of options is determined based on historical experience of similar awards, giving consideration to the contractual terms of the share-based awards, vesting schedules and post-vesting terminations. The risk-free interest rates are based on the US Treasury yield curve, with a remaining term approximately equal to the expected term used in the option pricing model. The fair value of each restricted stock unit award is estimated based on the market price of the underlying common stock on the date of the grant. The fair value of restricted stock unit awards that include market-based performance conditions is estimated on the date of grant using a Monte Carlo simulation model, based on the market price of the underlying common stock, expected performance measurement period, expected stock price volatility and expected risk-free interest rate. We estimate forfeitures at the time of grant and revise our estimate in subsequent periods if actual forfeitures differ from those estimates. Revenue Recognition Our revenues to date have been generated primarily through collaboration agreements and, to a lesser extent, toll manufacturing agreements. Our collaboration agreements may contain multiple elements including commercialization rights, services (joint steering committee and research and development services) and manufactured products. 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. We recognize revenue when (i) persuasive evidence of an arrangement exists, (ii) delivery has occurred and title has passed, (iii) the price is fixed or determinable and (iv) collectability is reasonably assured. Any advance payments we receive in excess of amounts earned are classified as deferred revenues. We previously deferred recognition of product sales and the related costs at the time we sold BELVIQ to our collaborators because we did not have the ability to estimate the amount of product that could have been returned to us and, as such, recognized revenues and the related costs from net product sales when our collaborators shipped BELVIQ to their distributors. In December 2016, primarily pursuant to a change in the terms of the Eisai Agreement (see Note 9), we determined that we now have the ability to reasonably estimate the amount of returns and thus now recognize revenue and the related cost from product sales when we ship BELVIQ to our collaborators. In December 2016, we recognized revenues and the related costs on net product sales which had been previously deferred. We evaluate deliverables in a multiple-element arrangement to determine whether each deliverable represents a separate unit of accounting. A deliverable constitutes a separate unit of accounting when it has standalone value to the customer. If the delivered element does not have standalone value without one of the undelivered elements in the arrangement, we combine such elements and account for them as a single unit of accounting. We allocate the consideration to each unit of accounting at the inception of the arrangement based on the relative selling price. To determine the selling price of a separate deliverable, we use the hierarchy as prescribed in Accounting Standards Codification Topic 605-25 based on vendor-specific objective evidence, or VSOE, third-party evidence, or TPE, or best estimate of selling price, or BESP. VSOE is based on the price charged when the element is sold separately and is the price actually charged for that deliverable. TPE is determined based on third-party evidence for a similar deliverable when sold separately. BESP is the estimated selling price at which we would transact a sale if the elements of collaboration and license arrangements were sold on a stand-alone basis to the buyer. Non-refundable upfront payments received under our collaboration agreements for commercialization rights have been deferred as such rights have not been deemed to have standalone value without the ongoing services required under the agreement. Such amounts are recognized as revenues on a straight-line basis over the period in which we expect to perform the services. In December 2016, we recognized a portion of the previously unrecognized non-refundable upfront payments received from Eisai as revenues in the amount of arrangement consideration allocated to the unit of accounting delivered to Eisai under the Eisai Agreement (see Note 9). Amounts we receive as reimbursement for our research and development expenditures are recognized as revenue as the services are performed. Under the milestone method, we recognize revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is an event (i) that can be achieved in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved and (iii) that would result in additional payments being due us. A milestone payment is considered substantive when the consideration payable to us for each milestone (a) is consistent with our performance necessary to achieve the milestone or the increase in value to the collaboration resulting from our performance, (b) relates solely to our past performance and (c) is reasonable relative to all of the other deliverables and payments under the arrangement. In making this assessment, we consider all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether any portion of the milestone consideration is related to future performance or deliverables. Other contingent-based payments received are recognized when earned. We also manufacture drug products under toll manufacturing agreements. Upon the customer’s acceptance of drug products manufactured by us under these agreements, we recognize toll manufacturing revenues. Research and Development Expenses Research and development expenses, which consist primarily of salaries and other personnel costs, clinical trial costs and preclinical study fees, manufacturing costs for non-commercial products, and the development of earlier-stage programs and technologies, are expensed as incurred when these expenditures have no alternative future uses. We accrue clinical trial expenses based on work performed. In determining the amount to accrue, we rely on estimates of total costs incurred based on enrollment, the completion of trials and other events. We follow this method because we believe reasonably dependable estimates of the costs applicable to various stages of a clinical trial can be made. However, the actual costs and timing of clinical trials are highly uncertain, subject to risks and may change depending on a number of factors. Differences between the actual clinical trial costs and the estimated clinical trial costs that we have accrued in any prior period are recognized in the subsequent period in which the actual costs become known. Historically, these differences have not been material; however, material differences could occur in the future. Payments made to reimburse collaborators for our share of their research and development activities are recorded as research and development expenses, and are recognized as the work is performed. Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and circumstances from non-owner sources. We report components of comprehensive loss in the period in which they are recognized. For the years ended December 31, 2016, 2015, and 2014, comprehensive loss consisted of net loss and foreign currency translation gains and losses. Net Loss Per Share We calculate basic and diluted net loss 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 have excluded from our calculation of diluted net loss per share all potentially dilutive in-the-money (i) stock options, (ii) restricted stock unit awards, or RSUs, (iii) Total Stockholder Return, or TSR, performance restricted stock unit, or PRSU, awards, (iv) unvested restricted stock in our deferred compensation plan and (v) our previously outstanding warrants, and our diluted net loss per share is the same as our basic net loss per share. The table below presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share for the years presented, in thousands. Years ended December 31, 2016 2015 2014 Stock options 24,947 17,030 15,530 Warrants — 19 370 RSUs and unvested restricted stock 210 547 476 Total 25,157 17,596 16,376 Because the market condition for the PRSUs was not satisfied at December 31, 2016, 2015, and 2014, such securities are excluded from the table above. Income Taxes We use the asset and liability method of accounting for income taxes. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Our deferred tax assets and liabilities are determined using the enacted tax rates expected to be in effect for the years in which those tax assets are expected to be realized. The realization of our deferred tax assets is dependent upon our ability to generate sufficient future taxable income. We establish a valuation allowance when it is more-likely-than-not the future realization of all or some of the deferred tax assets will not be achieved. The evaluation of the need for a valuation allowance is performed on a jurisdiction-by-jurisdiction basis, and includes a review of all available evidence, both positive and negative. The impact of an uncertain income tax position is recognized at the largest amount that is more-likely-than-not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained.

Fair Value Disclosures

Fair Value Disclosures	12 Months Ended
	Dec. 31, 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 December 31, 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 $ 46,371 $ 46,371 $ — $ — 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 $ — $ — (1) Included in cash and cash equivalents on our consolidated balance sheets.

Short-term Investments and Avai

Short-term Investments and Available-for-Sale Securities	12 Months Ended
	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Short-term Investments and Available-for-Sale Securities	3. Short-term Investments and Available-for-Sale Securities We held an investment in TaiGen Biotechnology Co., Ltd., or TaiGen, that, from December 31, 2011, to January 17, 2014, had a cost basis of zero due to prior impairment charges. On January 17, 2014, TaiGen completed an initial public offering and its common stock began to trade on the GreTai Securities Listed Market, under the name “TaiGen Biopharmaceuticals Holding Limited.” Such market is deemed to be comparable to a US over-the-counter market such that the fair value of our former investment in TaiGen, which previously had been accounted for as a cost method investment with a cost basis of zero, became readily determinable. Accordingly, on January 17, 2014, we recorded our former investment in TaiGen of 29.6 million shares based on its fair value of approximately $49.1 million. We began recording our former investment in TaiGen at fair value based on the trading price of TaiGen’s common stock, and the remaining former investment was revalued on each balance sheet date. Gains and losses on the sale of available-for-sale securities are determined using the specific-identification method. During the year ended December 31, 2014, we sold all of our shares of TaiGen and recorded a realized gain of $49.6 million.

Balance Sheet Details

Balance Sheet Details	12 Months Ended
	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Details	4. Balance Sheet Details Inventory consisted of the following, in thousands: December 31, 2016 2015 Raw materials $ 2,553 $ 2,487 Work in process 3,943 2,781 Finished goods at Arena GmbH 212 165 Finished goods at Eisai — 3,309 Finished goods at Ildong — 760 Total inventory $ 6,708 $ 9,502 The carrying value of finished goods at Eisai and Ildong at December 31, 2015, represented inventory sold to Eisai and Ildong, respectively, which had not yet been sold through to their distributors at December 31, 2015. We previously deferred recognition of revenue and the related costs at the time we sold BELVIQ to our collaborators because we did not have the ability to estimate the amount of product that could have been returned to us and, as such, recognized revenues and the related costs from net product sales when our collaborators shipped BELVIQ to their distributors. In December 2016, we determined that we now have the ability to reasonably estimate the amount of returns and thus now recognize revenue and the related cost from product sales when we ship BELVIQ to our collaborators. In December 2016, we recognized revenues and the related costs on net product sales which had been previously deferred. Land, property and equipment, net consisted of the following, in thousands: December 31, 2016 2015 Land $ 7,809 $ 8,131 Building and capital improvements 58,609 74,663 Leasehold improvements 17,769 18,025 Machinery and equipment 16,801 53,790 Computers and software 5,737 15,893 Furniture and office equipment 1,631 2,227 108,356 172,729 Less accumulated depreciation and amortization (64,528 ) (100,901 ) Land, property and equipment, net $ 43,828 $ 71,828 Intangibles consisted of the following, in thousands: December 31, 2016 2015 Acquired manufacturing production licenses – gross $ 2,357 $ 12,958 Acquired manufacturing production licenses – accumulated amortization — (5,183 ) Intangibles, net $ 2,357 $ 7,775 The Eisai Agreement entered on December 28, 2016, results in a significant change in our expected use of our Zofingen facility. We have agreed to manufacture and supply all of Eisai’s requirements (or specified minimum quantities if such quantities are greater than Eisai’s requirements), subject to certain exceptions, for BELVIQ for an initial two-year period. Eisai may extend this initial period for an additional six months upon payment of an exercise fee. Eisai will pay us agreed-upon prices to deliver BELVIQ during this period. Based on our estimate of future cash flows that are directly associated with our Zofingen facility, we determined that long-lived assets with a carrying amount of $32.9 million were no longer recoverable and were in fact impaired and wrote them down to their estimated fair value of $11.1 million. Fair value was based on an estimate of the net proceeds we would receive upon disposition of the asset group to a market participant. This estimate is a Level 3 input under Accounting Standards Codification Topic 820, Fair Value Measurement Following the impairment write-down, the carrying value of long-lived assets located in the United States and Switzerland were $35.1 million and $11.1 million, respectively, at December 31, 2016. The carrying value of long-lived assets located in the United States and Switzerland were $41.5 million and $38.1 million, respectively, at December 31, 2015. We capitalize into inventory amortization expense related to the manufacturing of BELVIQ. Such amortization will subsequently be recognized as cost of product sales when the related inventory is sold. Using the exchange rate in effect on December 31, 2016, we expect to record amortization of $1.2 million per year through 2018 for our manufacturing facility production licenses. Accounts payable and other accrued liabilities consisted of the following, in thousands: December 31, 2016 2015 Accounts payable $ 5,977 $ 2,078 Accrued compensation 4,820 5,118 Accrued workforce reduction expense 62 1,793 Other accrued liabilities 1,257 1,138 Total accounts payable and other accrued liabilities $ 12,116 $ 10,127

Agreements with Siegfried

Agreements with Siegfried	12 Months Ended
	Dec. 31, 2016
Contractors [Abstract]
Agreements with Siegfried	5. Agreements with Siegfried In January 2008, we acquired from Siegfried certain drug product facility assets, including manufacturing facility production licenses, fixtures, equipment, other personal property and real estate assets in Zofingen, Switzerland, under an asset purchase agreement. These assets are being used to manufacture and package lorcaserin as well as certain drug products for Siegfried. From time to time, we may also use this facility to manufacture and package tablets and capsules for other of our programs or for other entities. In connection with this transaction, we also entered into a long-term supply agreement for the active pharmaceutical ingredient of lorcaserin, a toll manufacturing agreement and a technical services agreement with Siegfried. For the years ended December 31, 2016, 2015, and 2014, we recognized expenses of $1.4 million, $1.3 million, and $2.5 million, respectively, for services incurred under the technical services agreement. The technical services agreement provides us with administrative and other services to operate the facility. The real estate assets we acquired in January 2008 pursuant to the asset purchase agreement consisted of approximately 67,000 square feet of space in a building that consists of approximately 134,000 square feet of space along with an option to purchase the remaining Siegfried-occupied portion of the building along with the underlying land at a price of CHF 15.0 million, plus an inflation adjustment. Siegfried also had the option to sell us such remaining portion of the building with the underlying land at a price of CHF 8.0 million, plus an inflation adjustment. In July 2014, Siegfried provided us notice of its exercise of the option to sell us the remaining Siegfried-occupied portion of the building with the underlying land. In December 2014, we took title of the remaining portion of the building with the underlying land, and in July 2015 we paid the purchase price of CHF 8.2 million to Siegfried. In connection with the exercise of the option, we lease this building space back to Siegfried for an annual base rent amount of CHF 0.4 million. Siegfried has the right to partially or fully terminate this lease with six months’ notice. Siegfried has an annual option to extend the lease for an additional year with the last extension term ending on December 31, 2019. At any time during the extension terms, we have the right to partially or fully terminate this lease with six months’ notice, but with a termination date no earlier than December 31, 2017.

Derivative Liabilities

Derivative Liabilities	12 Months Ended
	Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivative Liabilities	6. Derivative Liabilities In June 2006 and August 2008, we issued seven-year warrants, which we refer to as the Series B Warrants, to purchase 829,856 and 1,106,344 shares of our common stock, respectively, at an exercise price of $15.49 and $7.71 per share, respectively. As a result of the warrants’ anti-dilution provision and certain of our subsequent equity issuances, the number of shares issuable upon exercise of the warrants increased and the exercise price decreased. In August 2015, the August 2008 Series B Warrant, which was recorded as a current derivative liability of $0.5 million on our consolidated balance sheet at December 31, 2014, expired pursuant to its terms. Therefore, we recorded a gain in our consolidated statement of operations and comprehensive loss for the year ended December 31, 2015. The warrants were revalued on each balance sheet date, with changes in the fair value between reporting periods recorded in the interest and other income (expense) section of our consolidated statements of operations and comprehensive loss.

Commitments

Commitments	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments	7. Commitments We occupy four properties in California under sale and leaseback agreements. The terms of these leases stipulate annual increases in monthly rental payments of 2.5%. We accounted for our sale and leaseback transactions using the financing method. Under the financing method, the book value of the properties and related accumulated depreciation remain on our balance sheet and no sale is recognized. The sales price of the properties is recorded as a financing obligation, and a portion of each lease payment is recorded as interest expense. We recorded interest expense of $6.4 million, $6.7 million, and $6.9 million for the years ended December 31, 2016, 2015, and 2014, respectively, related to these leases. We expect interest expense related to our facilities to total $37.5 million from December 31, 2016, through the remaining terms of the leases in fiscal year 2027. At December 31, 2016, the total financing obligation for these facilities was $65.3 million. The aggregate residual value of the facilities at the end of the lease terms is $10.0 million. We lease an additional property in California under an operating lease, which expires in May 2027, and contains a purchase option and stipulates annual increases in monthly rental payments of 2.5%. We further lease commercial space in various facilities in Zofingen, Switzerland that can be terminated with 12-month written notice under an agreement that expires in 2032. We also lease a separate office space in Zofingen under an operating lease which expires in August 2020 and another office space in Zug, Switzerland under and operating lease which expires in September 2020. In accordance with the lease terms for certain of our properties, we are required to maintain deposits for the benefit of the landlord throughout the term of the leases. A total of $0.9 million and $0.8 million were recorded in other non-current assets on our consolidated balance sheets at December 31, 2016, and 2015, respectively, related to such leases. We recognize rent expense on a straight-line basis over the term of each lease. Rent expense of $1.2 million, $1.1 million and $1.1 million was recognized for the years ended December 31, 2016, 2015, and 2014, respectively. At December 31, 2016, the future minimum lease payments under our existing financing and operating lease obligation are as follows, in thousands: Year ending December 31, Financing Obligations Operating Leases 2017 $ 8,712 $ 1,259 2018 9,731 1,353 2019 8,053 1,376 2020 8,254 1,266 2021 8,461 976 Thereafter 49,613 5,723 Total minimum lease payments 92,824 $ 11,953 Less amounts representing interest (37,548 ) Add amounts representing residual value 9,990 Lease financing obligations 65,266 Less current portion (3,518 ) $ 61,748 In May 2016, we entered into an agreement to sublease one of our US properties to a third party, which commenced in August 2016 and expires in May 2027. The terms of the sublease stipulate annual increases in monthly rental payments of 3.19%. We recognize rent income on a straight-line basis over the term of the sublease. Expected minimum rental payments to be received under the sublease are as follows: Year ending December 31, 2017 $ 714 2018 737 2019 760 2020 784 2021 809 Thereafter 4,846 Total $ 8,650

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	8. Stockholders’ Equity Equity Compensation Plans. On June 10, 2013, our stockholders approved our 2013 Long-Term Incentive Plan, or 2013 LTIP. Upon such approval, our 2012 Long-Term Incentive Plan, or 2012 LTIP, was terminated. However, notwithstanding such termination or the previous termination of our 2009 Long-Term Incentive Plan, 2006 Long-Term Incentive Plan, as amended, 2002 Equity Compensation Plan, Amended and Restated 2000 Equity Compensation Plan, and Amended and Restated 1998 Equity Compensation Plan (together with the 2012 LTIP, the “Prior Plans”), all outstanding awards under the Prior Plans will continue to be governed under the terms of the Prior Plans. The number of shares of common stock authorized for issuance under the 2013 LTIP may be increased by the number of shares subject to any stock awards under the Prior Plans that are forfeited, expire or otherwise terminate without the issuance of such shares and would otherwise be returned to the share reserve under the Prior Plans but for their termination and as otherwise provided in the 2013 LTIP. The 2013 LTIP provides for the grant of a total of 30 million shares of our common stock (subject to adjustment for certain corporate events), as (i) decreased for grants made under the Prior Plans between December 31, 2012, and the approval of the 2013 LTIP and (ii) increased by the number of shares subject to any stock awards under the Prior Plans that, between December 31, 2012, and the approval of the 2013 LTIP, are forfeited, expire or settled for cash and as otherwise provided in the 2013 LTIP. Shares under the 2013 LTIP may be granted as incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and performance awards. Subject to certain limited exceptions, stock options and stock appreciation rights granted under the 2013 LTIP reduce the available number of shares by one share for every share issued while awards other than stock options and stock appreciation rights granted under the 2013 LTIP reduce the available number of shares by 1.25 shares for every share issued. In addition, shares that are released from awards granted under the Prior Plans or the 2013 LTIP because the awards expire, are forfeited or are settled for cash will increase the number of shares available under the 2013 LTIP by one share for each share released from a stock option or stock appreciation right and by 1.25 shares for each share released from awards other than stock options and stock appreciation rights. Stock options granted under the 2013 LTIP generally vest 25% a year for 4 years and are exercisable for up to 7 years from the date of grant. The recipient of a restricted stock award has all rights of a stockholder at the date of grant, subject to certain restrictions on transferability and a risk of forfeiture. Restricted stock unit awards generally vest over one or 4 years from the date of grant. The minimum performance period under a performance award is 12 months. Neither the exercise price of an option nor the grant price of a stock appreciation right may be less than 100% of the fair market value of the common stock on the date such equity award is granted, except in specified situations. The 2013 LTIP prohibits option and stock appreciation right repricings (other than to reflect stock splits, spin-offs or certain other corporate events) without stockholder approval. In 2003, we set up a deferred compensation plan for our executive officers, whereby executive officers elected to contribute their shares of restricted stock into the plan. There were 62,501 and 79,169 shares of restricted stock in the plan at December 31, 2016, and 2015, respectively. The following table summarizes our stock option activity under the Prior Plans and the 2013 LTIP, or collectively, our Equity Compensation Plans, for the year ended December 31, 2016, in thousands (except per share data): Options Weighted- Average Exercise Price Weighted-Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2015 16,407 $ 5.01 Granted 17,899 $ 1.61 Exercised (116 ) $ 1.55 Forfeited/cancelled/expired (8,990 ) $ 3.83 Outstanding at December 31, 2016 25,200 $ 3.03 4.28 $ 40 Vested and expected to vest at December 31, 2016 23,945 $ 3.10 4.17 $ 40 Vested and exercisable at December 31, 2016 12,808 $ 4.15 2.37 $ 40 The aggregate intrinsic value in the above table is calculated as the difference between the closing price of our common stock at December 31, 2016, of $1.42 per share and the exercise price of stock options that had strike prices below the closing price. The intrinsic value of all stock options exercised during the years ended December 31, 2016, 2015, and 2014, was less than $0.1 million, $2.2 million, and $2.7 million, respectively. During the year ended December 31, 2016, cash of $0.2 million was received from stock option exercises and cash of $0.2 million was received from stock purchases under the employee stock purchase plans. There is no tax impact related to share-based compensation or stock option exercises because we are in a net operating loss position with a full valuation allowance on our deferred tax assets. Subsequent to the year end, we granted an additional 14.0 million stock options to our employees and directors under the 2013 LTIP. The following table summarizes activity with respect to our time-based RSUs under our Equity Compensation Plans for the year ended December 31, 2016, in thousands (except per share data): RSUs Weighted-Average Grant-Date Fair Value Aggregate Intrinsic Value Unvested at December 31, 2015 273 $ 4.67 Granted — Vested (233 ) $ 4.75 Forfeited/cancelled (15 ) $ 4.23 Unvested at December 31, 2016 25 $ 4.26 Outstanding at December 31, 2016 539 $ 5.05 $ 2,723 The total fair value of RSUs vested during the years ended December 31, 2016, 2015, and 2014, was $1.1 million, $2.1 million, and $1.8 million, respectively. The weighted-average estimated grant-date fair value of RSUs granted during the years ended December 31, 2015, and 2014, was $4.11 and $5.23, respectively. No RSUs were granted in 2016. In March 2015, March 2014 and March 2013, we granted our executive officers PRSU awards. The PRSUs may be earned and converted into outstanding shares of our common stock based on the TSR of our common stock relative to the TSR over a three-year performance period beginning March 1 of the year granted of the NASDAQ Biotechnology Index. In the aggregate, the target number of shares of common stock that could be earned under the PRSUs granted in March 2015, March 2014 and March 2013 were originally 745,000, 695,000 and 780,000, respectively; however, the actual number of shares that could be earned ranges from 0% to 200% of such amounts. In addition, there is a cap on the number of shares that could be earned under the PRSUs equal to six times the grant-date fair value of each award, and funding is capped at 100% if the absolute 3-year TSR is negative even if performance is above the median. As these awards contain a market condition, we used a Monte Carlo simulation model to estimate the grant-date fair value, which totaled $3.4 million, $5.0 million and $5.9 million for the March 2015, 2014 and March 2013 grants, respectively. The grant-date fair value is recognized as compensation expense over the performance period as service is provided; no compensation expense is recognized for service not provided in case of separation from the Company. There is no adjustment of compensation expense recognized for service performed regardless of the number of PRSUs, if any, that ultimately vest. In February 2016, the remaining PRSUs granted 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 February 2017, the remaining PRSUs granted in March 2014 were forfeited without any earnout based on the TSR of our common stock relative to the TSR NASDAQ Biotechnology Index over the three-year performance period that began on March 1, 2014. Of the target number of shares of 745,000 for the March 2015 grants, 355,556 have been cancelled due to management changes during the years ended December 31, 2016, and 2015 (see Note 13). All the other PRSUs granted in March 2015 were outstanding and unvested at December 31, 2016. Employee Stock Purchase Plan. In June 2015, our stockholders approved our 2009 Employee Stock Purchase Plan, as amended, or 2009 ESPP. Under the 2009 ESPP substantially all employees can choose to have up to 15% of their annual compensation withheld to purchase up to 625 shares of common stock per purchase period, subject to certain limitations. The shares of common stock may be purchased over an offering period with a maximum duration of 24 months and at a price of not less than 85% of the lesser of the fair market value of the common stock on (i) the first trading day of the applicable offering period or (ii) the last trading day of the applicable three-month purchase period. Under applicable accounting guidance, the 2009 ESPP is considered a compensatory plan. At December 31, 2016, a total of 1,115,188 shares of common stock were available for issuance under the 2009 ESPP. During the years ended December 31, 2016, 2015, and 2014, 141,397, 327,950, and 304,085 shares, respectively, were purchased under the 2009 ESPP. Share-based Compensation. We estimate the grant-date fair value of all of our share-based awards in determining our share-based compensation expense. Our share-based awards include (i) stock options, (ii) options to purchase stock granted under our employee stock purchase plan, (iii) RSUs, and (iv) PRSU awards. The table below sets forth the weighted-average assumptions and estimated fair value of stock options we granted under our Equity Compensation Plans during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate 1.4 % 1.8 % 1.8 % Dividend yield 0 % 0 % 0 % Expected volatility 79 % 80 % 81 % Expected life (years) 4.81 6.08 6.17 Weighted-average estimated fair value per share of stock options granted $ 1.02 $ 2.55 $ 4.37 The table below sets forth the assumptions and estimated fair value of the options to purchase stock granted under our employee stock purchase plan for multiple offering periods during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate 0.2% - 1.2% 0.0% - 1.0% 0.0% - 0.6% Dividend yield 0% 0% 0% Expected volatility 75% - 82% 52% - 78% 53% - 81% Expected life (years) .25 - 2.0 .25 - 2.0 .25 - 2.0 Range of fair value per share of options granted under employee stock purchase plan $0.78 to $0.92 $0.78 to $2.94 $1.37 to $4.22 The table below sets forth the assumptions and estimated fair value of PRSU awards granted during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate — 1.1 % 0.7 % Dividend yield — 0 % 0 % Expected volatility — 75 % 78 % Performance period (years) — 2.97 2.99 Estimated fair value per share of PRSUs granted — $ 4.50 $ 7.16 We recognized share-based compensation expense as follows for the years presented, in thousands, except per share data: Years ended December 31, 2016 2015 2014 Cost of product sales $ 45 $ 29 $ — Research and development 5,615 7,582 7,118 General and administrative 4,425 6,710 6,391 Restructuring charges 1,032 142 — Total share-based compensation expense and impact on net loss $ 11,117 $ 14,463 $ 13,509 Impact on net loss per share, basic and diluted $ 0.05 $ 0.06 $ 0.06 Total share-based compensation capitalized into inventory $ 170 $ 173 $ 81 We capitalize into inventory share-based compensation related to awards granted to employees involved with the manufacturing of BELVIQ. Such compensation will subsequently be recognized as cost of product sales when the related inventory is sold. The table below sets forth our total unrecognized estimated compensation expense at December 31, 2016, by type of award and the weighted-average remaining requisite service period over which such expense is expected to be recognized: Unrecognized Expense (in thousands) Remaining Weighted-Average Recognition Period (in years) Unvested stock options $ 10,381 3.14 RSUs 85 1.66 PRSUs 289 0.60 Common Stock Reserved for Future Issuance. The following shares of our common stock are reserved for future issuance at December 31, 2016, in thousands: Equity Compensation Plans 45,515 2009 ESPP 1,115 Deferred compensation plan 63 Total 46,693

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	9. Collaborations Lorcaserin collaborations. Eisai. In July 2010, we granted Eisai exclusive commercialization rights for lorcaserin solely in the United States and its territories and possessions. In May 2012, we and Eisai entered into the first amended and restated agreement, which expanded Eisai’s exclusive commercialization rights to include most of North and South America. In November 2013, we and Eisai entered into the second amended and restated agreement, or Second Amended Agreement, which expanded Eisai’s exclusive commercialization rights for lorcaserin to all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. On December 28, 2016, we and Eisai amended and restated the terms of the Second Amended Agreement by entering into the Eisai Agreement, which was determined to be a material modification of the Second Amended Agreement. Under the Eisai Agreement, we identified the following significant deliverables to Eisai which each qualify as a separate unit of accounting: • An exclusive royalty-bearing license or transfer of intellectual property, or License, to commercialize lorcaserin world-wide relating to certain patents, regulatory approvals, samples, records, know-how related to lorcaserin, trademarks and domain names related to the lorcaserin brand names. We also assigned to Eisai our rights under the commercial lorcaserin distribution agreements with Ildong for South Korea, CYB for Taiwan and Teva for Israel. This is collectively referred to as the License Deliverable. • Bulk inventory and precursor material for manufacturing lorcaserin, or Inventory Deliverable. • A manufacturing and supply commitment for two years commencing December 28, 2016, or Manufacturing and Supply Commitment Deliverable. The following table summarizes the revenues we recognized under our collaboration with Eisai for the periods presented, in thousands: Years ended December 31, 2016 2015 2014 Net product sales $ 19,196 $ 14,236 $ 15,983 Amortization of upfront payments 66,014 7,541 7,630 Milestone payments 12,000 — 500 Reimbursement of development expenses 1,295 1,538 10,037 Reimbursement of patent and trademark expenses 392 426 444 Subtotal other Eisai collaboration revenue 79,701 9,505 18,611 Total $ 98,897 $ 23,741 $ 34,594 Royalty payments. Pursuant to the Eisai 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 We did not earn or recognize any revenue from these royalty payments in the year ended December 31, 2016. We expect to record revenues from those royalty payments in the period in which the net sales upon which the royalties are calculated occur as reported to us by Eisai. Upfront payments. Prior to the Eisai Agreement, we received from Eisai total upfront payments of $115.0 million under prior agreements. 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. The Eisai Agreement effectively eliminated our obligation to continue performing the development and regulatory activities required in the Second Amended Agreement. Therefore, on December 28, 2016, $58.5 million of deferred revenues from these upfront payments was allocated to the value of the License provided to Eisai and recognized as revenue in 2016. The remaining portion, $20.9 million, was deferred as of December 31, 2016. Milestone payments. In July 2016, the US Food and Drug Administration, or FDA, approved the New Drug Application for BELVIQ XR. We earned from Eisai a $10.0 million substantive milestone payment from this achievement. 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. We earned from Eisai a $1.0 million substantive milestone payment from this achievement. In December 2016, the Brazilian Health Surveillance Agency provided regulatory approval in Brazil for BELVIQ. We earned from Eisai a $1.0 million substantive milestone payment from this achievement. In addition to the $12.0 million in milestones mentioned above and the other $86.5 million in milestones previously achieved since we entered the original agreement in 2010, we are eligible to receive a substantive commercial milestone of $25.0 million upon the achievement of global net sales of lorcaserin for a calendar year first exceeding $250.0 million. Product purchase price and inventory purchase. We manufacture lorcaserin at our facility in Zofingen, Switzerland. Under the Eisai Agreement, we have agreed to manufacture and supply, and Eisai has agreed to purchase from us, all of Eisai’s requirements (or specified minimum quantities if such quantities are greater than Eisai’s requirements), subject to certain exceptions, for lorcaserin for development and commercial use for an initial two-year period. The initial period may be extended by Eisai for an additional six months upon payment of an extension fee of CHF 2.0 million. Eisai will pay us agreed upon prices to deliver finished drug product during this time. Additionally, Eisai has agreed to pay up to CHF 13.0 million in manufacturing support payments during the initial two-year period supply period, and pay up to CHF 6.0 million in manufacturing support payments during the six-month extension period, if the extension option is exercised by Eisai. On December 28, 2016, Eisai paid us $10.0 million to acquire our entire inventory of bulk lorcaserin and the precursor materials for manufacturing lorcaserin. This payment was included in the arrangement consideration allocated to the units of accounting under the Eisai Agreement. We expect this inventory will remain at our Zofingen, Switzerland facility for us to use to manufacture finished drug product in order to meet Eisai’s requirements during the initial two-year period and, if applicable, the six-month extension period. The inventory that is not expected to be used to manufacture finished drug product will be physically transferred to Eisai upon the earlier of Eisai’s request to transfer or the end of the manufacturing and supply commitment period. Under the Second Amended Agreement, we sold lorcaserin to Eisai for Eisai’s commercialization in the United States for a purchase price of 31.5% of Eisai’s aggregate annual net product sales (which are the gross invoiced sales less certain deductions described in the Second Amended Agreement), or the Product Purchase Price. The amount that Eisai paid us for lorcaserin product supply was based on Eisai’s estimated price at the time the order was shipped, which was Eisai’s estimate of the Eisai Product Purchase Price, and was subject to change on April 1 and October 1 of each year. The Eisai Product Purchase Price for the product Eisai sold under the Second Amended Agreement was lower than the estimated price that Eisai paid us for such product, primarily due to an increase in deductions from savings cards and returns, partially offset by a decrease in vouchers. At the end of Eisai’s fiscal year (March 31), the estimated price paid to us for product that Eisai sold to its distributors was compared to the Eisai Product Purchase Price of such product, and the difference was refunded back to Eisai for the overpayments. The $9.1 million classified as Payable to Eisai on our consolidated balance sheet at December 31, 2016, relates to product sold by Eisai to its distributors from April 1, 2015, through March 31, 2016. Under the Eisai Agreement, we will not refund to Eisai any net overpayment which would have been otherwise due to Eisai under the Second Amended Agreement for product we sold to Eisai under the Second Amended Agreement which Eisai did not sell to its distributors on or before March 31, 2016. For product which Eisai sold to its distributors from April 1, 2016, through December 28, 2016, we recognized the net overpayment which would have been otherwise due to Eisai under the Second Amended Agreement of $2.0 million as revenues and included this amount in net product sales for the year ended December 31, 2016. We previously deferred recognition of revenue and the related cost at the time we sold lorcaserin to Eisai because we did not have the ability to estimate the amount of product that could have been returned to us and thus recognized revenues and the related costs from net product sales when Eisai shipped BELVIQ to its distributors. Pursuant to a change in the terms of the Eisai Agreement, we determined that we now have the ability to reasonably estimate the amount of returns and thus will now recognize revenue and the related cost from product sales when we ship BELVIQ to Eisai. On December 28, 2016, we recognized revenues of $6.7 million and costs of $1.9 million on net product sales which had been previously deferred. Allocation of Eisai Agreement arrangement consideration to the units of accounting. The total arrangement consideration of $115.6 million primarily consists of (i) the December 28, 2016, balances of deferred revenues from the upfront payments received under the prior Eisai agreements and the distribution agreements with Ildong, CYB and Teva; (ii) the $10.0 million payment received from Eisai on December 28, 2016; and (iii) the product purchase payments and manufacturing support payments we expect to receive from Eisai for the initial two-year manufacturing and supply commitment period. All of the deliverables were determined to have standalone value and to meet the criteria to be accounted for as separate units of accounting. Factors considered in the determination included, among other things, for the license, the manufacturing experience and capabilities of Eisai and their sublicense rights, and for the remaining deliverables the fact that they are not proprietary and can be provided by other vendors. The total arrangement consideration was allocated to the units of accounting on the basis of their relative estimated selling prices as follows: • $64.0 million was allocated to the License Deliverable. As the License Deliverable was delivered on December 28, 2016, this amount was recognized as other Eisai collaboration revenue for the year ended December 31, 2016. • $30.8 million was allocated to the Inventory Deliverable. Title to this entire inventory passed to Eisai on December 28, 2016. However, none of this inventory was physically transferred from the manufacturing facility, and there is no fixed schedule for delivery given some will be delivered on a continuous basis as we perform under the manufacturing commitment while the rest will be physically transferred to Eisai upon request by Eisai or upon the end of the manufacturing and supply commitment period. Also, the risks of ownership for this inventory have not been fully passed to Eisai as we will continue to have financial responsibility for any loss, damage or destruction which occurs while in our possession. Therefore, none of the arrangement consideration allocated to this deliverable was recognized as revenue and none of the carrying value of this inventory was recognized as cost of product sales for the year ended December 31, 2016. • $20.8 million was allocated to the Manufacturing and Supply Commitment Deliverable. This deliverable will be provided over 2017 and 2018 as product is shipped to Eisai. Therefore, none of the arrangement consideration allocated to this deliverable was recognized as revenue for the year ended December 31, 2016. The consolidated balance sheet at December 31, 2016, includes deferred revenues of $30.8 million (primarily comprised of the deferred portion of the previously received upfront payments and the $10.0 million payment received from Eisai on December 28, 2016), and inventory of $4.4 million, which is the carrying value of the product under the Inventory Deliverable. These balances are expected to be recognized in subsequent periods as this inventory is used in the manufacture and supply of lorcaserin to Eisai over the commitment period. The estimated selling price represents the price at which we would contract if the deliverable was sold regularly on a standalone basis. The estimated selling price for each unit of accounting was determined as follows: • The estimated selling price for the License Deliverable was determined using an income approach that estimates the net present value of royalties Eisai is expected to earn under the Eisai Agreement as compared to the Second Amended Agreement, net of the development costs we are no longer obligated to spend. This model includes several assumptions, including the potential market for lorcaserin in each relevant jurisdiction, probabilities of obtaining regulatory approval in additional jurisdictions, the impact of competition, the potential impact of Eisai’s ongoing development and regulatory activities related to lorcaserin, and the appropriate discount rate. • The estimated selling price for the Inventory Deliverable was determined by considering the historical cost of the precursor materials, adjusted for any changes in market condition and supplier relationships. We believe that the Eisai Agreement pricing represents pricing that would be charged if it were sold on a standalone basis. • The estimated selling price for the Manufacturing and Supply Commitment Deliverable was determined to be the aggregate product purchase payments we expect to receive from Eisai for the initial two-year manufacturing and supply commitment period. As noted above, we believe that the Eisai Agreement pricing represents pricing that would be charged if it were sold on a standalone basis. Development payments. As part of the US approval of BELVIQ, the FDA, is requiring the evaluation of the effect of long-term treatment with BELVIQ on the incidence of major adverse cardiovascular events, or MACE, in overweight and obese patients with cardiovascular disease or multiple cardiovascular risk factors (which is the FDA-required portion of the cardiovascular outcomes trial), as well as the conduct of postmarketing studies to assess the safety and efficacy of BELVIQ for weight management in obese pediatric and adolescent patients. Under the Second Amended Agreement, Eisai and we were responsible for 90% and 10%, respectively, of the cost for the FDA-required portion of the cardiovascular outcomes trial, or CVOT, 50% and 50%, respectively, of the non-FDA portion of the studies and we were also obligated to share the cost of FDA-required studies in obese pediatric patients and for additional clinical studies in other territories. Under the Eisai Agreement, Eisai is solely responsible for all costs and expenses in connection with further development of lorcaserin from and after July 1, 2016, and we were relieved of any obligations under the Second Amended Agreement to pay our share of future development costs of lorcaserin. Accordingly, on December 28, 2016, we recorded a reduction of research and development expenses which would have been otherwise due to Eisai under the Second Amended Agreement of $3.7 million for the period from July 1, 2016, through December 28, 2016. For the years ended December 31, 2016, 2015, and 2014, we recognized expenses of $7.3 million (net of the aforementioned $3.7 million reduction), $16.2 million and $35.3 million, respectively, for external clinical study fees related to lorcaserin and internal non-commercial manufacturing costs primarily related to lorcaserin. Certain other terms. Eisai and we will each bear 50% of all future expenses and losses arising from any potential product liability claims during a specified period after the date of the Eisai Agreement. Thereafter, we and Eisai will each bear 50% of all expenses and losses arising from any alleged defective manufacturing of lorcaserin by Arena GmbH under the Eisai Agreement, and Eisai will be solely responsible for any expenses and losses associated with other product liability claims. We may terminate the Transaction Agreement with respect to the United States, the European Union, China and Japan, (collectively, the Major Markets) if Eisai permanently ceases development and commercialization of lorcaserin products in such Major Market, or in its entirety if Eisai permanently ceases development and commercialization of lorcaserin products. We may also terminate the Transaction Agreement if Eisai challenges any patent currently controlled by us related to lorcaserin, if Eisai is debarred under the United States Federal Food, Drug, and Cosmetic Act, or if Eisai is in material breach of the standstill provisions. Eisai may terminate the Transaction Agreement if, as a result of its change of control, it would be in breach of certain competition restrictions. In the event the Transaction Agreement is terminated by us due to Eisai’s failure to develop and commercialize lorcaserin products, Eisai’s challenging of any of the licensed patents or Eisai’s debarment or material breach of the standstill provisions, or by Eisai after a change of control that would result in Eisai being in breach of certain competition restrictions, Eisai will grant us an exclusive, royalty-free license to certain patent rights and know-how necessary or useful for the development and commercialization of lorcaserin products, re-assign the assets purchased by Eisai under the Eisai Agreement, and provide certain other transition assistance. Ildong Pharmaceutical Co., Ltd. In November 2012, we and Ildong entered into the Marketing and Supply Agreement, or Ildong Agreement. Under this agreement, we granted Ildong exclusive rights to commercialize BELVIQ in South Korea for weight loss or weight management in obese and overweight patients. We also provided certain services and manufacture and sold BELVIQ to Ildong. As noted above, the Ildong Agreement was assigned to Eisai pursuant to the Eisai Agreement on December 28, 2016. In connection with entering into the Ildong Agreement, we received from Ildong an upfront payment of $5.0 million, less withholding taxes. Revenues from this upfront payment were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, this payment was recognized ratably as revenue over the period in which we expected the services to be rendered. The assignment of the Ildong Agreement pursuant to the Eisai Agreement effectively eliminated our obligation to continue performing the development and regulatory activities required in the Ildong Agreement. Therefore, on December 28, 2016, the $3.5 million of deferred revenues from this upfront payment was allocated to the value of the License provided to Eisai and recognized as revenue in 2016. In February 2015, we earned a substantive milestone payment of $3.0 million upon the approval of BELVIQ for marketing in South Korea for weight management. We received the payment, less withholding taxes, in March 2015. On December 15, 2016, we earned a substantive milestone payment of $0.3 million upon the parties agreeing to include BELVIQ XR as an additional product under the Ildong Agreement. We recognized the milestone revenue in December 2016 and received the payment, less withholding taxes, in February 2017. We will pay 50% of this milestone to Eisai pursuant to the Eisai Agreement. Under the Ildong Agreement, we manufactured BELVIQ at our facility in Zofingen, Switzerland, and sold BELVIQ to Ildong for a purchase price starting at the higher of the defined minimum amount or 35% of Ildong’s annual net product sales (which are the gross invoiced sales less certain deductions described in the Ildong Agreement), or the Ildong Product Purchase Price. The Ildong Product Purchase Price increased on a tiered basis up to the higher of the defined minimum amount or 45% on the portion of annual net product sales exceeding $15.0 million. Since the inception of commercial sales of BELVIQ in South Korea in 2015, the Ildong Product Purchase Price equaled the defined minimum amount (which exceeded the amounts calculated using the applicable percentages for the applicable tiers of Ildong’s annual net product sales). We previously deferred recognition of revenue and the related cost at the time we sold BELVIQ to Ildong because we did not have the ability to estimate the amount of product that could have been returned to us and thus recognized revenues and the related costs from net product sales when Ildong shipped BELVIQ to its distributors. In December 2016, we determined that we now have the ability to reasonably estimate returns under the Ildong Agreement. Accordingly, we recognized revenues of $2.0 million and costs of $0.7 million in December 2016 on net product sales which had been previously deferred. For the years ended December 31, 2016, 2015, and 2014, we recognized revenues of $11.4 million, $8.9 million and $0.4 million, respectively, under the Ildong agreement. CY Biotech Company Limited. In July 2013, we entered into the CYB Agreement. 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, or TFDA. The CYB Agreement provided for us to perform certain services and to manufacture and sell BELVIQ to CYB. As noted above, the CYB Agreement was assigned to Eisai pursuant to the Eisai Agreement on December 28, 2016. In connection with entering into the CYB agreement, we received from CYB an upfront payment of $2.0 million, less withholding taxes. Revenues from this upfront payment were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, this payment was recognized ratably as revenue over the period in which we expected the services to be rendered. The assignment of the CYB Agreement pursuant to the Eisai Agreement effectively eliminated our obligation to continue performing the development and regulatory activities required in the CYB Agreement. Therefore, on December 28, 2016, the $1.7 million of deferred revenues from this upfront payment was allocated to the value of the License provided to Eisai and recognized as revenue in 2016. For the years ended December 31, 2016, 2015, and 2014, we recognized revenues of $1.8 million, $0.2 million, and $0.2 million, respectively, under this agreement. Abic Marketing Limited (Teva). In July 2014, we entered into the Teva Agreement. Under this agreement, 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. The Teva Agreement provided for us to perform certain services and to manufacture and sell BELVIQ to Teva. As noted above, the Teva Agreement was assigned to Eisai pursuant to the Eisai Agreement on December 28, 2016. We received from Teva an upfront payment of $0.5 million and a milestone payment of $0.3 million earned upon its application for regulatory approval of BELVIQ in Israel. Revenues from the upfront payment were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, this payment was recognized ratably as revenue over the period in which we expected the services to be rendered. The assignment of the Teva Agreement pursuant to the Eisai Agreement effectively eliminated our obligation to continue performing the development and regulatory activities required in the Teva Agreement. Therefore, on December 28, 2016, the $0.4 million of deferred revenues from this upfront payment was allocated to the value of the License provided to Eisai and recognized as revenue in 2016. For the years ended December 31, 2016, 2015, and 2014, we recognized revenues of $0.4 million, $0.1 million and $0.3 million, respectively, under the Teva Agreement. Other collaborations Nelotanserin - Axovant Sciences Ltd. In May 2015, we entered into the Axovant Agreement. In October 2015, Roivant Sciences, Ltd., or Roivant, assigned the exclusive rights to develop and commercialize nelotanserin to its subsidiary, Axovant. Under this agreement, 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. We received an upfront payment of $4.0 million, which was recorded as deferred revenues and is being recognized as revenue ratably over approximately five years, which is the period in which we expect to provide services under the arrangement. We will receive payments from 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 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. For the years ended December 31, 2016, and 2015, we recognized revenues of $2.1 million and $1.1 million, respectively, under this agreement. Orphan GPCR - Boehringer Ingelheim International GmbH. In December 2015, we and Boehringer Ingelheim entered into 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 this agreement, we granted Boehringer Ingelheim exclusive rights to our internally discovered, novel compounds and intellectual property for an orphan CNS receptor. We will jointly conduct research with Boehringer Ingelheim to identify additional drug candidates that are suitable for continued research and development as therapeutic compounds for various disease indications, with the initial focus expected to be psychiatric diseases such as schizophrenia. The agreement grants Boehringer Ingelheim exclusive worldwide rights to develop, manufacture and commercialize products resulting from the collaboration. In part consideration of the 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 January 2016, less $1.2 million of withholding taxes which was refunded to us in October 2016. Revenues from this upfront payment were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing participation in the joint research, and are being recognized ratably as revenues over the period in which we expect the services to be rendered, which is approximately two years. We are also 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. For the year ended December 31, 2016, we recognized revenues of $5.1 million under this agreement.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	10. Employee Benefit Plans 401(k) Plan. All of our US employees are eligible to participate in our defined contribution retirement plan that complies with Section 401(k) of the Internal Revenue Code, or IRC. We match 100% of each participant’s voluntary contributions, subject to a maximum of 6% of the participant’s eligible compensation. Our matching portion, which totaled $1.3 million, $1.9 million, and $1.6 million for the years ended December 31, 2016, 2015, and 2014, respectively, vests over a five-year period from the date of hire. Pension Plan. Arena GmbH contributes to a multiemployer defined benefit pension plan, established under an affiliated group of employers, for the purpose of providing mandatory occupational pension benefits for its employees. The risks of participating in a multiemployer plan are different from a single-employer plan in that (i) assets contributed to the multiemployer plan by one employer may be used to provide benefits to employees of other participating employers, (ii) if a participating employer stops contributing to the plan, the unfunded obligations of the plan may be borne by the remaining participating employers, (iii) if Arena GmbH elects to stop participating in the multiemployer plan, Arena GmbH may be required to pay the plan an amount based on the underfunded status of the plan, referred to as a withdrawal liability, and (iv) Arena GmbH has no involvement in the management of the multiemployer plan’s investments. We currently have no intention of withdrawing from the multiemployer plan. Our contributions to the multiemployer plan were $0.8 million, $0.7 million and $0.7 million for the years ended December 31, 2016, 2015, and 2014, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The following table summarizes our loss attributable to stockholders of Arena before benefit for income taxes by region for the years presented, in thousands: Years ended December 31, 2016 2015 2014 United States $ (10,268 ) $ (64,109 ) $ (16,607 ) Foreign (12,248 ) (43,870 ) (43,901 ) Total loss attributable to stockholders of Arena before income taxes $ (22,516 ) $ (107,979 ) $ (60,508 ) We have not recorded a benefit for income taxes for the years ended December 31, 2016, 2015, and 2014, because we have a full valuation allowance. Our effective income tax rate differs from the statutory federal rate of 34% for the years presented due to the following, in thousands: Years ended December 31, 2016 2015 2014 Benefit for income taxes at statutory federal rate $ (7,655 ) $ (36,713 ) $ (20,573 ) Change in federal and foreign valuation allowance 9,080 21,310 9,436 Permanent differences and other (5,931 ) 2,370 721 Share-based compensation expense 4,000 1,820 1,597 Foreign losses at lower effective rates 3,943 15,041 13,318 Research and development and Orphan Drug credits (3,437 ) (3,666 ) (2,992 ) State income tax, net of federal benefit and valuation allowance — — — Gain from valuation of derivative liabilities — (162 ) (1,507 ) Benefit for income taxes $ — $ — $ — The components of our net deferred tax assets are as follows, in thousands: December 31, 2016 2015 Deferred tax assets: Federal and California NOL carryforwards $ 255,317 $ 236,334 Federal and California research and development credit carryforwards 53,059 48,768 Share-based compensation expense 10,395 10,737 Deferred revenues 9,357 33,548 Depreciation 5,441 4,475 Foreign NOL carryforwards 5,108 7,060 Other, net 5,164 3,578 Total deferred tax assets 343,841 344,500 Deferred tax liabilities (228 ) (660 ) Net deferred tax assets 343,613 343,840 Valuation allowance (343,613 ) (343,840 ) Net deferred tax liabilities $ — $ — A valuation allowance is recorded against all of our deferred tax assets, as realization of such assets is not more-likely-than-not. The realization of our deferred tax assets is dependent upon future taxable income. Our ability to generate taxable income is analyzed regularly on a jurisdiction-by-jurisdiction basis. At such time as it is more-likely-than-not that we will generate taxable income in a jurisdiction, we will reduce or remove the valuation allowance. The valuation allowance decreased by $0.2 million from December 31, 2015, to December 31, 2016. At December 31, 2016, we had federal NOL carryforwards of $670.7 million that will begin to expire in 2023 unless previously utilized. At the same date, we had California NOL carryforwards of $528.3 million, which begin expiring in 2017 and foreign NOL carryforwards of $62.2 million, which begin expiring in 2017. At December 31, 2016, approximately $8.9 million of the federal and California NOL carryforwards related to stock option exercise windfalls, which will result in an increase to additional paid-in capital and a decrease in income taxes payable at the time such carryforwards are utilized. At December 31, 2016, we also had federal and California research and development tax credit carryforwards, net of reserves, of $31.4 million and $23.7 million, respectively. At December 31, 2016, we had a Federal Orphan Drug Credit carryforward of $6.0 million. Federal credit carryforwards will begin to expire after 2026 unless previously utilized. The California research and development credit carries forward indefinitely. Sections 382 and 383 of the IRC limit the utilization of tax attribute carryforwards that arise prior to certain cumulative changes in a corporation’s ownership. We have completed an IRC Section 382/383 analysis through 2015 and identified ownership changes that limit our utilization of tax attribute carryforwards. We reduced deferred tax assets associated with such tax attribute carryforwards to remove deferred tax assets that will expire prior to utilization. Pursuant to IRC Section 382 and 383, use of the Company’s net operating loss and research and development income tax credit carryforwards may be limited in the event of a future cumulative change in ownership of more than 50% within a three-year period. In accordance with authoritative guidance, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. The following table reconciles the beginning and ending amount of unrecognized tax benefits for the years presented, in thousands: Years ended December 31, 2016 2015 2014 Gross unrecognized tax benefits at the beginning of the year $ 5,619 $ 5,214 $ 4,629 Additions from tax positions taken in the current year 287 405 585 Additions from tax positions taken in prior years — — — Reductions from tax positions taken in prior years — — — Tax settlements — — — Gross unrecognized tax benefits at end of the year $ 5,906 $ 5,619 $ 5,214 Of our total unrecognized tax benefits at December 31, 2016, $4.5 million will impact our effective tax rate in the event the valuation allowance is removed. We do not anticipate that there will be a substantial change in unrecognized tax benefits within the next 12 months. Our practice is to recognize interest and/or penalties related to income tax matters in income tax expense. Because we have incurred net losses since our inception, we did not have any accrued interest or penalties included in our consolidated balance sheets at December 31, 2016, or 2015, and did not recognize any interest and/or penalties in our consolidated statements of operations and comprehensive loss for the years ended December 31, 2016, 2015, and 2014. We have elected the “with and without method – direct effects only”, prescribed in accordance with authoritative guidance, with respect to recognition of stock option windfall tax benefits within APIC and will utilize general NOLs to offset taxable income before utilization of NOLs attributable to windfall tax benefits. We are subject to income taxation in the United States at the Federal and state levels. All tax years are subject to examination by US and California tax authorities due to the carryforward of unutilized NOLs and tax credits. We are also subject to foreign income taxes in the countries in which we operate. To our knowledge, we are not currently under examination by any taxing authorities. At December 31, 2016, no foreign subsidiaries have accumulated earnings and, as such, there are no unrepatriated earnings. Our Swiss subsidiary, Arena GmbH, has been granted a conditional incentive tax holiday by the Canton of Aargau for its operations in Switzerland. Without a tax holiday or other tax incentives, the standard effective tax rate of a company located in Aargau is approximately 19%. As a result of the tax holiday and other tax incentives, we expect the effective tax rate for Arena GmbH to be approximately half of such rate. The tax holiday came into effect on January 1, 2013, and will continue for a period of up to 10 years, not to extend beyond December 31, 2022. As a result of foreign losses and a full valuation allowance, no net tax benefit was derived for the years ended December 31, 2016, 2015, and 2014, as a result of the tax holiday.

Legal Proceedings

Legal Proceedings	12 Months Ended
	Dec. 31, 2016
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 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 ® ® Waxman 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 ® release ® .

Management Changes

Management Changes	12 Months Ended
	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Management Changes	13. 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, 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 consolidated statement of operations and comprehensive loss for the year ended December 31, 2016. As of December 31, 2016, substantially all of these accrued benefits have been paid. 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 consolidated statement of operations and comprehensive loss for the year ended December 31, 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, to reserve an additional 800,000 shares of common stock for inducement 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, to reserve an additional 800,000 shares of common stock for inducement awards, which resulted in the total number of shares of common stock reserved for inducement awards during 2016 to be 5,400,000. 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. Dr. Behan's resignation follows our strategic shift in priorities to emphasize our proprietary clinical stage pipeline, which was announced in June 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 consolidated statement of operations and comprehensive loss for the year ended December 31, 2016. As of December 31, 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 paid in the first quarter of 2017. Termination of Senior Vice President of Operations and Head of Global Regulatory Affairs. In October 2016, our Board of Directors terminated without cause our former Senior Vice President and Head of Regulatory Affairs, Craig M. Audet. Mr. Audet is entitled to receive the following termination benefits: (1) a cash severance payment of approximately $0.5 million (subject to applicable withholdings); (2) compensation in lieu of continuation of health insurance coverage for a period of 12 months; (3) acceleration of the stock options and RSUs (other than PRSUs) held by Mr. Audet 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) 24 months (but not beyond the original contractual life of the option). We recorded a charge of $1.0 million in the fourth 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 consolidated statement of operations and comprehensive loss for the year ended December 31, 2016. As of December 31, 2016, there are remaining accruals for these benefits of $0.5 million included in accounts payable and other accrued expenses, which we expect to pay in the second quarter of 2017.

Restructuring Activities

Restructuring Activities	12 Months Ended
	Dec. 31, 2016
Restructuring And Related Activities [Abstract]
Restructuring Activities	14. Restructuring Activities 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 also 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 December 31, 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 December 31, 2016, substantially all of this charge has been paid. 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 substantially completed by the end of the January 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 December 31, 2016, substantially all of this charge has been paid.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	15. Quarterly Financial Data (Unaudited) The following tables present selected quarterly financial data for the years presented, in thousands, except per share data: 2016 Quarter ended December 31 Quarter ended September 30 Quarter ended June 30 Quarter ended March 31 Revenues $ 85,374 $ 19,242 $ 9,512 $ 9,847 Operating costs and expenses 47,245 29,099 35,735 29,042 Net income (loss) 38,314 (12,479 ) (27,183 ) (21,548 ) Net income (loss) attributable to stockholders of Arena 38,572 (12,357 ) (27,183 ) (21,548 ) Net income (loss) attributable to stockholders of Arena per share, basic and diluted 0.16 (0.05 ) (0.11 ) (0.09 ) 2015 Quarter December 31 Quarter ended September 30 Quarter ended June 30 Quarter ended March 31 Revenues $ 7,751 $ 9,138 $ 9,181 $ 12,256 Operating costs and expenses 37,045 34,319 36,160 34,000 Net loss (30,459 ) (26,418 ) (26,807 ) (24,295 ) Net loss per share, basic and diluted (0.13 ) (0.11 ) (0.11 ) (0.10 )

Beacon Discovery, Inc.

Beacon Discovery, Inc.	12 Months Ended
	Dec. 31, 2016
Variable Interest Entity Disclosure [Abstract]
Beacon Discovery, Inc.	16. Beacon Discovery, Inc. On September 1, 2016, we entered into a 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 we issued to Beacon. The outstanding principal amount and all accrued or unpaid interest thereon (calculated at a simple interest rate of 7% per annum) shall be due and payable on the earlier of (i) August 31, 2022 or (ii) Beacon receiving cumulative cash proceeds of $10 million 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 considered a variable interest entity. Arena does not own any equity interest in Beacon; however, as these agreements provide us the controlling financial interest in Beacon, we consolidate Beacon’s balances and activity within our consolidated financial statements. The noncontrolling interest attributable to Beacon presented on our 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 the assets and liabilities of Beacon which are included in our consolidated balance sheet at December 31, 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 December 31, 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 $ 311 Prepaid expense and other current assets 28 Land, property and equipment, net 705 Total assets of Beacon that can only be used to settle obligation of Beacon $ 1,044 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 86 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 86

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events On January 4, 2017, we entered into an Equity Distribution Agreement with Citigroup Global Markets Inc. (Citigroup), pursuant to which we may sell and issue shares of our common stock having an aggregate offering price of up to $50 million from time to time through Citigroup, as our sales agent (the ATM Offering). Sales of the shares under the Equity Distribution Agreement may be made in transactions that are deemed to be “at-the-market” equity offerings as defined in Rule 415 under the Securities Act of 1933, as amended, including sales made by means of ordinary brokers’ transactions, including on the NASDAQ Stock Market. Subject to the terms and conditions of the Equity Distribution Agreement, Citigroup will use its reasonable efforts to sell the shares from time to time based upon our instructions (including any price, time or size limits or other parameters or conditions we may impose). We will pay Citigroup a commission of up to 3.0% of the gross sales price of any shares sold under the Equity Distribution Agreement. As of March 10, 2017, we sold 2,017,301 shares of Equity Distribution 3.2 million 0.1 million 46.8 million Equity Distribution

The Company and Summary of Si24

The Company and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with the U.S. generally accepted accounting principles, or GAAP, and reflect all of our activities, including those of our wholly owned subsidiaries. All material intercompany accounts and transactions have been eliminated in consolidation. The accompanying consolidated financial statements include the balances and activity of Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 16). 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 consolidated balance sheets. 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 consolidated statements of operations and comprehensive loss.
Liquidity	Liquidity It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several 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 and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions. Our prospects are subject to the risks and uncertainties frequently encountered by companies in the early stages of development and commercialization, especially those companies in rapidly evolving and technologically advanced industries such as the biotechnology field. Our future viability approvals novel, small molecule drugs We incurred net losses of $22.9 million, $108.0 million and $60.5 million for the years ended December 31, 2016, 2015, and 2014, respectively. Additionally, we have used net cash of $62.1 million, $98.1 million and $101.4 million to fund our operating activities for years ended December 31, 2016, 2015, and 2014, respectively. We believe our plans to raise additional cash from outside sources and, if necessary, our cost containment efforts are sufficient to allow us to continue operations for the next twelve months. Our plans include pursuing additional cash through strategic corporate partnerships and possibly engaging in future sales of equity or debt. There is no guarantee that adequate funds will be available when needed from equity financing or additional debt, development and commercialization partnerships, increased results of operations, or from other sources, or on terms acceptable to us. If our efforts to obtain sufficient additional funds are not successful, we would be required to delay, scale back, or eliminate some or all of our research or development, manufacturing operations, administrative operations, and clinical or regulatory activities, which could negatively affect our ability to achieve certain corporate goals.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Revenue recognition. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers The new guidance 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 the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018, and not restate prior periods. The Company has continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard. Other. 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. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases modified In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash
Use of Estimates	Use of Estimates The preparation of financial statements in conformity 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.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist of cash and highly liquid investments with remaining maturities of three months or less when purchased.
Inventory	Inventory Inventory is stated at the lower of cost or market. We determine cost, which includes amounts related to materials, labor and overhead, using a first-in, first-out basis. We evaluate our inventory each period to identify potential obsolete, excess or otherwise non-saleable items. If non-saleable items are observed and there are no alternate uses for the inventory, we will record a write-down to net realizable value in the period that the decline in value is first recognized.
Concentrations of Risk	Concentrations of Risk 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 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. BELVIQ has been exclusively sold in the United States and South Korea by Eisai and Ildong, respectively, which are the only jurisdictions for which BELVIQ has been commercially sold. 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: Years ended December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 79.8 % 61.9 % 93.6 % Ildong Agreement (see Note 9) 9.2 % 23.2 % 1.0 % Toll manufacturing agreements 3.3 % 11.1 % 4.0 % Other collaboration agreements 7.7 % 3.8 % 1.4 % Total percentage of revenues 100.0 % 100.0 % 100.0 % Percentages of our total accounts receivable are as follows: December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 93.1 % 77.5 % 93.1 % Toll manufacturing agreements 2.1 % 9.6 % 0.0 % Ildong Agreement (see Note 9) 2.0 % 1.3 % 0.4 % Other collaboration agreements 2.8 % 11.6 % 6.5 % Total percentage of accounts receivable 100.0 % 100.0 % 100.0 % We purchase raw materials, starting materials, intermediates, API, excipients and other materials from commercial sources. To decrease the risk of an interruption to our supply, when we believe it is reasonable for us to do so, we source these materials from multiple suppliers so that, in general, the loss of any one source of supply would not have a material adverse effect on commercial production. However, currently we have only one or a limited number of suppliers for some of these materials. The loss of a primary source of supply would potentially delay our production. Our facility in Zofingen, Switzerland is currently the only manufacturer of finished drug product for BELVIQ. Eisai maintains a safety stock of BELVIQ to help mitigate risks related to having only one manufacturer of finished drug product.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated over the estimated useful lives of the assets (generally 3 to 15 years) using the straight-line method. Buildings are stated at cost and depreciated over an estimated useful life of approximately 20 years using the straight-line method. Leasehold improvements are stated at cost and amortized over the shorter of the estimated useful lives of the assets or the lease term using the straight-line method. Capital improvements are stated at cost and amortized over the estimated useful lives of the underlying assets using the straight-line method. On December 31, 2016, for the property and equipment located at our Zofingen, Switzerland facility, we recorded an impairment charge of $17.2 million, changed our estimate of the useful life for these assets to be two remaining years and expect to depreciate the remaining carrying value of these assets using the straight-line method over this period pursuant to the Eisai Agreement (see Note 9).
Intangibles	Intangibles Intangible assets consist of our manufacturing facility production licenses we acquired from Siegfried in January 2008. Through December 2016, we amortized these assets using the straight-line method over their estimated useful life of 20 years. On December 31, 2016, we recorded an impairment charge of $4.6 million for these assets, changed our estimate of the useful life for these assets to be two remaining years and expect to amortize the remaining carrying value of these assets using the straight-line method over this period pursuant to the Eisai Agreement (see Note 9).
Long-lived Assets	Long-lived Assets If indicators of impairment exist, we assess the recoverability of the affected long-lived assets by determining whether the carrying value of such assets can be recovered through undiscounted cash flows. If impairment is indicated, we measure the impairment loss by comparing the fair value to the carrying value of the asset.
Deferred Rent	Deferred Rent For financial reporting purposes, rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under lease agreements is recorded as deferred rent in the liability section of our consolidated balance sheets.
Derivative Liabilities	Derivative Liabilities We account for warrants and other derivative financial instruments as either equity or liabilities based upon the characteristics and provisions of each instrument. Warrants classified as equity are recorded as additional paid-in capital on our consolidated balance sheets and no further adjustments to their valuation are made. Warrants classified as derivative liabilities and other derivative financial instruments that require separate accounting as liabilities are recorded on our consolidated balance sheets at their fair value on the date of issuance and are revalued on each balance sheet date until such instruments are exercised or expire, with changes in the fair value between reporting periods recorded as other income or expense. There were no warrants classified as derivative liabilities as of December 31, 2016, and 2015.
Foreign Currency	Foreign Currency The functional currency of our wholly owned subsidiaries in Switzerland, Arena GmbH and Arena Pharmaceuticals Development GmbH is the Swiss franc. Accordingly, all assets and liabilities of these subsidiaries are translated to US dollars based on the applicable exchange rate on the balance sheet date. Revenue and expense components are translated to US dollars at weighted-average exchange rates in effect during the period. Gains and losses resulting from foreign currency translation are reported as a separate component of accumulated other comprehensive income or loss in the equity section of our consolidated balance sheets. Foreign currency transaction gains and losses, which are primarily the result of remeasuring US dollar-denominated receivables and payables at Arena GmbH, are recorded in the interest and other income (expense) section of our consolidated statements of operations and comprehensive loss. For the year ended December 31, 2016, we recognized foreign currency transaction gains, net of $0.9 million. For the year ended December 31, 2015, we recognized foreign currency transaction gains, net of $2.0 million. For the year ended December 31, 2014, we recognized foreign currency transaction losses, net of $2.2 million.
Share-based Compensation	Share-based Compensation Our share-based awards are measured at fair value and recognized over the requisite service or performance period. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option pricing model, based on the market price of the underlying common stock, expected life, expected stock price volatility and expected risk-free interest rate. Expected volatility is computed using a combination of historical volatility for a period equal to the expected term and implied volatilities from traded options to buy our common stock, with historical volatility being weighted at 75%. The expected life of options is determined based on historical experience of similar awards, giving consideration to the contractual terms of the share-based awards, vesting schedules and post-vesting terminations. The risk-free interest rates are based on the US Treasury yield curve, with a remaining term approximately equal to the expected term used in the option pricing model. The fair value of each restricted stock unit award is estimated based on the market price of the underlying common stock on the date of the grant. The fair value of restricted stock unit awards that include market-based performance conditions is estimated on the date of grant using a Monte Carlo simulation model, based on the market price of the underlying common stock, expected performance measurement period, expected stock price volatility and expected risk-free interest rate. We estimate forfeitures at the time of grant and revise our estimate in subsequent periods if actual forfeitures differ from those estimates.
Revenue Recognition	Revenue Recognition Our revenues to date have been generated primarily through collaboration agreements and, to a lesser extent, toll manufacturing agreements. Our collaboration agreements may contain multiple elements including commercialization rights, services (joint steering committee and research and development services) and manufactured products. 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. We recognize revenue when (i) persuasive evidence of an arrangement exists, (ii) delivery has occurred and title has passed, (iii) the price is fixed or determinable and (iv) collectability is reasonably assured. Any advance payments we receive in excess of amounts earned are classified as deferred revenues. We previously deferred recognition of product sales and the related costs at the time we sold BELVIQ to our collaborators because we did not have the ability to estimate the amount of product that could have been returned to us and, as such, recognized revenues and the related costs from net product sales when our collaborators shipped BELVIQ to their distributors. In December 2016, primarily pursuant to a change in the terms of the Eisai Agreement (see Note 9), we determined that we now have the ability to reasonably estimate the amount of returns and thus now recognize revenue and the related cost from product sales when we ship BELVIQ to our collaborators. In December 2016, we recognized revenues and the related costs on net product sales which had been previously deferred. We evaluate deliverables in a multiple-element arrangement to determine whether each deliverable represents a separate unit of accounting. A deliverable constitutes a separate unit of accounting when it has standalone value to the customer. If the delivered element does not have standalone value without one of the undelivered elements in the arrangement, we combine such elements and account for them as a single unit of accounting. We allocate the consideration to each unit of accounting at the inception of the arrangement based on the relative selling price. To determine the selling price of a separate deliverable, we use the hierarchy as prescribed in Accounting Standards Codification Topic 605-25 based on vendor-specific objective evidence, or VSOE, third-party evidence, or TPE, or best estimate of selling price, or BESP. VSOE is based on the price charged when the element is sold separately and is the price actually charged for that deliverable. TPE is determined based on third-party evidence for a similar deliverable when sold separately. BESP is the estimated selling price at which we would transact a sale if the elements of collaboration and license arrangements were sold on a stand-alone basis to the buyer. Non-refundable upfront payments received under our collaboration agreements for commercialization rights have been deferred as such rights have not been deemed to have standalone value without the ongoing services required under the agreement. Such amounts are recognized as revenues on a straight-line basis over the period in which we expect to perform the services. In December 2016, we recognized a portion of the previously unrecognized non-refundable upfront payments received from Eisai as revenues in the amount of arrangement consideration allocated to the unit of accounting delivered to Eisai under the Eisai Agreement (see Note 9). Amounts we receive as reimbursement for our research and development expenditures are recognized as revenue as the services are performed. Under the milestone method, we recognize revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is an event (i) that can be achieved in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance, (ii) for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved and (iii) that would result in additional payments being due us. A milestone payment is considered substantive when the consideration payable to us for each milestone (a) is consistent with our performance necessary to achieve the milestone or the increase in value to the collaboration resulting from our performance, (b) relates solely to our past performance and (c) is reasonable relative to all of the other deliverables and payments under the arrangement. In making this assessment, we consider all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether any portion of the milestone consideration is related to future performance or deliverables. Other contingent-based payments received are recognized when earned. We also manufacture drug products under toll manufacturing agreements. Upon the customer’s acceptance of drug products manufactured by us under these agreements, we recognize toll manufacturing revenues.
Research and Development Expenses	Research and Development Expenses Research and development expenses, which consist primarily of salaries and other personnel costs, clinical trial costs and preclinical study fees, manufacturing costs for non-commercial products, and the development of earlier-stage programs and technologies, are expensed as incurred when these expenditures have no alternative future uses. We accrue clinical trial expenses based on work performed. In determining the amount to accrue, we rely on estimates of total costs incurred based on enrollment, the completion of trials and other events. We follow this method because we believe reasonably dependable estimates of the costs applicable to various stages of a clinical trial can be made. However, the actual costs and timing of clinical trials are highly uncertain, subject to risks and may change depending on a number of factors. Differences between the actual clinical trial costs and the estimated clinical trial costs that we have accrued in any prior period are recognized in the subsequent period in which the actual costs become known. Historically, these differences have not been material; however, material differences could occur in the future. Payments made to reimburse collaborators for our share of their research and development activities are recorded as research and development expenses, and are recognized as the work is performed.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and circumstances from non-owner sources. We report components of comprehensive loss in the period in which they are recognized. For the years ended December 31, 2016, 2015, and 2014, comprehensive loss consisted of net loss and foreign currency translation gains and losses.
Net Loss Per Share	Net Loss Per Share We calculate basic and diluted net loss 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 have excluded from our calculation of diluted net loss per share all potentially dilutive in-the-money (i) stock options, (ii) restricted stock unit awards, or RSUs, (iii) Total Stockholder Return, or TSR, performance restricted stock unit, or PRSU, awards, (iv) unvested restricted stock in our deferred compensation plan and (v) our previously outstanding warrants, and our diluted net loss per share is the same as our basic net loss per share. The table below presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share for the years presented, in thousands. Years ended December 31, 2016 2015 2014 Stock options 24,947 17,030 15,530 Warrants — 19 370 RSUs and unvested restricted stock 210 547 476 Total 25,157 17,596 16,376 Because the market condition for the PRSUs was not satisfied at December 31, 2016, 2015, and 2014, such securities are excluded from the table above.
Income Taxes	Income Taxes We use the asset and liability method of accounting for income taxes. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Our deferred tax assets and liabilities are determined using the enacted tax rates expected to be in effect for the years in which those tax assets are expected to be realized. The realization of our deferred tax assets is dependent upon our ability to generate sufficient future taxable income. We establish a valuation allowance when it is more-likely-than-not the future realization of all or some of the deferred tax assets will not be achieved. The evaluation of the need for a valuation allowance is performed on a jurisdiction-by-jurisdiction basis, and includes a review of all available evidence, both positive and negative. The impact of an uncertain income tax position is recognized at the largest amount that is more-likely-than-not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained.
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.

The Company and Summary of Si25

The Company and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share	The table below presents the weighted-average number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share for the years presented, in thousands. Years ended December 31, 2016 2015 2014 Stock options 24,947 17,030 15,530 Warrants — 19 370 RSUs and unvested restricted stock 210 547 476 Total 25,157 17,596 16,376
Sales Revenue, Net
Percentages of Total Revenues and Accounts Receivable	Percentages of our total revenues are as follows: Years ended December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 79.8 % 61.9 % 93.6 % Ildong Agreement (see Note 9) 9.2 % 23.2 % 1.0 % Toll manufacturing agreements 3.3 % 11.1 % 4.0 % Other collaboration agreements 7.7 % 3.8 % 1.4 % Total percentage of revenues 100.0 % 100.0 % 100.0 %
Accounts Receivable
Percentages of Total Revenues and Accounts Receivable	Percentages of our total accounts receivable are as follows: December 31, 2016 2015 2014 Eisai Agreement (see Note 9) 93.1 % 77.5 % 93.1 % Toll manufacturing agreements 2.1 % 9.6 % 0.0 % Ildong Agreement (see Note 9) 2.0 % 1.3 % 0.4 % Other collaboration agreements 2.8 % 11.6 % 6.5 % Total percentage of accounts receivable 100.0 % 100.0 % 100.0 %

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 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 $ 46,371 $ 46,371 $ — $ — 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 $ — $ — (1) Included in cash and cash equivalents on our consolidated balance sheets.

Balance Sheet Details (Tables)

Balance Sheet Details (Tables)	12 Months Ended
	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Components of Inventory	Inventory consisted of the following, in thousands: December 31, 2016 2015 Raw materials $ 2,553 $ 2,487 Work in process 3,943 2,781 Finished goods at Arena GmbH 212 165 Finished goods at Eisai — 3,309 Finished goods at Ildong — 760 Total inventory $ 6,708 $ 9,502
Land, Property and Equipment, Net	Land, property and equipment, net consisted of the following, in thousands: December 31, 2016 2015 Land $ 7,809 $ 8,131 Building and capital improvements 58,609 74,663 Leasehold improvements 17,769 18,025 Machinery and equipment 16,801 53,790 Computers and software 5,737 15,893 Furniture and office equipment 1,631 2,227 108,356 172,729 Less accumulated depreciation and amortization (64,528 ) (100,901 ) Land, property and equipment, net $ 43,828 $ 71,828
Intangibles	Intangibles consisted of the following, in thousands: December 31, 2016 2015 Acquired manufacturing production licenses – gross $ 2,357 $ 12,958 Acquired manufacturing production licenses – accumulated amortization — (5,183 ) Intangibles, net $ 2,357 $ 7,775
Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following, in thousands: December 31, 2016 2015 Accounts payable $ 5,977 $ 2,078 Accrued compensation 4,820 5,118 Accrued workforce reduction expense 62 1,793 Other accrued liabilities 1,257 1,138 Total accounts payable and other accrued liabilities $ 12,116 $ 10,127

Commitments (Tables)

Commitments (Tables)	12 Months Ended
	Dec. 31, 2016
Operating Leased Assets [Line Items]
Future Minimum Lease Payments	At December 31, 2016, the future minimum lease payments under our existing financing and operating lease obligation are as follows, in thousands: Year ending December 31, Financing Obligations Operating Leases 2017 $ 8,712 $ 1,259 2018 9,731 1,353 2019 8,053 1,376 2020 8,254 1,266 2021 8,461 976 Thereafter 49,613 5,723 Total minimum lease payments 92,824 $ 11,953 Less amounts representing interest (37,548 ) Add amounts representing residual value 9,990 Lease financing obligations 65,266 Less current portion (3,518 ) $ 61,748
Sublease Agreement
Operating Leased Assets [Line Items]
Expected Minimum Rental Payments to be Received under Sublease	Expected minimum rental payments to be received under the sublease are as follows: Year ending December 31, 2017 $ 714 2018 737 2019 760 2020 784 2021 809 Thereafter 4,846 Total $ 8,650

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes our stock option activity under the Prior Plans and the 2013 LTIP, or collectively, our Equity Compensation Plans, for the year ended December 31, 2016, in thousands (except per share data): Options Weighted- Average Exercise Price Weighted-Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2015 16,407 $ 5.01 Granted 17,899 $ 1.61 Exercised (116 ) $ 1.55 Forfeited/cancelled/expired (8,990 ) $ 3.83 Outstanding at December 31, 2016 25,200 $ 3.03 4.28 $ 40 Vested and expected to vest at December 31, 2016 23,945 $ 3.10 4.17 $ 40 Vested and exercisable at December 31, 2016 12,808 $ 4.15 2.37 $ 40
Summary of Restricted Stock Units Activity	The following table summarizes activity with respect to our time-based RSUs under our Equity Compensation Plans for the year ended December 31, 2016, in thousands (except per share data): RSUs Weighted-Average Grant-Date Fair Value Aggregate Intrinsic Value Unvested at December 31, 2015 273 $ 4.67 Granted — Vested (233 ) $ 4.75 Forfeited/cancelled (15 ) $ 4.23 Unvested at December 31, 2016 25 $ 4.26 Outstanding at December 31, 2016 539 $ 5.05 $ 2,723
Weighted-Average Assumptions and Estimated Fair Value of Stock Options	The table below sets forth the weighted-average assumptions and estimated fair value of stock options we granted under our Equity Compensation Plans during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate 1.4 % 1.8 % 1.8 % Dividend yield 0 % 0 % 0 % Expected volatility 79 % 80 % 81 % Expected life (years) 4.81 6.08 6.17 Weighted-average estimated fair value per share of stock options granted $ 1.02 $ 2.55 $ 4.37
Assumptions and Estimated Fair Value of Options to Purchase Stock granted Under Employee Stock Purchase Plan	The table below sets forth the assumptions and estimated fair value of the options to purchase stock granted under our employee stock purchase plan for multiple offering periods during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate 0.2% - 1.2% 0.0% - 1.0% 0.0% - 0.6% Dividend yield 0% 0% 0% Expected volatility 75% - 82% 52% - 78% 53% - 81% Expected life (years) .25 - 2.0 .25 - 2.0 .25 - 2.0 Range of fair value per share of options granted under employee stock purchase plan $0.78 to $0.92 $0.78 to $2.94 $1.37 to $4.22
Assumptions and Estimated Fair Value of Performance Restricted Stock Unit Awards	The table below sets forth the assumptions and estimated fair value of PRSU awards granted during the years presented: Years ended December 31, 2016 2015 2014 Risk-free interest rate — 1.1 % 0.7 % Dividend yield — 0 % 0 % Expected volatility — 75 % 78 % Performance period (years) — 2.97 2.99 Estimated fair value per share of PRSUs granted — $ 4.50 $ 7.16
Share Based Compensation Expense	We recognized share-based compensation expense as follows for the years presented, in thousands, except per share data: Years ended December 31, 2016 2015 2014 Cost of product sales $ 45 $ 29 $ — Research and development 5,615 7,582 7,118 General and administrative 4,425 6,710 6,391 Restructuring charges 1,032 142 — Total share-based compensation expense and impact on net loss $ 11,117 $ 14,463 $ 13,509 Impact on net loss per share, basic and diluted $ 0.05 $ 0.06 $ 0.06 Total share-based compensation capitalized into inventory $ 170 $ 173 $ 81
Total Unrecognized Estimated Compensation Expense	The table below sets forth our total unrecognized estimated compensation expense at December 31, 2016, by type of award and the weighted-average remaining requisite service period over which such expense is expected to be recognized: Unrecognized Expense (in thousands) Remaining Weighted-Average Recognition Period (in years) Unvested stock options $ 10,381 3.14 RSUs 85 1.66 PRSUs 289 0.60
Common Stock Reserved for Future Issuance	The following shares of our common stock are reserved for future issuance at December 31, 2016, in thousands: Equity Compensation Plans 45,515 2009 ESPP 1,115 Deferred compensation plan 63 Total 46,693

Collaborations (Tables)

Collaborations (Tables)	12 Months Ended
	Dec. 31, 2016
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues Recognized under Eisai Agreement	The following table summarizes the revenues we recognized under our collaboration with Eisai for the periods presented, in thousands: Years ended December 31, 2016 2015 2014 Net product sales $ 19,196 $ 14,236 $ 15,983 Amortization of upfront payments 66,014 7,541 7,630 Milestone payments 12,000 — 500 Reimbursement of development expenses 1,295 1,538 10,037 Reimbursement of patent and trademark expenses 392 426 444 Subtotal other Eisai collaboration revenue 79,701 9,505 18,611 Total $ 98,897 $ 23,741 $ 34,594

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Summary of Loss Before Provision (Benefit) for Income Taxes by Region	The following table summarizes our loss attributable to stockholders of Arena before benefit for income taxes by region for the years presented, in thousands: Years ended December 31, 2016 2015 2014 United States $ (10,268 ) $ (64,109 ) $ (16,607 ) Foreign (12,248 ) (43,870 ) (43,901 ) Total loss attributable to stockholders of Arena before income taxes $ (22,516 ) $ (107,979 ) $ (60,508 )
Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes	Our effective income tax rate differs from the statutory federal rate of 34% for the years presented due to the following, in thousands: Years ended December 31, 2016 2015 2014 Benefit for income taxes at statutory federal rate $ (7,655 ) $ (36,713 ) $ (20,573 ) Change in federal and foreign valuation allowance 9,080 21,310 9,436 Permanent differences and other (5,931 ) 2,370 721 Share-based compensation expense 4,000 1,820 1,597 Foreign losses at lower effective rates 3,943 15,041 13,318 Research and development and Orphan Drug credits (3,437 ) (3,666 ) (2,992 ) State income tax, net of federal benefit and valuation allowance — — — Gain from valuation of derivative liabilities — (162 ) (1,507 ) Benefit for income taxes $ — $ — $ —
Components of Net Deferred Tax Assets	The components of our net deferred tax assets are as follows, in thousands: December 31, 2016 2015 Deferred tax assets: Federal and California NOL carryforwards $ 255,317 $ 236,334 Federal and California research and development credit carryforwards 53,059 48,768 Share-based compensation expense 10,395 10,737 Deferred revenues 9,357 33,548 Depreciation 5,441 4,475 Foreign NOL carryforwards 5,108 7,060 Other, net 5,164 3,578 Total deferred tax assets 343,841 344,500 Deferred tax liabilities (228 ) (660 ) Net deferred tax assets 343,613 343,840 Valuation allowance (343,613 ) (343,840 ) Net deferred tax liabilities $ — $ —
Reconciliation of Unrecognized Tax Benefits	The following table reconciles the beginning and ending amount of unrecognized tax benefits for the years presented, in thousands: Years ended December 31, 2016 2015 2014 Gross unrecognized tax benefits at the beginning of the year $ 5,619 $ 5,214 $ 4,629 Additions from tax positions taken in the current year 287 405 585 Additions from tax positions taken in prior years — — — Reductions from tax positions taken in prior years — — — Tax settlements — — — Gross unrecognized tax benefits at end of the year $ 5,906 $ 5,619 $ 5,214

Quarterly Financial Data (Una32

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Data	The following tables present selected quarterly financial data for the years presented, in thousands, except per share data: 2016 Quarter ended December 31 Quarter ended September 30 Quarter ended June 30 Quarter ended March 31 Revenues $ 85,374 $ 19,242 $ 9,512 $ 9,847 Operating costs and expenses 47,245 29,099 35,735 29,042 Net income (loss) 38,314 (12,479 ) (27,183 ) (21,548 ) Net income (loss) attributable to stockholders of Arena 38,572 (12,357 ) (27,183 ) (21,548 ) Net income (loss) attributable to stockholders of Arena per share, basic and diluted 0.16 (0.05 ) (0.11 ) (0.09 ) 2015 Quarter December 31 Quarter ended September 30 Quarter ended June 30 Quarter ended March 31 Revenues $ 7,751 $ 9,138 $ 9,181 $ 12,256 Operating costs and expenses 37,045 34,319 36,160 34,000 Net loss (30,459 ) (26,418 ) (26,807 ) (24,295 ) Net loss per share, basic and diluted (0.13 ) (0.11 ) (0.11 ) (0.10 )

Beacon Discovery, Inc. (Tables)

Beacon Discovery, Inc. (Tables)	12 Months Ended
	Dec. 31, 2016
Beacon Discovery, Inc.
Variable Interest Entity [Line Items]
Summary of Assets and Liabilities Included in Consolidated Balance Sheet	The following table presents the assets and liabilities of Beacon which are included in our consolidated balance sheet at December 31, 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 December 31, 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 $ 311 Prepaid expense and other current assets 28 Land, property and equipment, net 705 Total assets of Beacon that can only be used to settle obligation of Beacon $ 1,044 Liabilities of Beacon for which creditors do not have recourse to the general credit of Arena Accounts payable and other accrued liabilities $ 86 Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena $ 86

The Company and Summary of Si34

The Company and Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2016USD ($)Segment	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Schedule Of Significant Accounting Policies [Line Items]
Number of business segments | Segment									1
Net loss	$ 38,314,000	$ (12,479,000)	$ (27,183,000)	$ (21,548,000)	$ (30,459,000)	$ (26,418,000)	$ (26,807,000)	$ (24,295,000)	$ (22,896,000)	$ (107,979,000)	$ (60,508,000)
Net cash used in operating activities									$ (62,139,000)	(98,113,000)	(101,398,000)
Intangible assets, estimated useful life									20 years
Warrants classified as derivative liabilities	$ 0				$ 0				$ 0	0
Foreign currency transaction gain (loss)									$ 900,000	$ 2,000,000	$ (2,200,000)
Weighted average volatility									75.00%
Eisai | Change of Estimate
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									2 years
Impairment charge of property and equipment									$ 17,200,000
Intangible assets, estimated useful life									2 years
Impairment charge of intangible assets									$ 4,600,000
Building
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									20 years
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									3 years
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life									15 years

The Company and Summary of Si35

The Company and Summary of Significant Accounting Policies - Percentages of Total Revenues (Detail) - Sales Revenue, Net	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Percentage of revenues	100.00%	100.00%	100.00%
Eisai Agreement
Concentration Risk [Line Items]
Percentage of revenues	79.80%	61.90%	93.60%
Ildong Agreement
Concentration Risk [Line Items]
Percentage of revenues	9.20%	23.20%	1.00%
Toll manufacturing agreements
Concentration Risk [Line Items]
Percentage of revenues	3.30%	11.10%	4.00%
Other collaboration agreements
Concentration Risk [Line Items]
Percentage of revenues	7.70%	3.80%	1.40%

The Company and Summary of Si36

The Company and Summary of Significant Accounting Policies - Percentages of Total Accounts Receivable (Detail) - Accounts Receivable	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Percentage of accounts receivable	100.00%	100.00%	100.00%
Eisai Agreement
Concentration Risk [Line Items]
Percentage of accounts receivable	93.10%	77.50%	93.10%
Toll manufacturing agreements
Concentration Risk [Line Items]
Percentage of accounts receivable	2.10%	9.60%	0.00%
Ildong Agreement
Concentration Risk [Line Items]
Percentage of accounts receivable	2.00%	1.30%	0.40%
Other collaboration agreements
Concentration Risk [Line Items]
Percentage of accounts receivable	2.80%	11.60%	6.50%

The Company and Summary of Si37

The Company and Summary of Significant Accounting Policies - Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share (shares)	25,157	17,596	16,376
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)	24,947	17,030	15,530
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss per share (shares)		19	370
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)	210	547	476

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	Dec. 31, 2016	Dec. 31, 2015
Assets:
Money market funds	$ 46,371	$ 113,080
Quoted Prices in Active Markets (Level 1)
Assets:
Money market funds	$ 46,371	$ 113,080

Short-term Investments and Av39

Short-term Investments and Available-for-Sale Securities - Additional Information (Detail) - TaiGen Biotechnology Co., Ltd. - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2014	Jan. 17, 2014	Dec. 31, 2011
Investment Holdings [Line Items]
Equity investment			$ 0
Investment, number of shares		29.6
Investment, fair value		$ 49,100,000
Realized gains from sale of shares	$ 49,600,000

Balance Sheet Details - Compone

Balance Sheet Details - Components of Inventory (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Inventory [Line Items]
Raw materials	$ 2,553	$ 2,487
Work in process	3,943	2,781
Total inventory	6,708	9,502
Arena GmbH
Inventory [Line Items]
Finished goods	$ 212	165
Eisai
Inventory [Line Items]
Finished goods		3,309
Ildong
Inventory [Line Items]
Finished goods		$ 760

Balance Sheet Details - Land, P

Balance Sheet Details - Land, Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	$ 108,356	$ 172,729
Less accumulated depreciation and amortization	(64,528)	(100,901)
Land, property and equipment, net	43,828	71,828
Land
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	7,809	8,131
Building and capital improvements
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	58,609	74,663
Leasehold improvements
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	17,769	18,025
Machinery and equipment
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	16,801	53,790
Computers and software
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	5,737	15,893
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Land, property and equipment, gross	$ 1,631	$ 2,227

Balance Sheet Details - Intangi

Balance Sheet Details - Intangibles (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]
Acquired manufacturing production licenses – gross	$ 2,357	$ 12,958
Acquired manufacturing production licenses – accumulated amortization		(5,183)
Intangibles, net	$ 2,357	$ 7,775

Balance Sheet Details - Additio

Balance Sheet Details - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 28, 2016	Dec. 31, 2016	Dec. 31, 2015
Finite Lived Intangible Assets [Line Items]
Impairment of long-lived assets		$ 21,766
Expected annual amortization expense 2017		1,200
Expected annual amortization expense 2018		1,200
United States
Finite Lived Intangible Assets [Line Items]
Long-lived assets		35,100	$ 41,500
Switzerland
Finite Lived Intangible Assets [Line Items]
Long-lived assets		11,100	$ 38,100
Eisai
Finite Lived Intangible Assets [Line Items]
Collaboration agreement initial term	2 years
Collaboration agreement extended initial term	6 months
Impairment of long-lived assets		32,900
Estimated fair value of long-lived assets		$ 11,100

Balance Sheet Details - Account

Balance Sheet Details - Accounts Payable and Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Accounts payable	$ 5,977	$ 2,078
Accrued compensation	4,820	5,118
Accrued workforce reduction expense	62	1,793
Other accrued liabilities	1,257	1,138
Total accounts payable and other accrued liabilities	$ 12,116	$ 10,127

Agreements with Siegfried - Add

Agreements with Siegfried - Additional Information (Detail) SFr in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2014CHF (SFr)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Jul. 31, 2015CHF (SFr)	Jan. 31, 2008CHF (SFr)ft²
Contractors [Abstract]
Expenses incurred under technical services agreement | $		$ 1.4	$ 1.3	$ 2.5
Asset purchase agreement, area of building acquired (square feet) | ft²						67,000
Asset purchase agreement, total area of building (square feet) | ft²						134,000
Company's optional purchase price of remaining portion of the building						SFr 15
Seller's optional selling price of remaining portion of the building						SFr 8
Real estate property purchase price					SFr 8.2
Annual base rent amount	SFr 0.4
Termination notice	6 months
Extended lease expiration date		Dec. 31, 2019
Early termination date lessor option		Dec. 31, 2017

Derivative Liabilities - Additi

Derivative Liabilities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2014	Aug. 31, 2008	Jun. 30, 2006
Derivative [Line Items]
Issuance of warrant to purchase stock, number of shares			1,106,344	829,856
Original exercise price of warrant ($ per share)			$ 7.71	$ 15.49
Deerfield Acceleration Right
Derivative [Line Items]
Fair value of embedded liabilities		$ 0.5
Warrants
Derivative [Line Items]
Term of instrument	7 years

Commitments - Additional Inform

Commitments - Additional Information (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2016Property	Dec. 31, 2016USD ($)Property	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Commitments [Line Items]
Number of properties under sale and leaseback agreements | Property		4
Sale and leaseback transaction, interest expense		$ 6,400	$ 6,700	$ 6,900
Less amounts representing interest		$ 37,548
Sale leaseback transaction, description		Through the remaining terms of the leases in fiscal year 2027
Sale and leaseback transaction, total financing obligation		$ 65,266
Add amounts representing residual value		9,990
Deposits related to leases		900	800
Rent expense		$ 1,200	$ 1,100	$ 1,100
Number of properties subleased | Property	1
Sublease commencement period	2016-08
Sublease expiration period	2027-05
Percentage of annual increase in monthly rental payment	3.19%
Properties under sale and leaseback agreements
Commitments [Line Items]
Percentage of annual increase in monthly rental payments		2.50%
Properties under operating lease | CALIFORNIA
Commitments [Line Items]
Percentage of annual increase in monthly rental payments		2.50%
Operating lease agreement expiration date		2027-05
Properties under operating lease | SWITZERLAND
Commitments [Line Items]
Operating lease agreement expiration date		2020-08
Lease termination period		12 months
Operating lease agreement expiration year		2,032
Properties under operating lease | Zug, Switzerland
Commitments [Line Items]
Operating lease agreement expiration date		2020-09

Commitments - Future Minimum Le

Commitments - Future Minimum Lease Payments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Obligations
2,017	$ 8,712
2,018	9,731
2,019	8,053
2,020	8,254
2,021	8,461
Thereafter	49,613
Total minimum lease payments	92,824
Less amounts representing interest	(37,548)
Add amounts representing residual value	9,990
Lease financing obligations	65,266
Current portion of lease financing obligations	(3,518)	$ (2,978)
Lease financing obligations, less current portion	61,748	$ 65,267
Operating Leases
2,017	1,259
2,018	1,353
2,019	1,376
2,020	1,266
2,021	976
Thereafter	5,723
Total minimum lease payments	$ 11,953

Commitments - Expected Minimum

Commitments - Expected Minimum Rental Payments to be Received under Sublease (Detail) - Sublease Agreement $ in Thousands	Dec. 31, 2016USD ($)
Operating Leased Assets [Line Items]
2,017	$ 714
2,018	737
2,019	760
2,020	784
2,021	809
Thereafter	4,846
Total	$ 8,650

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Mar. 15, 2017	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 10, 2013
Stockholders Equity [Line Items]
Common stock closing price ($ per share)					$ 1.42
Intrinsic value of stock options exercised						$ 2,200,000	$ 2,700,000
Proceeds from stock option exercises					$ 200,000
Tax impact from exercise of stock options					$ 0
Number of shares available for issuance					1,115,188
Issuance of common stock under employee stock purchase plan (in shares)					141,397	327,950	304,085
Maximum
Stockholders Equity [Line Items]
Intrinsic value of stock options exercised					$ 100,000
Restricted Stock Units (RSU)
Stockholders Equity [Line Items]
RSUs and restricted stock unvested (shares)					25,000	273,000
Total fair value of RSUs vested					$ 1,100,000	$ 2,100,000	$ 1,800,000
Estimated fair value per share of PRSUs granted ($ per share)						$ 4.11	$ 5.23
Units granted (shares)					0
Shares cancelled due to management changes					15,000
Performance restricted stock units
Stockholders Equity [Line Items]
Vesting period			3 years	3 years	3 years
Estimated fair value per share of PRSUs granted ($ per share)						$ 4.50	$ 7.16
Units granted (shares)		745,000	695,000	780,000
Multiplier of grant-date fair value for cap on number of PRSUs that may be granted					600.00%
Cap on percentage of funding if the absolute 3-year TSR is negative					100.00%
PRSU grant-date fair value		$ 3,400,000	$ 5,000,000	$ 5,900,000
Shares cancelled due to management changes					355,556
Performance restricted stock units | Minimum
Stockholders Equity [Line Items]
Number of shares to be awarded as a percentage of target amounts					0.00%
Performance restricted stock units | Maximum
Stockholders Equity [Line Items]
Number of shares to be awarded as a percentage of target amounts					200.00%
Deferred compensation plan
Stockholders Equity [Line Items]
RSUs and restricted stock unvested (shares)					62,501	79,169
Long Term Incentive Plan Twenty Thirteen
Stockholders Equity [Line Items]
Number of shares authorized								30,000,000
Stock options exercisable period					7 years
Long Term Incentive Plan Twenty Thirteen | Subsequent Event
Stockholders Equity [Line Items]
Additional stock options granted to our employees and directors	14,000,000
Long Term Incentive Plan Twenty Thirteen | Other Than Stock Options And Stock Appreciation Rights
Stockholders Equity [Line Items]
Reduction in number of shares available for grant for every share issued, percentage					125.00%
Increase in number of shares available for grant for every share released, percentage					125.00%
Long Term Incentive Plan Twenty Thirteen | Stock options
Stockholders Equity [Line Items]
Vesting percentage					25.00%
Vesting period					4 years
Long Term Incentive Plan Twenty Thirteen | Restricted Stock Units (RSU) | Minimum
Stockholders Equity [Line Items]
Vesting period					1 year
Long Term Incentive Plan Twenty Thirteen | Restricted Stock Units (RSU) | Maximum
Stockholders Equity [Line Items]
Vesting period					4 years
Long Term Incentive Plan Twenty Thirteen | Performance restricted stock units | Minimum
Stockholders Equity [Line Items]
Vesting period					12 months
Long Term Incentive Plan Twenty Thirteen | Stock Options and Stock Appreciation Rights (SARs) | Minimum
Stockholders Equity [Line Items]
Purchase price of common shares as a percentage of fair market value					100.00%
2009 ESPP
Stockholders Equity [Line Items]
Proceeds from stock issued under employee stock purchase plans					$ 200,000
Maximum Percentage of Annual Compensation contributable to ESPP					15.00%
Maximum number of shares allowed to purchase per PURCHASE period					625
Maximum ESPP offering period					24 months
2009 ESPP | Minimum
Stockholders Equity [Line Items]
Purchase price of common shares as a percentage of fair market value					85.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Stock Option Activity (Detail) - Stock options $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)$ / sharesshares
Number of stock options
Outstanding at December 31, 2015 | shares	16,407
Granted | shares	17,899
Exercised | shares	(116)
Forfeited/cancelled/expired | shares	(8,990)
Outstanding at December 31, 2016 | shares	25,200
Vested and expected to vest at December 31, 2016 | shares	23,945
Vested and exercisable at December 31, 2016 | shares	12,808
Weighted Average Exercise Price ($ per share)
Outstanding at December 31, 2015 | $ / shares	$ 5.01
Granted | $ / shares	1.61
Exercised | $ / shares	1.55
Forfeited/cancelled/expired | $ / shares	3.83
Outstanding at December 31, 2016 | $ / shares	3.03
Vested and expected to vest at December 31, 2016 | $ / shares	3.10
Vested and exercisable at December 31, 2016 | $ / shares	$ 4.15
Weighted-Average Remaining Contractual Term (in years)
Outstanding at December 31, 2016	4 years 3 months 11 days
Vested and expected to vest at December 31, 2016	4 years 2 months 1 day
Vested and exercisable at December 31, 2016	2 years 4 months 13 days
Aggregate Intrinsic Value
Outstanding at December 31, 2016 | $	$ 40
Vested and expected to vest at December 31, 2016 | $	40
Vested and exercisable at December 31, 2016 | $	$ 40

Stockholders' Equity - Summar52

Stockholders' Equity - Summary of Restricted Stock Units Activity (Detail) - Restricted Stock Units (RSU) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock Units (shares)
Unvested at December 31, 2015	273,000
Granted	0
Vested	(233,000)
Forfeited/cancelled	(15,000)
Unvested at December 31, 2016	25,000	273,000
Outstanding at December 31, 2016	539,000
Weighted-Average Grant-Date Fair Value ($ per share)
Unvested at December 31, 2015	$ 4.67
Granted		$ 4.11	$ 5.23
Vested	4.75
Forfeited/cancelled	4.23
Unvested at December 31, 2016	4.26	$ 4.67
Outstanding at December 31, 2016	$ 5.05
Aggregate Intrinsic Value
Outstanding at December 31, 2016	$ 2,723

Stockholders' Equity - Weighted

Stockholders' Equity - Weighted-Average Assumptions and Estimated Fair Value of Stock Options (Detail) - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.40%	1.80%	1.80%
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	79.00%	80.00%	81.00%
Expected life (years)	4 years 9 months 22 days	6 years 29 days	6 years 2 months 1 day
Weighted-average estimated fair value per share of stock options granted	$ 1.02	$ 2.55	$ 4.37

Stockholders' Equity - Assumpti

Stockholders' Equity - Assumptions and Estimated Fair Value of Options to Purchase Stock granted Under Employee Stock Purchase Plan (Detail) - 2009 ESPP - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate Minimum	0.20%	0.00%	0.00%
Risk-free interest rate Maximum	1.20%	1.00%	0.60%
Dividend yield	0.00%	0.00%	0.00%
Expected volatility Minimum	75.00%	52.00%	53.00%
Expected volatility Maximum	82.00%	78.00%	81.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	3 months	3 months	3 months
Range of fair value per share of options granted under employee stock purchase plan	$ 0.78	$ 0.78	$ 1.37
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (years)	2 years	2 years	2 years
Range of fair value per share of options granted under employee stock purchase plan	$ 0.92	$ 2.94	$ 4.22

Stockholders' Equity - Assump55

Stockholders' Equity - Assumptions and Estimated Fair Value of Performance Restricted Stock Unit Awards (Detail) - Performance restricted stock units - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.10%	0.70%
Dividend yield	0.00%	0.00%
Expected volatility	75.00%	78.00%
Performance period (years)	2 years 11 months 19 days	2 years 11 months 27 days
Estimated fair value per share of PRSUs granted ($ per share)	$ 4.50	$ 7.16

Stockholders' Equity - Share Ba

Stockholders' Equity - Share Based Compensation Expense (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 11,117	$ 14,463	$ 13,509
Impact on net loss per share, basic and diluted		$ 0.05	$ 0.06	$ 0.06
Total share-based compensation capitalized into inventory		$ 170	$ 173	$ 81
Cost of product sales
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		45	29
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		5,615	7,582	7,118
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		4,425	6,710	$ 6,391
Restructuring charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,000	$ 1,032	$ 142

Stockholders' Equity - Total Un

Stockholders' Equity - Total Unrecognized Estimated Compensation Expense (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Unvested stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense (in thousands)	$ 10,381
Remaining Weighted-Average Recognition Period (in years)	3 years 1 month 21 days
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense (in thousands)	$ 85
Remaining Weighted-Average Recognition Period (in years)	1 year 7 months 28 days
PRSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense (in thousands)	$ 289
Remaining Weighted-Average Recognition Period (in years)	7 months 6 days

Stockholders' Equity - Common S

Stockholders' Equity - Common Stock Reserved for Future Issuance (Detail) shares in Thousands	Dec. 31, 2016shares
Class of Stock [Line Items]
Common stock shares reserved for future issuance	46,693
Equity Compensation Plans
Class of Stock [Line Items]
Common stock shares reserved for future issuance	45,515
2009 ESPP
Class of Stock [Line Items]
Common stock shares reserved for future issuance	1,115
Deferred compensation plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance	63

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Dec. 28, 2016USD ($)	Dec. 15, 2016USD ($)	Dec. 31, 2016USD ($)	Oct. 31, 2016USD ($)	Jul. 31, 2016USD ($)	Jan. 31, 2016USD ($)	May 31, 2015USD ($)	Mar. 31, 2015USD ($)	Jul. 31, 2014USD ($)	Jul. 31, 2013USD ($)	Nov. 30, 2012USD ($)	Dec. 28, 2016USD ($)	Dec. 31, 2016USD ($)Milestone	Dec. 31, 2016CHF (SFr)Milestone	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenues			$ 2,167,000										$ 2,167,000		$ 87,617,000			$ 2,167,000
Net product sales													26,349,000		19,726,000	$ 15,983,000
Payable to Eisai			9,074,000										9,074,000		12,080,000			9,074,000
Recognized revenues on net product sales	$ 6,700,000
Cost of product sales	1,900,000												9,297,000		8,590,000	6,369,000
Total arrangement consideration													115,600,000
Payment received from Eisai	$ 10,000,000											$ 10,000,000
Inventory			6,708,000										6,708,000		9,502,000			6,708,000
Research and development													66,425,000		88,411,000	100,347,000
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from royalty payments													0
Recognized revenues													66,014,000		7,541,000	7,630,000
Recognized milestone revenue													12,000,000			500,000		86,500,000
Additional milestone payments on achievement			25,000,000										25,000,000					25,000,000
Net product sales													$ 19,196,000		14,236,000	15,983,000
Collaboration agreement initial term	2 years
Collaboration agreement extended initial term	6 months
Eisai | Arena GmbH
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of future expenses and losses arising from any potential product liability claims													50.00%	50.00%
Percentage of expenses and losses arising from any alleged defective manufacturing of product													50.00%	50.00%
Eisai | US Food and Drug Administration New Drug Application Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue					$ 10,000,000
Eisai | Federal Commission for the Protection Against Sanitary Risk Mexico Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue					$ 1,000,000
Eisai | Brazilian Health Surveillance Agency
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized milestone revenue			1,000,000
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 | 9.5% Royalty Rate | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount													$ 175,000,000
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 | 13.5% Royalty Rate | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount													$ 500,000,000
Eisai | 13.5% Royalty Rate | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount													$ 175,000,000
Eisai | 18.5% Royalty Rate
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate on annual net sales													18.50%	18.50%
Eisai | 18.5% Royalty Rate | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty portion of annual net sales amount													$ 500,000,000
Lorcaserin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales													250,000,000
Research and development													7,300,000		16,200,000	35,300,000
Ildong | BELVIQ
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments											$ 5,000,000
Recognized revenues													11,400,000		8,900,000	400,000
Recognized milestone revenue		$ 300,000						$ 3,000,000
Recognized revenues on net product sales			2,000,000
Cost of product sales			700,000
Purchase price range minimum											35.00%
Purchase price range maximum											45.00%
Annual net product sales threshold for maximum purchase price											$ 15,000,000
CY Biotech Company Limited
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments										$ 2,000,000
Recognized revenues													1,800,000		200,000		$ 200,000
TEVA Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments									$ 500,000
Recognized revenues													400,000		100,000	$ 300,000
Recognized milestone revenue													300,000
Roivant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments							$ 4,000,000
Recognized revenues													2,100,000		$ 1,100,000
Collaborative agreement revenue recognition period							5 years
Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			41,500,000										$ 41,500,000					41,500,000
Number of development milestone | Milestone													2	2
Number of regulatory milestone | Milestone													4	4
Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments						$ 7,500,000
Recognized revenues													$ 5,100,000
Additional milestone payments on achievement			251,000,000										$ 251,000,000					251,000,000
Collaborative agreement revenue recognition period						2 years
Number of development milestone | Milestone													3	3
Refund of withholding taxes				$ 1,200,000
Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of development milestone | Milestone													3	3
Number of regulatory milestone | Milestone													9	9
Number of commercial milestone | Milestone													4	4
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
Manufacturing and supply commitment period													2 years	2 years
Net product sales													$ 2,000,000
Payable to Eisai			9,100,000										9,100,000					9,100,000
Refund net overpayments													$ 0
Reduction of research and development expense												$ 3,700,000
Eisai Second Amended Agreement | United States
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Product purchase price													31.50%	31.50%
Eisai Second Amended Agreement | Eisai | US Food and Drug Administration New Drug Application Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses													90.00%	90.00%
Eisai Second Amended Agreement | Eisai | Non-US Food and Drug Administration New Drug Application Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses													50.00%	50.00%
Eisai Second Amended Agreement | Arena GmbH | US Food and Drug Administration New Drug Application Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses													10.00%	10.00%
Eisai Second Amended Agreement | Arena GmbH | Non-US Food and Drug Administration New Drug Application Approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of expenses													50.00%	50.00%
Prior to Eisai Agreement | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement upfront payments													$ 115,000,000
Eisai Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement initial term													2 years	2 years
Collaboration agreement extended initial term													6 months	6 months
Collaboration agreement extension fee | SFr														SFr 2,000,000
Proceeds from sale of inventory	$ 10,000,000
Eisai Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional manufacturing support payments | SFr														13,000,000
Manufacturing support payments to be received | SFr														SFr 6,000,000
Eisai Agreement | Upfront payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized revenues	58,500,000
Deferred revenues			20,900,000										$ 20,900,000					20,900,000
Eisai Agreement | Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of milestone payment													50.00%	50.00%
Eisai Agreement | Ildong | Upfront payments | BELVIQ
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized revenues	3,500,000
Eisai Agreement | CY Biotech Company Limited | Upfront payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized revenues	1,700,000
Eisai Agreement | TEVA Agreement | Upfront payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized revenues	$ 400,000
License Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition													$ 64,000,000
Inventory Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition													30,800,000
Consideration allocated to deliverable revenue													0
Carrying value of inventory recognized in cost of product sales													0
Deferred revenues			30,800,000										30,800,000					30,800,000
Inventory			4,400,000										4,400,000					4,400,000
Manufacturing and Supply Commitment Deliverable
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price of the deliverables for revenue recognition													20,800,000
Consideration allocated to deliverable revenue													0
Development Milestones | Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			4,000,000										4,000,000					4,000,000
Development Milestones | Boehringer Ingelheim
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			7,000,000										7,000,000					7,000,000
Development Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			30,000,000										30,000,000					30,000,000
Regulatory Milestones | Axovant
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			37,500,000										37,500,000					37,500,000
Regulatory Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			84,000,000										84,000,000					84,000,000
Commercial Milestones | Boehringer Ingelheim | Non-substantive
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments on achievement			$ 130,000,000										$ 130,000,000					$ 130,000,000

Collaborations - Revenues Recog

Collaborations - Revenues Recognized under Eisai Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended			84 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									$ 26,349	$ 19,726	$ 15,983
Total revenues	$ 85,374	$ 19,242	$ 9,512	$ 9,847	$ 7,751	$ 9,138	$ 9,181	$ 12,256	123,975	38,326	36,970
Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net product sales									19,196	14,236	15,983
Amortization of upfront payments									66,014	7,541	7,630
Milestone payments									12,000		500	$ 86,500
Subtotal other Eisai collaboration revenue									79,701	9,505	18,611
Total revenues									98,897	23,741	34,594
Eisai | Development expenses
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses									1,295	1,538	10,037
Eisai | Patent and trademark expenses
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses									$ 392	$ 426	$ 444

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information - 401 (K) Plan (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, percentage employer matching contribution of first 6% contributed	100.00%
Maximum employer matching contribution percentage per employee	6.00%
Employer matching contributions	$ 1.3	$ 1.9	$ 1.6
Defined contribution plan, employer matching contribution vesting period in Years	5 years

Employee Benefit Plans - Addi62

Employee Benefit Plans - Additional Information - Pension Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
Multi-employer plan, employer contributions	$ 0.8	$ 0.7	$ 0.7

Income Taxes - Summary of Loss

Income Taxes - Summary of Loss Before Provision (Benefit) for Income Taxes by Region (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
United States	$ (10,268)	$ (64,109)	$ (16,607)
Foreign	(12,248)	(43,870)	(43,901)
Total loss attributable to stockholders of Arena before income taxes	$ (22,516)	$ (107,979)	$ (60,508)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Line Items]
Benefit for income taxes	$ 0	$ 0	$ 0
Statutory federal rate	34.00%	34.00%	34.00%
Decrease in valuation allowance	$ (200,000)
Net operating loss and research and development tax credit limitation on use threshold percentage of ownership	50.00%
Period for net operating loss and research and development tax credit limitation on use threshold percentage of ownership	3 years
Unrecognized tax benefits, that would impact the effective tax rate	$ 4,500,000
Accrued interest or penalties	0	$ 0
Recognize interest and/or penalties expense	0	$ 0	$ 0
Foreign subsidiaries repatriated earnings	0
Foreign subsidiaries accumulated earnings	$ 0
Maximum
Income Tax Disclosure [Line Items]
Income tax holiday expiration term	10 years
Income tax holiday termination date	Dec. 31, 2022
California
Income Tax Disclosure [Line Items]
NOL carryforwards	$ 528,300,000
California | Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2,017
California | Stock Option Exercises
Income Tax Disclosure [Line Items]
NOL carryforwards	$ 8,900,000
California | Research and Development Tax Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	23,700,000
Foreign
Income Tax Disclosure [Line Items]
NOL carryforwards	$ 62,200,000
Foreign | Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2,017
SWITZERLAND
Income Tax Disclosure [Line Items]
Standard effective tax rate	19.00%
Federal
Income Tax Disclosure [Line Items]
NOL carryforwards	$ 670,700,000
Federal | Minimum
Income Tax Disclosure [Line Items]
NOL carryforwards expiration date, year	2,023
Tax credit carryforwards expiration year	2,026
Federal | Research and Development Tax Credit
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	$ 31,400,000
Federal | Orphan Drug Credit | Minimum
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	$ 6,000,000

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Benefit for income taxes at statutory federal rate	$ (7,655,000)	$ (36,713,000)	$ (20,573,000)
Change in federal and foreign valuation allowance	9,080,000	21,310,000	9,436,000
Permanent differences and other	(5,931,000)	2,370,000	721,000
Share-based compensation expense	4,000,000	1,820,000	1,597,000
Foreign losses at lower effective rates	3,943,000	15,041,000	13,318,000
Research and development and Orphan Drug credits	(3,437,000)	(3,666,000)	(2,992,000)
Gain from valuation of derivative liabilities		(162,000)	(1,507,000)
Benefit for income taxes	$ 0	$ 0	$ 0

Income Taxes - Components of Ne

Income Taxes - Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Federal and California NOL carryforwards	$ 255,317	$ 236,334
Federal and California research and development credit carryforwards	53,059	48,768
Share-based compensation expense	10,395	10,737
Deferred revenues	9,357	33,548
Depreciation	5,441	4,475
Foreign NOL carryforwards	5,108	7,060
Other, net	5,164	3,578
Total deferred tax assets	343,841	344,500
Deferred tax liabilities	(228)	(660)
Net deferred tax assets	343,613	343,840
Valuation allowance	$ (343,613)	$ (343,840)

Income Taxes - Reconciliation67

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits at the beginning of the year	$ 5,619	$ 5,214	$ 4,629
Additions from tax positions taken in the current year	287	405	585
Gross unrecognized tax benefits at end of the year	$ 5,906	$ 5,619	$ 5,214

Management Changes (Detail)

Management Changes (Detail) - USD ($) $ in Thousands

May 31, 2016

Oct. 31, 2016

Sep. 30, 2016

Aug. 31, 2016

Jul. 31, 2016

Jun. 30, 2016

Dec. 31, 2016

Sep. 30, 2016

Jun. 30, 2016

Dec. 31, 2016

Dec. 31, 2015

Dec. 31, 2014

Management Changes [Line Items]

Share-based compensation

$ 11,117

$ 14,463

$ 13,509

Chief Medical Officer

Management Changes [Line Items]

Cash severance payment

$ 500

Duration

12 months

Expense

$ 1,000

Share-based compensation

$ 400

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

Consulting agreement term

5 years

Benefits

$ 900

$ 900

Senior Vice President of Operations and Head of Global Regulatory Affairs

Management Changes [Line Items]

Cash severance payment

$ 500

Duration

12 months

Expense

1,000

Share-based compensation

400

Benefits

$ 500

$ 500

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 Executive Officer | Cliff Vesting After One Year

Management Changes [Line Items]

Vesting period

1 year

2013 Long-Term Incentive Plan as Amended | Chief Executive Officer | Vesting Years Two Through Four

Management Changes [Line Items]

Vesting period

3 years

2013 Long-Term Incentive Plan as Amended | Chief Financial Officer

Management Changes [Line Items]

Options granted (shares)

800,000

Additional shares authorized (shares)

800,000

Term

7 years

Vesting period

4 years

Vesting percentage

25.00%

2013 Long-Term Incentive Plan as Amended | Chief Financial Officer | Cliff Vesting After One Year

Management Changes [Line Items]

Vesting period

1 year

2013 Long-Term Incentive Plan as Amended | Chief Financial Officer | Vesting Years Two Through Four

Management Changes [Line Items]

Vesting period

3 years

2013 Long-Term Incentive Plan as Amended | Chief Business Officer

Management Changes [Line Items]

Options granted (shares)

800,000

Additional shares authorized (shares)

800,000

5,400,000

Term

7 years

Vesting period

4 years

Vesting percentage

25.00%

2013 Long-Term Incentive Plan as Amended | Chief Business Officer | Cliff Vesting After One Year

Management Changes [Line Items]

Vesting period

1 year

2013 Long-Term Incentive Plan as Amended | Chief Business Officer | Vesting Years Two Through Four

Management Changes [Line Items]

Vesting period

3 years

Restructuring Activities (Detai

Restructuring Activities (Detail) $ in Thousands	3 Months Ended			12 Months Ended
	Sep. 30, 2016USD ($)employee	Jun. 30, 2016USD ($)employee	Dec. 31, 2015USD ($)employee	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charge			$ 4,000	$ 6,346	$ 3,972
Non-cash share-based compensation expense				11,117	14,463	$ 13,509
Restructuring charges
Restructuring Cost and Reserve [Line Items]
Non-cash share-based compensation expense		$ 1,000		$ 1,032	$ 142
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
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

Quarterly Financial Data (Una70

Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 85,374	$ 19,242	$ 9,512	$ 9,847	$ 7,751	$ 9,138	$ 9,181	$ 12,256	$ 123,975	$ 38,326	$ 36,970
Operating costs and expenses	47,245	29,099	35,735	29,042	37,045	34,319	36,160	34,000	141,121	141,524	142,243
Net income (loss)	38,314	(12,479)	(27,183)	(21,548)	$ (30,459)	$ (26,418)	$ (26,807)	$ (24,295)	(22,896)	(107,979)	(60,508)
Net income (loss) attributable to stockholders of Arena	$ 38,572	$ (12,357)	$ (27,183)	$ (21,548)					$ (22,516)	$ (107,979)	$ (60,508)
Net income (loss) attributable to stockholders of Arena per share, basic and diluted	$ 0.16	$ (0.05)	$ (0.11)	$ (0.09)	$ (0.13)	$ (0.11)	$ (0.11)	$ (0.10)

Beacon Discovery, Inc. - Additi

Beacon Discovery, Inc. - Additional Information (Detail) - Beacon Discovery, Inc. - Variable Interest Entity, Primary Beneficiary	12 Months Ended
	Dec. 31, 2016USD ($)ft²
Variable Interest Entity [Line Items]
Promissory note interest rate	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
Sublease Agreement
Variable Interest Entity [Line Items]
Area of space subleased | ft²	15,000
Period of sublease	5 years

Beacon Discovery, Inc. - Summar

Beacon Discovery, Inc. - Summary of Assets and Liabilities Included in Consolidated Balance Sheet (Detail) - Variable Interest Entity, Primary Beneficiary - Beacon Discovery, Inc. $ in Thousands	Dec. 31, 2016USD ($)
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	$ 1,044
Total liabilities of Beacon for which creditors do not have recourse to the general credit of Arena	86
Cash and Cash Equivalents
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	311
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	28
Land, Property and Equipment, Net
Variable Interest Entity [Line Items]
Total assets of Beacon that can only be used to settle obligation of Beacon	705
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	$ 86

Subsequent Events (Detail)

Subsequent Events (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 10, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 04, 2017
Subsequent Event [Line Items]
Aggregate gross proceeds from sale of common stock		$ 370	$ 103,628	$ 5,225
Common Stock
Subsequent Event [Line Items]
Sale of common stock shares			21,000,000
Subsequent Event | Equity Distribution Agreement | Citigroup | ATM Offering
Subsequent Event [Line Items]
Aggregate gross proceeds from sale of common stock	$ 3,200
Commissions and other issuance costs	$ 100
Subsequent Event | Common Stock | Equity Distribution Agreement | Citigroup | ATM Offering
Subsequent Event [Line Items]
Sale of common stock shares	2,017,301
Subsequent Event | Maximum | Equity Distribution Agreement | Citigroup | ATM Offering
Subsequent Event [Line Items]
Aggregate value of common stock available	$ 46,800				$ 50,000
Percentage of commission on gross sales price					3.00%
Subsequent Event | Weighted Average | Common Stock | Equity Distribution Agreement | Citigroup | ATM Offering
Subsequent Event [Line Items]
Average market price per share	$ 1.56