Arena Pharmaceuticals (ARNA) 10-Q 6 Nov 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014	Nov. 03, 2014
Document Information [Line Items]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'ARNA	'
Entity Registrant Name	'ARENA PHARMACEUTICALS INC	'
Entity Central Index Key	'0001080709	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	220,114,995

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$188,328	$221,878	[1]
Accounts receivable	2,914	10,602	[1]
Inventory	11,861	12,759	[1]
Prepaid expenses and other current assets	5,974	3,571	[1]
Total current assets	209,077	248,810	[1]
Land, property and equipment, net	75,489	77,388	[1]
Intangibles, net	8,977	10,182	[1]
Other non-current assets	3,129	3,427	[1]
Total assets	296,672	339,807	[1]
Current liabilities:	'	'
Accounts payable and other accrued liabilities	4,159	6,000	[1]
Accrued clinical and preclinical study fees	4,486	1,317	[1]
Accrued compensation	4,699	4,205	[1]
Current portion of deferred revenues	19,869	37,861	[1]
Derivative liabilities	1,403	0	[1]
Current portion of lease financing obligations	2,379	2,056	[1]
Total current liabilities	56,046	70,744	[1]
Deferred rent	340	247	[1]
Deferred revenues, less current portion	95,092	101,329	[1]
Derivative liabilities	0	4,892	[1]
Lease financing obligations, less current portion	68,908	70,738	[1]
Commitments and contingencies	'	'	[1]
Stockholders' equity:	'	'
Common stock	22	22	[1]
Additional paid-in capital	1,308,735	1,293,840	[1]
Accumulated other comprehensive income	3,709	5,728	[1]
Accumulated deficit	-1,236,180	-1,207,733	[1]
Total stockholders' equity	76,286	91,857	[1]
Total liabilities and stockholders' equity	296,672	339,807	[1]
Eisai BELVIQ Agreement	'	'
Current liabilities:	'	'
Payable to Eisai	19,051	19,305	[1]
Current portion of deferred revenues	19,247	37,301
Deferred revenues, less current portion	$88,819	$95,102
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues:	'	'	'	'
Manufacturing services	$158	$443	$1,184	$2,181
Total revenues	8,164	3,578	27,779	74,878
Operating Costs and Expenses:	'	'	'	'
Cost of product sales	1,755	460	4,049	1,514
Cost of manufacturing services	81	632	1,124	3,286
Research and development	24,508	14,592	72,521	47,428
General and administrative	8,029	7,760	25,198	23,614
Total operating costs and expenses	34,373	23,444	102,892	75,842
Loss from operations	-26,209	-19,866	-75,113	-964
Interest and Other Income (Expense):	'	'	'	'
Interest income	16	28	69	68
Interest expense	-1,723	-1,768	-5,205	-5,333
Gain from valuation of derivative liabilities	2,593	4,100	3,489	10,101
Gain on sale of available-for-sale securities	16,276	0	49,553	0
Other	-1,625	306	-1,240	152
Total interest and other income, net	15,537	2,666	46,666	4,988
Net income (loss)	-10,672	-17,200	-28,447	4,024
Net income (loss) per share:	'	'	'	'
Basic	($0.05)	($0.08)	($0.13)	$0.02
Diluted	($0.05)	($0.08)	($0.13)	$0.02
Shares used in calculating net income (loss) per share:	'	'	'	'
Basic	219,866	218,316	219,592	217,923
Diluted	219,866	218,316	219,592	224,354
Comprehensive Income (Loss):	'	'	'	'
Net income (loss)	-10,672	-17,200	-28,447	4,024
Foreign currency translation gain (loss)	-1,626	1,303	-2,019	104
Unrealized loss on available-for-sale securities	-18,313	0	0	0
Comprehensive income (loss)	-30,611	-15,897	-30,466	4,128
Eisai BELVIQ Agreement	'	'	'	'
Revenues:	'	'	'	'
Net product sales	5,726	2,011	12,137	3,330
Other collaborative revenue	2,133	973	14,034	68,933
Total revenues	7,859	2,984	26,171	72,263
Other	'	'	'	'
Revenues:	'	'	'	'
Other collaborative revenue	$147	$151	$424	$434

Condensed_Consolidated_Cash_Fl

Condensed Consolidated Cash Flow Statements (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013
Operating Activities	'		'
Net income (loss)	($28,447)		$4,024
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:	'		'
Depreciation and amortization	6,434		5,763
Amortization of intangibles	394		305
Share-based compensation	9,941		6,542
Gain from valuation of derivative liabilities	-3,489		-10,101
Gain on sale of available-for-sale securities	-49,553		0
Amortization of prepaid financing costs	102		102
(Gain) loss on sale of equipment	172		-21
Changes in assets and liabilities:	'		'
Accounts receivable	7,279		-6,330
Inventory	211		-3,520
Prepaid expenses and other assets	-2,551		37
Accounts payable, payable to Eisai and accrued liabilities	3,420		17,823
Deferred revenues	-22,998		12,565
Deferred rent	93		91
Net cash provided by (used in) operating activities	-78,992		27,280
Investing Activities	'		'
Proceeds from sale of available-for-sale securities	49,553		0
Purchases of property and equipment	-6,444		-4,600
Proceeds from sale of equipment	47		60
Other non-current assets	209		467
Net cash provided by (used in) investing activities	43,365		-4,073
Financing Activities	'		'
Principal payments on lease financing obligations	-1,507		-1,216
Proceeds from issuance of common stock	4,892		2,685
Net cash provided by financing activities	3,385		1,469
Effect of exchange rate changes on cash	-1,308		-64
Net increase (decrease) in cash and cash equivalents	-33,550		24,612
Cash and cash equivalents at beginning of period	221,878	[1]	156,091
Cash and cash equivalents at end of period	$188,328		$180,703
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2014
Basis of Presentation	'
	1. Basis of Presentation
	The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc., which include our wholly owned subsidiaries, should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2013, as filed with the Securities and Exchange Commission, or SEC, from which we derived our balance sheet as of December 31, 2013. The accompanying financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year.
	In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU No. 2014-09 outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU No. 2014-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2016. ASU No. 2014-09 allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying ASU No. 2014-09 is recognized as an adjustment to the fiscal 2017 opening retained earnings balance. We have not yet selected an adoption method as we are currently evaluating the impact of ASU No. 2014-09 on our consolidated financial statements.
	In August 2014, 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.” Under GAAP, continuation of a reporting entity as a going concern is presumed as the basis for preparing financial statements unless and until the entity’s liquidation becomes imminent. Preparation of financial statements under this presumption is commonly referred to as the going concern basis of accounting. If and when an entity’s liquidation becomes imminent, financial statements should be prepared under the liquidation basis of accounting. Even when an entity’s liquidation is not imminent, there may be conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern. In those situations, financial statements should continue to be prepared under the going concern basis of accounting, but the amendments in ASU No. 2014-15 should be followed to determine whether to disclose information about the relevant conditions and events. The amendments in ASU No. 2014-15 are effective for the annual reporting period ending after December 15, 2016, and for annual and interim periods thereafter. We do not expect the adoption of ASU No. 2014-15 to have a material impact on our consolidated financial statements.
	The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.

Fair_Value_Disclosures

Fair Value Disclosures	9 Months Ended
	Sep. 30, 2014
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 as of September 30, 2014, and December 31, 2013, in thousands:
			Fair Value Measurements at September 30, 2014
			Balance			Quoted		Significant		Significant
						Prices in		Other		Unobservable
						Active		Observable		Inputs
						Markets		Inputs		(Level 3)
						(Level 1)		(Level 2)
	Assets:
	Money market funds1		$	163,900		$	163,900	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	1,403		$	0	$	1,403	$	0
	1	Included in cash and cash equivalents on our condensed consolidated balance sheet.
			Fair Value Measurements at December 31, 2013
			Balance			Quoted		Significant		Significant
						Prices in		Other		Unobservable
						Active		Observable		Inputs
						Markets		Inputs		(Level 3)
						(Level 1)		(Level 2)
	Assets:
	Money market funds1		$	208,833		$	208,833	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	4,892		$	0	$	4,892	$	0
	1	Included in cash and cash equivalents on our condensed consolidated balance sheet.

Shortterm_Investments_Availabl

Short-term Investments, Available-for-Sale	9 Months Ended
	Sep. 30, 2014
Short-term Investments, Available-for-Sale	'
	3. Short-term Investments, Available-for-Sale
	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 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 investment in TaiGen of 29.6 million shares based on its fair value of approximately $49.1 million, with the unrealized gain of $49.1 million recorded as a component of accumulated other comprehensive income (loss) in the stockholders’ equity section of our March 31, 2014, condensed consolidated balance sheet. We began recording our investment in TaiGen at fair value based on the trading price of TaiGen’s common stock, and the remaining investment was revalued on each balance sheet date, with any unrealized gains or losses recorded as a component of accumulated other comprehensive income (loss) in the stockholders’ equity section of our condensed consolidated balance sheets.
	Gains and losses on the sale of available-for-sale securities are determined using the specific-identification method. During the three and nine months ended September 30, 2014, we sold 10.6 million and 29.6 million shares of TaiGen, respectively, and recorded gains of $16.3 million and $49.6 million, respectively, which were reclassified out of accumulated other comprehensive income. The following table summarizes the components of the unrealized gain (loss) on available-for-sale securities we recognized related to our investment in TaiGen in the three and nine months ended September 30, 2014, in thousands:
		Three months ended			Nine months ended
		September 30, 2014			September 30, 2014
	Reclassification adjustment for realized gain on sale of available-for-sale securities	$	(16,276	)	$	(49,553	)
	Unrealized holding gain (loss) on available-for-sale securities		(2,037	)		49,553
	Unrealized loss on available-for-sale securities	$	(18,313	)	$	0
	As of September 30, 2014, we had sold the remaining equity we held in TaiGen and had no remaining amount recorded in accumulated other comprehensive income related to our former investment in TaiGen.

Inventory

Inventory	9 Months Ended
	Sep. 30, 2014
Inventory	'
	4. Inventory
	All of our inventory relates to BELVIQ, and consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,		December 31,
		2014		2013
	Raw materials	$	579	$	657
	Work in process		3,069		4,104
	Finished goods at Arena GmbH		4,597		0
	Finished goods at Eisai		3,616		7,998
	Total inventory	$	11,861	$	12,759

Land_Property_and_Equipment

Land, Property and Equipment	9 Months Ended
	Sep. 30, 2014
Land, Property and Equipment	'
	5. Land, Property and Equipment
	Land, property and equipment consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,			December 31,
		2014			2013
	Cost	$	166,015		$	165,373
	Less accumulated depreciation and amortization		(90,526	)		(87,985	)
	Land, property and equipment, net	$	75,489		$	77,388

Accounts_Payable_and_Other_Acc

Accounts Payable and Other Accrued Liabilities	9 Months Ended
	Sep. 30, 2014
Accounts Payable and Other Accrued Liabilities	'
	6. Accounts Payable and Other Accrued Liabilities
	Accounts payable and other accrued liabilities consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,		December 31,
		2014		2013
	Accounts payable	$	2,579	$	3,721
	Accrued expenses		1,391		1,477
	Loss provision		82		567
	Other accrued liabilities		107		235
	Total accounts payable and other accrued liabilities	$	4,159	$	6,000

Derivative_Liabilities

Derivative Liabilities	9 Months Ended
	Sep. 30, 2014
Derivative Liabilities	'
	7. Derivative Liabilities
	In August 2008, we issued a warrant to purchase 1,106,344 shares of our common stock at an exercise price of $7.71 per share that expires on August 14, 2015. As a result of the warrant’s anti-dilution provision and certain subsequent equity issuances at prices below the adjustment price of $6.72 defined in the warrant agreement, the number of shares issuable upon exercise of the warrant increased and the exercise price decreased. At September 30, 2014, the number of shares issuable upon exercise of the outstanding warrant was 1,965,418 at an exercise price of $4.34 per share. At September 30, 2014, the outstanding warrant was valued at $1.4 million and recorded as a current derivative liability on our condensed consolidated balance sheet. At December 31, 2013, the outstanding warrant was valued at $4.9 million and recorded as a long-term derivative liability on our condensed consolidated balance sheet.
	Our outstanding warrant is 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 condensed consolidated statements of operations and comprehensive income (loss).

Marketing_and_Supply_Agreement

Marketing and Supply Agreement with Eisai	9 Months Ended
	Sep. 30, 2014
Marketing and Supply Agreement with Eisai	'
	8. Marketing and Supply Agreement with Eisai
	In November 2013, our wholly owned subsidiary, Arena Pharmaceuticals GmbH, or Arena GmbH, and Eisai Inc. and Eisai Co., Ltd. (collectively with Eisai Inc., Eisai) entered into the Second Amended and Restated Marketing and Supply Agreement, or Eisai Agreement. The Eisai Agreement amended and restated the previous agreement and expanded Eisai’s exclusive commercialization rights for BELVIQ to all of the countries in the world, except for South Korea, Taiwan, Australia, New Zealand and Israel. BELVIQ is approved in the United States for chronic weight management in adults who are overweight with a comorbidity or obese, and was made available to patients by prescription in the United States by Eisai in June 2013. In addition to providing commercialization rights, which are subject to applicable regulatory approval, we provide Eisai with services related to development and regulatory activities, and manufacture and sell BELVIQ to Eisai. Under the Eisai Agreement, we have received an upfront payment and payments from sales of BELVIQ, and are entitled to receive payments from future sales of BELVIQ, milestone payments based on the achievement of regulatory filings and approvals, one-time purchase price adjustment payments and other payments.
	Prior to entering into the Eisai Agreement, Arena GmbH and Eisai Inc. entered into the original marketing and supply agreement in July 2010, under which we granted Eisai Inc. exclusive commercialization rights for BELVIQ solely in the United States and its territories and possessions. In May 2012, Arena GmbH and Eisai Inc. amended and restated such agreement by entering into the first amended agreement, which expanded Eisai Inc.’s exclusive commercialization rights to include most of North and South America.
	The following table summarizes the revenues we recognized under our collaboration with Eisai in the three and nine months ended September 30, 2014, and 2013, in thousands:
			Three months ended						Nine months ended
			September 30,						September 30,
			2014			2013			2014		2013
	Net product sales		$	5,726		$	2,011		$	12,137	$	3,330
	Amortization of upfront payments			1,885			861			5,745		2,584
	Reimbursement of research and development expenses			143			16			7,456		26
	Milestone payments			0			0			500		66,000
	Reimbursements of patent and trademark expenses			105			96			333		323
	Subtotal other Eisai collaborative revenue			2,133			973			14,034		68,933
	Total		$	7,859		$	2,984		$	26,171	$	72,263
	The following table summarizes the deferred revenues under our collaboration with Eisai as of September 30, 2014, and December 31, 2013, in thousands:
			September 30,			December 31,
			2014			2013
	Upfront payments		$	96,359		$	102,104
	Net product sales			11,707			30,299
	Total deferred revenues attributable to Eisai			108,066			132,403
	Less current portion			(19,247	)		(37,301	)
	Deferred revenues attributable to Eisai, less current portion		$	88,819		$	95,102
	Upfront and Milestone Payments
	In connection with entering into the Eisai Agreement, we received from Eisai an upfront payment of $60.0 million. This payment is in addition to the $50.0 million and $5.0 million in upfront payments we received from Eisai in connection with entering into the original agreement and the first amended agreement, respectively. Revenues from these upfront payments were deferred, as we determined that the exclusive rights did not have standalone value without our ongoing development and regulatory activities. Accordingly, these payments are recognized ratably as revenue over the periods in which we expect the services to be rendered, which are approximately 15 years for the Eisai Agreement and first amended agreement and 16 years for the original agreement. In addition to the upfront payments, we have received from Eisai a total of $86.5 million in milestones payments, and we are eligible to receive up to an aggregate of $176.0 million in additional regulatory and development milestone payments.
	Product Purchase Price and Purchase Price Adjustment Payments
	We manufacture BELVIQ at our facility in Switzerland, and sell BELVIQ to Eisai for Eisai’s commercialization in the United States and, subject to applicable regulatory approval, in the other territories under the Eisai Agreement (other than Europe, China and Japan) for a purchase price starting at 31.5% and 30.75%, respectively (and starting at 27.5% in Europe, China and Japan), of Eisai’s aggregate annual net product sales (which are the gross invoiced sales less certain deductions described in the Eisai Agreement), or the Product Purchase Price, in the respective territory. The Product Purchase Price will increase on a tiered basis in the United States and the other territories (other than Europe, China and Japan) to as high as 36.5% and 35.75%, respectively, on the portion of Eisai’s annual aggregate net product sales exceeding $750.0 million in all territories other than Europe, China and Japan. The Product Purchase Price will increase to 35% in Europe, China and Japan on the portion of Eisai’s annual aggregate net product sales exceeding $500.0 million in such territories. The Product Purchase Price is subject to reduction (for sales in a particular country), including in the event of generic competition in the applicable country. The revenue we recognize for BELVIQ product revenue related to redemption of vouchers is based on our cost of goods sold.
	In addition to payments for purchases of BELVIQ, we are eligible to receive up to an aggregate of $1.56 billion in one-time purchase price adjustment payments and other payments. These payments include up to an aggregate of $1.19 billion that are based on Eisai’s annual net product sales of BELVIQ in all of the territories under the Eisai Agreement on an aggregate basis, with the first and last amounts payable with annual net product sales of $250.0 million and $2.5 billion, respectively. Of these payments, Eisai will pay us a total of $330.0 million for annual net product sales of up to $1.0 billion. The $1.56 billion also includes $370.0 million in one-time purchase price adjustment payments we are eligible to receive based on annual net product sales in the non-US territories, comprised of $185.0 million based on Eisai’s annual net product sales in the non-US territories in North and South America and $185.0 million based on Eisai’s annual net product sales in the territories outside of North and South America. The first and last amounts are payable upon first achievement of annual net product sales of $100.0 million and $1.0 billion, respectively, with respect to each of the following areas: (i) the non-US territories in North and South America and (ii) the territories outside of North and South America. In addition, we are also eligible to receive certain payments by Eisai if certain annual minimum sales requirements in Mexico, Canada and Brazil are not met during the first ten years after initial commercial sale in such territories.
	The amount that Eisai pays us for BELVIQ product supply is based on Eisai’s estimated price at the time the order is shipped, which is Eisai’s estimate of the Product Purchase Price, and is subject to change on April 1 and October 1 of each year. Eisai’s estimate of the Product Purchase Price was changed as of October 1, 2013, and there was no further change as of April 1, 2014. At the end of Eisai’s fiscal year (March 31), the estimated price paid to us for product that Eisai sold to their distributors is compared to the Product Purchase Price of such product, and the difference is either refunded back to Eisai (for overpayments) or paid to us (for underpayments). On a monthly basis, Eisai provides us the total amount of net product sales for the month, details of the total deductions from gross to net product sales and the sales in units. We recognize our revenues monthly based on our percentage of Eisai’s monthly net product sales figures. When the revenues we recognize differ from the estimated price that Eisai paid us for such product, the difference is reclassified from deferred revenues to a receivable or payable account, as appropriate. We also adjust the deferred revenues balance for the product supply held at Eisai based on the most current net product sales figures provided to us, with the difference reclassified from deferred revenues to a receivable or payable account.
	In the three months ended September 30, 2014, we recognized total revenues from net product sales of BELVIQ of $5.7 million, of which $5.2 million related to sales at the Product Purchase Price and $0.5 million related to redemptions of vouchers. In the nine months ended September 30, 2014, we recognized total revenues from net product sales of BELVIQ of $12.1 million, of which $11.0 million related to sales at the Product Purchase Price and $1.1 million related to redemptions of vouchers. The Product Purchase Price for the product Eisai has sold to date was lower than the initial estimated price that Eisai paid us for such product, primarily because the price that Eisai paid us did not include deductions for the use of vouchers and savings cards or for certain items related to product launch. In September 2014, Eisai determined to include product sampling as part of its commercialization efforts and they allocated certain bottles of BELVIQ for the initial product sampling. Eisai initiated product sampling in October 2014. Under the Eisai Agreement, Eisai will pay us our cost of goods for these product samples. The allocation of BELVIQ bottles for product sampling reduced our deferred revenues and increased our payable to Eisai by $6.0 million at September 30, 2014. These excess payments related to the above deductions and product sampling, which reflect both the amounts Eisai has sold to date and the product supply remaining in Eisai’s inventory at September 30, 2014, are included in the $19.1 million classified as Payable to Eisai on our condensed consolidated balance sheet. On a quarterly basis, subsequent to the end of each calendar quarter, we will refund to Eisai the portion of these excess payments related to product sampling for product shipped to physicians during the quarter. On an annual basis, subsequent to the end of Eisai’s fiscal year, we will refund to Eisai the portion of these excess payments related to product sold by Eisai to their distributors through March 31.
	Development Payments
	In connection with the US approval of BELVIQ, the US Food and Drug Administration, or FDA, is requiring (i) an evaluation as part of the cardiovascular outcomes trial, or CVOT, 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 and (ii) the conduct of postmarketing studies to assess the safety and efficacy of BELVIQ for weight management in obese pediatric patients. In addition to the FDA-required studies, we and Eisai are prioritizing the development areas of smoking cessation, a once-daily formulation, co-administration with phentermine, as well as potentially exploring, including as part of the CVOT, BELVIQ’s effect on conversion to type 2 diabetes and improvements in cardiovascular outcomes.
	The chart below summarizes the general agreement regarding cost sharing between Eisai and us for significant development activities under the Eisai Agreement. In addition, Eisai or we may from time to time conduct approved development of BELVIQ at such party’s own expense. For example, Eisai is responsible for the expenses of the pilot study of 12-week duration to preliminarily assess BELVIQ and phentermine when co-administered.
	Eisai Second Amended and Restated Marketing and Supply Agreement: Cost Sharing for Development
			United States		Rest of	Remaining Territories
					North and South America
	BELVIQ for weight		Not Applicable		General	Up to total of $100.0 million -
	management				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%
	- Pre-approval*
					Certain stability work	Thereafter, Eisai: 100%
					Eisai: 50%; Arena: 50%
	BELVIQ for weight management		General—Eisai: 90%; Arena 10%		General	Up to total of $50.0 million -
	- Post-approval*				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%
			Non-FDA required portion of CVOT
			Up to $80.0 million -		Certain stability work	Thereafter, Eisai: 90%;
			Eisai: 50%; Arena: 50%		Eisai: 50%; Arena: 50%	Arena: 10%
			Thereafter, Eisai: 100%
			Certain pediatric studies
			Eisai: 50%; Arena: 50%
	Products other than BELVIQ for weight management		Up to a total of $250.0 million (as reduced by up to $80.0 million for non-FDA required portion of CVOT) -
	- Pre-approval		Eisai: 50%; Arena: 50%
	Products other than BELVIQ for weight management		Up to a total of $100.0 million in the aggregate across all additional products -
	- Post-approval		Eisai: 50%; Arena: 50%
			Thereafter, Eisai: 90%; Arena: 10%
	*	Development required by a regulatory authority, with the exception of the non-FDA required portions of the CVOT.
	Certain Other Terms
	Please refer to our Annual Report on Form 10-K for the year ended December 31, 2013, for additional information regarding termination, indemnification, product liability, certain limitations and other provisions included in the Eisai Agreement.

Sharebased_Activity

Share-based Activity	9 Months Ended
	Sep. 30, 2014
Share-based Activity	'
	9. Share-based Activity
	Share-based Compensation
	We recognized share-based compensation expense as follows, in thousands:
		Three months ended					Nine months ended
		September 30,					September 30,
		2014			2013		2014		2013
	Cost of product sales	$	0		$	0	$	0	$	17
	Research and development		1,753			1,188		5,276		3,047
	General and administrative		1,645			1,131		4,665		3,478
	Total share-based compensation expense	$	3,398		$	2,319	$	9,941	$	6,542
	Total share-based compensation expense capitalized into inventory	$	22		$	35	$	61	$	79
	Share-based Award Activity
	The following table summarizes our stock option activity during the nine months ended September 30, 2014, in thousands (except per share data):
		Options			Weighted-
					Average
					Exercise Price
	Outstanding at January 1, 2014		14,681		$	4.99
	Granted		2,126		$	6.78
	Exercised		(1,070	)	$	3.72
	Forfeited/cancelled/expired		(230	)	$	6.28
	Outstanding at September 30, 2014		15,507		$	5.3
	The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the nine months ended September 30, 2014, in thousands (except per share data):
		RSUs			Weighted-
					Average
					Grant-Date
					Fair Value
	Unvested at January 1, 2014		369		$	7.23
	Granted		178		$	6.5
	Vested		(110	)	$	7.88
	Forfeited/cancelled		0
	Unvested at September 30, 2014		437		$	6.77
	In March 2014, we granted our executive officers Total Stockholder Return, or TSR, performance restricted stock unit, or 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, 2014, of the NASDAQ Biotechnology Index. In the aggregate, the target number of shares of common stock that may be earned under the PRSUs is 695,000; however, the actual number of shares that may be earned ranges from 0% to 200% of such amount. In addition, there is a cap on the number of shares that can be earned under the PRSUs equal to six times the grant-date fair value of the 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 $5.0 million and will be recognized over the performance period. The table below sets forth the assumptions used to value the PRSUs granted in 2014 and the estimated grant-date fair value:
	Risk-free interest rate		0.7	%
	Dividend yield		0	%
	Expected volatility		78	%
	Remaining performance period (years)		2.99
	Estimated fair value per share of PRSUs granted	$	7.16
	All of the PRSUs granted to date were unvested at September 30, 2014.

Concentration_of_Credit_Risk_a

Concentration of Credit Risk and Major Customers	9 Months Ended
	Sep. 30, 2014
Concentration of Credit Risk and Major Customers	'
	10. Concentration of Credit Risk and Major Customers
	Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency and government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors.
	Eisai is the exclusive distributor and our only customer for BELVIQ in the United States, which is the only jurisdiction for which BELVIQ has received regulatory approval for marketing. We also produce drug products for Siegfried AG, or Siegfried, under a manufacturing services agreement, and all of our manufacturing services revenues are attributable to Siegfried.
	Percentages of our total revenues are as follows:
		Three months ended						Nine months ended
		September 30,						September 30,
		2014			2013			2014			2013
	Eisai marketing and supply agreement		96.3	%		83.4	%		94.2	%		96.5	%
	Manufacturing services agreement with Siegfried		1.9	%		12.4	%		4.3	%		2.9	%
	Other collaborative agreements		1.8	%		4.2	%		1.5	%		0.6	%
	Total percentage of revenues		100	%		100	%		100	%		100	%

Net_Loss_Per_Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2014
Net Loss Per Share	'
	11. 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.
	When we are in a net loss position, we exclude from our calculation of diluted net loss per share all potentially dilutive (i) stock options, (ii) RSUs, (iii) PRSUs, (iv) unvested restricted stock in our deferred compensation plan and (v) warrants, and our diluted net loss per share is the same as our basic net loss per share for such periods. When we are in a net income position, the weighted-average number of shares used to calculate our diluted net income per share includes the potential dilutive effect of in-the-money securities, as determined using the treasury stock method.
	The table below reconciles the number of shares used to calculate basic and diluted net income (loss) per share for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2014		2013		2014		2013
	Weighted-average shares outstanding		219,866		218,316		219,592		217,923
	Potentially dilutive common shares outstanding:
	Stock options		0		0		0		5,489
	Warrants		0		0		0		876
	RSUs and unvested restricted stock		0		0		0		66
	Weighted-average shares used to calculate diluted net income (loss) per share		219,866		218,316		219,592		224,354
	The table below presents the potentially dilutive securities that would have been included in the above calculation of diluted net income (loss) per share if they were not antidilutive for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2014		2013		2014		2013
	Stock options		11,614		4,796		12,185		3,757
	Warrants		55		694		490		0
	RSUs, PRSUs, and unvested restricted stock		508		79		214		0
	Total		12,177		5,569		12,889		3,757

Legal_Proceedings

Legal Proceedings	9 Months Ended
	Sep. 30, 2014
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 against us and certain of our current and former employees and directors on behalf of certain purchasers of our common stock. The complaints were brought as purported stockholder class actions, and, in general, include allegations that we and certain of our current and former employees and directors violated federal securities laws by making materially false and misleading statements regarding our BELVIQ program, thereby artificially inflating the price of our common stock. The plaintiffs sought unspecified monetary damages and other relief. On August 8, 2011, the Court consolidated the actions and appointed a lead plaintiff and lead counsel. On November 1, 2011, the lead plaintiff filed a consolidated amended complaint. On March 28, 2013, the Court dismissed the consolidated amended complaint without prejudice. On May 13, 2013, the lead plaintiff filed a second consolidated amended complaint. On November 5, 2013, the Court dismissed the second consolidated amended complaint without prejudice as to all parties except for Robert E. Hoffman, who was dismissed from the action with prejudice. On November 27, 2013, the lead plaintiff filed a motion for leave to amend the second consolidated amended complaint. On March 20, 2014, the Court denied plaintiff’s motion and dismissed the second consolidated amended complaint with prejudice. On April 18, 2014, the lead plaintiff filed a notice of appeal, and on August 27, 2014, the lead plaintiff filed his appellate brief in the US Court of Appeals for the Ninth Circuit. On October 24, 2014, we filed our answering brief in response to the lead plaintiff’s appeal. Due to the stage of these proceedings, we are not able to predict or reasonably estimate the ultimate outcome or possible losses relating to these claims.
	In addition to the class actions, a complaint involving similar legal and factual issues had been brought by an individual stockholder. On March 29, 2013, the Court dismissed the matter, in part without prejudice. On May 13, 2013, the individual stockholder filed a new amended complaint. On March 20, 2014, the Court dismissed the matter in part, and remanded the remaining claims to state court. On May 5, 2014, we and the individual stockholder agreed to resolve this dispute, and the state court dismissed the complaint with prejudice on May 16, 2014.

Commitment_to_Acquire_Real_Pro

Commitment to Acquire Real Property	9 Months Ended
	Sep. 30, 2014
Commitment to Acquire Real Property	'
	13. Commitment to Acquire Real Property
	In January 2008, under an asset purchase agreement, we acquired from Siegfried certain drug product facility and real estate assets in Zofingen, Switzerland, including approximately 67,000 square feet of space in a building that consists of approximately 134,000 square feet of space. These assets are being used to manufacture BELVIQ as well as certain drug products for Siegfried. Under such agreement, we had the option to purchase the remaining portion of the building we are occupying 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 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 portion of the building we are occupying at a price of CHF 8.0 million, plus an inflation adjustment. Under our agreement, we are required to purchase such remaining portion of the building in July 2015.

Basis_of_Presentation_Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2014
Basis of Presentation	'
	The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc., which include our wholly owned subsidiaries, should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2013, as filed with the Securities and Exchange Commission, or SEC, from which we derived our balance sheet as of December 31, 2013. The accompanying financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year.
New Accounting Guidance	'
	In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU No. 2014-09 outlines a comprehensive revenue recognition model and supersedes most current revenue recognition guidance. ASU No. 2014-09 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2016. ASU No. 2014-09 allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying ASU No. 2014-09 is recognized as an adjustment to the fiscal 2017 opening retained earnings balance. We have not yet selected an adoption method as we are currently evaluating the impact of ASU No. 2014-09 on our consolidated financial statements.
	In August 2014, 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.” Under GAAP, continuation of a reporting entity as a going concern is presumed as the basis for preparing financial statements unless and until the entity’s liquidation becomes imminent. Preparation of financial statements under this presumption is commonly referred to as the going concern basis of accounting. If and when an entity’s liquidation becomes imminent, financial statements should be prepared under the liquidation basis of accounting. Even when an entity’s liquidation is not imminent, there may be conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern. In those situations, financial statements should continue to be prepared under the going concern basis of accounting, but the amendments in ASU No. 2014-15 should be followed to determine whether to disclose information about the relevant conditions and events. The amendments in ASU No. 2014-15 are effective for the annual reporting period ending after December 15, 2016, and for annual and interim periods thereafter. We do not expect the adoption of ASU No. 2014-15 to have a material impact on our consolidated financial statements.
Use of Estimates	'
	The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.
Fair Value Measurements	'
	We measure our financial assets and liabilities at fair value, which is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.
	We use the following three-level valuation hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value our financial assets and liabilities:
	Level 1	-		Observable inputs such as unadjusted quoted prices in active markets for identical instruments.
	Level 2	-		Quoted prices for similar instruments in active markets or inputs that are observable for the asset or liability, either directly or indirectly.
	Level 3	-		Significant unobservable inputs based on our assumptions.
Investments	'
	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 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 investment in TaiGen of 29.6 million shares based on its fair value of approximately $49.1 million, with the unrealized gain of $49.1 million recorded as a component of accumulated other comprehensive income (loss) in the stockholders’ equity section of our March 31, 2014, condensed consolidated balance sheet. We began recording our investment in TaiGen at fair value based on the trading price of TaiGen’s common stock, and the remaining investment was revalued on each balance sheet date, with any unrealized gains or losses recorded as a component of accumulated other comprehensive income (loss) in the stockholders’ equity section of our condensed consolidated balance sheets.
	Gains and losses on the sale of available-for-sale securities are determined using the specific-identification method. During the three and nine months ended September 30, 2014, we sold 10.6 million and 29.6 million shares of TaiGen, respectively, and recorded gains of $16.3 million and $49.6 million, respectively, which were reclassified out of accumulated other comprehensive income. The following table summarizes the components of the unrealized gain (loss) on available-for-sale securities we recognized related to our investment in TaiGen in the three and nine months ended September 30, 2014, in thousands:
		Three months ended			Nine months ended
		September 30, 2014			September 30, 2014
	Reclassification adjustment for realized gain on sale of available-for-sale securities	$	(16,276	)	$	(49,553	)
	Unrealized holding gain (loss) on available-for-sale securities		(2,037	)		49,553
	Unrealized loss on available-for-sale securities	$	(18,313	)	$	0
	As of September 30, 2014, we had sold the remaining equity we held in TaiGen and had no remaining amount recorded in accumulated other comprehensive income related to our former investment in TaiGen.
Derivative Liabilities	'
	In August 2008, we issued a warrant to purchase 1,106,344 shares of our common stock at an exercise price of $7.71 per share that expires on August 14, 2015. As a result of the warrant’s anti-dilution provision and certain subsequent equity issuances at prices below the adjustment price of $6.72 defined in the warrant agreement, the number of shares issuable upon exercise of the warrant increased and the exercise price decreased. At September 30, 2014, the number of shares issuable upon exercise of the outstanding warrant was 1,965,418 at an exercise price of $4.34 per share. At September 30, 2014, the outstanding warrant was valued at $1.4 million and recorded as a current derivative liability on our condensed consolidated balance sheet. At December 31, 2013, the outstanding warrant was valued at $4.9 million and recorded as a long-term derivative liability on our condensed consolidated balance sheet.
	Our outstanding warrant is 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 condensed consolidated statements of operations and comprehensive income (loss).
Concentration of Credit Risk and Major Customers	'
	Financial instruments, which potentially subject us to concentrations of credit risk, consist primarily of cash and cash equivalents. We limit our exposure to credit loss by holding our cash primarily in US dollars or, from time to time, placing our cash and investments in US government, agency and government-sponsored enterprise obligations and in corporate debt instruments that are rated investment grade, in accordance with an investment policy approved by our Board of Directors.
	Eisai is the exclusive distributor and our only customer for BELVIQ in the United States, which is the only jurisdiction for which BELVIQ has received regulatory approval for marketing. We also produce drug products for Siegfried AG, or Siegfried, under a manufacturing services agreement, and all of our manufacturing services revenues are attributable to Siegfried.
	Percentages of our total revenues are as follows:
		Three months ended						Nine months ended
		September 30,						September 30,
		2014			2013			2014			2013
	Eisai marketing and supply agreement		96.3	%		83.4	%		94.2	%		96.5	%
	Manufacturing services agreement with Siegfried		1.9	%		12.4	%		4.3	%		2.9	%
	Other collaborative agreements		1.8	%		4.2	%		1.5	%		0.6	%
	Total percentage of revenues		100	%		100	%		100	%		100	%
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.
	When we are in a net loss position, we exclude from our calculation of diluted net loss per share all potentially dilutive (i) stock options, (ii) RSUs, (iii) PRSUs, (iv) unvested restricted stock in our deferred compensation plan and (v) warrants, and our diluted net loss per share is the same as our basic net loss per share for such periods. When we are in a net income position, the weighted-average number of shares used to calculate our diluted net income per share includes the potential dilutive effect of in-the-money securities, as determined using the treasury stock method.
	The table below reconciles the number of shares used to calculate basic and diluted net income (loss) per share for the periods presented, in thousands.
		Three months ended						Nine months ended
		September 30,						September 30,
		2014			2013			2014			2013
	Weighted-average shares outstanding		219,866			218,316			219,592			217,923
	Potentially dilutive common shares outstanding:
	Stock options		0			0			0			5,489
	Warrants		0			0			0			876
	RSUs and unvested restricted stock		0			0			0			66
	Weighted-average shares used to calculate diluted net income (loss) per share		219,866			218,316			219,592			224,354
	The table below presents the potentially dilutive securities that would have been included in the above calculation of diluted net income (loss) per share if they were not antidilutive for the periods presented, in thousands.
		Three months ended						Nine months ended
		September 30,						September 30,
		2014			2013			2014			2013
	Stock options		11,614			4,796			12,185			3,757
	Warrants		55			694			490			0
	RSUs, PRSUs, and unvested restricted stock		508			79			214			0
	Total		12,177			5,569			12,889			3,757

Fair_Value_Disclosures_Tables

Fair Value Disclosures (Tables)	9 Months Ended
	Sep. 30, 2014
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 as of September 30, 2014, and December 31, 2013, in thousands:
			Fair Value Measurements at September 30, 2014
			Balance		Quoted		Significant		Significant
					Prices in		Other		Unobservable
					Active		Observable		Inputs
					Markets		Inputs		(Level 3)
					(Level 1)		(Level 2)
	Assets:
	Money market funds1		$	163,900	$	163,900	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	1,403	$	0	$	1,403	$	0
	1	Included in cash and cash equivalents on our condensed consolidated balance sheet.
			Fair Value Measurements at December 31, 2013
			Balance		Quoted		Significant		Significant
					Prices in		Other		Unobservable
					Active		Observable		Inputs
					Markets		Inputs		(Level 3)
					(Level 1)		(Level 2)
	Assets:
	Money market funds1		$	208,833	$	208,833	$	0	$	0
	Liabilities:
	Warrant derivative liabilities		$	4,892	$	0	$	4,892	$	0
	1	Included in cash and cash equivalents on our condensed consolidated balance sheet.

Shortterm_Investments_Availabl1

Short-term Investments, Available-for-Sale (Tables)	9 Months Ended
	Sep. 30, 2014
Components of Unrealized Gain (Loss) on Available-for-Sale Securities	'
	The following table summarizes the components of the unrealized gain (loss) on available-for-sale securities we recognized related to our investment in TaiGen in the three and nine months ended September 30, 2014, in thousands:
		Three months ended			Nine months ended
		September 30, 2014			September 30, 2014
	Reclassification adjustment for realized gain on sale of available-for-sale securities	$	(16,276	)	$	(49,553	)
	Unrealized holding gain (loss) on available-for-sale securities		(2,037	)		49,553
	Unrealized loss on available-for-sale securities	$	(18,313	)	$	0

Inventory_Tables

Inventory (Tables)	9 Months Ended
	Sep. 30, 2014
Components of Inventory	'
	All of our inventory relates to BELVIQ, and consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,		December 31,
		2014		2013
	Raw materials	$	579	$	657
	Work in process		3,069		4,104
	Finished goods at Arena GmbH		4,597		0
	Finished goods at Eisai		3,616		7,998
	Total inventory	$	11,861	$	12,759

Land_Property_and_Equipment_Ta

Land, Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2014
Land, Property and Equipment	'
	Land, property and equipment consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,			December 31,
		2014			2013
	Cost	$	166,015		$	165,373
	Less accumulated depreciation and amortization		(90,526	)		(87,985	)
	Land, property and equipment, net	$	75,489		$	77,388

Accounts_Payable_and_Other_Acc1

Accounts Payable and Other Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2014
Accounts Payable and Other Accrued Liabilities	'
	Accounts payable and other accrued liabilities consisted of the following as of September 30, 2014, and December 31, 2013, in thousands:
		September 30,		December 31,
		2014		2013
	Accounts payable	$	2,579	$	3,721
	Accrued expenses		1,391		1,477
	Loss provision		82		567
	Other accrued liabilities		107		235
	Total accounts payable and other accrued liabilities	$	4,159	$	6,000

Marketing_and_Supply_Agreement1

Marketing and Supply Agreement with Eisai (Tables)	9 Months Ended
	Sep. 30, 2014
Revenues Recognized under Collaboration with Eisai	'
	The following table summarizes the revenues we recognized under our collaboration with Eisai in the three and nine months ended September 30, 2014, and 2013, in thousands:
			Three months ended						Nine months ended
			September 30,						September 30,
			2014			2013			2014		2013
	Net product sales		$	5,726		$	2,011		$	12,137	$	3,330
	Amortization of upfront payments			1,885			861			5,745		2,584
	Reimbursement of research and development expenses			143			16			7,456		26
	Milestone payments			0			0			500		66,000
	Reimbursements of patent and trademark expenses			105			96			333		323
	Subtotal other Eisai collaborative revenue			2,133			973			14,034		68,933
	Total		$	7,859		$	2,984		$	26,171	$	72,263
Deferred Revenues under Collaboration with Eisai	'
	The following table summarizes the deferred revenues under our collaboration with Eisai as of September 30, 2014, and December 31, 2013, in thousands:
			September 30,			December 31,
			2014			2013
	Upfront payments		$	96,359		$	102,104
	Net product sales			11,707			30,299
	Total deferred revenues attributable to Eisai			108,066			132,403
	Less current portion			(19,247	)		(37,301	)
	Deferred revenues attributable to Eisai, less current portion		$	88,819		$	95,102
Summary of Cost Sharing Allocation	'
	Eisai Second Amended and Restated Marketing and Supply Agreement: Cost Sharing for Development
			United States		Rest of	Remaining Territories
					North and South America
	BELVIQ for weight		Not Applicable		General	Up to total of $100.0 million -
	management				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%
	- Pre-approval*
					Certain stability work	Thereafter, Eisai: 100%
					Eisai: 50%; Arena: 50%
	BELVIQ for weight management		General—Eisai: 90%; Arena 10%		General	Up to total of $50.0 million -
	- Post-approval*				Eisai: 90%; Arena: 10%	Eisai: 50%; Arena: 50%
			Non-FDA required portion of CVOT
			Up to $80.0 million -		Certain stability work	Thereafter, Eisai: 90%;
			Eisai: 50%; Arena: 50%		Eisai: 50%; Arena: 50%	Arena: 10%
			Thereafter, Eisai: 100%
			Certain pediatric studies
			Eisai: 50%; Arena: 50%
	Products other than BELVIQ for weight management		Up to a total of $250.0 million (as reduced by up to $80.0 million for non-FDA required portion of CVOT) -
	- Pre-approval		Eisai: 50%; Arena: 50%
	Products other than BELVIQ for weight management		Up to a total of $100.0 million in the aggregate across all additional products -
	- Post-approval		Eisai: 50%; Arena: 50%
			Thereafter, Eisai: 90%; Arena: 10%
	*	Development required by a regulatory authority, with the exception of the non-FDA required portions of the CVOT.

Sharebased_Activity_Tables

Share-based Activity (Tables)	9 Months Ended
	Sep. 30, 2014
Share Based Compensation Expense	'
	We recognized share-based compensation expense as follows, in thousands:
		Three months ended					Nine months ended
		September 30,					September 30,
		2014			2013		2014		2013
	Cost of product sales	$	0		$	0	$	0	$	17
	Research and development		1,753			1,188		5,276		3,047
	General and administrative		1,645			1,131		4,665		3,478
	Total share-based compensation expense	$	3,398		$	2,319	$	9,941	$	6,542
	Total share-based compensation expense capitalized into inventory	$	22		$	35	$	61	$	79
Summary of Stock Option Activity	'
	The following table summarizes our stock option activity during the nine months ended September 30, 2014, in thousands (except per share data):
		Options			Weighted-
					Average
					Exercise Price
	Outstanding at January 1, 2014		14,681		$	4.99
	Granted		2,126		$	6.78
	Exercised		(1,070	)	$	3.72
	Forfeited/cancelled/expired		(230	)	$	6.28
	Outstanding at September 30, 2014		15,507		$	5.3
Summary of Restricted Stock Activity	'
	The following table summarizes activity with respect to our time-based restricted stock unit awards, or RSUs, during the nine months ended September 30, 2014, in thousands (except per share data):
		RSUs			Weighted-
					Average
					Grant-Date
					Fair Value
	Unvested at January 1, 2014		369		$	7.23
	Granted		178		$	6.5
	Vested		(110	)	$	7.88
	Forfeited/cancelled		0
	Unvested at September 30, 2014		437		$	6.77
Assumptions used to Value Performance Restricted Stock Units Granted and Estimated Grant Date Fair Value	'
	The table below sets forth the assumptions used to value the PRSUs granted in 2014 and the estimated grant-date fair value:
	Risk-free interest rate		0.7	%
	Dividend yield		0	%
	Expected volatility		78	%
	Remaining performance period (years)		2.99
	Estimated fair value per share of PRSUs granted	$	7.16

Concentration_of_Credit_Risk_a1

Concentration of Credit Risk and Major Customers (Tables)	9 Months Ended
	Sep. 30, 2014
Percentages of Total Revenues	'
	Percentages of our total revenues are as follows:
		Three months ended						Nine months ended
		September 30,						September 30,
		2014			2013			2014			2013
	Eisai marketing and supply agreement		96.3	%		83.4	%		94.2	%		96.5	%
	Manufacturing services agreement with Siegfried		1.9	%		12.4	%		4.3	%		2.9	%
	Other collaborative agreements		1.8	%		4.2	%		1.5	%		0.6	%
	Total percentage of revenues		100	%		100	%		100	%		100	%

Net_Loss_Per_Share_Tables

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2014
Reconciliation of Number of Shares Used to Calculate Basic and Diluted Net Income (Loss) Per Share	'
	The table below reconciles the number of shares used to calculate basic and diluted net income (loss) per share for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2014		2013		2014		2013
	Weighted-average shares outstanding		219,866		218,316		219,592		217,923
	Potentially dilutive common shares outstanding:
	Stock options		0		0		0		5,489
	Warrants		0		0		0		876
	RSUs and unvested restricted stock		0		0		0		66
	Weighted-average shares used to calculate diluted net income (loss) per share		219,866		218,316		219,592		224,354
Potentially Dilutive Securities Excluded from Calculation of Diluted Net Income (Loss) Per Share	'
	The table below presents the potentially dilutive securities that would have been included in the above calculation of diluted net income (loss) per share if they were not antidilutive for the periods presented, in thousands.
		Three months ended				Nine months ended
		September 30,				September 30,
		2014		2013		2014		2013
	Stock options		11,614		4,796		12,185		3,757
	Warrants		55		694		490		0
	RSUs, PRSUs, and unvested restricted stock		508		79		214		0
	Total		12,177		5,569		12,889		3,757

Financial_Assets_and_Liabiliti

Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value Measurements, Recurring, USD $)	Sep. 30, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Assets:	'		'
Money market funds	$163,900	[1]	$208,833	[1]
Liabilities:	'		'
Warrant derivative liabilities	1,403		4,892
Quoted Prices in Active Markets (Level 1)	'		'
Assets:	'		'
Money market funds	163,900	[1]	208,833	[1]
Liabilities:	'		'
Warrant derivative liabilities	0		0
Significant Other Observable Inputs (Level 2)	'		'
Assets:	'		'
Money market funds	0	[1]	0	[1]
Liabilities:	'		'
Warrant derivative liabilities	1,403		4,892
Significant Unobservable Inputs (Level 3)	'		'
Assets:	'		'
Money market funds	0	[1]	0	[1]
Liabilities:	'		'
Warrant derivative liabilities	$0		$0
[1]	Included in cash and cash equivalents on our condensed consolidated balance sheet.

Shortterm_Investments_Availabl2

Short-term Investments Available-for-Sale - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
Share data in Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2014	Mar. 31, 2014	Jan. 17, 2014	Dec. 31, 2011
					TaiGen Biotechnology Co., Ltd.	TaiGen Biotechnology Co., Ltd.	TaiGen Biotechnology Co., Ltd.	TaiGen Biotechnology Co., Ltd.	TaiGen Biotechnology Co., Ltd.
Investment Holdings [Line Items]	'	'	'	'	'	'	'	'	'
Equity investment	'	'	'	'	'	'	'	'	$0
Investment, shares	'	'	'	'	'	'	'	29.6	'
Investment, fair value	'	'	'	'	'	'	'	49,100,000	'
Investment, unrealized gain included in accumulated other comprehensive income (loss)	'	'	'	'	0	0	49,100,000	'	'
Investment, shares sold	'	'	'	'	10.6	29.6	'	'	'
Realized gains from sale of shares	$16,276,000	$0	$49,553,000	$0	$16,276,000	$49,553,000	'	'	'

Components_of_Unrealized_Gain_

Components of Unrealized Gain (Loss) on Available-for-sale Securities (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Unrealized gain (loss) on available-for-sale securities	($18,313)	$0	$0	$0
TaiGen Biotechnology Co., Ltd.	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Reclassification adjustment for realized gain on sale of available-for-sale securities	-16,276	'	-49,553	'
Unrealized holding gain (loss) on available-for-sale securities	-2,037	'	49,553	'
Unrealized gain (loss) on available-for-sale securities	($18,313)	'	$0	'

Components_of_Inventory_Detail

Components of Inventory (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Inventory [Line Items]	'	'
Raw materials	$579	$657
Work in process	3,069	4,104
Total inventory	11,861	12,759	[1]
Arena GmbH	'	'
Inventory [Line Items]	'	'
Finished goods	4,597	0
Eisai BELVIQ Agreement	'	'
Inventory [Line Items]	'	'
Finished goods	$3,616	$7,998
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Land_Property_and_Equipment_De

Land, Property and Equipment (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment [Line Items]	'	'
Cost	$166,015	$165,373
Less accumulated depreciation and amortization	-90,526	-87,985
Land, property and equipment, net	$75,489	$77,388	[1]
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Accounts_Payable_and_Other_Acc2

Accounts Payable and Other Accrued Liabilities (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accounts Payable and Accrued Liabilities, Current [Abstract]	'	'
Accounts payable	$2,579	$3,721
Accrued expenses	1,391	1,477
Loss provision	82	567
Other accrued liabilities	107	235
Total accounts payable and other accrued liabilities	$4,159	$6,000	[1]
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Derivative_Liabilities_Additio

Derivative Liabilities - Additional Information (Detail) (USD $)	1 Months Ended
In Thousands, except Share data, unless otherwise specified	Aug. 31, 2008	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]	'	'	'
Issuance of warrant to purchase stock, number of shares	1,106,344	'	'
Original exercise price of warrant	$7.71	'	'
Warrant expiration date	14-Aug-15	'	'
Revised exercise price of warrant	'	$4.34	'
Warrant outstanding	'	1,965,418	'
Adjustment price of warrant in anti-dilution provision	$6.72	'	'
Derivative liabilities, current	'	$1,403	$0	[1]
Derivative liabilities, non-current	'	$0	$4,892	[1]
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Marketing_and_Supply_Agreement2

Marketing and Supply Agreement with Eisai - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

1 Months Ended

45 Months Ended

1 Months Ended

45 Months Ended

1 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Nov. 30, 2013

Nov. 30, 2013

Nov. 30, 2013

Jul. 31, 2010

31-May-12

Nov. 30, 2013

Mar. 31, 2014

Nov. 30, 2013

Nov. 30, 2013

Nov. 30, 2013

Nov. 30, 2013

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Eisai BELVIQ Agreement

Original Agreement

Eisai First Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Eisai Second Amended Agreement

Product Purchase Price

Product Purchase Price

Voucher Redemption

Voucher Redemption

Product Sampling

UNITED STATES

Non US Territories Other Than Europe, China and Japan

Europe, China and Japan

All Territories

All Non-US territories

Non-US Territories in North and South America

Territories Outside of North and South America

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Collaborative agreement initiation date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'2010-07

'2012-05

'2013-11

'

'

'

'

'

Collaborative agreement upfront payments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$50,000,000

$5,000,000

$60,000,000

'

'

'

'

'

Collaborative Agreement Revenue Recognition Period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'16 years

'15 years

'15 years

'

'

'

'

'

Recognized revenues

8,164,000

3,578,000

27,779,000

74,878,000

7,859,000

2,984,000

26,171,000

72,263,000

'

'

'

'

'

'

'

'

'

'

'

'

86,500,000

'

'

'

'

Additional milestone payments on achievement

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

176,000,000

'

'

'

'

Purchase price range minimum

'

'

'

'

'

'

'

'

'

'

'

'

'

'

31.50%

30.75%

27.50%

'

'

'

'

'

'

'

'

Purchase price range maximum

'

'

'

'

'

'

'

'

'

'

'

'

'

'

36.50%

35.75%

35.00%

'

'

'

'

'

'

'

'

Annual net product sales threshold for maximum purchase price

'

'

'

'

'

'

'

'

'

'

'

'

'

'

750,000,000

750,000,000

500,000,000

'

'

'

'

'

'

'

'

Aggregate one-time purchase price adjustments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,560,000,000

'

1,190,000,000

'

'

'

First annual net sales threshold to earn purchase price adjustments in all territories

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

250,000,000

'

'

'

Last annual net sales threshold to earn purchase price adjustments in all territories

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,500,000,000

'

'

'

Portion of purchase price adjustment payments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

330,000,000

'

'

'

Annual net sales threshold to earn portion of purchase price adjustments in all territories

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,000,000,000

'

'

'

Additional one-time purchase price adjustment payments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

370,000,000

185,000,000

185,000,000

First annual net sales threshold to earn purchase price adjustments outside of the US

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100,000,000

100,000,000

Last annual net sales threshold to earn additional purchase price adjustments outside of the US

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,000,000,000

1,000,000,000

Net product sales

'

'

'

'

5,726,000

2,011,000

12,137,000

3,330,000

'

5,200,000

11,000,000

500,000

1,100,000

'

'

'

'

'

'

'

'

'

'

'

'

Payable to Eisai

'

'

'

'

19,051,000

'

19,051,000

'

19,305,000

[1]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reduction in deferred revenues

'

'

-22,998,000

12,565,000

'

'

'

'

'

'

'

'

'

-6,000,000

'

'

'

'

'

'

'

'

'

'

'

Increase in payable to Eisai

'

'

'

'

'

'

'

'

'

'

'

'

'

$6,000,000

'

'

'

'

'

'

'

'

'

'

'

[1]

The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Revenues_Recognized_under_Coll

Revenues Recognized under Collaboration with Eisai (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total revenues	$8,164	$3,578	$27,779	$74,878
Eisai BELVIQ Agreement	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Net product sales	5,726	2,011	12,137	3,330
Amortization of upfront payments	1,885	861	5,745	2,584
Milestone payments	0	0	500	66,000
Subtotal other Eisai collaborative revenue	2,133	973	14,034	68,933
Total revenues	7,859	2,984	26,171	72,263
Eisai BELVIQ Agreement | Research and development	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Reimbursement revenue	143	16	7,456	26
Eisai BELVIQ Agreement | Patents and Trademarks	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Reimbursement revenue	$105	$96	$333	$323

Deferred_Revenues_under_Collab

Deferred Revenues under Collaboration with Eisai (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred Revenue Arrangement [Line Items]	'	'
Less current portion	($19,869)	($37,861)	[1]
Deferred revenues, less current portion	95,092	101,329	[1]
Eisai BELVIQ Agreement	'	'
Deferred Revenue Arrangement [Line Items]	'	'
Total deferred revenues	108,066	132,403
Less current portion	-19,247	-37,301
Deferred revenues, less current portion	88,819	95,102
Eisai BELVIQ Agreement | Up-front Payment Arrangement	'	'
Deferred Revenue Arrangement [Line Items]	'	'
Total deferred revenues	96,359	102,104
Eisai BELVIQ Agreement | Sales Revenue, Goods, Net	'	'
Deferred Revenue Arrangement [Line Items]	'	'
Total deferred revenues	$11,707	$30,299
[1]	The balance sheet data at December 31, 2013, has been derived from audited financial statements at that date. It does not include, however, all of the information and notes required by US generally accepted accounting principles for complete financial statements.

Summary_of_Cost_Sharing_Alloca

Summary of Cost Sharing Allocation (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014
BELVIQ Product For Weight Management Pre Approval Up To One Hundred Million Dollars | Territories Outside of North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Aggregate expense cap on equal split of development expenses	$100	[1]
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Pre Approval Up To One Hundred Million Dollars | Territories Outside of North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Pre Approval More Than One Hundred Million Dollars | Territories Outside of North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	100.00%	[1]
BELVIQ Product For Weight Management Post Approval Up To Fifty Million Dollars | Territories Outside of North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Aggregate expense cap on equal split of development expenses	50	[1]
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval Up To Fifty Million Dollars | Territories Outside of North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval More Than Fifty Million Dollars | Territories Outside of North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	10.00%	[1]
BELVIQ Product For Weight Management Post Approval More Than Fifty Million Dollars | Territories Outside of North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	90.00%	[1]
BELVIQ Product For Weight Management Pre Approval | Non-US Territories in North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	10.00%	[1]
BELVIQ Product For Weight Management Pre Approval | Non-US Territories in North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	90.00%	[1]
BELVIQ Product For Weight Management Pre Approval Certain Stability Work | Non-US Territories in North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Pre Approval Certain Stability Work | Non-US Territories in North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval | Non-US Territories in North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	10.00%	[1]
BELVIQ Product For Weight Management Post Approval | Non-US Territories in North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	90.00%	[1]
BELVIQ Product For Weight Management Post Approval | UNITED STATES	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	10.00%	[1]
BELVIQ Product For Weight Management Post Approval | UNITED STATES | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	90.00%	[1]
BELVIQ Product For Weight Management Post Approval Certain Stability Work | Non-US Territories in North and South America	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval Certain Stability Work | Non-US Territories in North and South America | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product for Weight Management Post Approval Certain Pediatric Studies | UNITED STATES	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product for Weight Management Post Approval Certain Pediatric Studies | UNITED STATES | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval Non FDA Required Portion of Cardiovascular Outcomes Trial Up To Eighty Million Dollars | UNITED STATES	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Aggregate expense cap on equal split of development expenses	80	[1]
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval Non FDA Required Portion of Cardiovascular Outcomes Trial Up To Eighty Million Dollars | UNITED STATES | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%	[1]
BELVIQ Product For Weight Management Post Approval Non FDA Required Portion of Cardiovascular Outcomes Trial More than Eighty Million Dollars | UNITED STATES | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	100.00%	[1]
BELVIQ Product Not For Weight Management Pre-Approval	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%
BELVIQ Product Not For Weight Management Pre-Approval | Maximum	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Aggregate expense cap on equal split of development expenses	250
BELVIQ Product Not For Weight Management Pre-Approval | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%
BELVIQ Product Not For Weight Management Post-Approval Up To One Hundred Million Dollars	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Aggregate expense cap on equal split of development expenses	$100
Portion of expenses	50.00%
BELVIQ Product Not For Weight Management Post-Approval Up To One Hundred Million Dollars | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	50.00%
BELVIQ Product Not For Weight Management Post-Approval More Than One Hundred Million Dollars	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	10.00%
BELVIQ Product Not For Weight Management Post-Approval More Than One Hundred Million Dollars | Eisai BELVIQ Agreement	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'
Portion of expenses	90.00%
[1]	Development required by a regulatory authority, with the exception of the non-FDA required portions of the CVOT.

Share_Based_Compensation_Expen

Share Based Compensation Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$3,398	$2,319	$9,941	$6,542
Total share-based compensation expense capitalized into inventory	22	35	61	79
Cost of product sales	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	0	0	0	17
Research and development	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	1,753	1,188	5,276	3,047
General and administrative	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$1,645	$1,131	$4,665	$3,478

Summary_of_Stock_Option_Activi

Summary of Stock Option Activity (Detail) (USD $)	9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014
Number of stock options	'
Outstanding at January 1, 2014	14,681
Granted	2,126
Exercised	-1,070
Forfeited/cancelled/expired	-230
Outstanding at September 30, 2014	15,507
Weighted Average Exercise Price	'
Outstanding at January 1, 2014	$4.99
Granted	$6.78
Exercised	$3.72
Forfeited/cancelled/expired	$6.28
Outstanding at September 30, 2014	$5.30

Summary_of_Restricted_Stock_Ac

Summary of Restricted Stock Activity (Detail) (Restricted Stock Units (RSU), USD $)	9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2014
Restricted Stock Units (RSU)	'
Restricted Stock Units	'
Unvested at January 1, 2014	369
Granted	178
Vested	-110
Forfeited/cancelled	0
Unvested at September 30, 2014	437
Weighted-Average Grant-Date Fair Value	'
Unvested at January 1, 2014	$7.23
Granted	$6.50
Vested	$7.88
Forfeited/cancelled	'
Unvested at September 30, 2014	$6.77

ShareBased_Activity_Additional

Share-Based Activity - Additional Information (Detail) (Performance restricted stock units, USD $)	1 Months Ended
In Millions, except Share data in Thousands, unless otherwise specified	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
PRSUs granted	695
Multiplier of grant-date fair value for cap on number of PRSUs that may be granted	6
Vesting period	'3 years
Cap on percentage of funding if the absolute 3-year TSR is negative	100.00%
PRSU grant-date fair value	$5
Minimum	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Expected number of shares to be awarded as a percentage of target amounts	0.00%
Maximum	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Expected number of shares to be awarded as a percentage of target amounts	200.00%

Assumptions_used_to_Value_Perf

Assumptions used to Value Performance Restricted Stock Units Granted and Estimated Grant Date Fair Value (Detail) (Performance restricted stock units, USD $)	9 Months Ended
	Sep. 30, 2014
Performance restricted stock units	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Risk-free interest rate	0.70%
Dividend yield	0.00%
Expected volatility	78.00%
Remaining performance period (years)	'2 years 11 months 27 days
Estimated fair value per share of PRSUs granted	$7.16

Percentages_of_Total_Revenues_

Percentages of Total Revenues (Detail) (Revenue)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	100.00%	100.00%	100.00%	100.00%
Eisai marketing and supply agreement	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	96.30%	83.40%	94.20%	96.50%
Manufacturing services agreement with Siegfried	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	1.90%	12.40%	4.30%	2.90%
Other collaborative agreements	'	'	'	'
Concentration Risk [Line Items]	'	'	'	'
Percentage of revenues	1.80%	4.20%	1.50%	0.60%

Reconciliation_of_Number_of_Sh

Reconciliation of Number of Shares Used to Calculate Basic and Diluted Net Income (Loss) Per Share (Detail)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net income (loss) per share:	'	'	'	'
Weighted-average shares outstanding	219,866	218,316	219,592	217,923
Potentially dilutive common shares outstanding:	'	'	'	'
Weighted-average shares used to calculate diluted net income (loss) per share	219,866	218,316	219,592	224,354
Stock Options	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potentially dilutive common shares outstanding	0	0	0	5,489
Warrants	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potentially dilutive common shares outstanding	0	0	0	876
RSUs and Unvested Restricted Stock	'	'	'	'
Potentially dilutive common shares outstanding:	'	'	'	'
Potentially dilutive common shares outstanding	0	0	0	66

Potentially_Dilutive_Securitie

Potentially Dilutive Securities Excluded From Calculation of Diluted Net Income (Loss) Per Share (Detail)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Total antidilutive securities excluded from calculation of diluted net loss per share	12,177	5,569	12,889	3,757
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	11,614	4,796	12,185	3,757
Warrants	'	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Total antidilutive securities excluded from calculation of diluted net loss per share	55	694	490	0
RSUs, PRSUs, 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	508	79	214	0

Commitment_to_Acquire_Real_Pro1

Commitment to Acquire Real Property - Additional Information (Detail) (CHF)	Jul. 31, 2014	Jan. 31, 2008
In Millions, unless otherwise specified		sqft
Other Commitments [Line Items]	'	'
Asset purchase agreement, area of building acquired	'	67,000
Asset purchase agreement, total area of building	'	134,000
Company's optional purchase price of remaining portion of the building	'	15
Seller's optional selling price of remaining portion of the building	8	'