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BALANCE SHEETS - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2010	Dec. 31, 2009
Current assets
Cash and cash equivalents	$ 135,376	$ 112,572	$ 117,311	$ 207,075
Marketable securities	91,065	136,796
Accounts receivable from the sanofi-aventis Group	84,391	79,603
Accounts receivable - other	3,765	13,509
Prepaid expenses and other current assets	10,904	15,142
Total current assets	325,501	357,622
Restricted cash and marketable securities	7,520	7,518
Marketable securities	373,621	370,053
Property, plant, and equipment, at cost, net of accumulated depreciation and amortization	357,423	347,450
Other assets	10,228	6,789
Total assets	1,074,293	1,089,432
Current liabilities
Accounts payable and accrued expenses	58,602	53,658
Deferred revenue from sanofi-aventis, current portion	19,561	19,506
Deferred revenue - other, current portion	33,291	35,217
Facility lease obligations, current portion	798	675
Total current liabilities	112,252	109,056
Deferred revenue from sanofi-aventis	94,398	97,081
Deferred revenue - other	183,019	188,775
Facility lease obligations	159,353	159,355
Other long term liabilities	7,180	7,350
Total liabilities	$ 556,202	$ 561,617
Commitments and contingencies
Stockholders' equity
Preferred stock, $.01 par value; 30,000,000 shares authorized; issued and outstanding-none	$ 0	$ 0
Common Stock	89	87
Additional paid-in capital	1,609,185	1,575,780
Accumulated deficit	(1,089,010)	(1,045,563)
Accumulated other comprehensive loss	(2,175)	(2,491)
Total stockholders' equity	518,091	527,815	386,271	396,762
Total liabilities and stockholders' equity	1,074,293	1,089,432
Class A [Member]
Stockholders' equity
Common Stock	2	2
Total stockholders' equity	$ 2	$ 2	$ 2	$ 2

BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2011	Dec. 31, 2010
Stockholders' equity
Preferred stock, par value	$ 0.010	$ 0.010
Preferred stock, authorized	30,000,000	30,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common Stock, par value	$ 0.001	$ 0.001
Common Stock, authorized	160,000,000	160,000,000
Common Stock, issued	88,548,041	87,238,301
Common Stock, outstanding	88,548,041	87,238,301
Class A [Member]
Stockholders' equity
Common Stock, par value	$ 0.001	$ 0.001
Common Stock, authorized	40,000,000	40,000,000
Common Stock, issued	2,182,036	2,182,036
Common Stock, outstanding	2,182,036	2,182,036

STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2011	Mar. 31, 2010
Revenues
Sanofi-aventis collaboration revenue	$ 85,329	$ 68,671
Other collaboration revenue	12,481	13,087
Technology licensing	7,845	10,038
Net product sales	4,427	9,852
Contract research and other	2,122	1,886
Total Revenues	112,204	103,534
Expenses
Research and development	129,392	117,471
Selling, general, and administrative	23,411	14,223
Cost of goods sold	382	717
Total Expenses	153,185	132,411
Loss from operations	(40,981)	(28,877)
Other income (expense)
Investment income	1,037	439
Interest expense	(3,719)	(2,084)
Total Other Income (Expense)	(2,682)	(1,645)
Net loss before income tax benefit	(43,663)	(30,522)
Income tax benefit	(216)	0
Net loss	$ (43,447)	$ (30,522)
Net loss per share, basic and diluted	$ (0.490)	$ (0.380)
Weighted average shares outstanding, basic and diluted	89,162	81,169

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2011	Mar. 31, 2010
Statement [Line Items]
Beginning Balance	$ 527,815	$ 396,762
Issuance of Common Stock in connection with exercise of stock options, net of shares tendered	15,104	8,657
Issuance of Common Stock in connection with Company 401(k) Savings Plan contribution	3,405	2,867
Stock-based compensation charges	14,898	8,834
Net loss	(43,447)	(30,522)
Change in net unrealized gain (loss) on marketable securities	316	(327)
Ending Balance	$ 518,091	$ 386,271
Common Stock [Member]
Statement [Line Items]
Beginning Balance (in shares)	87,238	78,861
Beginning Balance	$ 87	$ 79
Issuance of Common Stock in connection with exercise of stock options, net of shares tendered (in shares)	1,218	685
Issuance of Common Stock in connection with exercise of stock options, net of shares tendered	$ 2	$ 1
Issuance of Common Stock in connection with Company 401(k) Savings Plan contribution (in shares)	92	111
Conversion of Class A Stock to Common Stock (in shares)		33
Ending Balance (in shares)	88,548	79,690
Ending Balance	$ 89	$ 80
Additional Paid-in Capital [Member]
Statement [Line Items]
Beginning Balance	1,575,780	1,336,732
Issuance of Common Stock in connection with exercise of stock options, net of shares tendered	15,102	8,656
Issuance of Common Stock in connection with Company 401(k) Savings Plan contribution	3,405	2,867
Stock-based compensation charges	14,898	8,834
Ending Balance	1,609,185	1,357,089
Accumulated Deficit [Member]
Statement [Line Items]
Beginning Balance	(1,045,563)	(941,095)
Net loss	(43,447)	(30,522)
Ending Balance	(1,089,010)	(971,617)
Accumulated Other Comprehensive Income (Loss) [Member]
Statement [Line Items]
Beginning Balance	(2,491)	1,044
Change in net unrealized gain (loss) on marketable securities	316	(327)
Ending Balance	(2,175)	717
Comprehensive Loss [Member]
Statement [Line Items]
Net loss	(43,447)	(30,522)
Change in net unrealized gain (loss) on marketable securities	316	(327)
Ending Balance	$ (43,131)	$ (30,849)

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Mar. 31, 2011	Mar. 31, 2010
Cash flows from operating activities
Net loss	$ (43,447)	$ (30,522)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	6,978	4,183
Non-cash compensation expense	14,801	8,834
Other non-cash charges and expenses	582	544
Changes in assets and liabilities
Decrease (increase) in accounts receivable	4,956	(6,290)
Decrease (increase) in prepaid expenses and other assets	2,286	(2,483)
(Decrease) increase in deferred revenue	(10,310)	3,513
Increase in accounts payable, accrued expenses, and other liabilities	13,574	12,643
Total adjustments	32,867	20,944
Net cash used in operating activities	(10,580)	(9,578)
Cash flows from investing activities
Purchases of marketable securities	(15,638)	(177,594)
Sales or maturities of marketable securities	58,151	63,936
Capital expenditures	(22,166)	(22,743)
Net cash provided by (used in) investing activities	20,347	(136,401)
Cash flows from financing activities
Proceeds in connection with facility lease obligations	0	47,544
Payments in connection with facility lease obligations	(89)	(555)
Net proceeds from the issuance of Common Stock	13,343	9,226
Payments in connection with capital lease obligation	(217)	0
Net cash provided by financing activities	13,037	56,215
Net increase (decrease) in cash and cash equivalents	22,804	(89,764)
Cash and cash equivalents at beginning of period	112,572	207,075
Cash and cash equivalents at end of period	$ 135,376	$ 117,311

ARCALYST (rilonacept) Product Revenue

3 Months Ended

Mar. 31, 2011

Notes to Financial Statements [Abstract]

ARCALYST (rilonacept) Product Revenue

2.	ARCALYST® (rilonacept) Product Revenue

In February 2008, the Company received marketing approval from the U.S. Food and Drug Administration (“FDA”) for ARCALYST® Injection for Subcutaneous Use for the treatment of Cryopyrin-Associated Periodic Syndromes (“CAPS”). The Company had limited historical return experience for ARCALYST® beginning with initial sales in 2008 through the end of 2009; therefore, ARCALYST® net product sales were deferred until the right of return no longer existed and rebates could be reasonably estimated. Effective in the first quarter of 2010, the Company determined that it had accumulated sufficient historical data to reasonably estimate both product returns and rebates of ARCALYST®. As a result, $4.8 million of previously deferred ARCALYST® net product sales were recognized as revenue in the first quarter of 2010. The effect of this change in estimate related to ARCALYST® net product sales revenue was to lower the Company’s net loss per share by $0.06 for the three months ended March 31, 2010.

ARCALYST® net product sales totaled $4.4 million and $9.9 million for the three months ended March 31, 2011 and 2010, respectively. ARCALYST® net product sales during the first three months of 2010 included $5.1 million of net product sales made during this period and $4.8 million of previously deferred net product sales, as described above. There was no deferred ARCALYST® net product sales revenue at March 31, 2011 or 2010.

Cost of goods sold related to ARCALYST® sales, which consisted primarily of royalties, totaled $0.4 million and $0.7 million for the three months ended March 31, 2011 and 2010, respectively. ARCALYST® shipments to the Company’s customers primarily consisted of supplies of inventory manufactured and expensed as research and development costs prior to 2008; therefore, the costs of these supplies were not included in costs of goods sold.

Marketable Securities

3 Months Ended

Mar. 31, 2011

Notes to Financial Statements [Abstract]

Marketable Securities

5.	Marketable Securities

Marketable securities at March 31, 2011 and December 31, 2010 consisted of debt securities, as detailed below, and equity securities, the aggregate fair value of which was $4.6 million and $3.6 million at March 31, 2011 and December 31, 2010, respectively, and the aggregate cost basis of which was $4.0 million at both March 31, 2011 and December 31, 2010. The Company also held restricted marketable securities at both March 31, 2011 and December 31, 2010, which consisted of debt securities, as detailed below, that collateralize (i) a letter of credit in connection with the Company’s lease of facilities in Tarrytown, New York and (ii) capital lease obligations.

The following tables summarize the amortized cost basis of debt securities included in marketable securities, the aggregate fair value of those securities, and gross unrealized gains and losses on those securities at March 31, 2011 and December 31, 2010. The Company classifies its debt securities, other than mortgage-backed securities, based on their contractual maturity dates. Maturities of mortgage-backed securities have been estimated based primarily on repayment characteristics and experience of the senior tranches that the Company holds.

	Amortized	Fair	Unrealized
At March 31, 2011	Cost Basis	Value	Gains	(Losses)	Net
Unrestricted
Maturities within one year
U.S. government obligations	$38,551	$38,593	$42		$42
U.S. government guaranteed corporate bonds	39,670	39,939	269		269
U.S. government guaranteed collateralized mortgage obligations	1,588	1,590	2		2
Municipal bonds	10,786	10,799	13		13
Mortgage-backed securities	146	144		$(2)	(2)
	90,741	91,065	326	(2)	324
Maturities between one and five years
U.S. government obligations	348,753	346,784	10	(1,979)	(1,969)
U.S. government guaranteed corporate bonds	15,546	15,503		(43)	(43)
Municipal bonds	6,546	6,552	6		6
	370,845	368,839	16	(2,022)	(2,006)
Maturities between five and six years
Mortgage-backed securities	281	144		(137)	(137)
	461,867	460,048	342	(2,161)	(1,819)

Restricted
Maturities within one year
U.S. government obligations	2,924	2,926	2		2

Maturities between one and three years
U.S. government obligations	4,138	4,115		(23)	(23)
	7,062	7,041	2	(23)	(21)

	$468,929	$467,089	$344	$(2,184)	$(1,840)

At December 31, 2010
Unrestricted
Maturities within one year
U.S. government obligations	$83,635	$83,684	$54	$(5)	$49
U.S. government guaranteed corporate bonds	48,173	48,531	358		358
U.S. government guaranteed collateralized mortgage obligations	2,027	2,131	104		104
Municipal bonds	1,597	1,603	6		6
Mortgage-backed securities	875	847		(28)	(28)
	136,307	136,796	522	(33)	489

	Amortized	Fair	Unrealized
At December 31, 2010 (continued)	Cost Basis	Value	Gains	(Losses)	Net
Maturities between one and five years
U.S. government obligations	352,345	350,683	64	(1,726)	(1,662)
U.S. government guaranteed corporate bonds	15,522	15,477		(45)	(45)
Mortgage-backed securities	110	38		(72)	(72)
	367,977	366,198	64	(1,843)	(1,779)
Maturities between five and seven years
Mortgage-backed securities	284	243		(41)	(41)
	504,568	503,237	586	(1,917)	(1,331)

Restricted
Maturities within one year
U.S. government obligations	2,922	2,921		(1)	(1)

Maturities between one and three years
U.S. government obligations	4,135	4,118		(17)	(17)
	7,057	7,039		(18)	(18)

	$511,625	$510,276	$586	$(1,935)	$(1,349)

At March 31, 2011 and December 31, 2010, marketable securities included an additional unrealized gain of $0.6 million and an unrealized loss of $0.4 million, respectively, related to one equity security in the Company’s marketable securities portfolio.

The following table shows the fair value of the Company’s marketable securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at March 31, 2011 and December 31, 2010. The debt securities listed at March 31, 2011, excluding mortgage-backed securities, mature at various dates through December 2013. The mortgage-backed securities listed at March 31, 2011 mature at various dates through February 2017.

	Less than 12 Months		12 Months or Greater		Total
At March 31, 2011	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Unrestricted
U.S. government obligations	$331,731	$(1,979)			$331,731	$(1,979)
U.S. government guaranteed corporate bonds	15,503	(43)			15,503	(43)
Mortgage-backed securities			$288	$(139)	288	(139)
	347,234	(2,022)	288	(139)	347,522	(2,161)

Restricted
U.S. government obligations	4,115	(23)			4,115	(23)
	4,115	(23)			4,115	(23)

	$351,349	$(2,045)	$288	$(139)	$351,637	$(2,184)

	Less than 12 Months		12 Months or Greater		Total
At December 31, 2010	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Unrestricted
U.S. government obligations	$340,444	$(1,731)			$340,444	$(1,731)
U.S. government guaranteed corporate bonds	19,005	(45)			19,005	(45)
Equity securities	3,612	(433)			3,612	(433)
Mortgage-backed securities			$1,128	$(141)	1,128	(141)
	363,061	(2,209)	1,128	(141)	364,189	(2,350)

Restricted
U.S. government obligations	6,154	(18)			6,154	(18)
	6,154	(18)			6,154	(18)

	$369,215	$(2,227)	$1,128	$(141)	$370,343	$(2,368)

Realized gains and losses are included as a component of investment income. For the three months ended March 31, 2011 and 2010, realized gains and losses on sales of marketable securities were not significant. In computing realized gains and losses, the Company computes the cost of its investments on a specific identification basis. Such cost includes the direct costs to acquire the security, adjusted for the amortization of any discount or premium.

The Company’s assets that are measured at fair value on a recurring basis, at March 31, 2011 and December 31, 2010, were as follows:

		Fair Value Measurements at Reporting Date Using
	Fair Value	Quoted Prices in Active Markets for Identical Assets (Level 1)	Significant Other Observable Inputs (Level 2)	Significant Unobservable Inputs (Level 3)
At March 31, 2011
Unrestricted
Available-for-sale marketable securities
U.S. government obligations	$385,377		$385,377
U.S. government guaranteed corporate bonds	55,442		55,442
U.S. government guaranteed collateralized mortgage obligations	1,590		1,590
Municipal bonds	17,351		17,351
Mortgage-backed securities	288		288
Equity securities	4,638	$4,638
	464,686	4,638	460,048

Restricted
Available-for-sale marketable securities
U.S. government obligations	7,041		7,041

	$471,727	$4,638	$467,089

		Fair Value Measurements at Reporting Date Using
	Fair Value	Quoted Prices in Active Markets for Identical Assets (Level 1)	Significant Other Observable Inputs (Level 2)	Significant Unobservable Inputs (Level 3)
At December 31, 2010
Unrestricted
Available-for-sale marketable securities
U.S. government obligations	$434,367		$434,367
U.S. government guaranteed corporate bonds	64,008		64,008
U.S. government guaranteed collateralized mortgage obligations	2,131		2,131
Municipal bonds	1,603		1,603
Mortgage-backed securities	1,128		1,128
Equity securities	3,612	$3,612
	506,849	3,612	503,237

Restricted
Available-for-sale marketable securities
U.S. government obligations	7,039		7,039

	$513,888	$3,612	$510,276

Marketable securities included in Level 2 were valued using a market approach utilizing prices and other relevant information, such as interest rates, yield curves, prepayment speeds, loss severities, credit risks and default rates, generated by market transactions involving identical or comparable assets. The Company considers market liquidity in determining the fair value for these securities. During the three months ended March 31, 2010, deterioration in the credit quality of a marketable security from one issuer subjected the Company to the risk of not being able to recover a portion of the security’s $1.1 million carrying value. As a result, the Company recognized a $0.1 million impairment charge related to this Level 2 marketable security, which the Company considered to be other-than-temporarily impaired. The Company did not record any charges for other-than-temporary impairment of its Level 2 marketable securities during the three months ended March 31, 2011.

The Company holds one Level 3 marketable security, which had no fair value at March 31, 2011 and December 31, 2010. This Level 3 security was valued using information provided by the Company’s investment advisors and other sources, including quoted bid prices which took into consideration the security’s lack of liquidity. There were no purchases, sales, or maturities of Level 3 marketable securities and no unrealized gains or losses related to Level 3 marketable securities for the three months ended March 31, 2011 and 2010. There were no transfers of marketable securities between Levels 1, 2, or 3 classifications during the three months ended March 31, 2011 and 2010.

On a quarterly basis, the Company reviews its portfolio of marketable securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost are other-than-temporary. With respect to debt securities, this review process also includes an evaluation of the Company’s (a) intent to sell an individual debt security or (b) need to sell the debt security before its anticipated recovery or maturity. With respect to equity securities, this review process includes an evaluation of the Company’s ability and intent to hold the securities until their full value can be recovered.

Legal Matters

3 Months Ended

Mar. 31, 2011

Notes to Financial Statements [Abstract]

Legal Matters

9.	Legal Matters

From time to time, the Company is a party to legal proceedings in the course of the Company’s business. The Company does not expect any such current ordinary course legal proceedings to have a material adverse effect on the Company’s business or financial condition. Legal costs associated with the Company’s resolution of legal proceedings are expensed as incurred.

As previously reported, on November 19, 2010, the Company filed a complaint against Genentech, Inc. in the U.S. District Court for the Southern District of New York seeking a declaratory judgment that no activities relating to VEGF Trap infringe any valid claim of certain Genentech patents referred to as the Davis-Smyth patents. On January 12, 2011, Genentech filed a motion to dismiss the complaint, arguing that the lawsuit was premature and thus the Court lacked subject matter jurisdiction. Upon the Company’s submission to the FDA of a Biologics License Application (“BLA”) for VEGF Trap-Eye for the treatment of wet AMD, the Company filed a second complaint against Genentech in the same court seeking the same declaratory relief. On April 7, 2011, the Company and Genentech entered into a Joint Stipulation, which was approved and executed by the Court on April 11, 2011. Pursuant to the Joint Stipulation, the Company voluntarily dismissed its original complaint in favor of proceeding with its second complaint, and Genentech agreed that it would not seek to transfer the case to another judicial district or move to dismiss the second complaint for lack of subject matter jurisdiction or otherwise under Rule 12(b) of the Federal Rules of Civil Procedure. On April 25, 2011, Genentech filed an answer to the second complaint, denying that the Company is entitled to the declaratory relief being sought by it, and asserting counterclaims that the Company's prior or planned activities relating to VEGF Trap have infringed or will infringe one or more claims of the Davis-Smyth patents. In its answer, Genentech requests a judgment against the Company for damages, including for willful infringement, and other relief as the Court deems appropriate. The Company believes Genentech’s counterclaims are without merit and intends to defend against them vigorously. As this matter is at a very early stage, at this time the Company is not able to predict the probability of the outcome or an estimate of loss, if any, related to this matter.

The Company has initiated patent-related actions against Genentech in Germany and the United Kingdom, and may initiate other actions in other countries outside the U.S.

Recently Issued Accounting Standards

3 Months Ended

Mar. 31, 2011

Notes to Financial Statements [Abstract]

Recently Issued Accounting Standards

10.	Recently Issued Accounting Standards

Multiple-deliverable revenue arrangements

During the first quarter of 2011 the Company adopted amended authoritative guidance issued by the Financial Accounting Standards Board (“FASB”) on multiple-deliverable revenue arrangements. The amended guidance provides greater ability to separate and allocate consideration to be received in a multiple-deliverable revenue arrangement by requiring the use of estimated selling prices to allocate the consideration, thereby eliminating the use of the residual method of allocation. The amended guidance also requires expanded qualitative and quantitative disclosures surrounding multiple-deliverable revenue arrangements. The Company is applying this amended guidance prospectively for new or materially modified arrangements, of which there were none during the three months ended March 31, 2011. The adoption of this guidance did not have a material impact on the Company’s financial statements.

Milestone method of revenue recognition

During the first quarter of 2011, the Company adopted amended authoritative guidance issued by FASB codifying the milestone method of revenue recognition as an acceptable revenue recognition model when a milestone is deemed to be substantive. The Company has historically accounted for milestones under the milestone method; as such the adoption of this guidance did not have a material impact on the Company’s financial statements.

In accordance with the Company’s accounting policy for recognition of revenue in connection with collaboration agreements, as previously disclosed in the Company’s financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2010, payments which are based on achieving a specific performance milestone, involving a degree of risk, are recognized as revenue when the milestone is achieved and the related payment is due and non-refundable, provided there is no future service obligation associated with that milestone. Substantive performance milestones typically consist of significant achievements in the development life-cycle of the related product candidate, such as completion of clinical trials, filing for approval with regulatory agencies, and receipt of approvals by regulatory agencies. In determining whether a payment is deemed to be a substantive performance milestone, the Company takes into consideration (i) the nature, timing, and value of significant achievements in the development life-cycle of the related development product candidate, (ii) the relative level of effort required to achieve the milestone, and (iii) the relative level of risk in achieving the milestone, taking into account the high degree of uncertainty in successfully advancing product candidates in a drug development program and in ultimately attaining an approved drug product. Payments for achieving milestones which are not considered substantive are accounted for as license payments and recognized over the related performance period.

The Company earns substantive performance milestone payments in connection with its collaboration agreements to develop and commercialize product candidates with the sanofi-aventis Group and Bayer HealthCare LLC. Descriptions of these collaboration agreements, including various financial terms and conditions, were provided in the Company’s financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2010. Under the Company’s collaboration agreement with sanofi-aventis to jointly develop and commercialize ZALTRAPTM (aflibercept), the Company may receive up to $400 million in substantive milestone payments upon receipt of specified marketing approvals, including up to $360 million in milestone payments related to the receipt of marketing approvals for up to eight ZALTRAPTM oncology and other indications in the U.S. or the European Union and up to $40 million related to the receipt of marketing approvals for up to five ZALTRAPTM oncology indications in Japan. Under the Company’s global, strategic collaboration with sanofi-aventis to discover, develop, and commercialize fully human monoclonal antibodies, for each drug candidate identified under the collaboration’s Discovery and Preclinical Development Agreement, sanofi-aventis has the option to license rights to the candidate under the collaboration’s License and Collaboration Agreement and co-develop the drug candidate with the Company through product approval. Under certain defined circumstances, upon exercising its option to license rights to particular candidates, sanofi-aventis must make a $10.0 million substantive milestone payment to the Company. Under the Company’s license and collaboration agreement with Bayer HealthCare LLC to globally develop, and commercialize outside the U.S., the Company’s VEGF Trap for the treatment of eye disease by local administration (“VEGF Trap-Eye”), the Company is eligible to receive up to $50 million in future substantive milestone payments related to marketing approvals of VEGF Trap-Eye in major market countries outside the U.S..

Entity Information - USD ($)	3 Months Ended
Entity Information - USD ($)	Mar. 31, 2011	Apr. 13, 2011
Entity Registrant Name	REGENERON PHARMACEUTICALS INC
Entity Central Index Key	0000872589
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float	$ 1,726,149,000
Entity Common Stock, Shares Outstanding		88,739,294
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q1

STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2011 USD ($)
Statement of Stockholders' Equity [Abstract]
Change in net unrealized gain (loss) on marketable securities, net of tax effect	$ 0.2

Document Information	3 Months Ended
Document Information	Mar. 31, 2011
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2011

Label	Element	Value
Class A [Member]
Conversion of Class A Stock to Common Stock (in shares)	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities	(33,000)
Shares, Issued	us-gaap_SharesIssued	2,182,000
Shares, Issued	us-gaap_SharesIssued	2,182,000
Shares, Issued	us-gaap_SharesIssued	2,245,000
Shares, Issued	us-gaap_SharesIssued	2,212,000