Collaboration and Contract Research Agreements (Details) (USD $)	3 Months Ended				12 Months Ended			1 Months Ended			12 Months Ended			1 Months Ended	12 Months Ended					12 Months Ended			3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended		12 Months Ended													3 Months Ended		12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jan. 31, 2006 ZALTRAP Agreement	Jan. 31, 2005 ZALTRAP Agreement	Sep. 30, 2003 ZALTRAP Agreement	Dec. 31, 2013 ZALTRAP Agreement	Dec. 31, 2012 ZALTRAP Agreement	Dec. 31, 2011 ZALTRAP Agreement	Nov. 30, 2007 Antibody Collaboration	Dec. 31, 2013 Antibody Collaboration family antibody	Dec. 31, 2012 Antibody Collaboration	Dec. 31, 2011 Antibody Collaboration	Nov. 30, 2009 Antibody Collaboration	Dec. 31, 2013 VelociGene Agreement	Dec. 31, 2013 Collaboration Agreement with Sanofi	Dec. 31, 2012 Collaboration Agreement with Sanofi	Dec. 31, 2011 Collaboration Agreement with Sanofi	Mar. 31, 2014 PDGFR-beta	Jun. 30, 2013 PDGFR-beta	Dec. 31, 2013 PDGFR-beta	Jun. 30, 2013 Ang2	Dec. 31, 2013 Ang2	Aug. 31, 2007 Bayer Collaboration Agreement - EYLEA outside the US	Oct. 31, 2006 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2012 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2011 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2010 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2009 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2007 Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US Marketing approval	Dec. 31, 2012 Bayer Collaboration Agreement - EYLEA outside the US Marketing approval	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US Pricing approval	Dec. 31, 2012 Bayer Collaboration Agreement - EYLEA outside the US Pricing approval	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US sales achievement - 1st tier	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US sales achievement - 2nd tier	Dec. 31, 2013 Bayer Collaboration Agreement - EYLEA outside the US sales achievement - 3rd tier	Mar. 31, 2014 Bayer Collaboration Agreement - PDGFR-beta outside the US	Jan. 31, 2014 Bayer Collaboration Agreement - PDGFR-beta outside the US	Dec. 31, 2013 ex US Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2013 Maximum Bayer Collaboration Agreement - EYLEA outside the US	Dec. 31, 2013 Minimum Bayer Collaboration Agreement - EYLEA outside the US
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total number of shares owned by the collaborator																				15,816,953
Company-incurred expenses associated with Sanofi collaborations																				$ 600,100,000	$ 405,800,000	$ 318,200,000
Amount of non-refundable payment received for collaboration agreement									25,000,000																				75,000,000
Number of shares of Common Stock purchased										2,799,552
Cash price of newly issued, unregistered shares of Common Stock purchased										45,000,000
Royalty based on percentage of annual sales in Japan											35.00%
Maximum amount of substantive milestone payments that may be received relating to marketing approvals											350,000,000																																20,000,000
Percentage of development expenses to be reimbursed due to profitability of collaboration agreement											50.00%				50.00%															50.00%
Development expenses incurred to date											446,000,000				879,000,000															276,000,000
Minimum advance notice required to terminate collaboration agreement											12 months				3 months																															12 months	6 months
Regeneron's share of losses in connection with commercialization of products by our collaborator											(30,810,000)	(25,634,000)	(9,321,000)																	101,494,000	0	0
Milestone payment	25,000,000	45,000,000	10,000,000	65,000,000							0	50,000,000	0															20,000,000		70,000,000	25,000,000	0	10,000,000	20,000,000		15,000,000	15,000,000	10,000,000	10,000,000	15,000,000	15,000,000	15,000,000
Percentage of development cost for the territory outside the United States to be reimbursed due to collaboration agreement																																											25.00%
Number of sales milestones																														3
Starting specified levels of annual sales at which sales milestone payments maybe received -1st tier																																								200,000,000					200,000,000
Royalty rate minimum																														33.50%
Royalty rate maximum																														40.00%
Starting specified levels of annual sales at which sales milestone payments maybe received - 2nd tier																																									300,000,000
Starting specified levels of annual sales at which sales milestone payments may be received - 3rd tier																																										400,000,000
Reimbursement of Regeneron research and development and other expenses											5,639,000	10,702,000	16,871,000		453,489,000	365,245,000	299,281,000													20,905,000	34,892,000	33,682,000
Up-front payments made to acquire full rights to antibodies															20,000,000	0	0							(10,000,000)		(10,000,000)
Other Research and Development Expense																							5,000,000
Substantive Milestone Payments																									35,000,000		5,000,000
Other collaboration revenue											9,682,000	13,268,000	9,932,000		12,111,000	10,233,000	9,846,000													27,890,000	10,207,000	9,390,000
Collaboration revenue					430,111,000	423,814,000	326,609,000				(15,489,000)	48,336,000	17,482,000		445,600,000	375,478,000	309,127,000
Accounts receivable from Sanofi	104,707,000		99,913,000		104,707,000	99,913,000									106,100,000	102,600,000
Expenses under the collaboration payable to collaborating party											1,400,000	2,700,000
Deferred revenue											18,200,000	14,800,000			71,200,000	78,700,000														36,400,000	36,500,000
Other Collaboration Revenue					220,289,000	70,099,000	43,072,000																							220,289,000	70,099,000	43,072,000
Annual funding maximum of research activities per amended agreement																		160,000,000
Additional years to extend the agreement															3 years
Substantive milestone payment to the Company															10,000,000
Number of therapeutic antibodies currently under development															7
Percentage of Trial Costs borne by collaborating party															80.00%
Percentage of Trial Costs borne by entity															20.00%
Percentage of trial costs to be reimbursed due to profitability of agreement															30.00%
Excess Share of profit not required to to be applied to reimburse development cost															10.00%
Starting share of profits outside the United States, based on sales, for collaborating party															65.00%
Starting share of profits outside the United States, based on sales, for Company															35.00%
Ending share of profits outside the United States, based on sales, for collaborating party															55.00%
Ending share of profits outside the United States, based on sales, for Company															45.00%
Share of losses outside the United States, for collaborating party															55.00%
Share of losses outside the United States, for Company															45.00%
Maximum amount of milestone payments that may be received if certain sales are achieved															250,000,000
Amount of funding of agreed-upon cost incurred to expand manufacturing capacity															30,000,000
Period of notice to opt-out of further development and/or commercialization															12 months
Period of notice to opt-out of further development upon entering of joint development product															30 days
Minimum payments expected to be received under the agreement																			21,500,000
Proceeds from Collaborators								25,000,000	25,000,000	80,000,000				85,000,000									2,500,000						75,000,000						20,000,000								25,500,000
Number of families of novel antibodies															2
Maximum amount of sales milestone payments if total sales achieve specific levels																														90,000,000															30,000,000
Starting specified levels of annual sales at which sales milestone payments may be received															1,000,000,000
Period for Achieving Sales Target for Milestone Payment, Rolling Basis															12 months															12 months
Maximum levels of annual sales for which sales milestone payments may be received																														1,000,000,000
Additional research and development expenses obligated to reimburse															17,600,000															15,300,000	21,900,000	47,800,000
Account Receivable from Bayer HealthCare																														63,200,000	2,800,000
Percentage of global development cost to be reimbursed due to collaboration agreement																																											50.00%
Opt-in payment																																											20,000,000
Future development milestone payments																																											17,500,000
Maximum percentage of outstanding shares that may be acquired, under 'standstill' provisions																																												20.00%
Amount of Milestone payments earned																																$ 10,000,000