Collaboration Agreements (Sanofi) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Collaboration Agreement [Line Items]
Period for Achieving Sales Target for Milestone Payment, Rolling Basis			12 months
Revenue from Related Parties	$ 219,694		$ 173,356
Research and development	470,112		343,113
ZALTRAP Agreement
Collaboration Agreement [Line Items]
Deferred Revenue, Revenue Recognized			14,900
Other Income	5,300		19,800
Reimbursement of Regeneron research and development expenses	0		686
Contracts Revenue	0		15,236
Revenue from Related Parties	0		15,922
Antibody Collaboration
Collaboration Agreement [Line Items]
Net profit (loss) from commercialization of products under collaboration agreement	(99,422)		(22,405)
Recognition of Deferred Revenue	2,965		2,561
Reimbursement of Regeneron research and development expenses	193,602		168,820
Contracts Revenue	73,274		8,458
Revenue from Related Parties	$ 170,419		157,434
Percentage of Trial Costs borne by collaborating party	80.00%
Percentage of Trial Costs borne by entity	20.00%
IO Discovery Agreement
Collaboration Agreement [Line Items]
Annual Funding Maximum of Research Activities Per Agreement					$ 150,000
IO Agreement
Collaboration Agreement [Line Items]
Deferred Revenue, Additions		$ 640,000
Immuno-oncology Agreement
Collaboration Agreement [Line Items]
Recognition of Deferred Revenue	$ 20,000		0
Reimbursement of Regeneron research and development expenses	29,275		0
Revenue from Related Parties	$ 49,275		0
PDGF
Collaboration Agreement [Line Items]
Percentage of repayment of development balance out of profits	50.00%
Praluent, sarilumab, and dupilumab | Antibody Collaboration
Collaboration Agreement [Line Items]
Research and development	$ 21,700		$ 25,000
Scenario, Forecast [Member] | Antibody Collaboration
Collaboration Agreement [Line Items]
Annual funding maximum of research activities per amended agreement				$ 130,000	$ 130,000