Collaboration Agreements (Sanofi) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			12 Months Ended
	Jul. 31, 2015	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2017
Collaboration Agreement [Line Items]
Accounts Receivable from Sanofi		$ 168,855				$ 193,684
Revenue from Related Parties		189,490	$ 210,367
Research and development		498,586	507,435
Maximum shares the collaborator could sell						1,400,000
Antibody Collaboration
Collaboration Agreement [Line Items]
Accounts Receivable from Sanofi		94,022				$ 121,001
Deferred Revenue From Related Party		132,991				117,682
Deferred Revenue, Period Increase (Decrease)		37,036
Deferred Revenue, Revenue Recognized		(21,727)
Net profit (loss) from commercialization of products under collaboration agreement		(74,874)	(108,402)
Annual funding maximum of research activities per amended agreement						130,000
Reimbursement of Regeneron research and development expenses		60,394	155,245
Reimbursement of Regeneron commercialization-related expenses		85,424	73,559
Other		17,330	11,286
Revenue from Related Parties		$ 88,274	131,688
Percentage of Trial Costs borne by collaborating party		80.00%
Percentage of Trial Costs borne by entity		20.00%
Maximum amount of sales milestone payments if total sales achieve specific levels		$ 250,000
IO Discovery Agreement
Collaboration Agreement [Line Items]
Up-front payment received	$ 265,000
License agreement term	5 years
Additional years to extend the agreement	3 years
IO License and Collaboration Agreement
Collaboration Agreement [Line Items]
Deferred Revenue, Additions	$ 375,000
Maximum amount of shared development costs		2,000,000
Increase amount of shared development costs		990,000
IO Agreement
Collaboration Agreement [Line Items]
Up-front payment received				$ 640,000
Immuno-oncology Agreement
Collaboration Agreement [Line Items]
Revenue, Remaining Performance Obligation		1,656,000
Accounts Receivable from Sanofi		70,887				59,274
Deferred Revenue From Related Party		505,235				$ 440,000
Deferred Revenue, Period Increase (Decrease)		93,643
Deferred Revenue, Revenue Recognized		(28,408)
Reimbursement of Regeneron research and development expenses		73,824	58,679
Reimbursement of Regeneron commercialization-related expenses		1,210	0
Other		26,182	20,000
Revenue from Related Parties		101,216	78,679
Praluent, Kevzara, and Dupixent [Member] | Antibody Collaboration
Collaboration Agreement [Line Items]
Research and development		$ 13,900	$ 25,000
Scenario, Forecast [Member] | Antibody Collaboration
Collaboration Agreement [Line Items]
Levels of twelve month sales at which sales milestone payments would be received					$ 1,000,000
Period for Achieving Sales Target for Milestone Payment, Rolling Basis					12 months
PD-1 | IO License and Collaboration Agreement
Collaboration Agreement [Line Items]
Maximum amount of sales milestone payments if total sales achieve specific levels	375,000
Levels of twelve month sales at which sales milestone payments would be received	$ 2,000,000
Period for Achieving Sales Target for Milestone Payment, Rolling Basis	12 months