Collaboration Agreements - Sanofi, Antibodies Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Research and development	$ 556,972	$ 529,749	$ 1,584,847	$ 1,547,159
Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Annual funding maximum of research activities per amended agreement					$ 130,000
Percentage of Trial Costs borne by collaborating party			80.00%
Percentage of Trial Costs borne by entity			20.00%
Treasury stock, shares acquired	10,766
Cost of Treasury stock shares received	$ 4,400
Maximum amount of sales milestone payments if total sales achieve specific levels			$ 250,000
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
Praluent, Kevzara, and Dupixent | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Research and development	$ 13,400	$ 22,600	$ 37,200	$ 68,200