Collaboration Agreements - Sanofi, Antibody Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Research and development	$ 663,400,000		$ 557,000,000.0		$ 2,353,500,000	$ 1,584,800,000
Cost of Treasury stock shares received	$ 48,700,000	$ 54,000,000.0	42,600,000	$ 37,600,000
Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Percentage of Trial Costs borne by collaborating party					80.00%
Percentage of Trial Costs borne by entity					20.00%
Treasury stock, shares acquired	103,761
Cost of Treasury stock shares received	$ 19,400,000
Maximum amount of sales milestone payments if total sales achieve specific levels					$ 250,000,000.0
Levels of twelve month sales at which sales milestone payments would be received					$ 1,000,000,000.0
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	$ 10,200,000		$ 13,400,000		$ 29,800,000	$ 37,200,000