Collaboration and License Agreements - Sanofi, Immuno-Oncology Narrative (Details) - USD ($) $ in Millions		1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2018	Aug. 31, 2018	Jul. 31, 2015	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015	Jan. 07, 2018
Disaggregation of Revenue [Line Items]
Maximum shares the collaborator could sell (in shares)	1,173,847			1,173,847	1,173,847			1,400,000
Cost of Treasury stock shares received					$ 80.2	$ 0.0
Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Excess share of profit not required to to be applied to reimburse development cost			10.00%
Contingent reimbursement obligation	$ 58.0			$ 58.0	$ 58.0
Maximum shares the collaborator could sell (in shares)	1,400,000			1,400,000	1,400,000
Treasury stock, shares acquired (in shares)					215,387
Cost of Treasury stock shares received					$ 75.8
Maximum amount of sales milestone payments if total sales achieve specific levels			$ 375.0
Upfront payment made		$ 20.0
Remaining performance obligation	$ 1,398.4			$ 1,398.4	1,398.4
Sanofi Collaboration Agreement, Immuno-oncology | IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received			265.0
Potential future R&D expenses			1,090.0
Funding maximum of research activities per agreement			$ 825.0
License agreement term			5 years
Additional years to extend the agreement			3 years
Sanofi Collaboration Agreement, Immuno-oncology | Amended IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Aggregate payment received with regards to amendment					461.9
Cumulative catch-up adjustment to revenue, modification of contract				(192.7)	135.0
Sanofi Collaboration Agreement, Immuno-oncology | BCMAxCD3 Program
Disaggregation of Revenue [Line Items]
Expenditure costs cap	70.0
Sanofi Collaboration Agreement, Immuno-oncology | MUC16xCD3 Program
Disaggregation of Revenue [Line Items]
Expenditure costs cap	50.0
Potential maximum expenditure	70.0			$ 70.0	$ 70.0
Optional payment to increase development cost funding	$ 20.0
Sanofi Collaboration Agreement, Immuno-oncology | IO Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received							$ 640.0
Sanofi Collaboration Agreement, Immuno-oncology | IO License and Collaboration Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received			$ 375.0
Excess share of profit not required to to be applied to reimburse development cost			10.00%
Period of notice to opt out of further development and or commercialization			12 months
Sanofi Collaboration Agreement, Immuno-oncology | IO License and Collaboration Agreement | PD-1
Disaggregation of Revenue [Line Items]
Levels of twelve month sales at which sales milestone payments would be received			$ 2,000.0
Period for achieving sales target for milestone payment, rolling basis			12 months
Through December 31, 2023 | Sanofi Collaboration Agreement, Immuno-oncology | Libtayo
Disaggregation of Revenue [Line Items]
Percentage of royalties to be paid		8.00%
January 1, 2024 Through December 31, 2026 | Sanofi Collaboration Agreement, Immuno-oncology | Libtayo
Disaggregation of Revenue [Line Items]
Percentage of royalties to be paid		2.50%