Collaboration Agreements Sanofi - Immuno-Oncology Narrative (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Jul. 31, 2015	Jun. 30, 2018	Dec. 31, 2015
Disaggregation of Revenue [Line Items]
Cost of Treasury stock shares received		$ 37.6
Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Maximum shares the collaborator could sell (in shares)		1,400,000
Treasury stock, shares acquired		121,601
Remaining performance obligation		$ 1,550.1
Sanofi Collaboration Agreement, Immuno-oncology | IO Discovery Agreement [Member]
Disaggregation of Revenue [Line Items]
Up-front payment received	$ 265.0
License agreement term	5 years
Additional years to extend the agreement	3 years
Sanofi Collaboration Agreement, Immuno-oncology | IO Agreement [Member]
Disaggregation of Revenue [Line Items]
Up-front payment received			$ 640.0
Sanofi Collaboration Agreement, Immuno-oncology | IO License and Collaboration Agreement [Member]
Disaggregation of Revenue [Line Items]
Up-front payment received	$ 375.0
Maximum amount of shared development costs		1,640.0
Increase amount of shared development costs		$ 990.0
Sanofi Collaboration Agreement, Immuno-oncology | IO License and Collaboration Agreement [Member] | PD-1 [Member]
Disaggregation of Revenue [Line Items]
Maximum amount of sales milestone payments if total sales achieve specific levels	375.0
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