Collaboration Agreements - Sanofi, Immuno-Oncology Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Cost of Treasury stock shares received	$ 54.0	$ 37.6
Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Maximum shares the collaborator could sell (in shares)			1,042,732		1,400,000
Cost of Treasury stock shares received			$ 44.0	$ 37.6
Revenue, performance obligation amount			$ 1,251.1
Sanofi Collaboration Agreement, Immuno-oncology | Libtayo
Disaggregation of Revenue [Line Items]
Treasury stock, shares acquired			106,972	121,601
Sanofi Collaboration Agreement, Immuno-oncology | Amended IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Aggregate payment received with regards to amendment					$ 461.9