Collaboration Agreements - Sanofi, Immuno-Oncology Narrative (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Cost of Treasury stock shares received	$ 48.7	$ 54.0	$ 42.6	$ 37.6
Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Maximum shares the collaborator could sell (in shares)	869,828				869,828		1,400,000
Cost of Treasury stock shares received	$ 29.2				$ 73.3	$ 75.8
Revenue, performance obligation amount	$ 1,196.9				$ 1,196.9
Sanofi Collaboration Agreement, Immuno-oncology | Libtayo
Disaggregation of Revenue [Line Items]
Treasury stock, shares acquired	69,143				210,733	215,387
Sanofi Collaboration Agreement, Immuno-oncology | Amended IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Aggregate payment received with regards to amendment							$ 461.9