Collaboration Agreements - Sanofi, Immuno-Oncology Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Cost of Treasury stock shares received	$ 54.0
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
Revenue, performance obligation amount	$ 1,324.7
Sanofi Collaboration Agreement, Immuno-oncology | Libtayo
Disaggregation of Revenue [Line Items]
Treasury stock, shares acquired	106,972
Sanofi Collaboration Agreement, Immuno-oncology | Amended IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Aggregate payment received with regards to amendment		$ 461.9