Collaboration Agreements - Sanofi, Immuno-Oncology Narrative (Details) - Sanofi Collaboration Agreement, Immuno-oncology - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Aug. 31, 2018	Jul. 31, 2015	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2015
Disaggregation of Revenue [Line Items]
Maximum shares the collaborator could sell (in shares)			1,400,000		1,400,000
Treasury stock, shares acquired			93,786	121,601	215,387
Cost of Treasury stock shares received			$ 38.1	$ 37.6	$ 75.8
Maximum amount of sales milestone payments if total sales achieve specific levels		$ 375.0
Remaining performance obligation			1,562.9		1,562.9
Libtayo
Disaggregation of Revenue [Line Items]
Up-front payment received	$ 20.0
IO Discovery Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received		$ 265.0
License agreement term		5 years
Additional years to extend the agreement		3 years
IO Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received						$ 640.0
IO License and Collaboration Agreement
Disaggregation of Revenue [Line Items]
Up-front payment received		$ 375.0
Maximum amount of shared development costs			1,640.0		1,640.0
Increase amount of shared development costs			$ 990.0		$ 990.0
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 | Libtayo
Disaggregation of Revenue [Line Items]
Percentage of royalties to be paid	8.00%
January 1, 2024 Through December 31, 2026 | Libtayo
Disaggregation of Revenue [Line Items]
Percentage of royalties to be paid	2.50%