Statement of Cash Flows - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Accrued capital expenditures	$ 100.5		$ 40.5		$ 100.5	$ 40.5	$ 54.5	$ 41.8
Cost of Treasury stock shares received	48.7	$ 54.0	$ 42.6	$ 37.6
Sanofi Collaboration Agreement, Immuno-oncology
Condensed Cash Flow Statements, Captions [Line Items]
Cost of Treasury stock shares received	$ 29.2				$ 73.3	$ 75.8
Libtayo | Sanofi Collaboration Agreement, Immuno-oncology
Condensed Cash Flow Statements, Captions [Line Items]
Treasury stock, shares acquired	69,143				210,733	215,387