Business Overview and Summary of Significant Accounting Policies - Schedule of Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues	$ 2,422.9	$ 2,294.0	$ 1,952.0	$ 1,828.2	$ 1,863.5	$ 1,743.7	$ 1,577.8	$ 1,372.6	$ 8,497.1	$ 6,557.6	$ 5,145.6
Research and development									2,735.0	2,450.0	1,468.8
Selling, general, and administrative									1,346.0	1,341.9	1,127.2
Cost of Goods and Services Sold										402.8	237.5
Other operating (income) expense, net									(280.4)	(209.2)	(402.3)
Operating expenses	$ 1,255.9	$ 1,240.9	$ 1,295.6	$ 1,128.1	$ 1,187.8	$ 1,005.2	$ 1,262.2	$ 892.6	$ 4,920.5	4,347.8	2,611.2
Sanofi
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										403.6	(125.7)
Bayer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										1,145.6	1,036.1
Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										174.0	129.0
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										7,863.4	6,710.8
Research and development										3,036.6	2,186.1
Selling, general, and administrative										1,834.8	1,556.2
Cost of Goods and Services Sold										419.9	254.1
Other operating (income) expense, net										0.0	0.0
Operating expenses										5,653.6	4,176.4
As Previously Reported | Sanofi
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										1,426.8	1,111.1
As Previously Reported | Bayer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										1,188.8	1,076.7
As Previously Reported | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										413.4	416.8
Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										(1,305.8)	(1,565.2)
Research and development										(586.6)	(717.3)
Selling, general, and administrative										(492.9)	(429.0)
Cost of Goods and Services Sold										(17.1)	(16.6)
Other operating (income) expense, net										(209.2)	(402.3)
Operating expenses										(1,305.8)	(1,565.2)
Adjustments | Sanofi
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										(1,023.2)	(1,236.8)
Adjustments | Bayer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										(43.2)	(40.6)
Adjustments | Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues										$ (239.4)	$ (287.8)