Collaboration, License, and Other Agreements - Schedule of Collaboration Revenue Earned From Bayer (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
Disaggregation of Revenue [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
Bayer
Disaggregation of Revenue [Line Items]
Revenues										1,145.6	1,036.1
Bayer | Product and service, other
Disaggregation of Revenue [Line Items]
Revenues									1,186.1	1,145.6	1,036.1
Outside United States | Bayer | Regeneron's net profit in connection with commercialization of EYLEA outside the United States
Disaggregation of Revenue [Line Items]
Revenues									1,107.9	1,091.4	992.3
Outside United States | Bayer | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues									78.2	54.2	43.8
Outside United States | Bayer | Reimbursement of development expenses
Disaggregation of Revenue [Line Items]
Revenues									46.7	23.0	11.2
Outside United States | Bayer | Regeneron's obligation for its share of Bayer research and development expenses
Disaggregation of Revenue [Line Items]
Revenues									(35.8)	(20.1)	(0.5)
Outside United States | Bayer | Reimbursement of other expenses
Disaggregation of Revenue [Line Items]
Revenues									$ 7.4	$ 19.0	$ 28.9