Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 2,294.0	$ 1,743.7	$ 6,074.2	$ 4,694.1
Research and development expense	(684.6)	(526.0)	(1,990.5)	(1,897.6)
Bayer | Product and service, other
Disaggregation of Revenue [Line Items]
Revenues	299.9	293.6	825.5	834.8
Outside United States | Bayer | Regeneron's net profit in connection with commercialization of EYLEA outside the United States
Disaggregation of Revenue [Line Items]
Revenues	287.9	275.0	772.6	793.3
Outside United States | Bayer | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	12.0	18.6	52.9	41.5
Outside United States | Bayer | Reimbursement of development expenses
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	11.5	5.0	34.3	15.6
Outside United States | Bayer | Regeneron's obligation for its share of Bayer research and development expenses
Disaggregation of Revenue [Line Items]
Research and development expense	(12.9)	(7.0)	(26.3)	(13.6)
Outside United States | Bayer | Reimbursement of other expenses
Disaggregation of Revenue [Line Items]
Cost of collaboration and contract manufacturing	$ 2.0	$ 3.7	$ 5.3	$ 16.6