Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,828.2	$ 1,372.6
Research and development expense	(583.9)	(486.1)
Bayer | Product and service, other
Disaggregation of Revenue [Line Items]
Revenues	281.4	264.0
Outside United States | Bayer | Regeneron's net profit in connection with commercialization of EYLEA outside the United States
Disaggregation of Revenue [Line Items]
Revenues	253.8	249.3
Outside United States | Bayer | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	27.6	14.7
Outside United States | Bayer | Reimbursement of development expenses
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	12.0	2.6
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	(8.1)	(4.6)
Outside United States | Bayer | Reimbursement of other expenses
Disaggregation of Revenue [Line Items]
Cost of collaboration and contract manufacturing	$ 1.7	$ 8.8