Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 2,528.7	$ 1,828.2
Bayer | Outside United States | Collaboration revenue | Regeneron's net profit in connection with commercialization of EYLEA outside the United States
Disaggregation of Revenue [Line Items]
Revenues	308.9	253.8
Bayer | Outside United States | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	13.9	27.6
Bayer | Outside United States | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	10.8	12.0
Bayer | Outside United States | Research and development expense | Regeneron's obligation for its share of Bayer research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	$ (12.5)	$ (8.1)