Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 3,452.8	$ 2,294.0	$ 11,120.0	$ 6,074.2
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	351.0	287.9	995.3	772.6
Bayer | Outside United States | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	14.0	12.0	41.6	52.9
Bayer | Outside United States | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	14.5	11.5	35.2	34.3
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	$ (8.0)	$ (12.9)	$ (31.4)	$ (26.3)