Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 5,138.5	$ 1,952.0	$ 7,667.2	$ 3,780.2
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	335.4	230.9	644.3	484.7
Bayer | Outside United States | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	13.7	13.3	27.6	40.9
Bayer | Outside United States | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	9.9	10.8	20.7	22.8
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	$ (10.9)	$ (5.3)	$ (23.4)	$ (13.4)