Collaboration Agreements - Amounts Recognized in Statement of Operations with Bayer (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,952.0	$ 1,577.8	$ 3,780.2	$ 2,950.4
Research and development expense	(722.0)	(885.5)	(1,305.9)	(1,371.6)
Bayer | Product and service, other
Disaggregation of Revenue [Line Items]
Revenues	244.2	277.2	525.6	541.2
Outside United States | Bayer | Regeneron's net profit in connection with commercialization of EYLEA outside the United States
Disaggregation of Revenue [Line Items]
Revenues	230.9	269.0	484.7	518.3
Outside United States | Bayer | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	13.3	8.2	40.9	22.9
Outside United States | Bayer | Reimbursement of development expenses
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	10.8	8.0	22.8	10.6
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	(5.3)	(2.0)	(13.4)	(6.6)
Outside United States | Bayer | Reimbursement of other expenses
Disaggregation of Revenue [Line Items]
Cost of collaboration and contract manufacturing	$ 1.6	$ 4.1	$ 3.3	$ 12.9