Collaboration, License, and Other Agreements - Amounts Recognized In Statement of Operations with Sanofi (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)
Regeneron's share of profits in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	171.9	38.8	342.8	11.0
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	100.6	43.9	180.7	58.4
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	3.0	0.0	5.1	0.0
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	51.3	81.8	128.9	156.3
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	47.0	36.5	86.9	82.9
Regeneron's obligation for its share of Sanofi research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Research and development expense	(24.9)	(12.2)	(41.6)	(19.6)
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	86.0	121.1	177.2	237.7
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	14.3	1.8	24.7	4.0
Regeneron's obligation for Sanofi's share of Libtayo U.S. gross profits | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Cost of goods sold	(28.2)	(19.0)	(55.0)	(31.4)
Amounts recognized in connection with up-front payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Other operating income	20.5	29.0	37.0	55.3
Outside United States | Regeneron's share of profits in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	$ (6.4)	$ (6.9)	$ (12.6)	$ (11.6)