Collaboration, License, and Other Agreements - Schedule of Collaboration Revenue Earned From Sanofi (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,422.9	$ 2,294.0	$ 1,952.0	$ 1,828.2	$ 1,863.5	$ 1,743.7	$ 1,577.8	$ 1,372.6	$ 8,497.1	$ 6,557.6	$ 5,145.6
Product and service, other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									785.2	209.3	(227.0)
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									(25.7)	(21.7)	(12.4)
Sales-based milestone earned | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									50.0	0.0	0.0
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									368.0	216.0	113.7
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									8.9	0.0	0.0
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									226.7	277.7	265.3
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									166.2	163.0	311.8
Regeneron's obligation for its share of Sanofi research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									(77.6)	(46.0)	(47.7)
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									359.4	479.9	417.2
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									64.7	10.3	8.9
Regeneron's obligation for its share of Sanofi other expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									(21.5)	(12.8)	(16.1)
Regeneron's obligation for Sanofi's share of Libtayo U.S. gross profits | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									(119.1)	(78.2)	(6.8)
Amounts recognized in connection with up-front and development milestone payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									$ 210.6	$ 92.7	$ 243.8