Collaboration and License Agreements - Schedule of Collaboration Revenue Earned From Sanofi (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 2,169.5	$ 2,048.4	$ 1,933.7	$ 1,711.8	$ 1,927.8	$ 1,663.5	$ 1,608.0	$ 1,511.5	$ 7,863.4	$ 6,710.8	$ 5,872.2
Product and service, other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									1,172.1	559.0	553.7
Product and service, other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									254.7	552.1	323.5
Product and service, other | Sanofi
Disaggregation of Revenue [Line Items]
Revenues									1,426.8	1,111.1	877.2
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									277.7	265.3	508.4
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									163.0	311.8	240.0
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									479.9	417.2	368.8
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									10.3	8.9	7.0
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									206.7	127.6	35.1
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									209.3	(227.0)	(442.6)
Amounts recognized in connection with up-front payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									92.7	243.8	80.0
Other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues									(1.5)	(24.1)	84.0
Other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues									$ (11.3)	$ (12.4)	$ (3.5)