Collaboration Agreements - Schedule of Collaboration Revenue Earned From Sanofi (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,933.7	$ 1,608.0	$ 3,645.5	$ 3,119.5
Product and service, other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	278.2	131.1	454.4	219.3
Product and service, other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	70.9	106.7	141.1	207.9
Product and service, other | Sanofi
Disaggregation of Revenue [Line Items]
Revenues	349.1	237.8	595.5	427.2
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	81.8	64.5	156.3	124.9
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	36.5	77.0	82.9	150.9
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	121.1	103.7	237.7	189.1
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	1.7	2.1	4.0	3.2
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	38.8	(68.8)	11.0	(143.7)
Other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	36.5	31.7	49.4	49.0
Other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	$ 32.7	$ 27.6	$ 54.2	$ 53.8