Collaboration Agreements - Schedule of Collaboration Revenue Earned From Sanofi (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,048.4	$ 1,663.5	$ 5,693.9	$ 4,783.0
Product and service, other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	349.3	174.1	803.7	393.3
Product and service, other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	54.9	82.2	196.0	290.2
Product and service, other | Sanofi
Disaggregation of Revenue [Line Items]
Revenues	404.2	256.3	999.7	683.5
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	60.2	76.2	216.5	201.0
Reimbursement of Regeneron research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	38.0	74.8	120.9	225.7
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	111.6	103.7	349.3	292.8
Reimbursement of Regeneron commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	3.0	3.2	7.0	6.5
Reimbursement for Regeneron's manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	78.5	40.3	133.3	94.4
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	94.2	(38.9)	105.2	(182.6)
Other | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	4.8	(7.2)	(0.6)	(12.3)
Other | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	(4.6)	(3.7)	(5.7)	(7.2)
Amounts recognized in connection with up-front payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	$ 18.5	$ 7.9	$ 73.8	$ 65.2