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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 5,138.5	$ 1,952.0	$ 7,667.2	$ 3,780.2
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Regeneron's share of profits in connection with commercialization
Disaggregation of Revenue [Line Items]
Revenues	327.6	171.9	588.2	342.8
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	110.9	100.6	216.5	180.7
Sanofi Collaboration Agreement, Antibody | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	46.5	51.3	77.1	128.9
Sanofi Collaboration Agreement, Antibody | Research and development expense | Regeneron's obligation for its share of Sanofi research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	(10.6)	(24.9)	(22.5)	(41.6)
Sanofi Collaboration Agreement, Antibody | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	78.3	86.0	137.9	177.2
Sanofi Collaboration Agreement, Immuno-oncology | Collaboration revenue | Regeneron's share of profits in connection with commercialization | Outside United States
Disaggregation of Revenue [Line Items]
Revenues	(3.5)	(6.4)	(9.6)	(12.6)
Sanofi Collaboration Agreement, Immuno-oncology | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	2.7	3.0	7.4	5.1
Sanofi Collaboration Agreement, Immuno-oncology | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	22.5	47.0	44.4	86.9
Sanofi Collaboration Agreement, Immuno-oncology | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	20.7	14.3	39.2	24.7
Sanofi Collaboration Agreement, Immuno-oncology | Selling, general and administrative expense | Regeneron's obligation for its share of Sanofi commercial expenses
Disaggregation of Revenue [Line Items]
Revenues	(10.9)	(4.4)	(18.6)	(8.9)
Sanofi Collaboration Agreement, Immuno-oncology | Cost of goods sold | Regeneron's obligation for Sanofi's share of Libtayo U.S. gross profits
Disaggregation of Revenue [Line Items]
Revenues	(34.4)	(28.2)	(64.8)	(55.0)
Sanofi Collaboration Agreement, Immuno-oncology | Other operating income | Amounts recognized in connection with up-front payments received
Disaggregation of Revenue [Line Items]
Revenues	$ 20.7	$ 20.5	$ 43.6	$ 37.0