Collaboration, License, and Other Agreements - Amounts Recognized In Statement of Operations with Sanofi (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 3,452.8	$ 2,294.0	$ 11,120.0	$ 6,074.2
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Regeneron's share of profits (losses) in connection with commercialization
Disaggregation of Revenue [Line Items]
Revenues	387.0	212.8	975.2	555.6
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Sales-based milestone earned
Disaggregation of Revenue [Line Items]
Revenues	50.0	50.0	50.0	50.0
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	144.7	94.3	361.2	275.0
Sanofi Collaboration Agreement, Antibody | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	45.2	45.5	122.3	174.4
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	(10.3)	(17.5)	(32.8)	(59.1)
Sanofi Collaboration Agreement, Antibody | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	79.0	83.2	216.9	260.4
Sanofi Collaboration Agreement, Immuno-oncology | Collaboration revenue | Regeneron's share of profits (losses) in connection with commercialization | Outside United States
Disaggregation of Revenue [Line Items]
Revenues	(3.0)	(4.7)	(12.6)	(17.3)
Sanofi Collaboration Agreement, Immuno-oncology | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	3.1	0.9	10.5	6.0
Sanofi Collaboration Agreement, Immuno-oncology | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	21.8	49.8	66.2	136.7
Sanofi Collaboration Agreement, Immuno-oncology | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	22.8	14.5	62.0	39.2
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	(9.4)	(6.1)	(28.0)	(15.0)
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	(34.6)	(31.5)	(99.4)	(86.5)
Sanofi Collaboration Agreement, Immuno-oncology | Other operating income | Amounts recognized in connection with up-front payments received
Disaggregation of Revenue [Line Items]
Revenues	$ (47.7)	$ 20.0	$ (4.1)	$ 57.0