Collaboration, License, and Other Agreements - Amounts Recognized In Statement of Operations with Sanofi (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 2,528.7	$ 1,828.2
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Regeneron's share of profits in connection with commercialization
Disaggregation of Revenue [Line Items]
Revenues	260.6	170.9
Sanofi Collaboration Agreement, Antibody | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	105.6	80.1
Sanofi Collaboration Agreement, Antibody | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	30.1	77.6
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	(11.9)	(16.7)
Sanofi Collaboration Agreement, Antibody | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	60.4	91.2
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	(6.1)	(6.2)
Sanofi Collaboration Agreement, Immuno-oncology | Collaboration revenue | Reimbursement for manufacturing of commercial supplies
Disaggregation of Revenue [Line Items]
Revenues	4.7	2.1
Sanofi Collaboration Agreement, Immuno-oncology | Research and development expense | Reimbursement of research and development expenses
Disaggregation of Revenue [Line Items]
Revenues	21.9	39.9
Sanofi Collaboration Agreement, Immuno-oncology | Selling, general and administrative expense | Reimbursement of commercialization-related expenses
Disaggregation of Revenue [Line Items]
Revenues	18.5	10.4
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	(30.4)	(26.8)
Sanofi Collaboration Agreement, Immuno-oncology | Other operating income | Amounts recognized in connection with up-front payments received
Disaggregation of Revenue [Line Items]
Revenues	$ 22.9	$ 16.5