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, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Revenues	$ 4,290.7	$ 3,675.6	$ 7,896.1	$ 6,704.3
Sanofi Collaboration Agreement, Antibody \| Regeneron's share of profits \| Income Statement Location [Axis]: us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax
Disaggregation of Revenue [Line Items]
Revenues	2,032.6	1,282.1	3,483.4	2,300.2
Sanofi Collaboration Agreement, Antibody \| Reimbursement for manufacturing of commercial supplies \| Income Statement Location [Axis]: us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax
Disaggregation of Revenue [Line Items]
Revenues	141.7	161.5	296.0	326.6
Sanofi Collaboration Agreement, Antibody \| Regeneron's obligation for its share of Sanofi R&D expenses, net of reimbursement of R&D expenses \| Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpense
Disaggregation of Revenue [Line Items]
Revenues	(19.4)	(18.0)	(36.1)	(33.5)
Sanofi Collaboration Agreement, Antibody \| Reimbursement of commercialization-related expenses \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Disaggregation of Revenue [Line Items]
Revenues	$ 188.8	$ 194.0	$ 372.9	$ 353.2