Collaboration Agreements - Amounts Recognized In Statement of Operations with Sanofi (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,828.2	$ 1,372.6
Research and development expense	(583.9)	(486.1)
Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	170.9	(27.8)
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	80.1	14.5
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	2.1	0.0
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	77.6	74.5
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	39.9	46.4
Regeneron's obligation for its share of Sanofi research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Research and development expense	(16.7)	(7.4)
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	91.2	116.6
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	10.4	2.2
Regeneron's obligation for Sanofi's share of Libtayo U.S. gross profits | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Cost of goods sold	(26.8)	(12.4)
Amounts recognized in connection with up-front payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Other operating income	16.5	26.3
Outside United States | Regeneron's share of profits (losses) in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	$ (6.2)	$ (4.7)