Collaboration, License, and Other Agreements - Amounts Recognized In Statement of Operations with Sanofi (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 2,294,000	$ 1,743,700	$ 6,074,200	$ 4,694,100
Research and development expense	(684,600)	(526,000)	(1,990,500)	(1,897,600)
Regeneron's share of profits in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	212,800	94,200	555,600	105,200
Sales-based milestone earned | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	50,000	0	50,000	0
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Revenues	94,300	85,400	275,000	143,800
Reimbursement for manufacturing of commercial supplies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	900	0	6,000	0
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	45,500	60,200	174,400	216,500
Reimbursement of research and development expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Research and development expense	49,800	38,000	136,700	120,900
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	(17,500)	(10,200)	(59,100)	(29,800)
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	83,200	111,600	260,400	349,300
Reimbursement of commercialization-related expenses | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Reduction of Selling, general, and administrative expense	14,500	3,000	39,200	7,000
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	(31,500)	(20,100)	(86,500)	(51,500)
Amounts recognized in connection with up-front payments received | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Other operating income	20,000	18,500	57,000	73,800
Outside United States | Regeneron's share of profits in connection with commercialization of antibodies | Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Revenues	$ (4,700)	$ (4,600)	$ (17,300)	$ (16,200)