Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 01, 2018	Jun. 30, 2017
Entity Information [Line Items]
Entity Registrant Name	REGENERON PHARMACEUTICALS INC
Entity Central Index Key	0000872589
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 50,337,000,000
Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		105,785,444
Class A Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		1,911,354