Cover Cover - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Jan. 31, 2020
Jun. 28, 2019
Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 0-19034    
Entity Incorporation, State or Country Code NY    
Entity Tax Identification Number 13-3444607    
Entity Address, Address Line One 777 Old Saw Mill River Road    
Entity Address, City or Town Tarrytown,    
Entity Address, State or Province NY    
Entity Address, Postal Zip Code 10591-6707    
City Area Code 914    
Local Phone Number 847-7000    
Title of 12(b) Security Common Stock - par value $.001 per share    
Trading Symbol REGN    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 32,929
Documents Incorporated by Reference

Specified portions of the Registrant's definitive proxy statement to be filed in connection with solicitation of proxies for its 2020 Annual Meeting of Shareholders are incorporated by reference into Part III of this Form 10-K. Exhibit index is located on pages 76 to 81 of this filing.

Entity Central Index Key 0000872589    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Class A Stock      
Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   1,848,970  
Common Stock      
Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   108,170,839