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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 09, 2024
Jun. 30, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-36407    
Entity Registrant Name ALNYLAM PHARMACEUTICALS, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 77-0602661    
Entity Address, Address Line One 675 West Kendall Street    
Entity Address, Address Line Two Henri A. Termeer Square    
Entity Address, City or Town Cambridge    
Entity Address, State or Province MA    
Entity Address, Postal Zip Code 02142    
City Area Code 617    
Local Phone Number 551-8200    
Title of 12(b) Security Common Stock, $0.01 par value per share    
Trading Symbol ALNY    
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    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 23,650
Entity Common Stock, Shares Outstanding   125,945,793  
Documents Incorporated by Reference
Portions of the registrant’s definitive proxy statement for its 2024 annual meeting of stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year end of December 31, 2023, are incorporated by reference into Part II, Item 5 and Part III of this Form 10-K.
    
Amendment Flag false    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Entity Central Index Key 0001178670