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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 04, 2020
Jun. 30, 2019
Cover [Abstract]      
Entity Registrant Name Alkermes plc.    
Entity Central Index Key 0001520262    
Document Type 10-K    
Document Period End Date Dec. 31, 2019    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 3,507,584,798
Entity Common Stock, Shares Outstanding   157,787,433  
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Trading Symbol ALKS    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity File Number 001-35299    
Entity Incorporation, State or Country Code L2    
Entity Tax Identification Number 98-1007018    
Entity Address, Address Line One Connaught House    
Entity Address, Address Line Two 1 Burlington Road    
Entity Address, City or Town Dublin 4    
Entity Address, Country IE    
Entity Address, Postal Zip Code D04 C5Y6    
City Area Code 353    
Local Phone Number 1-772-8000    
Document Annual Report true    
Document Transition Report false    
Title of 12(b) Security Ordinary shares, $0.01 par value    
Security Exchange Name NASDAQ    
Documents Incorporated by Reference

Portions of the definitive proxy statement for our 2020 Annual General Meeting of Shareholders are incorporated by reference into Part III of this report.