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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Apr. 22, 2022
Jun. 30, 2021
Cover [Abstract]      
Entity Registrant Name Alkermes plc.    
Entity Central Index Key 0001520262    
Document Type 10-K/A    
Document Period End Date Dec. 31, 2021    
Amendment Flag true    
Current Fiscal Year End Date --12-31    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 3,918,149,321
Entity Common Stock, Shares Outstanding   163,426,943  
Document Fiscal Year Focus 2021    
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    
Amendment Description This Amendment No. 1 to Annual Report on Form 10-K/A (this “Amendment”), amends the Annual Report on Form 10-K for the fiscal year ended December 31, 2021 (Commission File Number 001-35299), as filed by the registrant with the U.S. Securities and Exchange Commission (the “SEC”) on February 16, 2022 (the “2021 Annual Report”). The principal purpose of this Amendment is to amend Part III of the 2021 Annual Report to include the information that was intended to be incorporated therein by reference to the registrant’s definitive proxy statement for its 2022 annual general meeting of shareholders, and to update certain of the information included on the cover page of the 2021 Annual Report and in the list of exhibits included and Item 15 of the Exhibit Index of the 2021 Annual Report. This Amendment hereby amends the cover page, Part III (Items 10 through 14) and Part IV (Item 15) of the 2021 Annual Report. In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), new certifications by the registrant’s principal executive officer and principal financial officer are filed as exhibits to this Amendment.   No attempt has been made in this Amendment to modify or update the other disclosures presented in the 2021 Annual Report. This Amendment does not reflect events occurring after the filing of the 2021 Annual Report (i.e., those events occurring after February 16, 2022) or modify or update those disclosures that may be affected by subsequent events. Accordingly, this Amendment should be read in conjunction with the 2021 Annual Report and the registrant’s other filings with the SEC.    
Auditor Name PricewaterhouseCoopers LLP    
Auditor Location Boston, Massachusetts    
Auditor Firm ID 238    
Documents Incorporated by Reference

None.