| Document and Entity Information | Apr. 29, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | Alkermes plc. |
| Amendment Flag | false |
| Entity Central Index Key | 0001520262 |
| Document Type | 8-K |
| Document Period End Date | Apr. 29, 2021 |
| Entity Incorporation State Country Code | L2 |
| Entity File Number | 001-35299 |
| 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 |
| Written Communications | false |
| Soliciting Material | true |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Ordinary shares, $0.01 par value |
| Trading Symbol | ALKS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
