| Cover Page Statement | Feb. 04, 2021 |
| Cover [Abstract] | |
| Title of 12(g) Security | Common Stock, par value $0.0001 per share |
| Entity Emerging Growth Company | false |
| Written Communications | false |
| Entity Incorporation, State or Country Code | DE |
| Entity Central Index Key | 0000899866 |
| Document Type | 8-K |
| Document Period End Date | Feb. 4, 2021 |
| Entity Registrant Name | ALEXION PHARMACEUTICALS, INC. |
| Entity Address, Address Line One | 121 Seaport Boulevard |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02210 |
| City Area Code | 475 |
| Local Phone Number | 230-2596 |
| Amendment Flag | false |
| Entity File Number | 000-27756 |
| Entity Tax Identification Number | 13-3648318 |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Trading Symbol | ALXN |
| Security Exchange Name | NASDAQ |
