| Cover Page Statement | Jul. 24, 2019 | Jun. 30, 2019 |
| Cover page. | ||
| 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 | Jul. 24, 2019 | |
| 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 |
