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Document and Entity Information Document - USD ($)
 12 Months Ended
Dec. 31, 2019
Jan. 29, 2020
Jun. 28, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 0-27756    
Entity Registrant Name ALEXION PHARMACEUTICALS, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 13-3648318    
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    
Title of 12(b) Security Common Stock $0.0001 par value    
Trading Symbol ALXN    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 28,243,087,610
Entity Common Stock, Shares Outstanding   221,400,872  
Documents Incorporated by Reference
Portions of the registrant’s Definitive Proxy Statement to be used in connection with its 2020 Annual Meeting of Stockholders currently anticipated to be held on May 13, 2020, are incorporated by reference into Part III of this report.

    
Entity Central Index Key 0000899866    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false