v2.4.0.8
Document and Entity Information Document (USD $)
12 Months Ended
Dec. 31, 2014
Feb. 03, 2015
Jun. 30, 2014
Entity Information [Line Items]      
Entity Registrant Name ALEXION PHARMACEUTICALS INC    
Entity Central Index Key 0000899866    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2014    
Document Fiscal Year Focus 2014    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   202,148,509  
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 30,706,252,188