v2.4.0.8
Document and Entity Information Document (USD $)
12 Months Ended
Dec. 31, 2013
Feb. 03, 2014
Jun. 30, 2013
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, 2013    
Document Fiscal Year Focus 2013    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   197,830,376  
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 17,833,350,643