v2.4.0.6
Document and Entity Information (USD $)
12 Months Ended
Dec. 31, 2011
Feb. 11, 2012
Jun. 30, 2011
Document and Entity Information [Abstract]      
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, 2011    
Document Fiscal Year Focus 2011    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   185,993,409  
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 8,659,265,185