v2.4.0.8
Document and Entity Information Document
3 Months Ended
Mar. 31, 2014
Apr. 23, 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-Q  
Document Period End Date Mar. 31, 2014  
Document Fiscal Year Focus 2014  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   197,796,456