v3.5.0.2
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2016
Jul. 29, 2016
Document and Entity Information    
Entity Registrant Name MOMENTA PHARMACEUTICALS INC  
Entity Central Index Key 0001235010  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Document Type 10-Q  
Document Period End Date Jun. 30, 2016  
Document Fiscal Year Focus 2016  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   70,744,044