v3.8.0.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2018
May 02, 2018
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 Mar. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   77,588,904