Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Oct. 29, 2018
Document and Entity Information    
Entity Registrant Name AMAG PHARMACEUTICALS INC.  
Entity Central Index Key 0000792977  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Current Reporting Status Yes  
Entity Common Stock, Shares Outstanding (in shares)   34,535,659