v3.5.0.2
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2016
Nov. 29, 2016
Document and Entity Information    
Entity Registrant Name Ra Pharmaceuticals, Inc.  
Entity Central Index Key 0001481512  
Document Type 10-Q  
Document Period End Date Sep. 30, 2016  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status No  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   22,546,165
Document Fiscal Year Focus 2016  
Document Fiscal Period Focus Q3