v3.8.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Mar. 02, 2018
Jun. 30, 2017
Document and Entity Information      
Entity Registrant Name Ra Pharmaceuticals, Inc.    
Entity Central Index Key 0001481512    
Document Type 10-K    
Document Period End Date Dec. 31, 2017    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer No    
Entity Filer Category Accelerated Filer    
Entity Public Float     $ 264.7
Entity Common Stock, Shares Outstanding   32,286,684  
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY