v3.3.1.900
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Mar. 04, 2016
Jun. 30, 2015
Document and Entity Information      
Entity Registrant Name COLLEGIUM PHARMACEUTICAL, INC    
Entity Central Index Key 0001267565    
Document Type 10-K    
Document Period End Date Dec. 31, 2015    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 189.0
Entity Common Stock, Shares Outstanding   23,507,347  
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus FY