v3.10.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Jan. 31, 2019
Jun. 30, 2018
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, 2018    
Amendment Flag false    
Entity Shell Company false    
Current Fiscal Year End Date --12-31    
Entity Emerging Growth Company false    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Small Business false    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 725
Entity Common Stock, Shares Outstanding   33,305,011  
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY