Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document and Entity Information    
Entity Registrant Name Collegium Pharmaceutical, Inc.  
Entity Central Index Key 0001267565  
Document Type 10-Q  
Document Period End Date Jun. 30, 2019  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Emerging Growth Company false  
Entity Small Business false  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   33,470,530
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Document Quarterly Report true  
Document Transition Report false  
Entity File Number 001-37372  
Entity Incorporation, State or Country Code VA  
Entity Tax Identification Number 03-0416362  
Entity Address, Address Line One 100 Technology Center Drive  
Entity Address, City or Town Stoughton  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02072  
City Area Code 781  
Local Phone Number 713-3699  
Title of 12(b) Security Common Stock  
Trading Symbol COLL  
Security Exchange Name NASDAQ  
Entity Interactive Data Current Yes  
Entity Shell Company false