| Document and Entity Information | Feb. 25, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 25, 2021 |
| Entity File Number | 001-37372 |
| Entity Registrant Name | COLLEGIUM PHARMACEUTICAL, INC. |
| Entity Incorporation, State or Country Code | VA |
| Entity Tax Identification Number | 03-0416362 |
| Entity Address, Address Line One | 100 Technology Center Drive |
| Entity Address, Adress Line Two | Suite 300 |
| Entity Address, State or Province | MA |
| Entity Address, City or Town | Stoughton |
| Entity Address, Postal Zip Code | 02072 |
| City Area Code | 781 |
| Local Phone Number | 713-3699 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.001 per share |
| Trading Symbol | COLL |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001267565 |
| Amendment Flag | false |
