| 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 | 001-38150 |
| Entity Registrant Name | Kala Pharmaceuticals, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 27-0604595 |
| Entity Address, Address Line One | 490 Arsenal Way |
| Entity Address, Adress Line Two | Suite 120 |
| Entity Address, City or Town | Watertown |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02472 |
| City Area Code | 781 |
| Local Phone Number | 996-5252 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | KALA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001479419 |
| Amendment Flag | false |
