Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 06, 2018
Document and Entity Information    
Entity Registrant Name Kala Pharmaceuticals, Inc.  
Entity Central Index Key 0001479419  
Trading Symbol kala  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   33,795,656
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3