v3.19.3
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Dec. 05, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Trading Symbol CABA  
Entity Registrant Name CABALETTA BIO, INC.  
Entity Current Reporting Status No  
Entity Central Index Key 0001759138  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   24,034,022
Entity File Number 001-39103  
Entity Tax Identification Number 82-1685768  
Entity Address, Address Line One 2929 Arch Street  
Entity Address, Address Line Two Suite 600  
Entity Address, City or Town Philadelphia  
Entity Address, State or Province PA  
Entity Address, Postal Zip Code 19104  
City Area Code 267  
Local Phone Number 759-3100  
Entity Interactive Data Current Yes  
Title of 12(b) Security Common Stock, par value $0.00001 per share  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Document Quarterly Report true  
Document Transition Report false