v3.25.2
Cover - shares
 6 Months Ended
Jun. 30, 2025
Aug. 11, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 001-40215  
Entity Registrant Name Instil Bio, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 83-2072195  
Entity Address, Address Line One 3963 Maple Avenue, Suite 350  
Entity Address, City or Town Dallas  
Entity Address, State or Province TX  
Entity Address, Postal Zip Code 75219  
City Area Code 972  
Local Phone Number 499-3350  
Title of 12(b) Security Common Stock, $0.000001 par value per share  
Trading Symbol TIL  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   6,750,716
Amendment Flag false  
Entity Central Index Key 0001789769  
Current Fiscal Year End Date --12-31  
Document Fiscal Period Focus Q2  
Document Fiscal Year Focus 2025