v3.26.1
Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Mar. 25, 2026
Jun. 30, 2025
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Current Fiscal Year End Date --12-31    
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 Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction false    
Entity Shell Company false    
Entity Public Float     $ 83.4
Entity Common Stock, Shares Outstanding   6,781,976  
Documents Incorporated by Reference
DOCUMENTS INCORPORATED BY REFERENCE
Portions of the registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission (SEC) subsequent to the date hereof pursuant to Regulation 14A in connection with the registrant’s 2026 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K. Such proxy statement will be filed with the SEC not later than 120 days after the conclusion of the registrant’s fiscal year ended December 31, 2025.
    
Amendment Flag false    
Entity Central Index Key 0001789769    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2025