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Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Feb. 17, 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 001-36866    
Entity Registrant Name Summit Therapeutics Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 37-1979717    
Entity Address, Address Line One 601 Brickell Key Drive, Suite 1000    
Entity Address, City or Town Miami    
Entity Address, State or Province FL    
Entity Address, Postal Zip Code 33131    
City Area Code 305    
Local Phone Number 203-2034    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol SMMT    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 2,483.8
Entity Common Stock, Shares Outstanding   775,372,700  
Documents Incorporated by Reference
Portions of the registrant’s definitive proxy statement relating to the registrant’s 2026 annual meeting of stockholders to be filed hereafter are incorporated by reference into Part III of this Annual Report on Form 10-K. The registrant’s definitive proxy statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.
    
Amendment Flag false    
Entity Central Index Key 0001599298    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY