| Document and Entity Information | May 31, 2024 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001599298 |
| Document Type | 8-K |
| Document Period End Date | May 31, 2024 |
| Entity Registrant Name | Summit Therapeutics Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-36866 |
| Entity Tax Identification Number | 37-1979717 |
| Entity Address, Address Line One | 601 Brickell Key Drive |
| Entity Address, Address Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.01 par value per share |
| Trading Symbol | SMMT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
