| Cover | Oct. 07, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 7, 2021 |
| Entity File Number | 001-36866 |
| Entity Registrant Name | Summit Therapeutics Inc. |
| Entity Central Index Key | 0001599298 |
| Entity Tax Identification Number | 37-1979717 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | One Broadway, 14th Floor |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 617 |
| Local Phone Number | 514-7149 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, $0.01 par value per share |
| Trading Symbol | SMMT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Information, Former Legal or Registered Name | Not Applicable |
