Nature of Business and Operations and Recent Events	12 Months Ended
Nature of Business and Operations and Recent Events	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business and Operations and Recent Events	Nature of Business and Operations and Recent Events Nature of Business and Operations The Company is a biopharmaceutical company focused on the discovery, development, and commercialization of patient-, physician-, caregiver- and societal-friendly medicinal therapies intended to improve quality of life, increase life expectancy, and resolve serious unmet needs. The Company's novel mechanism pipeline of product candidates is designed with the goal to become the patient-friendly, new-era standard-of-care medicines, and to work in harmony with the human microbiome. Currently, the Company's lead product candidate, ridinilazole, is a novel first-in-class drug that is engaged in a global Phase III clinical trial program. On December 20, 2021, the Company announced topline results for the Phase III Ri-CoDIFy study evaluating ridinilazole for treating patients suffering from Clostridioides difficile infection, also known as C. difficile infection, or CDI. The Company's second product candidate, SMT-738, was announced in May 2021 for combating multidrug resistant infections, specifically Carbapenem-resistant Enterobacteriaceae (“CRE”) infections. SMT-738 is the first of a novel class of precision antibiotics that has entered into preclinical development. The Company intends to expand its portfolio by developing further new mechanism, new era product offerings that are designed to work in harmony with the human gut microbiome in the therapeutic areas of oncology and infectious diseases. On September 18, 2020, Summit Therapeutics Inc. ("Summit"), a Delaware corporation, became the successor issuer to Summit Therapeutics plc, a public limited company incorporated under the laws of England and Wales with the Registrar of Companies of England and Wales, United Kingdom ("U.K."), for certain purposes under both the Securities Act of 1933, as amended, and the Securities Exchange Act of 1934, as amended, or the Exchange Act. Such succession occurred pursuant to a statutory scheme of arrangement under U.K. law pursuant to which all Summit Therapeutics plc outstanding ordinary shares were exchanged on a five-for-one basis for newly issued shares of Summit common stock and Summit became the holding company of Summit Therapeutics plc (the predecessor registrant and former holding company) and its subsidiaries (which is referred to as the "Redomiciliation Transaction"). Concurrently, Summit Therapeutics plc was converted into a private limited company under the laws of England and Wales and renamed Summit Therapeutics Limited. In addition, the warrants and stock options to purchase shares of Summit Therapeutics plc were canceled and replacement warrants and stock options to purchase common stock in Summit Therapeutics Inc. were issued. The scheme of arrangement was accounted for as an exchange of equity interests among entities under common control. All assets and liabilities of Summit Therapeutics plc were assumed by Summit, resulting in the retention of the historical basis of accounting as if they had always been combined for accounting purposes and the historical consolidated financial statements of Summit Therapeutics plc became the historical consolidated financial statements of Summit Therapeutics Inc. All share and per share data for periods prior to the Redomiciliation Transaction in the financial statements were retroactively reflected to be presented as shares of the Company's common stock, par value $0.01 per share. Recent Events On May 12, 2021, the Company closed its rights offering, which was fully subscribed. The Company received aggregate gross proceeds from the rights offering of $75,000 from the sale of 14,312,976 shares of its common stock at a price per share of $5.24. Issuance costs associated with the rights offering were immaterial. In connection with the closing of the rights offering, a promissory note, dated April 20, 2021, was issued by the Company in favor of the Company's Chairman, Chief Executive Officer, and the beneficial owner of approximately 70% of its outstanding common stock prior to this rights offering, Robert W. Duggan, in the principal amount of $55,000, matured and became due and the Company repaid all principal and accrued interest thereunder using a portion of the proceeds from the rights offering. On August 11, 2021, based on a thorough review of the design and enrollment status of its two ongoing blinded Phase III Ri-CoDIFy trials, the Company announced that it combined its two blinded pivotal Phase III clinical trials evaluating ridinilazole versus vancomycin into a single study and presented this decision to the United States ("U.S.") Food and Drug Administration (the "FDA") as such. During September 2021, the Company received feedback from the FDA that the FDA did not agree with the change to the primary endpoint that the Company proposed and subsequently implemented in its ongoing Phase III Ri-CoDIFy studies when combining the trials. On December 20, 2021, the Company announced topline results for the Phase III Ri-CoDIFy study evaluating ridinilazole, for the treatment of and Sustained Clinical Response (“SCR”), as defined below, for patients suffering from C. difficile infection ("C. diff. infection" or "CDI"). The study showed that ridinilazole resulted in a numerically higher SCR rate than vancomycin, but did not meet the study’s primary endpoint for superiority. The pivotal Phase III clinical trial consisted of two Phase III clinical trials combined into a single study, designed to assess, as the primary endpoint, the superiority of ridinilazole compared to vancomycin in SCR, which is defined as clinical response of the treated episode of CDI and no recurrence of CDI through 30 days after the end of treatment. Additional endpoints included safety, tolerability, analyses of the gut microbiome and metabolome, in addition to quality of life and health economic outcome measures. We are in the process of evaluating the future path forward with respect to ridinilazole, including potential partnership opportunities. On March 10, 2022, the Company’s Chief Executive Officer, Robert W. Duggan, entered into a Note Purchase Agreement (the “2022 Note”), pursuant to which he has loaned the Company $25,000 in exchange for the issuance by the Company of an unsecured promissory note in the amount of $25,000. The 2022 Note is to accrue interest at a rate per annum equal to the prime rate as reported in the Wall Street Journal, which is 3.25% as of the effective date. The 2022 Note becomes due upon the earlier of (i) the consummation of a registered public offering with net proceeds of no less than $25,000 or (ii) 18 months from the date of issuance of the 2022 Note.