Description of Business	6 Months Ended
Description of Business	Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Overview SELLAS Life Sciences Group, Inc. (the "Company" or "SELLAS") is a late-stage clinical biopharmaceutical company focused on novel cancer immunotherapeutics for a broad range of cancer indications. SELLAS’ lead product candidate, galinpepimut-S ("GPS"), is a cancer immunotherapeutic agent licensed from Memorial Sloan Kettering Cancer Center ("MSK") and targets the Wilms Tumor 1 ("WT1") protein, which is present in 20 or more cancer types. Based on its mechanism of action as a directly immunizing agent, GPS has potential as a monotherapy or in combination with other immunotherapeutic agents to address a broad spectrum of hematologic, or blood, cancers and solid tumor indications. SELLAS’ second product candidate, nelipepimut-S ("NPS"), is a cancer immunotherapy targeting the human epidermal growth factor receptor 2 ("HER2") expressing cancers with potential for the treatment of patients with early stage breast cancer with low to intermediate HER2 expression, otherwise known as HER2 1+ or 2+, which includes triple negative breast cancer ("TNBC") patients, following standard of care. As used in this Quarterly Report on Form 10-Q, the words the "Company," and "SELLAS" refer to SELLAS Life Sciences Group, Inc. and its consolidated subsidiaries following the completion of the business combination with Galena Biopharma, Inc., a Delaware corporation ("Galena"), and SELLAS Life Sciences Group, Ltd., a privately held Bermuda exempted company ("Private SELLAS"), in December 2017. This business combination is referred to as the Merger. Upon completion of the Merger, the Company's name changed from "Galena Biopharma, Inc." to "SELLAS Life Sciences Group, Inc." and the Company's financial statements became those of Private SELLAS. On March 11, 2020, the World Health Organization declared the outbreak of a new coronavirus to be a “pandemic”. The COVID-19 pandemic continues to present substantial public health and economic challenges around the world which have impacted, and will continue to impact, millions of individuals and business worldwide. Efforts to contain the spread of the coronavirus since March 2020 have led to travel bans and restrictions, quarantines, shelter-in-place orders and shutdowns. The Company is continuously monitoring the impact of the pandemic on its clinical development programs. The full extent to which the COVID-19 pandemic directly or indirectly impacts the Company's business, results of operations and financial condition will depend on future developments that are highly uncertain, subject to change and cannot be predicted with confidence, including the actions taken to contain or treat COVID-19, the ultimate overall duration of the pandemic, the emergence of new variants of COVID-19, the emergence of new geographic hotspots where the coronavirus is spreading more rapidly, and continued or new travel restrictions, quarantines, social distancing and business closure requirements in the United States and in other countries, as well as the effectiveness of actions taken globally to contain and treat the coronavirus, including the availability of safe and effective vaccines and the uptake thereof and whether existing vaccines are effective with respect to new variants. In particular, the continued spread of the coronavirus globally could adversely impact the Company's clinical trial operations and could have an adverse impact on our business and the financial results.