Organization and Description of Business	12 Months Ended
Organization and Description of Business	Dec. 31, 2018
Accounting Policies [Abstract]
Organization and Description of Business	Organization and Description of Business Overview SELLAS Life Sciences Group, Inc. (the "Company," or “SELLAS”) is a clinical-stage biopharmaceutical company focused on the development of novel cancer immunotherapeutics for a broad range of indications. Merger of Galena Biopharma, Inc. and SELLAS Life Sciences Group Ltd. As used in this annual report on Form 10-K, the words the “Company,” and “SELLAS” refer to SELLAS Life Sciences Group, Inc. and its consolidated subsidiaries following completion of the business combination with Galena Biopharma, Inc., a Delaware corporation ("Galena"). This business combination is referred to as the Merger. On December 29, 2017, Galena completed the Merger with SELLAS Life Sciences Group, Ltd., a privately held Bermuda exempted company ("Private SELLAS"), in accordance with the terms of the Agreement and Plan of Merger and Reorganization, dated as of August 7, 2017 and amended November 5, 2017 (the "Merger Agreement"), among the Galena, Sellas Intermediate Holdings I, Inc., Sellas Intermediate Holdings II, Inc., Galena Bermuda Merger Sub, Ltd., and Private SELLAS. As a result of the Merger, the Company’s business is now substantially comprised of the business of Private SELLAS, and although the Company is considered the legal acquiror of Private SELLAS, for accounting purposes, Private SELLAS is considered to have acquired the Company in the Merger. Consequently, the Merger was accounted for as a reverse acquisition and the Private SELLAS financial statements became the Company's financial statements. Immediately prior to the Merger, Galena effected a 1-for-30 reverse stock split of its outstanding common stock, par value $0.0001 per share. Under the terms of the Merger Agreement, Galena issued shares of its common stock to Private SELLAS’ securityholders at an exchange ratio of 43.9972 shares of its common stock in exchange for each common share of Private SELLAS outstanding immediately prior to the Merger. The Company also assumed all of the restricted stock units (“RSUs”) issued and outstanding under the Private SELLAS Stock Incentive Plan #1, and all of the issued and outstanding warrants of Private SELLAS. Accordingly, such RSUs will now be settled in, and such warrants now are exercisable for, shares of the Company’s common stock. Accordingly, immediately after the Merger, there were 5,766,891 shares of the Company’s common stock outstanding, with the former Private SELLAS securityholders owning 67.5% of the Company’s fully diluted common stock, and the Company’s pre-Merger securityholders owning the remaining 32.5%. The number of shares and per share amounts of common stock in the accompanying consolidated financial statements and notes to the consolidated financial statements have been restated to give retroactive effect to the common stock conversion ratio and reverse stock split for all periods presented, including common stock options, RSUs, and common stock warrants. Upon completion of the Merger, the Company’s name changed from “Galena Biopharma, Inc.” to “SELLAS Life Sciences Group, Inc.," the Company’s financial statements became those of Private SELLAS, and the Company's common stock began trading on The Nasdaq Capital Market under the new ticker symbol “SLS” on January 2, 2018.