Basis of Presentation and Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to U.S. GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Updates of the Financial Accounting Standards Board ("FASB").
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the financial statements of the Company and its wholly owned subsidiaries. All significant intercompany accounts and transactions have been eliminated upon consolidation. Unless the context otherwise indicates, reference in these notes to the "Company" refer to SELLAS Life Sciences Group, Inc., and its wholly owned subsidiaries, Private SELLAS, SLSG Limited, LLC, Sellas Life Sciences Limited, and Apthera, Inc. The functional currency of the Company's non-U.S. operations is the U.S. dollar.
Reclassification	Reclassification Certain prior year amounts have been reclassified to conform to current year presentation. These reclassifications had no effect on the Company's loss from operations, net loss, and net loss per share.
Time-Vested Restricted Stock Units	Time-Vested Restricted Stock Units During the six months ended June 30, 2021, the Board of Directors granted restricted stock units ("RSUs") to employees that vest based on continuous service. Time-vested RSUs awarded to employees vest one-fourth per year annually over four years, provided the employee remains employed with the Company. The fair values of the RSUs are measured on the date of grant and are based on the Company's closing stock price on such date. Compensation expense for RSUs with only service conditions is recognized straight-line over the applicable service period. The Company accounts for forfeitures of RSUs when they occur. Previously recognized compensation expense for forfeited RSUs are reversed in the period the RSUs are forfeited.
Net Loss Per Share	Net Loss Per Share Net loss per share is computed by dividing net loss applicable to common stockholders by the weighted average number of shares of common stock outstanding during each period. Diluted loss per share includes the effect, if any, from the potential exercise or conversion of securities, such as warrants, stock options and unvested restricted stock that would result in the issuance of incremental shares of common stock. In computing the basic and diluted net loss per share, the weighted average number of shares remains the same for both calculations due to the fact that when a net loss exists, dilutive shares are not included in the calculation as the impact is anti-dilutive.
Recent Accounting Standards Adopted and Recent Accounting Standards Not Yet Adopted	Recent Accounting Standards Adopted and Recent Accounting Standards Not Yet Adopted Recent Accounting Standards Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes which, among other things, eliminates certain exceptions in the current rules regarding the approach for intraperiod tax allocations and the methodology for calculating income taxes in an interim period, and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The standard was adopted by the Company on January 1, 2021. This new standard did not have a material impact on the Company’s financial statements. Recent accounting standards not yet adopted In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity which, among other things, simplifies the accounting models for the allocation of proceeds attributable to the issuance of a convertible debt instrument. As a result, after adopting the ASU’s guidance, entities will not separately present in equity an embedded conversion feature in such debt. Instead, they will account for a convertible debt instrument wholly as debt, and for convertible preferred stock wholly as preferred stock (i.e., as a single unit of account), unless (i) a convertible instrument contains features that require bifurcation as a derivative under ASC 815 or (ii) a convertible debt instrument was issued at a substantial premium. The standard becomes effective for the Company in the first quarter of 2022 and early adoption is permitted. The Company is currently evaluating the potential impact of the adoption of this standard on its consolidated financial statements.