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Organization and Description of Business (Narrative) (Details) | Dec. 29, 2017 shares | Dec. 28, 2017 $ / shares | Sep. 30, 2018 $ / shares shares | Dec. 31, 2017 $ / shares shares
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Schedule Of Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Common stock, par value (usd per share) |  |  | $ 0.0001 | $ 0.0001
Common stock, shares outstanding (in shares) | shares | 5,766,891 |  | 20,501,476 | 5,766,891
Galena Biopharma, Inc |  |  |  |
Schedule Of Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Reverse stock split ratio |  | 0.0333 |  |
Common stock, par value (usd per share) |  | $ 0.0001 |  |
Private SELLAS Security Holders |  |  |  |
Schedule Of Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Common stock ownership percentage | 67.50% |  |  |
Private SELLAS Security Holders | Galena Biopharma, Inc |  |  |  |
Schedule Of Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Common stock conversion ratio | 43.9972 |  |  |
Pre-Merger Securityholders |  |  |  |
Schedule Of Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Common stock ownership percentage | 32.50% |  |  |

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