Organization and Description of Business (Narrative) (Details)	Dec. 29, 2017 shares	Dec. 28, 2017 $ / shares	Mar. 31, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares
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		6,154,377	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%