Organization and Description of Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares	Dec. 29, 2017 $ / shares shares	Dec. 31, 2016 $ / shares shares
Organization And Description Of Business [Line Items]
Reverse stock split, shares	0.03
Reverse stock split, description	Immediately prior to the Merger, the Galena effected a 1-for-30 reverse stock split of the Company’s outstanding common stock.
Common stock shares outstanding | shares	5,766,891	5,766,891	1,268,489
Common stock, par value (USD per share)	$ 0.0001	$ 0.0001	$ 0.0001
Private SELLAS [Member]
Organization And Description Of Business [Line Items]
Common stock conversion ratio	43.9972
Percentage of fully diluted common stock		67.50%
Common stock, par value (USD per share)		$ 10.00
Pre-Merger Securityholders [Member]
Organization And Description Of Business [Line Items]
Percentage of fully diluted common stock		32.50%