Supplementary Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected quarterly financial data
License and collaboration revenue - related party	$ 11,282	$ 8,781							$ 20,063
Loss from operations	(3,513)	(4,980)	$ (15,131)	$ (14,273)	$ (11,397)	$ (7,138)	$ (7,091)	$ (7,040)	(37,897)	$ (32,666)	$ (14,794)
Net loss	$ (3,052)	$ (4,825)	$ (14,979)	$ (14,101)	$ (11,248)	$ (7,084)	$ (7,098)	$ (11,747)	$ (36,957)	$ (37,177)	$ (14,858)
Net loss per share of common stock attributable to common stockholders, basic and diluted	$ (0.15)	$ (0.29)	$ (0.89)	$ (0.84)	$ (0.67)	$ (0.87)	$ (19.07)	$ (40.96)	$ (2.09)	$ (5.80)	$ (59.32)