Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information (unaudited)
Research and development expenses	$ 8,087	$ 9,148	$ 7,232	$ 3,927	$ 3,344	$ 2,642	$ 2,817	$ 3,573	$ 28,394	$ 12,376	$ 22,005
General and administrative expenses	2,187	2,073	2,338	2,187	1,593	1,571	1,691	1,812	8,785	6,667	6,237
Net loss	$ (10,113)	$ (11,110)	$ (9,504)	$ (5,999)	$ (4,738)	$ (4,170)	$ (4,552)	$ (5,438)	$ (36,726)	$ (18,898)	$ (28,643)
Net loss per share, basic and diluted	$ (0.24)	$ (0.27)	$ (0.24)	$ (0.15)	$ (0.12)	$ (0.15)	$ (0.21)	$ (0.26)	$ (0.90)	$ (0.80)	$ (1.47)