Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information (unaudited)
Research and development expenses	$ 12,512	$ 11,572	$ 10,010	$ 8,872	$ 8,087	$ 9,148	$ 7,232	$ 3,927	$ 42,966	$ 28,394
General and administrative expenses	2,960	2,327	2,502	3,667	2,187	2,073	2,338	2,187	11,456	8,785
Net loss	$ (15,409)	$ (13,806)	$ (12,423)	$ (12,483)	$ (10,113)	$ (11,110)	$ (9,504)	$ (5,999)	$ (54,121)	$ (36,726)
Net loss per share, basic and diluted	$ (0.25)	$ (0.26)	$ (0.24)	$ (0.24)	$ (0.24)	$ (0.27)	$ (0.24)	$ (0.15)	$ (0.99)	$ (0.90)