Selected Quarterly Financial Data (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
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 476	$ 944	$ 2,182	$ 4,784	$ 1,192	$ 1,631	$ 718	$ 462	$ 8,386	$ 4,003
Research and development expenses	(8,234)	(7,399)	(8,375)	(6,413)	(4,998)	(5,018)	(5,487)	(4,120)	(30,421)	(19,623)
General and administrative expenses	(2,695)	(2,696)	(2,668)	(2,990)	(2,548)	(2,502)	(2,581)	(3,281)	(11,049)	(10,912)
Other income (expense), net	136	140	147	(42)	(32)	55	38	(41)
Net loss	(10,317)	(9,011)	(8,714)	(4,661)	(6,386)	(5,834)	(7,312)	(6,980)	(32,703)	(26,512)
Net loss available to common stockholders	$ (10,317)	$ (9,011)	$ (8,714)	$ (4,661)	$ (6,386)	$ (5,834)	$ (7,312)	$ (6,985)	$ (32,703)	$ (26,517)
Net loss per share, basic and diluted (in dollars per share)	$ (0.33)	$ (0.29)	$ (0.29)	$ (0.18)	$ (0.29)	$ (0.27)	$ (0.34)	$ (0.60)	$ (1.10)	$ (1.38)