Net Loss per Share - Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (7,172)	$ (7,213)	$ (9,491)	$ (7,588)	$ (14,385)	$ (17,079)
Denominator:
Weighted average common shares outstanding—basic and diluted.	26,453,428		14,737,236		20,666,988	14,734,775
Weighted average pre-funded warrants to purchase common stock.	1,072,637				539,281
Total	27,526,065		14,737,236		21,206,269	14,734,775
Net loss per share —basic and diluted	$ (0.26)		$ (0.64)		$ (0.68)	$ (1.16)