CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Public offering	At-the-market	2018 Stock purchase agreement	2019 Stock purchase agreement	Common Stock	Common Stock Public offering	Common Stock At-the-market	Common Stock 2018 Stock purchase agreement	Common Stock 2019 Stock purchase agreement	Additional Paid-in Capital	Additional Paid-in Capital Public offering	Additional Paid-in Capital At-the-market	Additional Paid-in Capital 2018 Stock purchase agreement	Additional Paid-in Capital 2019 Stock purchase agreement	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017						22,568,006
Beginning balance at Dec. 31, 2017	$ 38,510					$ 226					$ 131,877					$ 0	$ (93,593)
Stockholders’ deficit
Issuance of common stock, net of issuance cost (in shares)							7,414,996	372,852	150,987
Issuance of common stock, net of issuance cost		$ 37,845	$ 2,657	$ 935			$ 74	$ 4	$ 2			$ 37,771	$ 2,653	$ 933
Shares issued for cash upon the exercise of stock options (in shares)						280,178
Shares issued for cash upon the exercise of stock options	182					$ 2					180
Issuance of common stock for cash under employee stock purchase plan (in shares)						52,444
Issuance of common stock for cash under employee stock purchase plan	242										242
Share-based compensation expense	3,679										3,679
Change in unrealized gain on investments	(3)															(3)
Net loss	$ (32,703)																(32,703)
Ending balance (in shares) at Dec. 31, 2018	30,839,463					30,839,463
Ending balance at Dec. 31, 2018	$ 51,344					$ 308					177,335					(3)	(126,296)
Stockholders’ deficit
Issuance of common stock, net of issuance cost (in shares)								658,000	606,364	2,557,544
Issuance of common stock, net of issuance cost			$ 758	$ 483	$ 890			$ 7	$ 6	$ 26			$ 751	$ 477	$ 864
Shares issued for cash upon the exercise of stock options (in shares)						144,252
Shares issued for cash upon the exercise of stock options	87					$ 1					86
Issuance of common stock for cash under employee stock purchase plan (in shares)						56,253
Issuance of common stock for cash under employee stock purchase plan	110					$ 1					109
Share-based compensation expense	3,970										3,970
Reclassification of warrant liability from equity	(18)										(18)
Change in unrealized gain on investments	3															3
Net loss	$ (41,873)																(41,873)
Ending balance (in shares) at Dec. 31, 2019	34,861,876					34,861,876
Ending balance at Dec. 31, 2019	$ 15,754					$ 349					$ 183,574					$ 0	$ (168,169)