CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Loss	Accumulated deficit	Private Financing	Private Financing Common stock	Private Financing Additional paid-in capital	Public Offering	Public Offering Common stock	Public Offering Additional paid-in capital	At-the-market Offering	At-the-market Offering Common stock	At-the-market Offering Additional paid-in capital	Private placement	Private placement Common stock	Private placement Additional paid-in capital	Redeemable Convertible Preferred Stock [Member]
Beginning balance (in shares) at Dec. 31, 2016		833,744
Beginning balance at Dec. 31, 2016	$ (61,921)	$ 8	$ 5,147		$ (67,076)
Beginning balance (in shares) at Dec. 31, 2016																		18,584,390
Beginning balance at Dec. 31, 2016																		$ 76,976
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock to redemption value																		$ 5
Conversion of preferred stock to common stock (in shares)																		(18,584,390)
Conversion of preferred stock to common stock																		$ (76,981)
Ending balance (in shares) at Feb. 13, 2017																		0
Ending balance at Feb. 13, 2017																		$ 0
Beginning balance (in shares) at Dec. 31, 2016		833,744
Beginning balance at Dec. 31, 2016	(61,921)	$ 8	5,147		(67,076)
Stockholders’ deficit
Issuance of common stock in public offering, net of issuance cost (in shares)							6,359,628						840,534
Issuance of common stock in public offering, net of issuance cost						$ 39,156	$ 64	$ 39,092				$ 7,603	$ 8	$ 7,595
Accretion of redeemable convertible preferred stock to redemption value	(5)				(5)
Issuance of common stock upon reverse merger (in shares)		1,024,960
Issuance of common stock upon reverse merger	204	$ 10	194
Reclassification of warrant liability to equity	51		51
Issuance of common stock upon cashless exercise of warrant (in shares)		16,387
Shares issued for cash upon the exercise of stock options (in shares)		412,894
Shares issued for cash upon the exercise of stock options	307	$ 5	302
Issuance of common stock for cash under employee stock purchase plan (in shares)		13,193
Issuance of common stock for cash under employee stock purchase plan	110		110
Issuance of warrant classified as equity	127		127
Share-based compensation expense	2,409		2,409
Change in unrealized loss on investments	0
Net loss	$ (26,512)				(26,512)
Ending balance (in shares) at Dec. 31, 2017	22,568,006	22,568,006
Ending balance at Dec. 31, 2017	$ 38,510	$ 226	131,877	$ 0	(93,593)
Beginning balance (in shares) at Dec. 31, 2016																		18,584,390
Beginning balance at Dec. 31, 2016																		$ 76,976
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock to redemption value																		$ 5
Conversion of preferred stock to common stock (in shares)		13,066,666																(18,584,390)
Conversion of preferred stock to common stock	76,981	$ 131	76,850															$ (76,981)
Ending balance (in shares) at Feb. 13, 2017																		0
Ending balance at Feb. 13, 2017																		$ 0
Stockholders’ deficit
Issuance of common stock in public offering, net of issuance cost (in shares)										7,414,996			372,852			150,987
Issuance of common stock in public offering, net of issuance cost									$ 37,845	$ 74	$ 37,771	$ 2,657	$ 4	$ 2,653	$ 935	$ 2	$ 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
Issuance of warrant classified as equity	0
Share-based compensation expense	3,679		3,679
Change in unrealized loss 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)
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of preferred stock to common stock	$ 0