CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Adjustment From Adoption of ASC 842	Public offering	2019 Stock purchase agreement	2017 At-the-market	Common Stock	Common Stock Public offering	Common Stock 2019 Stock purchase agreement	Common Stock 2017 At-the-market	Additional Paid-in Capital	Additional Paid-in Capital Public offering	Additional Paid-in Capital 2019 Stock purchase agreement	Additional Paid-in Capital 2017 At-the-market	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit	Accumulated Deficit Adjustment From Adoption of ASC 842
Beginning balance (in shares) at Dec. 31, 2018						30,839,463
Beginning 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)									47,589
Issuance of common stock, net of issuance cost					$ 134				$ 1				$ 133
Issuance of common stock for cash under employee stock purchase plan (in shares)						34,167
Issuance of common stock for cash under employee stock purchase plan	84									84
Share-based compensation expense	1,018									1,018
Change in unrealized gain on investments	5													5
Net loss	(11,629)														(11,629)
Ending balance (in shares) at Mar. 31, 2019						30,921,219
Ending balance at Mar. 31, 2019	40,956					$ 309				178,570				2	(137,925)
Beginning balance (in shares) at Dec. 31, 2018						30,839,463
Beginning balance at Dec. 31, 2018	51,344					$ 308				177,335				(3)	(126,296)
Stockholders’ deficit
Change in unrealized gain on investments	14
Net loss	(20,525)
Ending balance (in shares) at Jun. 30, 2019						31,037,363
Ending balance at Jun. 30, 2019	33,437					$ 310				179,937				11	(146,821)
Beginning balance (in shares) at Dec. 31, 2018						30,839,463
Beginning balance at Dec. 31, 2018	$ 51,344					$ 308				177,335				(3)	(126,296)
Ending balance (in shares) at Dec. 31, 2019	34,861,876					34,861,876
Ending balance at Dec. 31, 2019	$ 15,754	$ (3)				$ 349				183,574				0	(168,169)	$ (3)
Stockholders’ deficit
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Beginning balance (in shares) at Mar. 31, 2019						30,921,219
Beginning balance at Mar. 31, 2019	$ 40,956					$ 309				178,570				2	(137,925)
Stockholders’ deficit
Issuance of common stock, net of issuance cost (in shares)						96,652
Issuance of common stock, net of issuance cost	278					$ 1				277
Shares issued for cash upon the exercise of stock options (in shares)						19,492
Shares issued for cash upon the exercise of stock options under an equity incentive plan	15									15
Share-based compensation expense	1,075									1,075
Change in unrealized gain on investments	9													9
Net loss	(8,896)														(8,896)
Ending balance (in shares) at Jun. 30, 2019						31,037,363
Ending balance at Jun. 30, 2019	$ 33,437					$ 310				179,937				11	(146,821)
Beginning balance (in shares) at Dec. 31, 2019	34,861,876					34,861,876
Beginning balance at Dec. 31, 2019	$ 15,754	(3)				$ 349				183,574				0	(168,169)	(3)
Stockholders’ deficit
Issuance of common stock, net of issuance cost (in shares)							15,000,000	2,200,000	975,063
Issuance of common stock, net of issuance cost			$ 13,881	$ 4,049	$ 670		$ 150	$ 22	$ 10		$ 13,731	$ 4,027	$ 660
Shares issued for cash upon the exercise of stock options (in shares)						17,238
Shares issued for cash upon the exercise of stock options under an equity incentive plan	13									13
Issuance of common stock for cash under employee stock purchase plan (in shares)						23,171
Issuance of common stock for cash under employee stock purchase plan	16									16
Share-based compensation expense	664									664
Net loss	(8,044)														(8,044)
Ending balance (in shares) at Mar. 31, 2020						53,077,348
Ending balance at Mar. 31, 2020	$ 27,000					$ 531				202,685				0	(176,216)
Beginning balance (in shares) at Dec. 31, 2019	34,861,876					34,861,876
Beginning balance at Dec. 31, 2019	$ 15,754	$ (3)				$ 349				183,574				0	(168,169)	$ (3)
Stockholders’ deficit
Issuance of common stock for cash under employee stock purchase plan (in shares)						145,061
Change in unrealized gain on investments	0
Net loss	$ (14,479)
Ending balance (in shares) at Jun. 30, 2020	53,077,348					53,077,348
Ending balance at Jun. 30, 2020	$ 21,171					$ 531				203,291				0	(182,651)
Beginning balance (in shares) at Mar. 31, 2020						53,077,348
Beginning balance at Mar. 31, 2020	27,000					$ 531				202,685				0	(176,216)
Stockholders’ deficit
Issuance costs related to public offering			$ (17)								$ (17)
Share-based compensation expense	623									623
Change in unrealized gain on investments	0
Net loss	$ (6,435)														(6,435)
Ending balance (in shares) at Jun. 30, 2020	53,077,348					53,077,348
Ending balance at Jun. 30, 2020	$ 21,171					$ 531				$ 203,291				$ 0	$ (182,651)