CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]
Balance at Dec. 31, 2018	$ 37,633	$ 95	$ 445,192	$ (408,493)	$ 839
Balance, shares at Dec. 31, 2018		95,372,236
Net loss	(46,383)			(46,383)
Other comprehensive income	1				1
Issuance of stock, net of issue costs	20,755	$ 12	20,743
Issuance of stock, net of issue costs, shares		12,234,495
Exercise of stock options	308	$ 1	307
Exercise of stock options, shares		166,760
Vesting of stock units	(87)		(87)
Vesting of stock units, shares		255,753
Stock-based compensation	3,711		3,711
Balance at Sep. 30, 2019	15,938	$ 108	469,866	(454,876)	840
Balance, shares at Sep. 30, 2019		108,029,244
Balance at Jun. 30, 2019	28,210	$ 106	466,493	(439,229)	840
Balance, shares at Jun. 30, 2019		106,297,792
Net loss	(15,647)			(15,647)
Issuance of stock, net of issue costs	2,412	$ 2	2,410
Issuance of stock, net of issue costs, shares		1,707,995
Vesting of stock units	(14)		(14)
Vesting of stock units, shares		23,457
Stock-based compensation	977		977
Balance at Sep. 30, 2019	15,938	$ 108	469,866	(454,876)	840
Balance, shares at Sep. 30, 2019		108,029,244
Balance at Dec. 31, 2019	$ 8,330	$ 109	472,667	(465,286)	840
Balance, shares at Dec. 31, 2019	109,417,322	109,417,322
Net loss	$ (29,925)			(29,925)
Issuance of stock, net of issue costs	24,701	$ 21	24,680
Issuance of stock, net of issue costs, shares		21,029,465
Employee stock purchase plan	294	$ 1	293
Employee stock purchase plan, shares		336,975
Vesting of stock units	(90)		(90)
Vesting of stock units, shares		638,084
Stock-based compensation	4,284		4,284
Balance at Sep. 30, 2020	$ 7,594	$ 131	501,834	(495,211)	840
Balance, shares at Sep. 30, 2020	131,421,846	131,421,846
Balance at Jun. 30, 2020	$ 4,188	$ 125	494,633	(491,410)	840
Balance, shares at Jun. 30, 2020		125,197,899
Net loss	(3,801)			(3,801)
Issuance of stock, net of issue costs	4,711	$ 6	4,705
Issuance of stock, net of issue costs, shares		6,029,465
Employee stock purchase plan	107		107
Employee stock purchase plan, shares		175,315
Vesting of stock units, shares		19,167
Stock-based compensation	2,389		2,389
Balance at Sep. 30, 2020	$ 7,594	$ 131	$ 501,834	$ (495,211)	$ 840
Balance, shares at Sep. 30, 2020	131,421,846	131,421,846