CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income	Total
Balance, amount at Dec. 31, 2018	$ 6	$ 481,357	$ (365,512)	$ (38)	$ 115,813
Balance, shares at Dec. 31, 2018	62,516,627
Issuance of common stock under employee stock purchase plan, amount		198			198
Issuance of common stock under employee stock purchase plan, shares	83,046
Issuance of common stock for services, amount		311			311
Issuance of common stock for services, shares	113,136
Issuance of common stock upon exercise of options, amount		7			7
Issuance of common stock upon exercise of options, shares	2,451
Issuance of common stock upon vesting of restricted stock units, shares	85,609
Stock-based compensation		3,845			3,845
Unrealized gains on available-for-sale securities				54	54
Net loss			(51,611)		(51,611)
Balance, amount at Jun. 30, 2019	$ 6	485,718	(417,123)	16	68,617
Balance, shares at Jun. 30, 2019	62,800,869
Balance, amount at Mar. 31, 2019	$ 6	483,479	(391,656)	12	91,841
Balance, shares at Mar. 31, 2019	62,600,443
Issuance of common stock for services, amount		311			311
Issuance of common stock for services, shares	113,136
Issuance of common stock upon exercise of options, amount		5			5
Issuance of common stock upon exercise of options, shares	1,681
Issuance of common stock upon vesting of restricted stock units, shares	85,609
Stock-based compensation		1,923			1,923
Unrealized gains on available-for-sale securities				4	4
Net loss			(25,467)		(25,467)
Balance, amount at Jun. 30, 2019	$ 6	485,718	(417,123)	16	68,617
Balance, shares at Jun. 30, 2019	62,800,869
Balance, amount at Dec. 31, 2019	$ 9	647,078	(460,452)	20	186,655
Balance, shares at Dec. 31, 2019	88,817,741
Issuance of common stock under employee stock purchase plan, amount		375			375
Issuance of common stock under employee stock purchase plan, shares	75,804
Issuance of common stock for services, amount		310			310
Issuance of common stock for services, shares	42,403
Issuance of common stock upon exercise of options, amount		420			420
Issuance of common stock upon exercise of options, shares	204,615
Stock-based compensation		5,622			5,622
Unrealized gains on available-for-sale securities				297	297
Net loss			(47,329)		(47,329)
Balance, amount at Jun. 30, 2020	$ 9	653,805	(507,781)	317	146,350
Balance, shares at Jun. 30, 2020	89,140,563
Balance, amount at Mar. 31, 2020	$ 9	650,617	(482,825)	(44)	167,757
Balance, shares at Mar. 31, 2020	89,035,096
Issuance of common stock for services, amount		310			310
Issuance of common stock for services, shares	42,403
Issuance of common stock upon exercise of options, amount		204			204
Issuance of common stock upon exercise of options, shares	63,064
Stock-based compensation		2,674			2,674
Unrealized gains on available-for-sale securities				361	361
Net loss			(24,956)		(24,956)
Balance, amount at Jun. 30, 2020	$ 9	$ 653,805	$ (507,781)	$ 317	$ 146,350
Balance, shares at Jun. 30, 2020	89,140,563