CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income Income (Loss)	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 upon exercise of options, amount		2			2
Issuance of common stock upon exercise of options, shares	770
Stock-based compensation		1,922			1,922
Unrealized (losses) gains on available-for-sale securities				50	50
Net loss			(26,144)		(26,144)
Balance, amount at Mar. 31, 2019	$ 6	483,479	(391,656)	12	91,841
Balance, shares at Mar. 31, 2019	62,600,443
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 upon exercise of options, amount		216			216
Issuance of common stock upon exercise of options, shares	141,551
Stock-based compensation		2,948			2,948
Unrealized (losses) gains on available-for-sale securities				(64)	(64)
Net loss			(22,373)		(22,373)
Balance, amount at Mar. 31, 2020	$ 9	$ 650,617	$ (482,825)	$ (44)	$ 167,757
Balance, shares at Mar. 31, 2020	89,035,096