STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock.	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 3	$ 280,840	$ (34)	$ (170,473)	$ 110,336
Balance (in shares) at Dec. 31, 2018	29,323,930
Increase (Decrease) in Stockholders' Deficit
Common stock issued on exercise of stock options		1			1
Common stock issued on exercise of stock options (in shares)	2,970
Vesting of restricted stocks issued upon early exercise of stock options		5			5
Stock-based compensation expense		1,953			1,953
Unrealized gain (loss) on marketable securities			45		45
Net loss				(11,643)	(11,643)
Balance at Mar. 31, 2019	$ 3	282,799	11	(182,116)	100,697
Balance (in shares) at Mar. 31, 2019	29,326,900
Balance at Dec. 31, 2019	$ 3	288,224	29	(217,145)	71,111
Balance (in shares) at Dec. 31, 2019	27,953,233
Increase (Decrease) in Stockholders' Deficit
Stock-based compensation expense		1,845			1,845
Unrealized gain (loss) on marketable securities			14		14
Net loss				(12,935)	(12,935)
Balance at Mar. 31, 2020	$ 3	$ 290,069	$ 43	$ (230,080)	$ 60,035
Balance (in shares) at Mar. 31, 2020	27,953,233