CONSOLIDATED 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 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, 2018	$ 3	280,840	(34)	(170,473)	110,336
Balance (in shares) at Dec. 31, 2018	29,323,930
Increase (Decrease) in Stockholders' Deficit
Net loss					(24,621)
Balance at Jun. 30, 2019	$ 3	284,706	39	(195,094)	89,654
Balance (in shares) at Jun. 30, 2019	29,348,400
Balance at Mar. 31, 2019	$ 3	282,799	11	(182,116)	100,697
Balance (in shares) at Mar. 31, 2019	29,326,900
Increase (Decrease) in Stockholders' Deficit
Common stock issued on exercise of stock options		5			5
Common stock issued on exercise of stock options (in shares)	21,500
Vesting of restricted stocks issued upon early exercise of stock options		4			4
Stock-based compensation expense		1,898			1,898
Unrealized gain on marketable securities			28		28
Net loss				(12,978)	(12,978)
Balance at Jun. 30, 2019	$ 3	284,706	39	(195,094)	89,654
Balance (in shares) at Jun. 30, 2019	29,348,400
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 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
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
Net loss					(23,546)
Balance at Jun. 30, 2020	$ 3	291,475	53	(240,691)	50,840
Balance (in shares) at Jun. 30, 2020	28,034,733
Balance at Mar. 31, 2020	$ 3	290,069	43	(230,080)	60,035
Balance (in shares) at Mar. 31, 2020	27,953,233
Increase (Decrease) in Stockholders' Deficit
Common stock issued on exercise of stock options		23			23
Common stock issued on exercise of stock options (in shares)	81,500
Stock-based compensation expense		1,383			1,383
Unrealized gain on marketable securities			10		10
Net loss				(10,611)	(10,611)
Balance at Jun. 30, 2020	$ 3	$ 291,475	$ 53	$ (240,691)	$ 50,840
Balance (in shares) at Jun. 30, 2020	28,034,733