STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Follow On Public Offering Common Stock.	Follow On Public Offering Additional Paid-In Capital	Follow On Public Offering	At-the-market offering Common Stock.	At-the-market offering Additional Paid-In Capital	At-the-market offering	Common Stock.	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balance at Dec. 31, 2016							$ 2
Balance at Dec. 31, 2016								$ 200,709	$ (39)	$ (67,871)	$ 132,801
Balance (in shares) at Dec. 31, 2016							20,922,428
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock					$ 711	$ 711
Issuance of stock (in shares)				52,569
Common stock issued on exercise of stock options								731			731
Common stock issued on exercise of stock options (in shares)							66,253
Vesting of restricted stocks issued upon early exercise of stock options								28			28
Stock-based compensation expense								6,229			6,229
Unrealized gain (loss) on marketable securities									(2)		(2)
Net loss										(55,663)	(55,663)
Balance at Dec. 31, 2017							$ 2	208,408	(41)	(123,534)	84,835
Balance (in shares) at Dec. 31, 2017							21,041,250
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock	$ 1	$ 64,876	$ 64,877
Issuance of stock (in shares)	8,117,647
Common stock issued on exercise of stock options								393			393
Common stock issued on exercise of stock options (in shares)							165,033
Vesting of restricted stocks issued upon early exercise of stock options								28			28
Stock-based compensation expense								7,135			7,135
Unrealized gain (loss) on marketable securities									7		7
Net loss										(46,939)	(46,939)
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
Retirement of common stock in exchange for common stock warrant								(5,030)			(5,030)
Retirement of common stock in exchange for common stock warrant (in shares)							(1,458,000)
Issuance of common stock warrant in exchange for retirement of common stock								5,030			5,030
Common stock issued on exercise of stock options								24			24
Common stock issued on exercise of stock options (in shares)							87,303
Vesting of restricted stocks issued upon early exercise of stock options								12			12
Stock-based compensation expense								7,348			7,348
Unrealized gain (loss) on marketable securities									63		63
Net loss										(46,672)	(46,672)
Balance at Dec. 31, 2019							$ 3	$ 288,224	$ 29	$ (217,145)	$ 71,111
Balance (in shares) at Dec. 31, 2019							27,953,233