CONDENSED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Gain (Loss) [Member]
Beginning Balance at Dec. 31, 2017	$ (7,041)					$ 5,394	$ (12,435)	$ 105,144
Beginning Balance (shares) at Dec. 31, 2017		21,302,972	1,937,984
Beginning Balance at Dec. 31, 2017		$ 10,508	$ 5,000
Beginning Balance (shares) at Dec. 31, 2017					3,804,643
Stock-based compensation	135					135
Series C convertible preferred stock				$ 21,000
Series C convertible preferred stock (shares)				4,606,267
Issuance costs for Series C preferred stock				$ (7)
Net loss	(1,847)						(1,847)
Ending Balance at Mar. 31, 2018	(8,753)					5,529	(14,282)
Ending Balance (Shares) at Mar. 31, 2018		21,302,972	1,937,984	4,606,267
Ending Balance at Mar. 31, 2018		$ 10,508	$ 5,000	$ 20,993
Ending Balance (Shares) at Mar. 31, 2018					3,804,643			105,144
Beginning Balance at Dec. 31, 2017	(7,041)					5,394	(12,435)	$ 105,144
Beginning Balance (shares) at Dec. 31, 2017		21,302,972	1,937,984
Beginning Balance at Dec. 31, 2017		$ 10,508	$ 5,000
Beginning Balance (shares) at Dec. 31, 2017					3,804,643
Unrealized gain (loss) on marketable securities	(7)
Net loss	(13,070)
Ending Balance at Sep. 30, 2018	96,354				$ 3	121,863	(25,505)		$ (7)
Ending Balance (Shares) at Sep. 30, 2018					25,801,515			105,144
Beginning Balance at Mar. 31, 2018	(8,753)					5,529	(14,282)
Beginning Balance (shares) at Mar. 31, 2018		21,302,972	1,937,984	4,606,267
Beginning Balance at Mar. 31, 2018		$ 10,508	$ 5,000	$ 20,993
Beginning Balance (shares) at Mar. 31, 2018					3,804,643			105,144
Stock-based compensation	779					779
Conversion of preferred stock into common stock	36,501	$ (10,508)	$ (5,000)	$ (20,993)	$ 2	36,499
Conversion of preferred stock into common stock (shares)		(21,302,972)	(1,937,984)	(4,606,267)	16,246,872
Issuance of common stock in connection with IPO, net of offering costs	78,381				$ 1	78,380
Issuance of common stock in connection with IPO, net of offering costs (shares)					5,750,000
Net loss	(5,317)						(5,317)
Ending Balance at Jun. 30, 2018	101,591				$ 3	121,187	(19,599)
Ending Balance (Shares) at Jun. 30, 2018					25,801,515			105,144
Stock-based compensation	683					683
Offering costs	(7)					(7)
Unrealized gain (loss) on marketable securities	(7)								(7)
Net loss	(5,906)						(5,906)
Ending Balance at Sep. 30, 2018	96,354				$ 3	121,863	(25,505)		(7)
Ending Balance (Shares) at Sep. 30, 2018					25,801,515			105,144
Beginning Balance at Dec. 31, 2018	89,429				$ 3	122,526	(33,083)		(17)
Beginning Balance (shares) at Dec. 31, 2018					25,809,900			105,144
Stock-based compensation	780					780
Exercise of stock options	3					3
Exercise of stock options (in shares)					3,729
Unrealized gain (loss) on marketable securities	28								28
Net loss	(7,479)						(7,479)
Adoption of ASU 2018-07 (See Note 2)						(98)	98
Ending Balance at Mar. 31, 2019	82,761				$ 3	123,211	(40,464)		11
Ending Balance (Shares) at Mar. 31, 2019					25,813,629			105,144
Beginning Balance at Dec. 31, 2018	89,429				$ 3	122,526	(33,083)		(17)
Beginning Balance (shares) at Dec. 31, 2018					25,809,900			105,144
Unrealized gain (loss) on marketable securities	44
Net loss	(20,570)
Ending Balance at Sep. 30, 2019	71,653				$ 3	125,178	(53,555)		27
Ending Balance (Shares) at Sep. 30, 2019					25,849,941			105,144
Beginning Balance at Mar. 31, 2019	82,761				$ 3	123,211	(40,464)		11
Beginning Balance (shares) at Mar. 31, 2019					25,813,629			105,144
Stock-based compensation	846					846
Exercise of stock options	212					212
Exercise of stock options (in shares)					31,812
Unrealized gain (loss) on marketable securities	27								27
Net loss	(7,001)						(7,001)
Ending Balance at Jun. 30, 2019	76,845				$ 3	124,269	(47,465)		38
Ending Balance (Shares) at Jun. 30, 2019					25,845,441			105,144
Stock-based compensation	905					905
Exercise of stock options	4					4
Exercise of stock options (in shares)					4,500
Unrealized gain (loss) on marketable securities	(11)								(11)
Net loss	(6,090)						(6,090)
Ending Balance at Sep. 30, 2019	$ 71,653				$ 3	$ 125,178	$ (53,555)		$ 27
Ending Balance (Shares) at Sep. 30, 2019					25,849,941			105,144