CONDENSED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS (DEFICIT) (unaudited) - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2017						$ 89,634
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2017						9,132,024
Temporary equity, Accretion of redeemable preferred stock to redemption value						$ 16
Redeemable convertible preferred stock, ending balance at Mar. 31, 2018						$ 89,650
Redeemable convertible preferred stock, ending balance, shares at Mar. 31, 2018						9,132,024
Beginning balance at Dec. 31, 2017	$ (97,416)	$ 9	$ (24)	$ 1,664	$ (99,065)
Beginning balance, shares at Dec. 31, 2017		775,780	(29,967)
Stock-based compensation	104			104
Accretion of redeemable preferred stockto redemption value	(16)			(16)
Net loss	(4,403)				(4,403)
Ending balance at Mar. 31, 2018	(101,731)	$ 9	$ (24)	1,752	(103,468)
Ending balance, shares at Mar. 31, 2018		775,780	(29,967)
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2017						$ 89,634
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2017						9,132,024
Redeemable convertible preferred stock, ending balance at Jun. 30, 2018						$ 89,667
Redeemable convertible preferred stock, ending balance, shares at Jun. 30, 2018						9,132,024
Beginning balance at Dec. 31, 2017	(97,416)	$ 9	$ (24)	1,664	(99,065)
Beginning balance, shares at Dec. 31, 2017		775,780	(29,967)
Net loss	(11,077)
Ending balance at Jun. 30, 2018	(108,360)	$ 9	$ (24)	1,797	(110,142)
Ending balance, shares at Jun. 30, 2018		775,780	(29,967)
Redeemable convertible preferred stock, beginning balance at Mar. 31, 2018						$ 89,650
Redeemable convertible preferred stock, beginning balance, shares at Mar. 31, 2018						9,132,024
Temporary equity, Accretion of redeemable preferred stock to redemption value						$ 17
Redeemable convertible preferred stock, ending balance at Jun. 30, 2018						$ 89,667
Redeemable convertible preferred stock, ending balance, shares at Jun. 30, 2018						9,132,024
Beginning balance at Mar. 31, 2018	(101,731)	$ 9	$ (24)	1,752	(103,468)
Beginning balance, shares at Mar. 31, 2018		775,780	(29,967)
Stock-based compensation	62			62
Accretion of redeemable preferred stockto redemption value	(17)			(17)
Net loss	(6,674)				(6,674)
Ending balance at Jun. 30, 2018	(108,360)	$ 9	$ (24)	1,797	(110,142)
Ending balance, shares at Jun. 30, 2018		775,780	(29,967)
Beginning balance at Dec. 31, 2018	$ 50,927	$ 25	$ (24)	173,837	(122,911)
Beginning balance, shares at Dec. 31, 2018	24,498,275	24,528,242	(29,967)
Issuance of common stock warrants	$ 210			210
Exercises of stock options, shares		150
Stock-based compensation	238			238
Net loss	(7,293)				(7,293)
Ending balance at Mar. 31, 2019	44,082	$ 25	$ (24)	174,285	(130,204)
Ending balance, shares at Mar. 31, 2019		24,528,392	(29,967)
Beginning balance at Dec. 31, 2018	$ 50,927	$ 25	$ (24)	173,837	(122,911)
Beginning balance, shares at Dec. 31, 2018	24,498,275	24,528,242	(29,967)
Net loss	$ (16,525)
Ending balance at Jun. 30, 2019	$ 81,609	$ 29	$ (24)	221,040	(139,436)
Ending balance, shares at Jun. 30, 2019	28,697,640	28,727,607	(29,967)
Beginning balance at Mar. 31, 2019	$ 44,082	$ 25	$ (24)	174,285	(130,204)
Beginning balance, shares at Mar. 31, 2019		24,528,392	(29,967)
Issuance of common stock warrants	86			86
Issuance of common stock in public offering, net	46,277	$ 4		46,273
Issuance of common stock in public offering, net, shares		4,124,475
Exercises of stock options	126			126
Exercises of stock options, shares		74,740
Stock-based compensation	270			270
Net loss	(9,232)				(9,232)
Ending balance at Jun. 30, 2019	$ 81,609	$ 29	$ (24)	$ 221,040	$ (139,436)
Ending balance, shares at Jun. 30, 2019	28,697,640	28,727,607	(29,967)