STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Convertible Preferred Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance at Dec. 31, 2017	$ 1,327	$ 1	$ 7,305	$ 119,106		$ (125,085)
Common stock, outstanding (in shares) at Dec. 31, 2017		1,272,360
Preferred stock, outstanding (in shares) at Dec. 31, 2017				76,235,050
Stock options exercised	186		186
Stock options exercised (in shares)		106,739
Stock-based compensation expense	55		55
Net loss	(6,501)					(6,501)
Balance at Mar. 31, 2018	(4,933)	$ 1	7,546	$ 119,106		(131,586)
Common stock, outstanding (in shares) at Mar. 31, 2018		1,379,099
Preferred stock, outstanding (in shares) at Mar. 31, 2018				76,235,050
Balance at Dec. 31, 2018	$ 163,999	$ 23	310,941		$ (52)	(146,913)
Common stock, outstanding (in shares) at Dec. 31, 2018	23,401,675	23,401,675
Stock options exercised	$ 166	$ 1	165
Stock options exercised (in shares)	100,089	100,089
Issuance of common stock	$ 58		58
Issuance of common stock (in shares)		1,246
Stock-based compensation expense	1,391		1,391
Other comprehensive loss	68				68
Net loss	(8,266)					(8,266)
Balance at Mar. 31, 2019	$ 157,416	$ 24	$ 312,555		$ 16	$ (155,179)
Common stock, outstanding (in shares) at Mar. 31, 2019	23,503,010	23,503,010