Statements of Stockholders' Equity - USD ($) $ in Thousands	Series A Preferred Stock [Member] Preferred Stock [Member]	Series A Preferred Stock [Member] Additional Paid-in Capital [Member]	Series A Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2014				16,448,455
Balance at Dec. 31, 2014				$ 16	$ 192,324	$ (109,874)	$ (6)	$ 82,460
Exercise of common stock options (in shares)				45,413
Exercise of common stock options					88			88
Issuance of common stock upon ESPP purchase (in shares)				7,632
Issuance of common stock upon ESPP purchase					75			75
Stock-based compensation expense					2,164			2,164
Unrealized (loss) gain on available-for-sale investments							(5)	(5)
Net loss						(21,377)		(21,377)
Balance (in shares) at Dec. 31, 2015				16,501,500
Balance at Dec. 31, 2015				$ 16	194,651	(131,251)	(11)	63,405
Exercise of common stock options (in shares)				97,521
Exercise of common stock options				$ 1	196			197
Issuance of common stock upon ESPP purchase (in shares)				4,538
Issuance of common stock upon ESPP purchase					11			11
Stock-based compensation expense					3,343			3,343
Unrealized (loss) gain on available-for-sale investments							11	11
Net loss						(28,526)		(28,526)
Balance (in shares) at Dec. 31, 2016				16,603,559
Balance at Dec. 31, 2016				$ 17	198,201	(159,777)		$ 38,441
Exercise of common stock options (in shares)				74,001				74,001
Exercise of common stock options					108			$ 108
Issuance of common stock upon ESPP purchase (in shares)				100,358
Issuance of common stock upon ESPP purchase					130			130
Stock-based compensation expense					3,227			3,227
Unrealized (loss) gain on available-for-sale investments								(20)
Net loss						(29,964)		(29,964)
Issuance of stock, net of issuance costs (in shares)	22,000			896,811
Issuance of stock, net of issuance costs	$ 14,776	$ 6,747	$ 21,523	$ 1	1,287			1,288
Accretion of Series A Convertible Preferred Stock	$ 6,747				(6,747)
Other comprehensive loss							(14)	(14)
Balance (in shares) at Dec. 31, 2017	22,000			17,674,729
Balance at Dec. 31, 2017	$ 21,523			$ 18	$ 202,953	$ (189,741)	$ (14)	$ 34,739