CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2014	$ 5	$ 200,034	$ (10)	$ (150,489)	$ 49,540
Balance (in shares) at Dec. 31, 2014	4,979
Increase (Decrease) in Stockholders' Equity
Issuance of common stock for exercise of options, employee stock purchase plan and release of awards		1,772			1,772
Issuance of common stock for exercise of options, employee stock purchase plan and release of awards (in shares)	133
Issuance of common stock, net of issuance costs	$ 2	21,366			21,368
Issuance of common stock, net of issuance costs (in shares)	1,437
Stock-based compensation		3,650			3,650
Change in unrealized gain/(loss) on available-for-sale investments			(6)		(6)
Net loss				(31,483)	(31,483)
Balance at Dec. 31, 2015	$ 7	226,822	(16)	(181,972)	44,841
Balance (in shares) at Dec. 31, 2015	6,549
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs		364			364
Issuance of common stock, net of issuance costs (in shares)	81
Stock-based compensation		2,155			2,155
Change in unrealized gain/(loss) on available-for-sale investments			$ 16		16
Net loss				(32,048)	(32,048)
Balance at Dec. 31, 2016	$ 7	229,341		(214,020)	$ 15,328
Balance (in shares) at Dec. 31, 2016	6,630				6,630
Increase (Decrease) in Stockholders' Equity
Issuance of common stock for exercise of options, employee stock purchase plan and release of awards		(20)			$ (20)
Issuance of common stock for exercise of options, employee stock purchase plan and release of awards (in shares)	113
Issuance of common stock, net of issuance costs	$ 1	1,986			1,987
Issuance of common stock, net of issuance costs (in shares)	1,325
Stock-based compensation		719			719
Net loss				(10,668)	(10,668)
Balance at Dec. 31, 2017	$ 8	$ 232,026		$ (224,688)	$ 7,346
Balance (in shares) at Dec. 31, 2017	8,068				8,068