Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2015	$ 108,813	$ 66	$ 304,781	$ (196,023)	$ (11)
Beginning Balance, shares at Dec. 31, 2015		6,605,849
Stock-based compensation	2,909		2,909
Stock options exercised, value	191		191
Stock options exercised, shares		13,577
Common stock issued pursuant to employee stock purchase plan, value	155		155
Common stock issued pursuant to employee stock purchase plan, shares		6,481
Issuance of common stock, net of $403 in offering costs, value	3,997	$ 1	3,996
Issuance of common stock, net of $403 in offering costs, shares		120,672
Unrealized comprehensive gain (loss) on marketable securities	(43)				(43)
Net loss	(42,854)			(42,854)
Ending Balance at Dec. 31, 2016	73,168	$ 67	312,032	(238,877)	(54)
Ending Balance, shares at Dec. 31, 2016		6,746,579
Stock-based compensation	4,036		4,036
Common stock issued pursuant to employee stock purchase plan, value	35		35
Common stock issued pursuant to employee stock purchase plan, shares		5,971
Issuance of common stock pursuant to settlement of restricted stock units, value		$ 1	(1)
Issuance of common stock pursuant to settlement of restricted stock units, shares		56,406
Shares surrendered by employees to pay taxes related to settlement of restricted stock	(16)		(16)
Shares surrendered by employees to pay taxes related to settlement of restricted stock, shares		(3,270)
Unrealized comprehensive gain (loss) on marketable securities	38				38
Net loss	(29,497)			(29,497)
Ending Balance at Dec. 31, 2017	$ 47,764	$ 68	$ 316,086	$ (268,374)	$ (16)
Ending Balance, shares at Dec. 31, 2017		6,805,686