Condensed Consolidated Statements Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 1,340	$ 36	$ 267,421	$ (266,117)
Balance (in shares) at Dec. 31, 2017		35,948,900
Stock-based compensation expense	1,381		1,381
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan	5		5
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		70,983
Prior year accumulated deficit adjustment from ASC 606 implementation	100			100
Net loss	(12,913)			(12,913)
Balance at Mar. 31, 2018	(10,087)	$ 36	268,807	(278,930)
Balance (in shares) at Mar. 31, 2018		36,019,883
Balance at Dec. 31, 2018	$ 11,387	$ 44	328,514	(317,171)
Balance (in shares) at Dec. 31, 2018	44,175,441	44,175,441
Stock-based compensation expense	$ 2,033		2,033
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		163,802
Change in fair value of warrants upon modification	147		147
Net loss	(15,147)			(15,147)
Balance at Mar. 31, 2019	$ (1,580)	$ 44	$ 330,694	$ (332,318)
Balance (in shares) at Mar. 31, 2019	44,339,243	44,339,243