Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2014	$ 1	$ 106,172	$ (100,255)	$ 5,918
Beginning Balance, (in shares) at Dec. 31, 2014	23,453,000
Share-based compensation expense		4,666		4,666
Issuance of common stock in public offerings		14,480		14,480
Issuance of stock in public offering (in shares)	2,388,245
Fair value of derivative warrant liability reclassified to additional paid-in capital		16,121		16,121
Issuance of common stock upon cashless exercise of warrants (in shares)	25,052
Issuance of common stock upon exercise of warrants		7,789		7,789
Issuance of common stock upon exercise of warrants (in shares)	1,379,575
Issuance of common stock upon exercise of stock options		1,068		1,068
Issuance of common stock upon exercise of stock options (in shares)	316,177
Fractional shares issued due to reverse stock split	1,514
Issuance of common stock to 401(k) plan	$ 201	201		201
Issuance of common stock to 401(k) plan (in shares)	17,437
Net loss			(33,314)	(33,314)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2015	$ 1	150,497	(133,569)	16,929
Ending Balance, (in shares) at Dec. 31, 2015	27,555,948
Share-based compensation expense		5,063		5,063
Issuance of common stock and warrants in public offerings, net of $2,040 issuance costs		29,905		29,905
Issuance of common stock and warrants in public offerings, net of $2,040 issuance costs (in shares)	4,293,333
Issuance of common stock for services (in shares)	365
Issuance of common stock upon cashless exercise of warrants (in shares)	4,979
Issuance of common stock upon exercise of stock options		191		191
Issuance of common stock upon exercise of stock options (in shares)	135,205
Issuance of common stock under ESPP		91		91
Issuance of common stock under ESPP (in shares)	16,729
Issuance of common stock to 401(k) plan	$ 208	208		208
Issuance of common stock to 401(k) plan (in shares)	37,528
Net loss			(23,438)	(23,438)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2016	$ 1	185,955	(157,007)	$ 28,949
Ending Balance, (in shares) at Dec. 31, 2016	32,044,087			32,044,087
Share-based compensation expense		4,106		$ 4,106
Issuance of common stock on warrant exchange		3,537		3,537
Issuance of common stock on warrant exchange (in shares)	2,021,419
Issuance of common stock for services (in shares)	350
Issuance of common stock upon exercise of warrants		3		3
Issuance of common stock upon exercise of warrants (in shares)	3,464
Issuance of common stock upon exercise of stock options		26		26
Issuance of common stock upon exercise of stock options (in shares)	89,387
Issuance of common stock under ESPP		51		51
Issuance of common stock under ESPP (in shares)	17,750
Issuance of common stock to 401(k) plan	$ 183	183		183
Issuance of common stock to 401(k) plan (in shares)	98,319
Net loss			(26,745)	(26,745)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2017	$ 1	194,016	(183,907)	$ 10,110
Ending Balance, (in shares) at Dec. 31, 2017	34,274,776			34,274,776
Cumulative Adjustment on adoption of ASU 2016-09. | ASU 2016-09		$ 155	$ (155)