Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Direct Registered Offering	Convertible Preferred Stock	Convertible Preferred Stock Direct Registered Offering	Additional Paid-in Capital	Additional Paid-in Capital Direct Registered Offering	Additional Paid-in Capital Convertible Preferred Stock	Additional Paid-in Capital Convertible Preferred Stock Direct Registered Offering	Common Stock	Common Stock Direct Registered Offering	Accumulated Deficit	Accumulated Deficit Direct Registered Offering
Balance at Dec. 31, 2017	$ 1,987				$ 321,174				$ 666		$ (319,853)
Balance, shares at Dec. 31, 2017									67,653,974
Issuance of common stock from option exercises	378				375				$ 3
Issuance of common stock from option exercises, shares									265,647
Issuance of common stock from releases of restricted stock units, shares									12,468
Stock-based compensation	1,190				1,190
Issuance of common stock, value		$ 30,093				$ 30,052				$ 41
Issuance of common stock, shares										4,090,000
Issuance of preferred Series A stock upon direct registered offering, value		22,207		$ 30		2,728		$ 22,177				$ (2,728)
Issuance of preferred Series A stock upon direct registered offering, share				3,000,581
Issuance of common stock warrants in connection with direct registered offering		$ 6,623				$ 6,623
Net loss	(4,814)										(4,814)
Balance at Mar. 31, 2018	57,664				362,142		$ 22,177		$ 710		(327,395)
Temporary equity balance , shares at Mar. 31, 2018			3,000,581
Temporary equity balance at Mar. 31, 2018			$ 30
Balance, shares at Mar. 31, 2018									72,022,089
Balance at Dec. 31, 2018	167,309				565,334				$ 952		(398,977)
Balance, shares at Dec. 31, 2018									96,332,023
Issuance of common stock from option exercises	$ 2,884				2,877				$ 7
Issuance of common stock from option exercises, shares	588,120								588,120
Stock-based compensation	$ 4,552				4,552
Issuance of common stock from warrant exercises, shares									12,955
Reclassification of warrant liability to additional paid-in capital upon warrant exercises	92				92
Net loss	(33,361)										(33,361)
Balance at Mar. 31, 2019	$ 141,476				$ 572,855				$ 959		$ (432,338)
Balance, shares at Mar. 31, 2019									96,933,098