Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Total
Beginning Balance at Dec. 31, 2018	$ 82	$ 429,727	$ (388,274)	$ (9)	$ 41,526
Balance (in shares) at Dec. 31, 2018	82,323,413
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		754			754
Exercise of stock options		21			21
Exercise of stock options (in shares)	30,225
Issuance of warrants		347			347
Issuance of common stock, net of issuance costs	$ 10	8,886			8,896
Issuance of common stock, net of issuance costs (in shares)	10,000,000
Unrealized gain on marketable securities				12	12
Net loss			(5,169)		(5,169)
Ending Balance at Mar. 31, 2019	$ 92	439,735	(393,443)	$ 3	46,387
Balance (in shares) at Mar. 31, 2019	92,353,638
Beginning Balance at Dec. 31, 2019	$ 94	443,129	(413,145)		$ 30,078
Balance (in shares) at Dec. 31, 2019	94,213,760				94,213,760
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		891			$ 891
Issuance of common stock, net of issuance costs	$ 5	3,546			$ 3,551
Issuance of common stock, net of issuance costs (in shares)	4,816,244				4,800,000
Net loss			(5,725)		$ (5,725)
Ending Balance at Mar. 31, 2020	$ 99	$ 447,566	$ (418,870)		$ 28,795
Balance (in shares) at Mar. 31, 2020	99,030,004				99,030,004