Note 1 - Nature of Operations and Summary of Significant Accounting Policies - Basic and Diluted Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Net loss	$ (4,542)	$ (8,784)	$ (5,739)	$ (4,958)	$ (13,326)	$ (10,697)
Common stock warrants						(388)
Total net loss for purpose of calculating diluted net loss per common share	$ (4,542)		$ (5,739)		$ (13,326)	$ (11,085)
Weighted average number of common shares outstanding - basic (in shares)	255,643,828		255,149,140		255,643,828	254,206,926
Common stock warrants (in shares)						11,315,870
Total shares for purpose of calculating diluted net loss per common share (in shares)	255,643,828		255,149,140		255,643,828	265,522,796
Basic (in dollars per share)	$ (0.02)		$ (0.02)		$ (0.05)	$ (0.04)
Diluted (in dollars per share)	$ (0.02)		$ (0.02)		$ (0.05)	$ (0.04)