Note 9 - Stockholders' Equity - Activity of Stock Option Plan (Details)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Outstanding, options (in shares) | shares	7,024,275
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 0.29
Granted, options (in shares) | shares
Granted, weighted average exercise price, options (in dollars per share) | $ / shares
Exercised, options (in shares) | shares
Exercised , weighted average exercise price, options (in dollars per share) | $ / shares
Forfeited or expired, options (in shares) | shares	(138,000)
Forfeited or expired, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.01
Outstanding, options (in shares) | shares	6,886,275
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 0.24
Exercisable, options (in shares) | shares	1,890,618
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 0.71