Note 8 - Stockholders' Equity - Warrant Activity (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Outstanding (in shares) | shares	17,265,208
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.99
Warrants expired (in shares) | shares	(83,420)
Warrants expired (in dollars per share) | $ / shares	$ 56.20
Warrants exercised (in shares) | shares	(500,000)
Warrants exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 0.01
Outstanding (in shares) | shares	16,681,788
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.81
Exercisable (in shares) | shares	15,983,259
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 2.58