Note 7 - Stockholders' Equity - Warrant Activity (Details)	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Outstanding (in shares) | shares	1,716,066
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 25.23
Warrants expired (in shares) | shares
Warrants expired, weighted average exercise price (in dollars per share) | $ / shares
Warrants exercised (in shares) | shares	(107,866)
Warrants exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 0.53
Outstanding (in shares) | shares	1,608,200
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 26.89
Exercisable (in shares) | shares	1,538,347
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 24.47