Note 7 - Stockholders' Equity - Warrant Activity (Details)	9 Months Ended
	Sep. 30, 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	(599,582)
Warrants exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 2.81
Outstanding (in shares) | shares	1,116,484
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 37.27
Exercisable (in shares) | shares	1,046,631
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 34.42