Note 7 - Stockholders' Equity - Warrant Activity (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Balance (in shares) | shares	4,828,723
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 9.37
Warrants granted (in shares) | shares	304,818
Warrants granted, weighted average exercise price (in dollars per share) | $ / shares	$ 2.68
Warrants exercised (in shares) | shares
Warrants exercised (cashless), weighted average exercise price (in dollars per share) | $ / shares
Balance (in shares) | shares	5,133,541
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 8.97
Exercisable at March 31, 2018 (in shares) | shares	4,130,194
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 9.60