Note 7 - Stockholders' Equity - Warrant Activity (Details)	6 Months Ended
	Jun. 30, 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	9,471,485
Warrants granted, weighted average exercise price (in dollars per share) | $ / shares	$ 0.67
Warrants exercised (in shares) | shares
Warrants exercised (cashless), weighted average exercise price (in dollars per share) | $ / shares
Balance (in shares) | shares	14,300,208
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 3.61
Exercisable at June 30, 2018 (in shares) | shares	13,296,861
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 3.40