Note 8 - Share-based Compensation - Summary of Warrant Activity (Details) - $ / shares	1 Months Ended			12 Months Ended
	Dec. 31, 2018	Nov. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Warrants outstanding, balance (in shares)				2,011,528	1,313,360
Warrants outstanding, exercise price, balance (in dollars per share)				$ 28.40	$ 4.82
Class of Warrant or Right, Issued During Period				33,720	698,168
Warrants issued, weighted average exercise price (in dollars per share)				$ 5.56	$ 4.90
Warrants exercised (in shares)	(30,000)	(156,250)	(250,002)	(436,252)
Warrants exercised, weighted average exercise price (in dollars per share)	$ (5)			$ (4.70)
Warrants outstanding, balance (in shares)	1,608,996			1,608,996	2,011,528
Warrants outstanding, exercise price, balance (in dollars per share)	$ 4.71			$ 4.71	$ 28.40