Note 10 - Warrants (Details Textual) - $ / shares	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2016	Mar. 31, 2014
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 4.33	$ 4.35	$ 4.50
Warrants to Purchase Common Stock [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)					6,910,283
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)					$ 4.75
Warrants in Connection with WAVI Credit Facility [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				550,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)				$ 1.75