Note 6 - Warrants (Details Textual) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 09, 2018	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Class of Warrant or Right, Outstanding		4,863,158	4,863,158	6,688,849
Class of Warrant or Right, Exercise Price of Warrants or Rights		$ 4.41	$ 4.41	$ 4.50
Class of Warrant or Right, Exercised During Period		1,813,691	1,825,691
Class of Warrant or Right, Exercised During Period, Exercise Price		$ 4.75	$ 4.75
Proceeds from Warrant Exercises		$ 8.6	$ 8.7
Subsequent Event [Member]
Class of Warrant or Right, Exercised During Period	274,798
Class of Warrant or Right, Exercised During Period, Exercise Price	$ 4.75
Proceeds from Warrant Exercises	$ 1.3