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Note 11 - Equity Incentive Plans (Details Textual) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	6 Months Ended
	Apr. 23, 2019	Nov. 30, 2016	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Jul. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant			186,410		200,650
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value			$ 0.55	$ 3.59
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount			$ 1.4
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition			2 years 299 days
The 2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant						0
The 2016 Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	0
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period		10 years
The 2019 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,600,000