Note 10 - Stockholders' Equity (Details Textual) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended	19 Months Ended
	May 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Jun. 30, 2021	May 19, 2020	Dec. 13, 2019
Proceeds from Issuance of Common Stock, Net				$ 6,832,000	$ 5,214,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period (in shares)						503,050
Share-based Payment Arrangement, Expense		$ 2,099,000	$ 314,000	$ 2,357,000	$ 381,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares)				0
Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period (in shares)	490,000
Executive Officer [Member] | Share-based Payment Arrangement, Option [Member]
Share-based Payment Arrangement, Accelerated Cost		$ 2,008,000
At The Market Offering Agreement [Member]
Maximum Offering Price for Issuance of Common Stock								$ 15,280,313	$ 4,400,000
Stock Issued During Period, Shares, New Issues (in shares)				2,976,832			5,597,484
Proceeds from Issuance of Common Stock							$ 15,280,000
Shares Issued, Price Per Share (in dollars per share)		$ 2.73		$ 2.73			$ 2.73
Proceeds from Issuance of Common Stock, Net				$ 6,832,000			$ 14,568,000
Payments of Stock Issuance Costs				$ 224,000			$ 712,000