Note 8 - Equity Offerings - Summary of Sales Under Offering (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Apr. 28, 2017
Average price per share (in dollars per share)					$ 1.00
Net proceeds	$ 0	$ 13,525
The 2019 ATM Offering [Member]
Total shares of common stock sold (in shares)	19,120,037
Average price per share (in dollars per share)	$ 1.47
Gross proceeds	$ 28,100		$ 11,600	$ 7,200
Commissions earned by Cantor	843
Net proceeds	$ 27,257		$ 11,200	$ 7,000