Note 8 - Equity Offerings - Summary of Sales Under Offering (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Apr. 28, 2017
Average price per share (in dollars per share)			$ 1.00
Net proceeds	$ 0	$ 13,525
The 2019 ATM Offering and 2020 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
Commissions earned by Cantor	843
Net proceeds	$ 27,257
The 2021 ATM Offering [Member]
Total shares of common stock sold (in shares)	331,811
Average price per share (in dollars per share)	$ 3.47
Gross proceeds	$ 1,152
Commissions earned by Cantor	34
Net proceeds	$ 1,118