Note 5 - Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Total Assets	$ 35,106	$ 15,619
Fair Value, Inputs, Level 1 [Member]
Total Assets	35,106	15,619
Fair Value, Inputs, Level 2 [Member]
Total Assets
Fair Value, Inputs, Level 3 [Member]
Total Assets
Money Market Funds [Member]
Cash equivalents	32,077	6,518
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	32,077	6,518
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents
US Treasury Securities [Member]
Cash equivalents	3,029	9,101
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	3,029	9,101
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents