Note 7 - Fair Value Measurements - Fair Value of Financial Assets Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Marketable securities	$ 25,920
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	65,637	$ 72,225
Marketable securities	7,602	0
Total	73,531	127,603
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents	292	55,378
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Marketable securities	18,318	0
Total	18,318	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents	0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents	0	0
Marketable securities	0	0
Total	0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents	$ 0	$ 0