Note 5 - Investments, Available-for-sale - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Amortized Cost	$ 94,438	$ 117,944
Unrealized Gain	0	1
Unrealized Loss	(798)	(248)
Fair Value	93,640	117,697
Short-term investments	78,913	80,262
Long-term investments	14,727	37,435
Mature in less than one year, amortized cost	79,453
Mature in less than one year, fair value	78,913
Mature in one to two years, amortized cost	14,985
Mature in one to two years, fair value	14,727
Total, amortized cost	94,438	117,944
Total, fair value	93,640	117,697
Corporate Debt Securities [Member]
Amortized Cost	50,314	67,239
Unrealized Gain	0	1
Unrealized Loss	(607)	(202)
Fair Value	49,707	67,038
Short-term investments	34,980	29,602
Long-term investments	14,727	37,435
Total, amortized cost	50,314	67,239
Total, fair value	49,707	67,038
Commercial Paper [Member]
Amortized Cost	44,124	50,705
Unrealized Gain	0	0
Unrealized Loss	(191)	(46)
Fair Value	43,933	50,659
Short-term investments	43,933	50,660
Long-term investments	0	0
Total, amortized cost	44,124	50,705
Total, fair value	$ 43,933	$ 50,659