Note 4 - Investments - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost	$ 88,167
Unrealized Gain	0
Unrealized Loss	(53)
Fair Value	88,114	$ 0
Short-term investments	57,972	0
Long-term investments	30,142	0
Mature in less than one year, amortized cost	57,997
Mature in less than one year, fair value	57,972
Mature in one to two years, amortized cost	30,170
Mature in one to two years, fair value	30,142
Total, amortized cost	88,167
Total, fair value	88,114	$ 0
Corporate Debt Securities [Member]
Amortized Cost	59,455
Unrealized Gain	0
Unrealized Loss	(41)
Fair Value	59,414
Short-term investments	29,272
Long-term investments	30,142
Total, amortized cost	59,455
Total, fair value	59,414
Commercial Paper [Member]
Amortized Cost	28,712
Unrealized Gain	0
Unrealized Loss	(12)
Fair Value	28,700
Short-term investments	28,700
Long-term investments	0
Total, amortized cost	28,712
Total, fair value	$ 28,700