Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost	$ 13,433	$ 28,211
Accrued Interest	17	25
Gross Unrealized Gains	4	6
Gross Unrealized Losses	0	(3)
Estimated Fair Value	13,454	28,239
US Government Agencies Debt Securities [Member]
Amortized Cost	10,675	24,986
Accrued Interest	6	14
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	(3)
Estimated Fair Value	10,684	25,000
Certificates of Deposit [Member]
Amortized Cost	2,758	3,225
Accrued Interest	11	11
Gross Unrealized Gains	1	3
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 2,770	$ 3,239