Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost	$ 84,076	$ 99,965
Accrued Interest	421	444
Gross Unrealized Gains	52	19
Gross Unrealized Losses	(141)	(316)
Estimated Fair Value	84,408	100,112
US Government Agencies Debt Securities [Member]
Amortized Cost	46,201	50,613
Accrued Interest	183	176
Gross Unrealized Gains	31	15
Gross Unrealized Losses	(55)	(131)
Estimated Fair Value	46,360	50,673
Corporate Debt Securities [Member]
Amortized Cost	34,316	45,793
Accrued Interest	226	254
Gross Unrealized Gains	15	4
Gross Unrealized Losses	(81)	(171)
Estimated Fair Value	34,476	45,880
Certificates of Deposit [Member]
Amortized Cost	3,559	3,559
Accrued Interest	12	14
Gross Unrealized Gains	6
Gross Unrealized Losses	(5)	(14)
Estimated Fair Value	$ 3,572	$ 3,559