Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost	$ 61,778	$ 99,965
Accrued Interest	309	444
Gross Unrealized Gains	82	19
Gross Unrealized Losses	(41)	(316)
Estimated Fair Value	62,128	100,112
US Government Agencies Debt Securities [Member]
Amortized Cost	32,488	50,613
Accrued Interest	144	176
Gross Unrealized Gains	50	15
Gross Unrealized Losses	(13)	(131)
Estimated Fair Value	32,669	50,673
Corporate Debt Securities [Member]
Amortized Cost	25,731	45,793
Accrued Interest	147	254
Gross Unrealized Gains	24	4
Gross Unrealized Losses	(27)	(171)
Estimated Fair Value	25,875	45,880
Certificates of Deposit [Member]
Amortized Cost	3,559	3,559
Accrued Interest	18	14
Gross Unrealized Gains	8
Gross Unrealized Losses	(1)	(14)
Estimated Fair Value	$ 3,584	$ 3,559