Note 1 - Significant Accounting Policies - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost	$ 38,222	$ 99,965
Accrued Interest	250	444
Gross Unrealized Gains	67	19
Gross Unrealized Losses	(7)	(316)
Estimated Fair Value	38,532	100,112
US Government Agencies Debt Securities [Member]
Amortized Cost	16,464	50,613
Accrued Interest	86	176
Gross Unrealized Gains	38	15
Gross Unrealized Losses	(2)	(131)
Estimated Fair Value	16,586	50,673
Corporate Debt Securities [Member]
Amortized Cost	19,360	45,793
Accrued Interest	151	254
Gross Unrealized Gains	19	4
Gross Unrealized Losses	(5)	(171)
Estimated Fair Value	19,525	45,880
Certificates of Deposit [Member]
Amortized Cost	2,398	3,559
Accrued Interest	13	14
Gross Unrealized Gains	10
Gross Unrealized Losses		(14)
Estimated Fair Value	$ 2,421	$ 3,559