Note 6 - Marketable Securities - Short-term and Long-term Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost	$ 75,271	$ 48,397
Unrealized Gains	167	112
Unrealized Losses	(255)	(22)
Fair Value	75,183	48,487
Bank Time Deposits [Member]
Amortized Cost	3,697	1,484
Unrealized Gains	31	22
Unrealized Losses	(2)	0
Fair Value	3,726	1,506
US States and Political Subdivisions Debt Securities [Member]
Amortized Cost	12,704	10,204
Unrealized Gains	19	9
Unrealized Losses	(14)	(3)
Fair Value	12,709	10,210
Corporate Debt Securities [Member]
Amortized Cost	35,100	26,944
Unrealized Gains	45	80
Unrealized Losses	(239)	(12)
Fair Value	34,906	27,012
US Government Agencies Debt Securities [Member]
Amortized Cost	20,801	6,274
Unrealized Gains	61	1
Unrealized Losses	0	(7)
Fair Value	20,862	6,268
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Amortized Cost	2,969	3,491
Unrealized Gains	11	0
Unrealized Losses	0	0
Fair Value	$ 2,980	$ 3,491