Note 6 - Marketable Securities - Short-term and Long-term Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost	$ 50,135	$ 70,329
Unrealized Gains	136	1
Unrealized Losses	(26)	(242)
Fair Value	50,245	70,088
Bank Time Deposits [Member]
Amortized Cost	1,975	4,208
Unrealized Gains	16	0
Unrealized Losses	(2)	(25)
Fair Value	1,989	4,183
US States and Political Subdivisions Debt Securities [Member]
Amortized Cost	6,651	17,509
Unrealized Gains	12	1
Unrealized Losses	(1)	(33)
Fair Value	6,662	17,477
Corporate Debt Securities [Member]
Amortized Cost	26,519	31,769
Unrealized Gains	105	0
Unrealized Losses	(11)	(96)
Fair Value	26,613	31,673
US Government Agencies Debt Securities [Member]
Amortized Cost	12,997	16,843
Unrealized Gains	2	0
Unrealized Losses	(12)	(88)
Fair Value	12,987	$ 16,755
US Treasury Securities [Member]
Amortized Cost	1,993
Unrealized Gains	1
Unrealized Losses	0
Fair Value	$ 1,994