Note 6 - Marketable Securities - Short-term and Long-term Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost	$ 33,425	$ 70,329
Unrealized Gains	134	1
Unrealized Losses	(16)	(242)
Fair Value	33,543	70,088
Bank Time Deposits [Member]
Amortized Cost	1,479	4,208
Unrealized Gains	22	0
Unrealized Losses	(1)	(25)
Fair Value	1,500	4,183
US States and Political Subdivisions Debt Securities [Member]
Amortized Cost	5,612	17,509
Unrealized Gains	11	1
Unrealized Losses	(3)	(33)
Fair Value	5,620	17,477
Corporate Debt Securities [Member]
Amortized Cost	16,503	31,769
Unrealized Gains	100	0
Unrealized Losses	(1)	(96)
Fair Value	16,602	31,673
US Government Agencies Debt Securities [Member]
Amortized Cost	8,340	16,843
Unrealized Gains	1	0
Unrealized Losses	(10)	(88)
Fair Value	8,331	$ 16,755
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Amortized Cost	1,491
Unrealized Gains	0
Unrealized Losses	(1)
Fair Value	$ 1,490