Note 6 - Marketable Securities - Balance of Held-to-maturity Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Due in one year or less	$ 53,201
Due after one year through five years	55,351
Total marketable securities	$ 108,552	$ 123,956