Note 4 - Investments (Details Textual) - USD ($) $ in Thousands |
Jun. 30, 2018 |
Dec. 31, 2017 |
---|---|---|
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax, Total | $ 0 | $ 0 |
Certificates of Deposit [Member] | ||
Available-for-sale Securities, Total | $ 13,300 | $ 24,000 |