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Note 4 - Marketable Securities - Available-for-sale Marketable Securities in Gross Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Estimated fair value, less than 12 months $ 5,997 $ 7,140
Gross unrealized holding losses, less than 12 months (56) (38)
Estimated fair value, more than 12 months 2,785 2,782
Gross unrealized holding losses, more than 12 months (49) (39)
Estimated fair value 8,782 9,922
Gross unrealized holding losses (105) (77)
Corporate Debt Securities [Member]    
Estimated fair value, less than 12 months 2,672 3,799
Gross unrealized holding losses, less than 12 months (24) (20)
Estimated fair value, more than 12 months 1,866 2,125
Gross unrealized holding losses, more than 12 months (38) (30)
Estimated fair value 4,538 5,924
Gross unrealized holding losses (62) (50)
US States and Political Subdivisions Debt Securities [Member]    
Estimated fair value, less than 12 months 3,325 3,341
Gross unrealized holding losses, less than 12 months (32) (18)
Estimated fair value, more than 12 months 919 657
Gross unrealized holding losses, more than 12 months (11) (9)
Estimated fair value 4,244 3,998
Gross unrealized holding losses $ (43) $ (27)