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Note 2 - Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Amortized cost $ 13,095 $ 26,491
Gross unrealized holding gains 20 70
Gross unrealized holding losses (77) (166)
Available-for-sale securities 13,038 26,395
Corporate Debt Securities [Member]    
Amortized cost 8,458 20,028
Gross unrealized holding gains 19 64
Gross unrealized holding losses (49) (122)
Available-for-sale securities 8,428 19,970
US States and Political Subdivisions Debt Securities [Member]    
Amortized cost 4,637 6,463
Gross unrealized holding gains 1 6
Gross unrealized holding losses (28) (44)
Available-for-sale securities $ 4,610 $ 6,425