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Note 4 - Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Amortized cost $ 10,653 $ 13,095
Gross unrealized holding gains 5 20
Gross unrealized holding losses (110) (77)
Available-for-sale securities 10,548 13,038
Corporate Debt Securities [Member]    
Amortized cost 6,040 8,458
Gross unrealized holding gains 5 19
Gross unrealized holding losses (73) (49)
Available-for-sale securities 5,972 8,428
US States and Political Subdivisions Debt Securities [Member]    
Amortized cost 4,613 4,637
Gross unrealized holding gains 0 1
Gross unrealized holding losses (37) (28)
Available-for-sale securities $ 4,576 $ 4,610