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Note 4 - Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Amortized cost $ 9,838 $ 4,760
Gross unrealized holding gains 116 1
Gross unrealized holding losses (8) (46)
Available-for-sale securities 9,946 4,715
Corporate Debt Securities [Member]    
Amortized cost 5,756 2,911
Gross unrealized holding gains 96 1
Gross unrealized holding losses (7) (31)
Available-for-sale securities 5,845 2,881
US States and Political Subdivisions Debt Securities [Member]    
Amortized cost 4,082 1,849
Gross unrealized holding gains 20
Gross unrealized holding losses (1) (15)
Available-for-sale securities $ 4,101 $ 1,834