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Note 4 - Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Amortized cost $ 3,851 $ 4,760
Gross unrealized holding gains 36 1
Gross unrealized holding losses (4) (46)
Available-for-sale securities 3,883 4,715
Corporate Debt Securities [Member]    
Amortized cost 2,331 2,911
Gross unrealized holding gains 25 1
Gross unrealized holding losses (4) (31)
Available-for-sale securities 2,352 2,881
US States and Political Subdivisions Debt Securities [Member]    
Amortized cost 1,520 1,849
Gross unrealized holding gains 11
Gross unrealized holding losses (15)
Available-for-sale securities $ 1,531 $ 1,834