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Note 4 - Marketable Securities - Available-for-sale Marketable Securities in Gross Unrealized Loss Position (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
Estimated fair value, less than 12 months $ 6,303
Gross unrealized holding losses, less than 12 months (58)
Estimated fair value, more than 12 months 2,952
Gross unrealized holding losses, more than 12 months (52)
Estimated fair value 9,255
Gross unrealized holding losses (110)
Corporate Debt Securities [Member]  
Estimated fair value, less than 12 months 3,106
Gross unrealized holding losses, less than 12 months (30)
Estimated fair value, more than 12 months 2,282
Gross unrealized holding losses, more than 12 months (43)
Estimated fair value 5,388
Gross unrealized holding losses (73)
US States and Political Subdivisions Debt Securities [Member]  
Estimated fair value, less than 12 months 3,197
Gross unrealized holding losses, less than 12 months (28)
Estimated fair value, more than 12 months 670
Gross unrealized holding losses, more than 12 months (9)
Estimated fair value 3,867
Gross unrealized holding losses $ (37)