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Note 3 - Fair Value Measurements (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash Equivalents, at Carrying Value $ 49,400,000    
Available-for-sale Securities, Current 27,796,000 $ 37,724,000  
Available-for-sale Securities, Gross Unrealized Gain 7,000 23,000  
Available-for-sale Securities, Gross Unrealized Loss 10,000 17,000 $ 40,000
Available-for-sale Securities, Gross Realized Gains 0 0 0
Available-for-sale Securities, Gross Realized Losses $ 0 $ 0 $ 0