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Note 3 - Fair Value Measurements (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Cash Equivalents, at Carrying Value, Total $ 9,100,000 $ 49,400,000  
Available-for-sale Securities, Current, Total 300,535,000 27,796,000  
Debt Securities, Available-for-sale, Realized Gain (Loss), Total 77,000 7,000 $ 23,000
Available-for-sale Securities, Gross Realized Gains 0 0 0
Available-for-sale Securities, Gross Realized Losses $ 0 $ 0 $ 0