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Note 5 - Fair Value Measurements - Fair Value of Financial Assets Measured on a Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Fair Value, Inputs, Level 1 [Member]    
Money market mutual fund $ 26,088 $ 68,962
Total 26,088 68,962
Fair Value, Inputs, Level 2 [Member]    
Money market mutual fund 0 0
Total 0 0
Fair Value, Inputs, Level 3 [Member]    
Money market mutual fund 0 0
Total $ 0 $ 0