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Note 3 - Fair Value Measurements - Fair Value of Assets Measured on a Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Assets:    
Money market funds $ 517 $ 5,358
Fair Value, Inputs, Level 1 [Member]    
Assets:    
Money market funds 517 5,358
Fair Value, Inputs, Level 2 [Member]    
Assets:    
Money market funds
Fair Value, Inputs, Level 3 [Member]    
Assets:    
Money market funds