Note 10 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale securities	$ 9,946	$ 4,715
Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	9,946	4,715
Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
Corporate Debt Securities [Member]
Available-for-sale securities	5,845	2,881
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	5,845	2,881
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	4,101	1,834
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	4,101	1,834
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities