Note 10 - Fair Value Measurements - Fair Value of Assets on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available for sale securities	$ 55,135	$ 58,012
Fair Value, Measurements, Recurring [Member]
Available for sale securities	55,135	58,012
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available for sale securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	55,135	58,012
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available for sale securities