Note 7 - Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Corporate bonds	$ 75,324	$ 88,754
Corporate Bonds [Member]
Corporate bonds	75,324	88,754
Corporate Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Corporate bonds	0	0
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Corporate bonds	75,324	88,754
Corporate Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Corporate bonds	$ 0	$ 0