Note 2 - Summary of Significant Accounting Policies - Fair Value of Items Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments in debt securities (short-term)	$ 4,415
Investments in debt securities (long-term)	7,074
Fair Value, Recurring [Member]
Investments in debt securities (short-term)	4,415
Investments in debt securities (long-term)	7,074
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Investments in debt securities (short-term)	705
Investments in debt securities (long-term)	908
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Investments in debt securities (short-term)	3,710
Investments in debt securities (long-term)	6,166
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Investments in debt securities (short-term)
Investments in debt securities (long-term)