Note 2 - Investment Securities - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash equivalents	$ 26,316	$ 26,052
Available-for-sale securities	7,605	9,523
Fair Value, Recurring [Member]
Available-for-sale securities	7,605	9,523
Total assets at fair value	13,916	13,606
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities	4,114	4,274
Total assets at fair value	10,425	8,357
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	3,491	5,249
Total assets at fair value	3,491	5,249
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
Total assets at fair value
Money Market Funds [Member] | Fair Value, Recurring [Member]
Cash equivalents	6,311	4,083
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	6,311	4,083
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents