Note 2 - Investment Securities - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash equivalents	$ 26,052	$ 14,184
Available-for-sale securities	9,523	21,728
Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	9,523	21,728
Total assets at fair value	13,606	21,854
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Total assets at fair value	4,083	126
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	9,523	21,728
Total assets at fair value	9,523	21,728
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities
Total assets at fair value
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents	4,083	126
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	4,083	126
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents