Note 3 - Investment Securities - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash equivalents	$ 14,184	$ 13,775
Available-for-sale securities	21,728	40,299
Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	21,728	40,299
Total assets at fair value	21,854	47,402
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities
Total assets at fair value	126	6,053
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	21,728	40,299
Total assets at fair value	21,728	41,349
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	126	6,053
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	126	6,053
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		1,050
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		1,050
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents