Note 3 - Summary of Significant Accounting Policies - Fair Value of Assets Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and cash equivalents	$ 1,182	$ 4,162
Short-term investments	12,441	18,428
Total assets at fair value	13,623	22,590
Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	1,182	4,162
Short-term investments	12,441	18,428
Total assets at fair value	13,623	22,590
Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents
Short-term investments
Total assets at fair value
Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents
Short-term investments
Total assets at fair value