Note 3 - Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Total	$ 32,633	$ 33,801
Cash Equivalents [Member]
Cash equivalents	11,662	28,876
US Government Agencies Debt Securities [Member]
Government securities	20,971	4,925
Fair Value, Inputs, Level 1 [Member]
Total	32,633	33,801
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Cash equivalents	11,662	28,876
Fair Value, Inputs, Level 1 [Member] | US Government Agencies Debt Securities [Member]
Government securities	20,971	4,925
Fair Value, Inputs, Level 2 [Member]
Total
Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]
Cash equivalents
Fair Value, Inputs, Level 2 [Member] | US Government Agencies Debt Securities [Member]
Government securities
Fair Value, Inputs, Level 3 [Member]
Total
Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]
Cash equivalents
Fair Value, Inputs, Level 3 [Member] | US Government Agencies Debt Securities [Member]
Government securities