Note 3 - Fair Value Measurement - Fair Value of Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Marketable securities	$ 1,796
Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents	15,109	$ 21,352
Marketable securities	1,796
Fair Value, Measurements, Recurring [Member] | U.S. States and Political Subdivisions Debt Securities [Member]
Marketable securities	1,796
Fair Value, Measurements, Recurring [Member] | Cash [Member]
Cash and cash equivalents	3,901
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents	8,711
Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Cash and cash equivalents	649
Fair Value, Measurements, Recurring [Member] | US Government Corporations and Agencies Securities [Member]
Cash and cash equivalents	1,848
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents	3,901	21,352
Marketable securities
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. States and Political Subdivisions Debt Securities [Member]
Marketable securities
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member]
Cash and cash equivalents	3,901
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Corporations and Agencies Securities [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents	11,208
Marketable securities	1,796
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. States and Political Subdivisions Debt Securities [Member]
Marketable securities	1,796
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents	8,711
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Cash and cash equivalents	649
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Corporations and Agencies Securities [Member]
Cash and cash equivalents	1,848
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Cash and cash equivalents
Marketable securities
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. States and Political Subdivisions Debt Securities [Member]
Marketable securities
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Corporations and Agencies Securities [Member]
Cash and cash equivalents