Note 3 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cash equivalents:
Cash Equivalents	$ 14,784	$ 13,775
Marketable investments:
Available-for-sale securities	35,692	40,299
Money Market Funds [Member]
Cash equivalents:
Cash Equivalents	1,636	6,053
Commercial Paper [Member]
Cash equivalents:
Cash Equivalents	6,445	1,050
Fair Value, Measurements, Recurring [Member]
Marketable investments:
Available-for-sale securities	35,392	40,299
Total assets at fair value	43,773	47,402
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Marketable investments:
Total assets at fair value	1,636	6,053
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable investments:
Available-for-sale securities	35,392	40,299
Total assets at fair value	42,137	41,349
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash Equivalents	1,636	6,053
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash Equivalents	1,636	6,053
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Cash equivalents:
Cash Equivalents	6,445	1,050
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash Equivalents	$ 6,445	$ 1,050