Note 5 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash equivalents:
Cash equivalents	$ 9,254	$ 6,393
Investments:
Investments	21,250	24,000
Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents	6,504	5,893
Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents	2,750	500
Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Money Market Funds [Member]
Cash equivalents:
Cash equivalents	6,504	5,893
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents	6,504	5,893
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Certificates of Deposit [Member]
Cash equivalents:
Cash equivalents	2,750	500
Investments:
Investments	21,250	24,000
Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents
Investments:
Investments
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents	2,750	500
Investments:
Investments	21,250	24,000
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Investments:
Investments