Note 5 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash equivalents	$ 60,962	$ 6,393
Investments	13,250	24,000
Fair Value, Inputs, Level 1 [Member]
Cash equivalents	60,962	5,893
Fair Value, Inputs, Level 2 [Member]
Cash equivalents		500
Money Market Funds [Member]
Cash equivalents	17,366	5,893
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	17,366	5,893
US Treasury Securities [Member]
Cash equivalents	43,596
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	43,596
Certificates of Deposit [Member]
Cash equivalents		500
Investments	13,250	24,000
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents		500
Investments	$ 13,250	$ 24,000