Note 5 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments	$ 71,222	$ 69,972
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member]
Cash equivalents	2,147	4,984
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	2,147	4,984
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents	2,147	4,984
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member]
Investments	71,221	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Investments	71,221	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Investments	$ 71,221	$ 69,972