Note 5 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments	$ 72,045	$ 69,972
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member]
Cash equivalents	1,305	4,984
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents	1,305	4,984
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents
Money Market Funds [Member] | Reported Value Measurement [Member]
Cash equivalents	1,305	4,984
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member]
Investments	72,045	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Investments	72,045	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Investments
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Investments
US Treasury Securities [Member] | Reported Value Measurement [Member]
Investments	$ 72,032	$ 69,972