Note 5 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments:
Investments	$ 69,972	$ 24,000
Estimate of Fair Value Measurement [Member]
Cash equivalents:
Cash equivalents		6,393
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents		5,893
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents		500
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Reported Value Measurement [Member]
Cash equivalents:
Cash equivalents		6,393
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member]
Cash equivalents:
Cash equivalents	4,984	5,893
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents	4,984	5,893
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents
Money Market Funds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Money Market Funds [Member] | Reported Value Measurement [Member]
Cash equivalents:
Cash equivalents	4,984	5,893
Certificates of Deposit [Member] | Estimate of Fair Value Measurement [Member]
Cash equivalents:
Cash equivalents		500
Investments:
Investments		24,000
Certificates of Deposit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash equivalents:
Cash equivalents
Investments:
Investments
Certificates of Deposit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash equivalents:
Cash equivalents		500
Investments:
Investments		24,000
Certificates of Deposit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Cash equivalents:
Cash equivalents
Investments:
Investments
Certificates of Deposit [Member] | Reported Value Measurement [Member]
Cash equivalents:
Cash equivalents		500
Investments:
Investments		$ 24,000
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member]
Investments:
Investments	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Investments:
Investments	69,972
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments
US Treasury Securities [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Investments:
Investments
US Treasury Securities [Member] | Reported Value Measurement [Member]
Investments:
Investments	$ 69,972