Note 4 - Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Money market fund	$ 6,393	$ 69,102
Investments	24,000	20,500
Fair Value, Inputs, Level 1 [Member]
Money market fund	5,893
Investments
Fair Value, Inputs, Level 2 [Member]
Money market fund	500	69,102
Investments		20,500
Fair Value, Inputs, Level 3 [Member]
Money market fund
Investments
Mutual Funds [Member]
Money market fund	5,893
Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Money market fund	5,893
Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Money market fund
Mutual Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Money market fund
Money Market Funds [Member]
Money market fund		68,352
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Money market fund
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Money market fund		68,352
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Money market fund
Certificates of Deposit [Member]
Money market fund	500	750
Investments	24,000	20,500
Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Money market fund
Investments
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Money market fund	500	750
Investments	24,000	20,500
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Money market fund
Investments