Note 4 - Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Money market funds	$ 136,052	$ 87,231
Total assets	136,052	87,231
Fair value	11,000	16,800
Total liabilities	11,000	16,800
Fair Value, Inputs, Level 1 [Member]
Money market funds	136,052	87,231
Total assets	136,052	87,231
Fair value
Total liabilities
Fair Value, Inputs, Level 2 [Member]
Money market funds
Total assets
Fair value
Total liabilities
Fair Value, Inputs, Level 3 [Member]
Money market funds
Total assets
Fair value	11,000	16,800
Total liabilities	$ 11,000	$ 16,800