Note 4 - Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Money market funds	$ 55,386	$ 136,052
Total assets	55,386	136,052
Fair value	4,300	11,000
Total liabilities	4,300	11,000
Fair Value, Inputs, Level 1 [Member]
Money market funds	55,386	136,052
Total assets	55,386	136,052
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	4,300	11,000
Total liabilities	$ 4,300	$ 11,000