Note 4 - Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Money market funds	$ 145,515	$ 87,231
Total assets	145,515	87,231
Contingent consideration liability	10,900	16,800
Total liabilities	10,900	16,800
Fair Value, Inputs, Level 1 [Member]
Money market funds	145,515	87,231
Total assets	145,515	87,231
Contingent consideration liability
Total liabilities
Fair Value, Inputs, Level 2 [Member]
Money market funds
Total assets
Contingent consideration liability
Total liabilities
Fair Value, Inputs, Level 3 [Member]
Money market funds
Total assets
Contingent consideration liability	10,900	16,800
Total liabilities	$ 10,900	$ 16,800