Note 9 - Defined Contribution Plan (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 0.4	$ 0.3	$ 0.2
Maximum [Member]
Defined Contribution Plan Maximum Percentage of Employee Voluntary Contribution		100.00%