Note 10 - Defined Contribution Plan (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 288,000	$ 262,000	$ 244,000
Maximum [Member]
Defined Contribution Plan Maximum Percentage of Employee Voluntary Contribution	100.00%