Note 15 - Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue	$ 16,200	$ 16,252	$ 16,890	$ 16,726	$ 16,794	$ 16,946	$ 17,406	$ 17,380	$ 66,068	$ 68,526	$ 68,944
Operating income	4,024	4,536	5,172	4,061	4,544	4,989	5,053	5,021	$ 17,793	$ 19,607	$ 18,813
Net income	$ 2,237	$ 2,326	$ 2,908	$ 1,996	$ 7,382	$ 1,589	$ 1,536	$ 1,608
Net income per common share - basic (in dollars per share)	$ 0.66	$ 0.69	$ 0.86	$ 0.59	$ 2.21	$ 0.67	$ 0.46	$ 0.48	$ 2.79	$ 3.62	$ 1.57
Net income per common share - diluted (in dollars per share)	$ 0.65	$ 0.67	$ 0.85	$ 0.58	$ 2.14	$ 0.65	$ 0.45	$ 0.47	$ 2.76	$ 3.52	$ 1.51