Note 16 - Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenue	$ 172,661	$ 170,464	$ 166,090	$ 161,929	$ 168,773	$ 171,346	$ 163,588	$ 161,287	$ 671,144	$ 664,994	$ 672,929
Operating income	14,533	14,869	14,776	14,125	15,237	14,354	14,161	17,311	58,303	61,063	51,006
Net income	$ 8,303	$ 8,437	$ 8,531	$ 8,193	$ 8,790	$ 8,410	$ 8,357	$ 10,188	$ 33,464	$ 35,745	$ 29,834
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.26	$ 0.26	$ 0.25	$ 0.26	$ 0.25	$ 0.25	$ 0.30	$ 1.03	$ 1.07	$ 0.89
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.26	$ 0.26	$ 0.25	$ 0.26	$ 0.25	$ 0.25	$ 0.30	$ 1.02	$ 1.06	$ 0.89