Note 16 - Quarterly Financial Data (Unaudited) (Details) - Quarterly Results of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Results of Operations [Abstract]
Operating revenue	$ 173,547	$ 171,550	$ 168,423	$ 159,409	$ 166,224	$ 167,103	$ 161,413	$ 164,474	$ 672,929	$ 659,214	$ 638,456
Operating income	16,392	13,042	12,932	8,640	12,683	13,653	13,121	12,538	51,006	51,995	45,853
Net income	$ 8,969	$ 7,652	$ 7,926	$ 5,287	$ 7,301	$ 7,977	$ 7,667	$ 7,202	$ 29,834	$ 30,147	$ 27,267
Basic earnings per common share (in Dollars per share)	$ 0.27	$ 0.23	$ 0.24	$ 0.16	$ 0.22	$ 0.24	$ 0.23	$ 0.22	$ 0.89	$ 0.91	$ 0.82
Diluted earnings per common share (in Dollars per share)	$ 0.27	$ 0.23	$ 0.24	$ 0.16	$ 0.22	$ 0.24	$ 0.23	$ 0.22	$ 0.89	$ 0.90	$ 0.82