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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Results of Operations [Abstract]
Operating revenue	$ 166,224	$ 167,103	$ 161,413	$ 164,474	$ 166,412	$ 163,606	$ 156,964	$ 151,474	$ 659,214	$ 638,456	$ 603,679
Operating income	12,683	13,653	13,121	12,538	13,211	9,840	13,283	9,519	51,995	45,853	43,030
Net income	$ 7,301	$ 7,977	$ 7,667	$ 7,202	$ 7,719	$ 6,518	$ 7,584	$ 5,446	$ 30,147	$ 27,267	$ 24,285
Basic earnings per common share (in Dollars per share)	$ 0.22	$ 0.24	$ 0.23	$ 0.22	$ 0.23	$ 0.20	$ 0.23	$ 0.16	$ 0.91	$ 0.82	$ 0.74
Diluted earnings per common share (in Dollars per share)	$ 0.22	$ 0.24	$ 0.23	$ 0.22	$ 0.23	$ 0.20	$ 0.23	$ 0.16	$ 0.90	$ 0.82	$ 0.73