Note 14 - Quarterly Financial Data (Unaudited) (Detail) - Quarterly results of operations: (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenue	$ 166,412	$ 163,606	$ 156,964	$ 151,474	$ 158,340	$ 156,348	$ 151,135	$ 137,856	$ 638,456	$ 603,679	$ 516,920
Operating income	13,211	9,840	13,283	9,519	12,951	11,111	11,359	7,609	45,853	43,030	35,289
Net income	$ 7,719	$ 6,518	$ 7,584	$ 5,446	$ 7,663	$ 6,333	$ 6,194	$ 4,095	$ 27,267	$ 24,285	$ 19,742
Basic earnings per common share (in Dollars per share)	$ 0.35	$ 0.30	$ 0.34	$ 0.25	$ 0.35	$ 0.29	$ 0.28	$ 0.19	$ 1.24	$ 1.10	$ 0.90
Diluted earnings per common share (in Dollars per share)	$ 0.35	$ 0.29	$ 0.34	$ 0.25	$ 0.35	$ 0.29	$ 0.28	$ 0.19	$ 1.23	$ 1.10	$ 0.90