Quarterly Results (unaudited) (Details) (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 (unaudited) [Abstract]
Operating revenues	$ 77,965	$ 77,513	$ 77,454	$ 76,010	$ 74,998	$ 72,876	$ 71,378	$ 68,823	$ 308,942	$ 288,075	$ 251,145
Operating income	12,436	13,262	14,500	15,209	9,297	5,407	11,137	8,817	55,407	34,658	32,290
Net income from continuing operations					5,058	1,415	5,722	4,408	29,586	16,603	13,538
Net income	$ 6,676	$ 6,717	$ 7,842	$ 8,351	$ 4,916	$ 1,361	$ 5,560	$ 4,466	$ 29,586	$ 16,303	$ 12,993
Net income from continuing operations per share - basic and diluted (in dollars per share)					$ 0.21	$ 0.06	$ 0.24	$ 0.19	$ 1.23	$ 0.69	$ 0.57
Net income per share - basic and diluted (in dollars per share)	$ 0.27	$ 0.28	$ 0.33	$ 0.35	$ 0.21	$ 0.06	$ 0.23	$ 0.19	$ 1.23	$ 0.68	$ 0.55