Quarterly Results (unaudited) (Details) (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
Quarterly Results (unaudited) [Abstract]
Operating revenues	$ 74,998	$ 72,876	$ 71,378	$ 68,823	$ 66,505	$ 62,657	$ 61,555	$ 60,428	$ 288,075	$ 251,145	$ 195,206
Operating income	9,297	5,407	11,137	8,817	8,035	9,170	7,994	7,091	34,658	32,290	36,331
Net income from continuing operations	5,058	1,415	5,722	4,408	3,825	3,615	3,038	3,060	16,603	13,538	18,774
Net income (loss)	$ 4,916	$ 1,361	$ 5,560	$ 4,466	$ 3,972	$ 3,002	$ 2,992	$ 3,027	$ 16,303	$ 12,993	$ 18,075
Net income from continuing operations per share - basic and diluted (in dollars per share)	$ 0.21	$ 0.06	$ 0.24	$ 0.19	$ 0.16	$ 0.15	$ 0.13	$ 0.13	$ 0.69	$ 0.57	$ 0.79
Net income per share - basic and diluted (in dollars per share)	$ 0.21	$ 0.06	$ 0.23	$ 0.19	$ 0.16	$ 0.13	$ 0.13	$ 0.13	$ 0.68	$ 0.55	$ 0.76