Quarterly Results (unaudited) (Details) (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 (unaudited) [Abstract]
Operating revenues	$ 82,810	$ 82,268	$ 81,416	$ 80,452	$ 77,965	$ 77,513	$ 77,454	$ 76,010	$ 326,946	$ 308,942	$ 288,075
Operating income	16,326	14,144	15,793	15,680	12,436	13,262	14,500	15,209	61,943	55,407	34,658
Net income from continuing operations									33,883	29,586	16,603
Net income	$ 8,649	$ 8,003	$ 8,615	$ 8,616	$ 6,676	$ 6,717	$ 7,842	$ 8,351	$ 33,883	$ 29,586	$ 16,303
Net income per share - basic (in dollars per share)	$ 0.36	$ 0.33	$ 0.36	$ 0.36	$ 0.27	$ 0.28	$ 0.33	$ 0.35	$ 1.41	$ 1.23	$ 0.68
Net income per share - diluted (in dollars per share)	$ 0.35	$ 0.33	$ 0.35	$ 0.36					$ 1.39	$ 1.23	$ 0.68