Quarterly Financial Information (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 Financial Information (Unaudited)
Operating revenues	$ 99,631	$ 97,252	$ 96,784	$ 97,072	$ 100,506	$ 97,682	$ 96,997	$ 95,965	$ 390,739	$ 391,150	$ 385,498
Operating income	7,821	6,620	7,892	8,138	10,046	7,914	15,226	8,585	30,471	41,771	45,856
Net income	$ 1,976	$ 1,516	$ 2,239	$ 2,368	$ 2,628	$ 2,061	$ 3,951	$ 1,848	$ 8,099	$ 10,488	$ 109,982
Basic	$ 0.190	$ 0.140	$ 0.210	$ 0.220	$ 0.250	$ 0.200	$ 0.380	$ 0.180	$ 0.76	$ 1.01	$ 10.74
Diluted	$ 0.170	$ 0.130	$ 0.200	$ 0.210	$ 0.230	$ 0.180	$ 0.360	$ 0.170	$ 0.72	$ 0.95	$ 10.32