Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 99,207	$ 100,905	$ 96,187	$ 97,114	$ 99,631	$ 97,252	$ 96,784	$ 97,072	$ 393,413	$ 390,739	$ 391,150
Operating income	3,888	4,195	5,260	5,919	7,821	6,620	7,892	8,138	19,262	30,471	41,771
Net Income (Loss)	$ (435)	$ 105	$ 455	$ 975	$ 1,976	$ 1,516	$ 2,239	$ 2,368	$ 1,100	$ 8,099	$ 10,488
Earnings (Loss) per share - basic (in dollars per share)	$ (0.040)	$ 0.010	$ 0.040	$ 0.090	$ 0.190	$ 0.140	$ 0.210	$ 0.220	$ 0.10	$ 0.76	$ 1.01
Earnings (Loss) per share - diluted (in dollars per share)	$ (0.040)	$ 0.010	$ 0.040	$ 0.090	$ 0.170	$ 0.130	$ 0.200	$ 0.210	$ 0.10	$ 0.72	$ 0.95
Pension settlement losses	$ 1,700	$ 4,100	$ 1,400	$ 900
CAF Phase II [Member]
Operating revenues		$ 2,200