Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								10 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, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Operating revenue	$ 46,582	$ 45,813	$ 43,857	$ 46,944	$ 39,564	$ 45,244	$ 40,108	$ 38,622	$ 10,783	$ 183,196	$ 163,538	$ 162,247
Operating income	5,090	4,557	4,487	5,252	3,645	6,342	5,002	4,666		19,386	19,655	20,968
Net Income	$ 2,525	$ 1,741	$ 1,738	$ 2,294	$ 1,574	$ 2,133	$ 2,348	$ 2,346	$ 939	$ 8,298	$ 8,401	$ 12,592
Basic earnings per share	$ 0.19	$ 0.13	$ 0.13	$ 0.17	$ 0.12	$ 0.16	$ 0.18	$ 0.18		$ 0.62	$ 0.63	$ 0.95
Diluted earnings per share	$ 0.19	$ 0.13	$ 0.13	$ 0.17	$ 0.12	$ 0.16	$ 0.18	$ 0.18	$ 0.07	$ 0.61	$ 0.63	$ 0.95
Dividends per share	$ 0.145	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.135	$ 0.135	$ 0.135		$ 0.565	$ 0.545	$ 0.525