Quarterly Financial Data Quarterly Financial Data (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 Financial [Abstract]
Operating Revenues	$ 19,489	$ 24,461	$ 21,348	$ 18,540	$ 15,426	$ 20,628	$ 17,359	$ 15,989	$ 83,838	$ 69,402	$ 66,408
Operating Income (Loss)	3,534	7,961	5,199	3,727	3,235	5,054	4,580	3,538	20,421	16,407	14,577
Net Income	$ 1,593	$ 5,974	$ 4,163	$ 1,910	$ 1,828	$ 3,734	$ 3,470	$ 2,268	$ 13,640	$ 11,300	$ 9,798
Basic (in dollars per share)	$ 0.16	$ 0.69	$ 0.48	$ 0.22	$ 0.21	$ 0.43	$ 0.41	$ 0.26	$ 1.55	$ 1.31	$ 1.14
Diluted (in dollars per share)	$ 0.16	$ 0.67	$ 0.47	$ 0.22	$ 210.00	$ 0.42	$ 0.40	$ 0.26	$ 1.53	$ 1.29	$ 1.13