Quarterly Operating Results - Unaudited (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 Operating Results - Unaudited Details
Operating Revenues	$ 27,079	$ 32,353	$ 27,401	$ 23,546	$ 23,300	$ 28,671	$ 26,102	$ 23,996	$ 110,379	$ 102,069	$ 102,735
Operating Income	5,717	10,843	7,210	3,877	4,179	8,516	6,738	4,768	27,647	24,201	26,597
Net Income	$ 2,726	$ 6,138	$ 3,725	$ 1,807	$ 2,049	$ 5,143	$ 3,625	$ 2,630	$ 14,396	$ 13,447	$ 14,330
Basic	$ 0.17	$ 0.39	$ 0.23	$ 0.11	$ 0.12	$ 0.33	$ 0.23	$ 0.17	$ 0.90	$ 0.85	$ 0.96
Diluted	$ 0.17	$ 0.39	$ 0.23	$ 0.11	$ 0.12	$ 0.32	$ 0.23	$ 0.17	$ 0.90	$ 0.84	$ 0.96