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
The summary of condensed quarterly results of operations for the years ended December
Operating revenues	$ 177,224	$ 170,857	$ 160,554	$ 138,051	$ 138,434	$ 145,576	$ 135,976	$ 119,640	$ 62,931	$ 35,215	$ 43,325
Gain (Loss) on Disposition of Assets	(306)	(701)	679	11	113	(638)	77	146	(317)	(302)	239
Earnings before income taxes	8,889	11,440	10,090	3,630	5,363	5,969	1,206	(4,452)	34,049	8,086	14,593
Net earnings	$ 3,441	$ 6,384	$ 6,054	$ 2,178	$ 3,218	$ 3,582	$ 723	$ (2,671)	$ 18,057	$ 4,852	$ 7,117
Net earnings per share
Basic	$ 0.22	$ 0.42	$ 0.40	$ 0.14	$ 0.21	$ 0.23	$ 0.05	$ (0.17)
Diluted	$ 0.22	$ 0.41	$ 0.39	$ 0.14	$ 0.20	$ 0.23	$ 0.05	$ (0.17)