Selected Quarterly Financial Information (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
Selected Quarterly Financial Information [Abstract]
Revenues	$ 331,582		$ 321,790		$ 327,785		$ 333,966		$ 347,341		$ 322,237		$ 287,829		$ 250,096		$ 1,315,123	$ 1,207,503	$ 870,671
Gross profit	(81,833)	[1]	95,921	[1]	(22,253)	[1]	95,912	[1]	96,192	[1]	94,306	[1]	85,780	[1]	76,078	[1]
Net income	$ (56,547)		$ 46,586		$ (19,302)		$ 52,439		$ 51,661		$ 53,360		$ 49,819		$ 41,027		$ 23,176	$ 195,867	$ 146,484
Basic	$ (1.18)	[2]	$ 0.97	[2]	$ (0.40)	[2]	$ 1.10	[2]	$ 1.08	[3]	$ 1.12	[3]	$ 1.05	[3]	$ 0.86	[3]	$ 0.48	$ 4.11	$ 3.10
Diluted	$ (1.18)		$ 0.97		$ (0.40)		$ 1.09		$ 1.08		$ 1.11		$ 1.04		$ 0.86		$ 0.48	$ 4.08	$ 3.09

[1] Gross profit excludes general and administrative expense, interest expense, gain (loss) on non-designated hedges and hedge ineffectiveness, and other income (loss).

[2] Plugging liability estimates were revised in March 2012 for updates in the cost of services used to plug wells over the preceding year. Although cost per well increased, a slight decrease in the inflation factor resulted in a decrease in estimated cost. Costs were reviewed each quarter resulting in no change to the March 2012 estimates.

[3] Due to the effect of rounding the basic earnings or diluted per share for the year’s four quarters does not equal annual earnings per share.