Selected Quarterly Financial Data (Unaudited) (Details) (USD $)	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
Income Tax Reconciliation, Tax Credits, Research	$ 7,200,000
Impairment of oil and natural gas properties																	79,000		625,564,000		0
Operating Revenues	259,750,000		244,369,000		229,221,000		241,492,000		240,600,000		238,763,000		242,709,000		197,486,000		974,832,000		919,558,000
Net (Loss) Earnings	38,499,000		18,126,000		81,016,000		33,898,000		(414,733,000)		134,001,000		105,166,000		(52,497,000)		171,539,000		(228,063,000)		82,524,000
Basic net earnings (loss) per share	$ 0.70	[1],[2],[3]	$ 0.33	[1],[2]	$ 1.47	[1],[2]	$ 0.62	[1],[2]	$ (7.62)	[1],[2],[4]	$ 2.45	[1],[2]	$ 1.93	[1],[2]	$ (0.98)	[1],[2]	$ 3.11	[1],[2]	$ (4.21)	[1],[2]	$ 1.54
Diluted net earnings (loss) per share	$ 0.69	[1],[2],[3]	$ 0.33	[1],[2]	$ 1.46	[1],[2]	$ 0.61	[1],[2]	$ (7.62)	[1],[2],[4]	$ 2.42	[1],[2]	$ 1.90	[1],[2]	$ (0.98)	[1],[2]	$ 3.09	[1],[2]	$ (4.21)	[1],[2]	$ 1.52
Oil and Gas Properties | Fair Value, Measurements, Nonrecurring | East Texas
Impairment of oil and natural gas properties									$ 625,000,000

[1] (4)The sum of the individual quarterly net earnings (loss) per common share amounts may not agree with year-to-date net earnings (loss) per common share as each quarterly computation is based on the weighted average number of common shares outstanding during that period. Potentially dilutive securities were included in the computation of diluted net earnings (loss) per common share for each quarter in which the Company reported net earnings.

[2] (3)Due to the volatility of commodity prices, the estimated fair value of the Company's commodity derivative instruments is subject to fluctuations. As a result, the Company may recognize significant unrealized gains and losses (non-cash changes in fair value) on commodity derivative instruments from period to period.

[3] (1)In the fourth quarter of 2012, the Company recorded a $7.2 million income tax benefit for research and development tax credits

[4] (2)In the fourth quarter of 2011, the Company recorded an impairment of $625.0 million related to its E. Texas natural gas assets, largely due to the impact of lower natural gas prices.