Fair Value Measurements (Details 2) (USD $)			3 Months Ended	12 Months Ended											12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2011 Net Profit Plan liability [Member]	Dec. 31, 2012 Net Profit Plan liability [Member] A		Dec. 31, 2011 Net Profit Plan liability [Member]		Dec. 31, 2010 Net Profit Plan liability [Member]		Dec. 31, 2008 Net Profit Plan liability [Member]	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2012 Net Profits Plan [Member]	Dec. 31, 2011 Net Profits Plan [Member]	Dec. 31, 2010 Net Profits Plan [Member]	Dec. 31, 2012 Convertible Debt [Member]		Dec. 31, 2011 Convertible Debt [Member]		Dec. 31, 2010 Convertible Debt [Member]	Dec. 31, 2012 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2011 6.625% Senior Notes Due 2019 [Member]	Feb. 07, 2011 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2010 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2011 6.50% Senior Notes Due 2021 [Member]	Nov. 09, 2011 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2010 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2023 [Member]		Jun. 29, 2012 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2011 6.50% Senior Notes Due 2023 [Member]		Dec. 31, 2010 6.50% Senior Notes Due 2023 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value																		$ 0	[1],[2]	$ 394,068,000	[1],[2]		$ 371,875,000	$ 359,408,000			$ 371,070,000	$ 360,283,000			$ 424,200,000	[3]		$ 0	[3]
Debt Instrument, Interest Rate, Stated Percentage																		3.50%		3.50%		3.50%	6.625%	6.625%	6.625%	0.00%	6.50%	6.50%	6.50%	0.00%	6.50%		6.50%	0.00%		0.00%
Discount rate to calculate liability pools currently in payout (as a percent)				12.00%
Discount rate to calculate liability pools not currently in payout (as a percent)				15.00%
Number of periods used for price assumptions of strip prices of liabilities				5
Period used for price assumptions of strip prices of liabilities (in years)				1
Percentage change in commodity prices (as a percent)				5.00%
Change in liability due to change in commodity prices by 5 percent				7,000,000
Percentage increase in discount rate (as a percent)				1.00%
Increase in liability due to a 1 percent decrease in discount rate				4,000,000
Decrease in liability due to a 1 percent increase in discount rate				3,000,000
Liability measured at fair value using Level 3 inputs
Net Profits Plan Liability: Beginning Balance			135,850,000	107,731,000		135,850,000				170,291,000
Net increase (decrease) in liability				(9,251,000)	[4]	2,269,000	[4]	14,063,000	[4]
Net settlements				(19,653,000)	[4],[5],[6]	(30,388,000)	[4],[5],[6]	(48,504,000)	[4],[5],[6]
Transfers in (out) of Level 3				0		0		0
Net Profits Plan Liability: Ending balance				78,827,000		107,731,000		135,850,000		170,291,000
Amount of cash payments related to divested properties															2,300,000	6,300,000	26,100,000
Amount paid for settlement of liabilities			2,600,000
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)	12.00%
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	$ 175,287,000	$ 120,966,000									$ 42,765,000	[7]	$ 15,809,000	[7]

[1] The 3.50% Senior Convertible Notes were settled during the second quarter of 2012. Please refer to Note 5 - Long-term Debt for additional discussion.

[2] The estimated fair value of the embedded contingent interest derivative was immaterial as of December 31, 2011.

[3] The 2023 Notes were issued on June 29, 2012.

[4] Net changes in the Net Profits Plan liability are shown in the Change in Net Profits Plan liability line item of the accompanying statements of operations.

[5] Settlements represent cash payments made or accrued under the Net Profits Plan. The amounts in the table include cash payments made or accrued under the Net Profits Plan of $2.3 million, $6.3 million, and $26.1 million relating to divestiture proceeds for the years ended December 31, 2012, 2011, and 2010 respectively.

[6] During 2011, the Company elected to cash out several Net Profits Plan pools with a $2.6 million direct payment. As a result, the Company reduced its Net Profits Plan liability by that amount. There was no impact on the accompanying statements of operations for the period ended December 31, 2011, related to these settlements.

[7] This represents a non-financial asset that is measured at fair value on a nonrecurring basis.