Fair Value Measurements (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014 A		Dec. 31, 2013		Dec. 31, 2012
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)	12.00%
Number of periods used for price assumptions of strip prices of liabilities	5
Proved Oil and Gas Property, Successful Effort Method	$ 7,348,436,000		$ 5,637,462,000
Unproved Oil and Gas Property, Successful Effort Method	532,498,000		271,100,000
Net Profit Plan liability [Member]
Liability measured at fair value using Level 3 inputs
Discount rate to calculate liability pools currently in payout (as a percent)	12.00%
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	2,600,000
Percent Change in Discount Rate for Sensitivity Analysis	1.00%
Sensitivity Analysis Change in Liability Due to Change in Discount Rate	1,100,000
Liability measured at fair value using Level 3 inputs
Net Profits Plan Liability: Beginning Balance	56,985,000		78,827,000		107,731,000
Net increase (decrease) in liability	(12,492,000)	[1]	3,527,000	[1]	(9,251,000)	[1]
Net settlements	(17,357,000)	[1],[2]	(25,369,000)	[1],[2]	(19,653,000)	[1],[2]
Transfers in (out) of Level 3	0		0		0
Net Profits Plan Liability: Ending balance	27,136,000		56,985,000		78,827,000
Cash Payments Made or Accrued under Profit Sharing Plan Related to Divested Property	8,300,000		10,300,000		2,300,000
6.625% Senior Notes Due 2019 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value	350,018,000		374,290,000
6.50% Senior Notes Due 2021 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value	343,000,000		373,625,000
6.125% Senior Notes Due 2022 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value	556,500,000	[3]	0	[3]
6.50% Senior Notes Due 2023 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value	379,000,000		422,000,000
5% Senior Notes Due 2024 [Member]
Liability measured at fair value using Level 3 inputs
Long-term Debt, Fair Value	$ 435,000,000		$ 475,315,000

[1] (1)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.

[2] (2)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 $8.3 million, $10.3 million, and $2.3 million relating to divestiture proceeds for the years ended December 31, 2014, 2013, and 2012, respectively.

[3] (1) The 2022 Notes were issued on November 17, 2014.