Fair Value Measurements Fair Value Measurement (Details 2) (USD $)	6 Months Ended	3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Net Profit Plan liability [Member]	Jun. 30, 2013 Net Profit Plan liability [Member]		Dec. 31, 2012 Net Profit Plan liability [Member]	Jun. 30, 2013 Net Profits Plan [Member]	Jun. 30, 2012 Net Profits Plan [Member]	Jun. 30, 2013 Net Profits Plan [Member]	Jun. 30, 2012 Net Profits Plan [Member]	Jun. 30, 2013 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 6.625% Senior Notes Due 2019 [Member]	Jun. 30, 2013 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2021 [Member]	Jun. 30, 2013 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2023 [Member]	Jun. 30, 2013 5.0% Senior Notes Due 2024 [Member]
Liabiliity measured at fair value using Level 3 inputs
Long-term Debt, Fair Value										$ 368,592,000	$ 371,875,000	$ 367,500,000	$ 371,070,000	$ 419,000,000	$ 424,200,000	$ 473,125,000	[1]
Net Profits Plan
Discount Rate Used to Calculate Currently in Payout Liabilities		12.00%	12.00%
Discount Rate Used to Calculate Not Currently in Payout Liabilities		15.00%	15.00%
Period Used for Price Assumptions of Strip Prices of Liabilities			1
Percent Change in Commodity Prices for Sensitivity Analysis			5.00%
Sensitivity Analysis Change in Liability, Due to Change in Commodity Prices		6,000,000	6,000,000
Percent Change in Discount Rate for Sensitivity Analysis			1.00%
Sensitivity Analysis Decrease in Liability Due to Change in Discount Rate		3,000,000	3,000,000
Cash Payments Made or Accrued under Profit Sharing Plan Related to Divested Property						2,600,000	1,400,000	2,600,000	1,700,000
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance		71,464,000	71,464,000		78,827,000
Net increase in liability			3,147,000	[2]
Net settlements			(10,510,000)	[2],[3]
Transfers in (out) of Level 3		$ 0
Proved Oil and Gas Properties
Discount Rate Used for Fair Value of Oil and Gas Properties	12.00%
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast	5		5

[1] The 2024 Notes were issued on May 20, 2013.

[2] Net changes in the Company’s Net Profits Plan liability are shown in the Change in Net Profits Plan liability line item of the accompanying statements of operations

[3] Settlements represent cash payments made or accrued under the Net Profits Plan. The Company accrued or made cash payments under the Net Profits Plan relating to divestiture proceeds of $2.6 million for the six months ended June 30, 2013