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Fair Value Measurements Fair Value Measurement (Details 2) (USD $) | 6 Months Ended | 3 Months Ended | 6 Months Ended |  | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  |
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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

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- Definition Amount of gain (loss) recognized in earnings, arising from liabilities measured at fair value on a recurring basis using unobservable inputs (level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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