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Fair Value Measurements (Details) (USD $) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 Recurring basis Level 1 | Dec. 31, 2012 Recurring basis Level 1 | Dec. 31, 2013 Recurring basis Level 1 Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Recurring basis Level 1 Not Designated as Hedging Instrument [Member] | Dec. 31, 2013 Recurring basis Level 2 | Dec. 31, 2012 Recurring basis Level 2 | Dec. 31, 2013 Recurring basis Level 2 Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Recurring basis Level 2 Not Designated as Hedging Instrument [Member] | Dec. 31, 2013 Recurring basis Fair Value, Inputs, Level 3 [Member] | Dec. 31, 2012 Recurring basis Fair Value, Inputs, Level 3 [Member] | Dec. 31, 2013 Recurring basis Fair Value, Inputs, Level 3 [Member] Not Designated as Hedging Instrument [Member] | Dec. 31, 2012 Recurring basis Fair Value, Inputs, Level 3 [Member] Not Designated as Hedging Instrument [Member] | Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 1 | Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 1 | Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Level 2 | Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Level 2 | Dec. 31, 2013 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member] | Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member] | Mar. 31, 2011 Net Profit Plan liability [Member] | Dec. 31, 2013 Net Profit Plan liability [Member]
Financial assets and liabilities measured at fair value on a recurring basis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent Decrease in Discount Rate for Sensitivity Analysis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00%
Payment to Settle, Net Profit Plan Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,600,000 |
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  |  |  |  |  |  | 0 | [1] | 0 | [1] |  |  |  |  | 52,510,000 | [1] | 54,339,000 | [1] |  |  |  |  | 0 | [1] | 0 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 52,510,000 | 54,339,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proved oil and gas properties | 5,637,462,000 | 5,401,684,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [2] | 0 | [2] | 0 | [2] | 0 | [2] | 62,178,000 | [2] | 209,959,000 | [2] |  |
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value | 271,100,000 | 175,287,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [2] | 0 | [2] | 0 | [2] | 0 | [2] | 3,280,000 | [2] | 42,765,000 | [2] |  |
Oil and gas properties held for sale, net of accumulated depletion, depreciation and amortization of $7,390 in 2013 and $20,676 in 2012 | 19,072,000 | 33,620,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [2] |  |  | 0 | [2] |  |  | 650,000 | [2] |  |  |  |
Liabilities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Liability, Fair Value, Gross Liability |  |  |  |  |  |  | 0 | [1] | 0 | [1] |  |  |  |  | 31,020,000 | [1] | 15,644,000 | [1] |  |  |  |  | 0 | [1] | 0 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Profits Plan liability | $ 56,985,000 | $ 78,827,000 | $ 0 | [1] | $ 0 | [1] |  |  |  |  | $ 0 | [1] | $ 0 | [1] |  |  |  |  | $ 56,985,000 | [1] | $ 78,827,000 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | This represents a financial asset or liability that is measured at fair value on a recurring basis.
[2] | This represents a non-financial asset that is measured at fair value on a nonrecurring basis.

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- Definition Long-lived assets that are held for sale apart from normal operations and anticipated to be sold in less than one year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 144 -Paragraph 46 -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 Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph a -Clause 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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959 + Details
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- Definition Oil and Gas properties with proved reserves under the successful effort method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Glossary Proved Properties -URI http://asc.fasb.org/extlink&oid=6522428 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6474861&loc=d3e64746-109465 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 11 -Subparagraph a -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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