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Supplemental Financial Information Exploration and Production Details 2 (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] |  |  |  |  |  |
Costs Incurred, Acquisition of Oil and Gas Properties with Proved Reserves | $ 3,999 | [1] | $ 89,733 | [1] | $ 3,879 | [1]
Costs Incurred, Acquisition of Unproved Oil and Gas Properties | 63,354 | [1] | 92,100 | [1] | 8,771 | [1]
Costs Incurred, Exploration Costs | 41,775 | [1] | 33,226 | [1] | 33,123 | [1]
Costs Incurred, Development Costs | 161,647 | [1] | 139,733 | [1] | 135,202 | [1]
Capital Expenditures Related to Oil and Natural Gas Producing Activities | 270,775 | [1] | 354,792 | [1] | 180,975 | [1]
Net change in property, plant and equipment related to future liabilitie for asset retirement obligations that are excluded | $ (1,800) |  | $ 11,100 |  | $ 2,000 |
[1] | Excludes net additions/(reductions) to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of natural gas and oil wells, as discussed in Note 10, of $(1.8) million, $11.1 million and $2.0 million for the years ended December 31, 2011, 2010 and 2009, respectively.

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- Definition Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=6821537&loc=d3e62038-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 18 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6821537&loc=d3e62014-109447 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59P -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. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59Q -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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