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Summary of Significant Accounting Policies (Details 4) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008 | Mar. 31, 2009
Natural gas and oil properties [Abstract] |  |  |  |  |
Discount rate used in calculating present value of future net cash flows from proved reserves (in hundredths) | 10.00% |  |  |  |
Noncash write-down of natural gas and oil properties | $ 0 | $ 0 | $ 620,000,000 |  |
Noncash write-downs of natural gas and oil production properties, after tax |  |  | 384,400,000 |  |
Additional write-downs of its natural gas and oil properties that would have been recognized without effects of cash flow hedges |  |  |  |  | 107,900,000
Additional write-downs of its natural gas and oil properties that would have been recognized without effects of cash flow hedges, after tax |  |  |  |  | 66,900,000
Natural gas and oil properties not subject to amortization [Abstract] |  |  |  |  |
Acquisition | 185,773,000 |  |  |  |
Development | 9,938,000 |  |  |  |
Exploration | 27,439,000 |  |  |  |
Capitalized interest | 9,312,000 |  |  |  |
Total costs not subject to amortization | 232,462,000 | 182,402,000 | 178,214,000 |  |
Acquisition Costs, Period Cost | 50,721,000 | 71,315,000 | 988,000 | 62,749,000 |
Development Costs, Period Cost | 9,689,000 | 156,000 | 2,000 | 91,000 |
Exploration Costs, Period Cost | 24,389,000 | 2,710,000 | 72,000 | 268,000 |
Interest Costs, Capitalized During Period | 3,539,000 | 3,096,000 | 44,000 | 2,633,000 |
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost | $ 88,338,000 | $ 77,277,000 | $ 1,106,000 | $ 65,741,000 |
Time period for evaluating the majority of costs not subject to amortization (in years) | five years |  |  |  |

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- Definition The capitalized costs incurred during the period (excluded from amortization) to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 4 -Section 10 -Paragraph c -Subparagraph 3(ii)(A) Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(c)(7)(ii)) -URI http://asc.fasb.org/extlink&oid=6949603&loc=d3e511914-122862 + Details
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