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Oil and natural gas properties (Details 6) (USD $) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Sep. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 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% |  | 10.00% |  |  |
NYMEX oil price | 94.71 | 94.97 | 94.71 |  |  |
Henry Hub natural gas price | 2.76 | 2.83 | 2.76 |  |  |
Write-downs of oil and natural gas properties | $ 231,700,000 | $ 160,100,000 | $ 391,800,000 | $ 0 | $ 0 |
Noncash write-downs of oil and natural gas properties, after tax | 145,900,000 | 100,900,000 | 246,800,000 |  |  |
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges | 20,800,000 | 19,500,000 | 20,800,000 |  |  |
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges, after tax | 13,100,000 | 12,300,000 | 13,100,000 |  |  |
Oil and natural gas properties not subject to amortization [Abstract] |  |  |  |  |  |
Acquisition | 144,521,000 |  | 144,521,000 |  |  |
Development | 7,415,000 |  | 7,415,000 |  |  |
Exploration | 36,246,000 |  | 36,246,000 |  |  |
Capitalized interest | 3,612,000 |  | 3,612,000 |  |  |
Total costs not subject to amortization | 191,794,000 |  | 191,794,000 | 232,462,000 | 182,402,000 |
Acquisition costs, period cost |  |  | 26,318,000 | 38,186,000 | 22,142,000 | 57,875,000
Development costs, period cost |  |  | 6,858,000 | 399,000 | 77,000 | 81,000
Exploration costs, period cost |  |  | 34,407,000 | 856,000 | 643,000 | 340,000
Capitalized interest costs, period costs |  |  | 1,297,000 | 757,000 | 439,000 | 1,119,000
Capitalized costs of unproved properties excluded from amortization, period cost |  |  | $ 68,880,000 | $ 40,198,000 | $ 23,301,000 | $ 59,415,000
Time period for evaluating the majority of costs not subject to amortization (in years) |  |  | five years |  |  |

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- Definition Discount rate used in calculating the present value of future net cash flows from proved reserves as mandated under the rules of the SEC. + References No definition available. + Details
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- Definition The dollar amount that would have been charged to income after tax had the effects of the cash flow hedges not been considered in calculating the ceiling limitation at an oil and gas producing company subject to the full cost rules. + References No definition available. + Details
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- Definition The after tax expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced. + References No definition available. + Details
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- Definition Time period for evaluating the majority of costs not subject to amortization and including the costs in the amortization base as the properties are evaluated and/or developed. + References No definition available. + Details
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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 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=21918352&loc=d3e511914-122862 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph c -Subparagraph 3(ii)(A) -Article 4 + Details
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- Definition The sum of the capitalized costs incurred, as of the balance sheet date, of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs. + References Reference 1: 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=21918352&loc=d3e511914-122862 + Details
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- Definition The capitalized costs incurred during the period (excluded from amortization) to obtain access to proved reserves and to provide facilities for extracting, treating, gathering and storing the oil and gas. + References Reference 1: 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=21918352&loc=d3e511914-122862 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph c -Subparagraph 3(ii)(A) -Article 4 + Details
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- Definition The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, to obtain access to proved reserves and to provide facilities for extracting, treating, gathering and storing the oil and gas. + References Reference 1: 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=21918352&loc=d3e511914-122862 + Details
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- Definition Amount that would have been charged to income had the effects of the cash flow hedges not been considered in calculating the ceiling limitation at an oil and gas producing company subject to the full cost rules. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 12.D.3(b)) -URI http://asc.fasb.org/extlink&oid=6601679&loc=d3e516453-122869 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 12 -Section D -Paragraph b -Subsection 3 + Details
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- Definition The capitalized costs incurred during the period (excluded from amortization) in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. Exploration costs also include costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, the maintenance of land and lease records, dry hole contributions and bottom hole contributions, costs of drilling and equipping exploratory wells and costs of drilling exploratory-type stratigraphic test wells. + References Reference 1: 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=21918352&loc=d3e511914-122862 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 10 -Paragraph a -Subparagraph 15 -Article 4 + Details
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