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Oil and natural gas properties (Details) (USD $) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Oil and Gas Property [Abstract] |  |  |  |
Discount rate used in calculating present value of future net cash flows from proved reserves |  | 10.00% |  | 10.00%
Write-down of oil and natural gas properties | $ 0 | $ 160,100,000 | $ 0 | $ 160,100,000
Write-down of oil and natural gas properties, after tax |  | 100,900,000 |  | 100,900,000
Effects of cash flow hedges not considered in calculating ceiling limitation, amount |  | 19,500,000 |  | 19,500,000
Effects of cash flow hedges not considered in calculating ceiling limitation amount, after tax |  | $ 12,300,000 |  | $ 12,300,000

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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 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=27015634&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 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 35 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28370743&loc=d3e66150-109466 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230 + Details
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