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Impairment fair value and ceiling test (Details 2) - Oil and natural gas properties [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Sep. 30, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Percent of oil and natural gas production to be sold |  |  |  | 90.00%
Discontinued operation, provision for loss (gain) on disposal, before income tax | $ 356.1 | $ 400.0 |  |
Discontinued operation, provision for loss (gain) on disposal, net of tax | 224.4 | $ 252.0 |  |
Disposal group - estimated transaction costs | 3.0 |  |  | $ 3.0
Disposal group - exit and disposal costs | 11.0 |  |  | 11.0
Disposal group - potential future lease payments | $ 12.0 |  |  | $ 12.0
Impairment of oil and gas properties |  |  | $ 500.4 |
Impairment of oil and gas properties, after tax |  |  | $ 315.3 |

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- Definition Disposal group - exit and disposal costs. + References No definition available. + Details
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- Definition Disposal group - potential future lease payments. + 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 before tax of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51717284&loc=d3e1361-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=51716988&loc=d3e957-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=51716988&loc=SL51721525-107759 + Details
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- Definition Amount after tax of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=51716988&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=51716988&loc=SL51721525-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5C -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=51717284&loc=SL51721675-107760 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51717284&loc=d3e1361-107760 + Details
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