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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Results of Operations for Producing Activities (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) usd_per_mcf | Dec. 31, 2016 USD ($) usd_per_mcf | Dec. 31, 2015 USD ($) usd_per_mcf
Results of Operations for Oil and Gas Producing Activities [Line Items] |  |  |
Revenues | $ 1,320,303,000 | $ 385,910,000 | $ 708,990,000
Production costs | (350,367,000) | (248,125,000) | (222,805,000)
Depletion | (358,792,000) | (243,098,000) | (335,288,000)
Impairment | 0 | (715,495,000) | (1,440,418,000)
Results of operations, before income taxes | 611,144,000 | (820,808,000) | (1,289,521,000)
Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |  |
Current | 3,362,000 | 0 | 0
Deferred | (3,602,000) | 0 | (220,201,000)
Income tax expense | (240,000) | 0 | (220,201,000)
Results of operations from producing activities | $ 611,384,000 | $ (820,808,000) | $ (1,069,320,000)
Depletion per Mcf of gas equivalent (usd per Mcfe) | usd_per_mcf | 0.90 | 0.92 | 1.68
Investment in Grizzly Oil Sands ULC |  |  |
Results of Operations for Oil and Gas Producing Activities [Line Items] |  |  |
Revenues | $ 0 | $ 0 | $ 1,436,000
Production costs | 0 | (13,000) | (1,549,000)
Depletion | 0 | 0 | (625,000)
Results of operations, before income taxes | 0 | (13,000) | (738,000)
Income Tax Expense (Benefit), Continuing Operations [Abstract] |  |  |
Income tax expense | 0 | 0 | 0
Results of operations from producing activities | $ 0 | $ (13,000) | $ (738,000)

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- Definition [Line Items] for Results of Operations for Oil and Gas Producing Activities [Table] + References No definition available. + 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 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2921-110230 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=66906256&loc=d3e66150-109466 Reference 3: 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=98514028&loc=d3e3602-108585 + Details
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- Definition Depreciation, depletion and amortization, and valuation provisions (impairments) related to oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 + Details
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- Definition Income before income taxes for oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 + Details
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- Definition Production (lifting) costs from oil and gas producing activities, including but not limited to lease operating expense, production and ad valorem taxes, and transportation expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 + Details
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- Definition Total revenues from oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62246-109447 + Details
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