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Derivative Instruments (Net (Loss) Gain on Derivative Instruments) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Derivative [Line Items] |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | $ (16,529) | $ 99,577
Natural gas derivatives |  |
Derivative [Line Items] |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | (9,696) | 86,277
Oil derivatives |  |
Derivative [Line Items] |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | (9,147) | 10,905
Natural gas liquids derivatives |  |
Derivative [Line Items] |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | $ 2,314 | $ 2,395

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- Definition Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 + Details
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