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Derivative Instruments (Net (Loss) Gain on Derivative Instruments) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Derivative [Line Items] |  |  |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | $ (22,860) | $ 35,281 | $ 141,588 | $ (44,376)
Natural gas derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | (7,077) | 33,167 | 135,868 | (43,454)
Oil derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | (6,571) | 1,708 | 12,477 | 362
Natural gas liquids derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net (loss) gain on natural gas, oil, and NGL derivatives | $ (9,212) | $ 406 | $ (6,757) | $ (1,284)

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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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