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Derivative Instruments - Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative [Line Items] |  |  |
Gain due to changes in fair value of derivative instruments | $ 213,679 | $ (174,506) | $ 203,513
Natural gas derivatives |  |  |
Derivative [Line Items] |  |  |
Gain due to changes in fair value of derivative instruments | 232,143 | (165,933) | 182,993
Oil derivatives |  |  |
Derivative [Line Items] |  |  |
Gain due to changes in fair value of derivative instruments | (3,350) | (5,387) | 19,201
Natural gas liquids derivatives |  |  |
Derivative [Line Items] |  |  |
Gain due to changes in fair value of derivative instruments | $ (15,114) | $ (3,186) | $ 1,319

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- Definition Amount of gain (loss)recognized in the income statement from the hedging ineffectiveness of a fair value hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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