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Derivative Instruments (Net Gain (Loss) on Derivative Instruments) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2017 | Mar. 31, 2016
Derivative [Line Items] |  |
Net gain on gas, oil, and NGL derivatives | $ 99,577 | $ 57,735
Natural gas derivatives |  |
Derivative [Line Items] |  |
Net gain on gas, oil, and NGL derivatives | 86,277 | 57,000
Oil derivatives |  |
Derivative [Line Items] |  |
Net gain on gas, oil, and NGL derivatives | 10,905 | 1,282
Natural gas liquids derivatives |  |
Derivative [Line Items] |  |
Net gain on gas, oil, and NGL derivatives | $ 2,395 | $ (547)

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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=64837297&loc=SL5618551-113959 + Details
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