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Derivative Instruments (Net Gain (Loss) on Derivative Instruments) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Derivative [Line Items] |  |  |  |
Net gain (loss) on natural gas, oil, and NGL derivatives | $ 64,871 | $ (137,392) | $ 164,448 | $ (79,657)
Natural gas derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net gain (loss) on natural gas, oil, and NGL derivatives | 56,668 | (133,621) | 142,945 | (76,621)
Oil derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net gain (loss) on natural gas, oil, and NGL derivatives | 8,143 | (2,628) | 19,048 | (1,346)
Natural gas liquids derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Net gain (loss) on natural gas, oil, and NGL derivatives | $ 60 | $ (1,143) | $ 2,455 | $ (1,690)

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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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