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Derivative Financial Instruments Gains and Losses (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative settlement (gain) loss |  | $ (21,234) | $ (329,478) | $ (512,566)
Net derivative (gain) loss |  | 26,414 | 250,633 | (408,831)
Oil Contracts [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative settlement (gain) loss |  | 31,176 | (243,102) | (362,219)
Net derivative (gain) loss |  | 71,502 | 85,370 | (191,165)
Gas Contracts [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative settlement (gain) loss | [1] | (87,857) | (94,936) | (123,180)
Net derivative (gain) loss |  | (76,315) | 81,060 | (189,734)
Gain on Early Settlement of Derivatives |  |  |  | 15,300
NGL Contracts [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative settlement (gain) loss |  | 35,447 | 8,560 | (27,167)
Net derivative (gain) loss |  | $ 31,227 | $ 84,203 | $ (27,932)
Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Derivative, Number of Instruments Held |  | 0 | 0 | 0
[1] | Gas derivative settlements for the year ended December 31, 2015, include $15.3 million of early settlements of futures contracts as a result of divesting assets in the Company’s Mid-Continent region.

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- Definition The number of derivative instruments of a particular group held by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959 + Details
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