| **Derivative Financial Instruments (Tables)**                                                                                                                | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
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| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| Schedule of Other Derivatives Not Designated as Hedging Instruments, Statements of Financial Performance and Financial Position, Location [Table Text Block] | The following table presents the balance sheet location and fair value amounts of our derivative instruments on the condensed consolidated balance sheets:     Fair ValueDerivative instruments: Condensed consolidated balance sheet line item June 30, 2017 December 31, 2016     (in thousands)Derivative assets:Current       Commodity derivative contracts  Fair value of derivatives $49,540 $8,490 Basis protection derivative contracts  Fair value of derivatives 2,565 301     52,105 8,791 Non-current       Commodity derivative contracts  Fair value of derivatives 15,051 1,123 Basis protection derivative contracts  Fair value of derivatives 1,346 1,263     16,397 2,386Total derivative assets   $68,502 $11,177        Derivative liabilities:Current       Commodity derivative contracts  Fair value of derivatives $9,943 $53,565 Basis protection derivative contracts  Fair value of derivatives 195 30     10,138 53,595 Non-current       Commodity derivative contracts  Fair value of derivatives 2,311 27,595     2,311 27,595Total derivative liabilities   $12,449 $81,190    The following table presents the impact of our derivative instruments on our condensed consolidated statements of operations:  Three Months Ended June 30, Six Months Ended June 30,Condensed consolidated statement of operations line item 2017 2016 2017 2016  (in thousands)Commodity price risk management gain, net         Net settlements $12,015 $53,301 $12,566 $120,132Net change in fair value of unsettled derivatives 45,917 (146,102) 126,070 (201,877)Total commodity price risk management gain, net $57,932 $(92,801) $138,636 $(81,745)         Net settlements of commodity derivatives decreased for the three and six months ended June 30, 2017 as compared to the three and six months ended June 30, 2016.  We entered into agreements for the derivative instruments that settled throughout 2016 prior to commodity prices becoming depressed in late 2014.  Substantially all of these higher-value agreements settled by the end of 2016.  Net settlements for the three and six months ended June 30, 2017 reflect derivative instruments entered into since 2015, which more closely approximate recent realized prices.  Based upon the forward strip pricing at June 30, 2017, we expect that settlements will continue to be substantially lower in 2017 on a relative basis as compared to those in 2016. All of our financial derivative agreements contain master netting provisions that provide for the net settlement of all contracts through a single payment in the event of early termination.  We have elected not to offset the fair value positions recorded on our condensed consolidated balance sheets.The following table reflects the impact of netting agreements on gross derivative assets and liabilities:As of June 30, 2017 Derivative instruments, recorded in condensed consolidated balance sheet, gross Effect of master netting agreements Derivative instruments, net  (in thousands)Asset derivatives:      Derivative instruments, at fair value $68,502 $(10,974) $57,528       Liability derivatives:      Derivative instruments, at fair value $12,449 $(10,974) $1,475       As of December 31, 2016 Derivative instruments, recorded in condensed consolidated balance sheet, gross Effect of master netting agreements Derivative instruments, net  (in thousands)Asset derivatives:      Derivative instruments, at fair value $11,177 $(10,930) $247       Liability derivatives:      Derivative instruments, at fair value $81,190 $(10,930) $70,260        |

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| - References
No definition available.
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| - Definition
Tabular disclosure for other derivative instruments not designated as hedging instruments of (a) the location and amount of gains and losses reported in the statement of financial performance and (b) the location and fair value amounts of the instruments reported in the statement of financial position.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959
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