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Acquisitions and Divestitures - Arena Acquisition - Final Valuation of Assets Acquired and Liabilities Assumed (Parenthetical) (Details) (Arena Acquisition, USD $)
In Thousands, unless otherwise specified
 0 Months Ended
Jul. 16, 2010
Business Acquisition [Line Items]  
Weighted average commodity prices, basis of price determination The prices utilized were based upon forward commodity strip prices, as of July 16, 2010, for the first four years and escalated for inflation at a rate of 2.5% annually beginning with the fifth year through the end of production, which was more than 50 years.
Commodity prices period valuation before escalation 4 years
Annual escalation factor for forward commodity strip prices beginning the fifth year through end of production 2.50%
Fair value allocated to oil and natural gas properties attributed to oil reserves 91.00%
Purchase price allocation, net deferred tax liability $ 454,500
Oil
  
Business Acquisition [Line Items]  
Commodity average price (in dollars per bbl for oil/dollars per mcf for natural gas) 105.58
Natural Gas
  
Business Acquisition [Line Items]  
Commodity average price (in dollars per bbl for oil/dollars per mcf for natural gas) 8.56