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Supplemental Oil And Gas Reserve Information - Unaudited (Narrative) (Details) (USD $)
 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2011
MBoe
Dec. 31, 2010
MBoe
Dec. 31, 2009
MBoe
Dec. 31, 2008
Nov. 30, 2010
Roosevelt County [Member]
Dec. 31, 2010
Richland County [Member]
Dec. 31, 2009
Kerogen [Member]
Dec. 31, 2009
Fidelity [Member]
Dec. 31, 2011
MBoe [Member]
Dec. 31, 2010
MBoe [Member]
Dec. 31, 2009
MBoe [Member]
Reserve Quantities [Line Items]                      
Purchases of reserves in place         715 764 1,511 484 111 1,479 1,995
Extensions, discoveries and other additions 43,168               43,168 24,473 8,950
Estimated extensions and discoveries of reserves associated with new wells 12,696 8,122 1,508                
Percentage of reserves produced from wells in certain formations 100.00% 99.00% 95.00%                
Percentage of proved undeveloped reserves 100.00% 100.00% 100.00%                
Additional proved undeveloped reserves 30,472 16,351 7,442                
Number of traded interests in non-operated properties   3                  
Remaining reserves                 (142) (123) (5)
Net positive revisions of previous estimates                 (288) 2,552 767
Percentage of revisions due to decrease in oil prices 0.72%                    
Percentage of revisions due to increase in oil prices   29.00%                  
Percentage of revisions due to higher working interests in proved wells 29.00%                    
Unweighted arithmetic average first-day-of-the-month prices for oil, price per Bbl $ 96.23 $ 79.40 $ 61.04                
Market price of oil       $ 44.60              
Unweighted arithmetic average first-day-of-the-month prices for natural gas, price per MMBtu $ 4.12 $ 4.38 $ 3.87                
Percentage of revisions to estimates due to other changes   42.00%                  
Discount rate of future cash flows 10.00%