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Natural Gas and Oil Properties (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Discount rate of natural gas and oil reserves 10.00%  
Period of time needed to calculate ceiling value of reserves 12 months  
Cash flow hedges impact on ceiling value $ 150.8 $ 354.8
Net capitalized costs of our natural gas and oil properties in excess of ceiling value   $ 496.4
West Texas Intermediate Oil [Member]
   
Full cost ceiling test, Price 88.13 92.17
Henry Hub Natural Gas [Member]
   
Full cost ceiling test, Price 3.44 3.15