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Natural Gas and Oil Properties (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Jun. 30, 2012
Sep. 30, 2013
West Texas Intermediate Oil [Member]
Sep. 30, 2012
West Texas Intermediate Oil [Member]
Sep. 30, 2013
Henry Hub Natural Gas [Member]
Sep. 30, 2012
Henry Hub Natural Gas [Member]
Discount rate of natural gas and oil reserves 10.00%            
Period of time needed to calculate ceiling value of reserves 12 months            
Full cost ceiling test, Price       91.56 91.48 3.60 2.83
Cash flow hedges impact on ceiling value $ 79.2 $ 330.6          
Net capitalized costs of our natural gas and oil properties in excess of ceiling value   $ 185.7 $ 496.4