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Property and Equipment (Details 1) (USD $)
12 Months Ended 48 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2008
Summary of unevaluated properties excluded from oil and natural gas properties being amortized        
Property acquisition costs $ 12,543,000 $ 560,314,000 $ 94,969,000 $ 49,566,000
Exploration and development 270,062,000 86,251,000 3,758,000 6,682,000
Capitalized interest 44,853,000 20,958,000 3,228,000 3,922,000
Total 327,458,000 667,523,000 101,955,000 60,170,000
Property acquisition costs 717,392,000      
Exploration and development 366,753,000      
Capitalized interest 72,961,000      
Total $ 1,157,106,000 $ 870,130,000