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Properties and Equipment, Net (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Properties and Equipment, Net    
Proved oil and gas properties $ 6,676,002 $ 6,362,570
Unproved oil and gas properties 365,186 375,428
Gathering and pipeline systems 239,925 239,958
Land, building and other equipment 97,082 94,243
Properties and Equipment, gross, total 7,378,195 7,072,199
Accumulated depreciation, depletion and amortization (2,667,626) (2,525,972)
Properties and Equipment, Net $ 4,710,569 $ 4,546,227