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PROPERTIES AND EQUIPMENT, NET (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
PROPERTIES AND EQUIPMENT, NET    
Proved oil and gas properties $ 6,522,753 $ 5,724,940
Unproved oil and gas properties 444,353 467,483
Gathering and pipeline systems 240,285 239,656
Land, building and other equipment 92,740 86,137
Properties and Equipment, gross, total 7,300,131 6,518,216
Accumulated depreciation, depletion and amortization (2,609,955) (2,207,239)
Properties and Equipment, Net $ 4,690,176 $ 4,310,977