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PROPERTIES AND EQUIPMENT, NET (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
PROPERTIES AND EQUIPMENT, NET    
Proved oil and gas properties $ 6,245,196 $ 5,724,940
Unproved oil and gas properties 458,047 467,483
Gathering and pipeline systems 240,062 239,656
Land, building and other equipment 90,690 86,137
Property, plant and equipment, gross, total 7,033,995 6,518,216
Accumulated depreciation, depletion and amortization (2,475,788) (2,207,239)
Properties and Equipment, Net $ 4,558,207 $ 4,310,977