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Properties and Equipment, Net (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 8,531,171 $ 7,984,979
Unproved oil and gas properties 451,332 492,208
Gathering and pipeline systems 242,371 241,272
Land, building and other equipment 114,809 109,758
Properties and equipment, gross, total 9,339,683 8,828,217
Accumulated depreciation, depletion and amortization (4,207,028) (3,902,506)
Properties and equipment, net $ 5,132,655 $ 4,925,711