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Properties and Equipment, Net (Details) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 8,701,932 $ 7,984,979
Unproved oil and gas properties 415,355 492,208
Gathering and pipeline systems 243,089 241,272
Land, building and other equipment 116,801 109,758
Properties and equipment, gross, total 9,477,177 8,828,217
Accumulated depreciation, depletion and amortization (4,335,773) (3,902,506)
Properties and equipment, net $ 5,141,404 $ 4,925,711