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Properties and Equipment, Net - Components of net property and equipment (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 8,821,146 $ 7,984,979
Unproved oil and gas properties 390,434 492,208
Gathering and pipeline systems 243,672 241,272
Land, building and other equipment 117,848 109,758
Properties and equipment, gross, total 9,573,100 8,828,217
Accumulated depreciation, depletion and amortization (4,596,221) (3,902,506)
Properties and equipment, Net $ 4,976,879 $ 4,925,711