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Properties and Equipment, Net - Components of net property and equipment (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 7,437,604 $ 8,821,146
Unproved oil and gas properties 260,543 390,434
Gathering and pipeline systems 187,846 243,672
Land, building and other equipment 84,462 117,848
Properties and equipment, gross, total 7,970,455 9,573,100
Accumulated depreciation, depletion and amortization (3,720,330) (4,596,221)
Properties and equipment, net $ 4,250,125 $ 4,976,879