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Properties and Equipment, Net - Components of Net Property and Equipment (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 4,932,512 $ 7,437,604
Unproved oil and gas properties 190,474 260,543
Gathering and pipeline systems 1,569 187,846
Land, building and other equipment 82,670 84,462
Properties and equipment, gross, total 5,207,225 7,970,455
Accumulated depreciation, depletion and amortization (2,135,021) (3,720,330)
Properties and equipment, net $ 3,072,204 $ 4,250,125