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Property and equipment - Company property and equipment (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]    
Evaluated oil and natural gas properties $ 6,432,913 $ 6,070,940
Less accumulated depletion and impairment (4,746,413) (4,657,466)
Evaluated oil and natural gas properties, net 1,686,500 1,413,474
Unevaluated properties not being depleted 156,815 175,865
Property and equipment, net 2,020,526 1,768,385
Midstream service assets    
Property, Plant and Equipment [Line Items]    
Midstream service assets 171,875 171,427
Less accumulated depreciation and impairment (37,048) (33,102)
Property and equipment, net 134,827 138,325
Depreciable other fixed assets    
Property, Plant and Equipment [Line Items]    
Depreciable other fixed assets 49,304 48,957
Less accumulated depreciation and impairment (25,179) (23,150)
Property and equipment, net 24,125 25,807
Land    
Property, Plant and Equipment [Line Items]    
Depreciable other fixed assets $ 18,259 $ 14,914