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Property and equipment - Company property and equipment (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Property, Plant and Equipment [Line Items]    
Evaluated oil and natural gas properties $ 7,953,141 $ 7,874,932
Less accumulated depletion and impairment (6,852,688) (6,817,949)
Evaluated oil and natural gas properties, net 1,100,453 1,056,983
Unevaluated oil and natural gas properties not being depleted 60,260 70,020
Midstream service assets, net 111,083 112,697
Depreciable other fixed assets, net 31,576 32,011
Property and equipment, net 1,303,372 1,271,711
Midstream service assets    
Property, Plant and Equipment [Line Items]    
Midstream service assets 182,405 181,718
Less accumulated depreciation and impairment (71,322) (69,021)
Depreciable other fixed assets    
Property, Plant and Equipment [Line Items]    
Less accumulated depreciation and impairment (24,937) (24,344)
Depreciable other fixed assets 37,612 37,454
Depreciable other fixed assets, net 12,675 13,110
Land    
Property, Plant and Equipment [Line Items]    
Depreciable other fixed assets, net $ 18,901 $ 18,901