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Property and equipment - Costs Incurred in the Acquisition, Exploration and Development of Oil and Natural Gas Properties (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Property acquisition costs:        
Evaluated $ 0 $ 0 $ 0 $ 7,586
Unevaluated 0 912 0 16,468
Exploration costs 15,313 3,374 19,270 10,084
Development costs 87,509 72,567 152,001 218,725
Total oil and natural gas properties incurred capital expenditures $ 102,822 $ 76,853 $ 171,271 $ 252,863