Property and equipment - Other property and equipment information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) $ / Boe	Jun. 30, 2019 USD ($) $ / Boe	Jun. 30, 2020 USD ($) $ / Boe	Jun. 30, 2019 USD ($) $ / Boe
Property, Plant and Equipment [Abstract]
Capitalized employee-related costs	$ 4,092	$ 3,430	$ 8,597	$ 10,112
Depletion expense of evaluated oil and natural gas properties	$ 63,305	$ 61,938	$ 121,057	$ 121,308
Depletion expense (USD per BOE) | $ / Boe	7.39	8.27	7.36	8.51