Disclosures about oil and natural gas activities (unaudited) (Changes in Standardized Measure of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]
Beginning of year	$ 686,366	$ 497,873	$ 528,781
Sale of oil and natural gas produced, net of production costs	(170,255)	(175,199)	(175,246)
Net changes in prices and production costs	(332,546)	95,430	125,795
Extensions and discoveries	114,199	192,105	136,887
Improved recoveries	0	0	0
Changes in future development costs	116,677	(2,424)	(4,879)
Development costs incurred during the period that reduced future development costs	38,270	6,277	37,912
Revisions of previous quantity estimates	(8,152)	79,192	68,428
Purchases and sales of reserves in place, net	0	(45,222)	(238,445)
Accretion of discount	58,668	36,386	24,267
Net change in income taxes	0	0	0
Changes in production rates and other	10,976	1,948	(5,627)
End of year	$ 514,203	$ 686,366	$ 497,873