Disclosures about oil and natural gas activities (unaudited) (Prices Used in Determining Future Net Revenues) (Details)	12 Months Ended
	Dec. 31, 2019 $ / bbl $ / Mcf	Dec. 31, 2018 $ / bbl $ / Mcf	Dec. 31, 2017 $ / bbl $ / Mcf
Oil
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price before field differentials used in determining future net revenues	55.69	65.56	51.34
Natural gas
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price before field differentials used in determining future net revenues | $ / Mcf	2.58	3.10	2.98
Natural gas liquids
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Price before field differentials used in determining future net revenues	16.21	25.56	24.17