Business exit costs (Details 6) - Discontinued operations, held-for-sale or disposed by sale [Member] - USD ($)	3 Months Ended	12 Months Ended		42 Months Ended
	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2018
Other restructuring [Member] | Refining [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs		$ 0
Other restructuring [Member] | Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs		$ 0		$ 10,500,000
Contract termination [Member] | Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 3,200,000		$ 900,000