Business exit costs (Details 8) - Exploration and production [Member] - Discontinued operations, held-for-sale or disposed by sale [Member] - USD ($)	3 Months Ended			12 Months Ended		30 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Other restructuring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs		$ 300,000		$ 5,600,000	$ 2,500,000	$ 10,500,000
Contract termination [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business exit costs	$ 3,200,000		$ 3,300,000	$ 900,000