Impairment fair value, ceiling test and transactions costs (Details 7) - USD ($)	3 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value impairment	$ 900,000	$ (1,400,000)	$ (1,600,000)	$ 356,100,000	$ 400,000,000.0
Fair value impairment after tax	600,000	$ (900,000)	$ (1,000,000.0)	$ 224,400,000	$ 252,000,000.0
Impairment of oil and gas properties						$ 500,400,000
Impairment of oil and gas properties, after tax						$ 315,300,000
Refining [Member] | Discontinued operations, held-for-sale or disposed by sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value impairment	251,900,000
Fair value impairment after tax	$ 156,700,000